5 INTERNET-DRAFT RTFM, Inc.
6 <draft-ietf-tls-rfc2246-bis-10.txt> April 2005 (Expires October 2005)
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35 Copyright (C) The Internet Society (1999-2004). All Rights Reserved.
39 This document specifies Version 1.1 of the Transport Layer Security
40 (TLS) protocol. The TLS protocol provides communications security
41 over the Internet. The protocol allows client/server applications to
42 communicate in a way that is designed to prevent eavesdropping,
43 tampering, or message forgery.
48 5 1.1 Requirements Terminology
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56 7 3. Goals of this document
57 7 4. Presentation language
58 8 4.1. Basic block size
63 10 4.6. Constructed types
65 12 4.7. Cryptographic attributes
67 14 5. HMAC and the pseudorandom function
68 14 6. The TLS Record Protocol
69 16 6.1. Connection states
71 19 6.2.1. Fragmentation
72 19 6.2.2. Record compression and decompression
73 20 6.2.3. Record payload protection
74 21 6.2.3.1. Null or standard stream cipher
75 22 6.2.3.2. CBC block cipher
76 22 6.3. Key calculation
77 25 7. The TLS Handshaking Protocols
78 26 7.1. Change cipher spec protocol
79 27 7.2. Alert protocol
80 27 7.2.1. Closure alerts
81 28 7.2.2. Error alerts
82 29 7.3. Handshake Protocol overview
83 32 7.4. Handshake protocol
84 36 7.4.1. Hello messages
85 37 7.4.1.1. Hello request
86 37 7.4.1.2. Client hello
87 38 7.4.1.3. Server hello
88 40 7.4.2. Server certificate
89 41 7.4.3. Server key exchange message
90 43 7.4.4. Certificate request
91 45 7.4.5. Server hello done
92 46 7.4.6. Client certificate
93 47 7.4.7. Client key exchange message
94 47 7.4.7.1. RSA encrypted premaster secret message
95 48 7.4.7.2. Client Diffie-Hellman public value
96 50 7.4.8. Certificate verify
98 51 8. Cryptographic computations
99 52 8.1. Computing the master secret
101 54 8.1.2. Diffie-Hellman
102 54 9. Mandatory Cipher Suites
103 54 A. Protocol constant values
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111 56 A.2. Change cipher specs message
112 57 A.3. Alert messages
113 57 A.4. Handshake protocol
114 58 A.4.1. Hello messages
115 58 A.4.2. Server authentication and key exchange messages
116 59 A.4.3. Client authentication and key exchange messages
117 60 A.4.4. Handshake finalization message
118 61 A.5. The CipherSuite
119 61 A.6. The Security Parameters
121 66 C. CipherSuite definitions
122 70 D. Implementation Notes
123 72 D.1 Random Number Generation and Seeding
124 72 D.2 Certificates and authentication
126 72 E. Backward Compatibility With SSL
127 73 E.1. Version 2 client hello
128 74 E.2. Avoiding man-in-the-middle version rollback
129 75 F. Security analysis
130 77 F.1. Handshake protocol
131 77 F.1.1. Authentication and key exchange
132 77 F.1.1.1. Anonymous key exchange
133 77 F.1.1.2. RSA key exchange and authentication
134 78 F.1.1.3. Diffie-Hellman key exchange with authentication
135 79 F.1.2. Version rollback attacks
136 79 F.1.3. Detecting attacks against the handshake protocol
137 80 F.1.4. Resuming sessions
138 80 F.1.5. MD5 and SHA
139 81 F.2. Protecting application data
141 81 F.4 Security of Composite Cipher Modes
142 82 F.5 Denial of Service
149 03-Dec-04 ekr@rtfm.com
150 * Removed export cipher suites
152 26-Oct-04 ekr@rtfm.com
153 * Numerous cleanups from Last Call comments
155 10-Aug-04 ekr@rtfm.com
156 * Added clarifying material about interleaved application data.
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164 27-Jul-04 ekr@rtfm.com
165 * Premature closes no longer cause a session to be nonresumable.
166 Response to WG consensus.
168 * Added IANA considerations and registry for cipher suites
169 and ClientCertificateTypes
171 26-Jun-03 ekr@rtfm.com
172 * Incorporated Last Call comments from Franke Marcus, Jack Lloyd,
173 Brad Wetmore, and others.
175 22-Apr-03 ekr@rtfm.com
176 * coverage of the Vaudenay, Boneh-Brumley, and KPR attacks
177 * cleaned up IV text a bit.
178 * Added discussion of Denial of Service attacks.
180 11-Feb-02 ekr@rtfm.com
181 * Clarified the behavior of empty certificate lists [Nelson Bolyard]
182 * Added text explaining the security implications of authenticate
184 * Cleaned up the explicit IV text.
185 * Added some more acknowledgement names
187 02-Nov-02 ekr@rtfm.com
188 * Changed this to be TLS 1.1.
189 * Added fixes for the Rogaway and Vaudenay CBC attacks
190 * Separated references into normative and informative
192 01-Mar-02 ekr@rtfm.com
193 * Tightened up the language in F.1.1.2 [Peter Watkins]
194 * Fixed smart quotes [Bodo Moeller]
195 * Changed handling of padding errors to prevent CBC-based attack
197 * Fixed certificate_list spec in the appendix [Aman Sawrup]
198 * Fixed a bug in the V2 definitions [Aman Sawrup]
199 * Fixed S 7.2.1 to point out that you don't need a close notify
200 if you just sent some other fatal alert [Andreas Sterbenz]
201 * Marked alert 41 reserved [Andreas Sterbenz]
202 * Changed S 7.4.2 to point out that 512-bit keys cannot be used for
203 signing [Andreas Sterbenz]
204 * Added reserved client key types from SSLv3 [Andreas Sterbenz]
205 * Changed EXPORT40 to "40-bit EXPORT" in S 9 [Andreas Sterbenz]
206 * Removed RSA patent statement [Andreas Sterbenz]
207 * Removed references to BSAFE and RSAREF [Andreas Sterbenz]
209 14-Feb-02 ekr@rtfm.com
210 * Re-converted to I-D from RFC
211 * Made RSA/3DES the mandatory cipher suite.
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218 * Added discussion of the EncryptedPMS encoding and PMS version number
220 * Removed the requirement in 7.4.1.3 that the Server random must be
221 different from the Client random, since these are randomly generated
222 and we don't expect servers to reject Server random values which
223 coincidentally are the same as the Client random.
224 * Replaced may/should/must with MAY/SHOULD/MUST where appropriate.
225 In many cases, shoulds became MUSTs, where I believed that was the
226 actual sense of the text. Added an RFC 2119 bulletin.
227 * Clarified the meaning of "empty certificate" message. [Peter Gutmann]
228 * Redid the CertificateRequest grammar to allow no distinguished names.
230 * Removed the reference to requiring the master secret to generate
231 the CertificateVerify in F.1.1 [Bodo Moeller]
232 * Deprecated EXPORT40.
233 * Fixed a bunch of errors in the SSLv2 backward compatible client hello.
237 The primary goal of the TLS Protocol is to provide privacy and data
238 integrity between two communicating applications. The protocol is
239 composed of two layers: the TLS Record Protocol and the TLS Handshake
240 Protocol. At the lowest level, layered on top of some reliable
241 transport protocol (e.g., TCP[TCP]), is the TLS Record Protocol. The
242 TLS Record Protocol provides connection security that has two basic
245 - The connection is private. Symmetric cryptography is used for
246 data encryption (e.g., DES [DES], RC4 [RC4], etc.). The keys for
247 this symmetric encryption are generated uniquely for each
248 connection and are based on a secret negotiated by another
249 protocol (such as the TLS Handshake Protocol). The Record
250 Protocol can also be used without encryption.
252 - The connection is reliable. Message transport includes a message
253 integrity check using a keyed MAC. Secure hash functions (e.g.,
254 SHA, MD5, etc.) are used for MAC computations. The Record
255 Protocol can operate without a MAC, but is generally only used in
256 this mode while another protocol is using the Record Protocol as
257 a transport for negotiating security parameters.
259 The TLS Record Protocol is used for encapsulation of various higher
260 level protocols. One such encapsulated protocol, the TLS Handshake
261 Protocol, allows the server and client to authenticate each other and
262 to negotiate an encryption algorithm and cryptographic keys before
263 the application protocol transmits or receives its first byte of
264 data. The TLS Handshake Protocol provides connection security that
265 has three basic properties:
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272 - The peer's identity can be authenticated using asymmetric, or
273 public key, cryptography (e.g., RSA [RSA], DSS [DSS], etc.). This
274 authentication can be made optional, but is generally required
275 for at least one of the peers.
277 - The negotiation of a shared secret is secure: the negotiated
278 secret is unavailable to eavesdroppers, and for any authenticated
279 connection the secret cannot be obtained, even by an attacker who
280 can place himself in the middle of the connection.
282 - The negotiation is reliable: no attacker can modify the
283 negotiation communication without being detected by the parties
284 to the communication.
286 One advantage of TLS is that it is application protocol independent.
287 Higher level protocols can layer on top of the TLS Protocol
288 transparently. The TLS standard, however, does not specify how
289 protocols add security with TLS; the decisions on how to initiate TLS
290 handshaking and how to interpret the authentication certificates
291 exchanged are left up to the judgment of the designers and
292 implementors of protocols which run on top of TLS.
294 This document is a revision of the TLS 1.0 [TLS1.0] protocol which
295 contains some small security improvements, clarifications, and
296 editorial improvements. The major changes are:
298 - The implicit Initialization Vector (IV) is replaced with an
300 IV to protect against CBC attacks [CBCATT].
302 - Handling of padding errors is changed to use the bad_record_mac
303 alert rather than the decryption_failed alert to protect against
306 - IANA registries are defined for protocol parameters.
308 - Premature closes no longer cause a session to be nonresumable.
310 - Additional informational notes were added for various new attacks
313 In addition, a number of minor clarifications and editorial
314 improvements were made.
318 1.1 Requirements Terminology
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326 Keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT" and
327 "MAY" that appear in this document are to be interpreted as described
332 The goals of TLS Protocol, in order of their priority, are:
334 1. Cryptographic security: TLS should be used to establish a secure
335 connection between two parties.
337 2. Interoperability: Independent programmers should be able to
338 develop applications utilizing TLS that will then be able to
339 successfully exchange cryptographic parameters without knowledge
340 of one another's code.
342 3. Extensibility: TLS seeks to provide a framework into which new
343 public key and bulk encryption methods can be incorporated as
344 necessary. This will also accomplish two sub-goals: to prevent
345 the need to create a new protocol (and risking the introduction
346 of possible new weaknesses) and to avoid the need to implement an
347 entire new security library.
349 4. Relative efficiency: Cryptographic operations tend to be highly
350 CPU intensive, particularly public key operations. For this
351 reason, the TLS protocol has incorporated an optional session
352 caching scheme to reduce the number of connections that need to
353 be established from scratch. Additionally, care has been taken to
354 reduce network activity.
356 3. Goals of this document
358 This document and the TLS protocol itself are based on the SSL 3.0
359 Protocol Specification as published by Netscape. The differences
360 between this protocol and SSL 3.0 are not dramatic, but they are
361 significant enough that TLS 1.1, TLS 1.0, and SSL 3.0 do not
362 interoperate (although each protocol incorporates a mechanism by
363 which an implementation can back down prior versions. This document
364 is intended primarily for readers who will be implementing the
365 protocol and those doing cryptographic analysis of it. The
366 specification has been written with this in mind, and it is intended
367 to reflect the needs of those two groups. For that reason, many of
368 the algorithm-dependent data structures and rules are included in the
369 body of the text (as opposed to in an appendix), providing easier
372 This document is not intended to supply any details of service
373 definition nor interface definition, although it does cover select
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380 areas of policy as they are required for the maintenance of solid
383 4. Presentation language
385 This document deals with the formatting of data in an external
386 representation. The following very basic and somewhat casually
387 defined presentation syntax will be used. The syntax draws from
388 several sources in its structure. Although it resembles the
389 programming language "C" in its syntax and XDR [XDR] in both its
390 syntax and intent, it would be risky to draw too many parallels. The
391 purpose of this presentation language is to document TLS only, not to
392 have general application beyond that particular goal.
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434 4.1. Basic block size
436 The representation of all data items is explicitly specified. The
437 basic data block size is one byte (i.e. 8 bits). Multiple byte data
438 items are concatenations of bytes, from left to right, from top to
439 bottom. From the bytestream a multi-byte item (a numeric in the
440 example) is formed (using C notation) by:
442 value = (byte[0] << 8*(n-1)) | (byte[1] << 8*(n-2)) |
445 This byte ordering for multi-byte values is the commonplace network
446 byte order or big endian format.
450 Comments begin with "/*" and end with "*/".
452 Optional components are denoted by enclosing them in "[[ ]]" double
455 Single byte entities containing uninterpreted data are of type
460 A vector (single dimensioned array) is a stream of homogeneous data
461 elements. The size of the vector may be specified at documentation
462 time or left unspecified until runtime. In either case the length
463 declares the number of bytes, not the number of elements, in the
464 vector. The syntax for specifying a new type T' that is a fixed
465 length vector of type T is
469 Here T' occupies n bytes in the data stream, where n is a multiple of
470 the size of T. The length of the vector is not included in the
473 In the following example, Datum is defined to be three consecutive
474 bytes that the protocol does not interpret, while Data is three
475 consecutive Datum, consuming a total of nine bytes.
477 opaque Datum[3]; /* three uninterpreted bytes */
478 Datum Data[9]; /* 3 consecutive 3 byte vectors */
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488 Variable length vectors are defined by specifying a subrange of legal
489 lengths, inclusively, using the notation <floor..ceiling>. When
490 encoded, the actual length precedes the vector's contents in the byte
491 stream. The length will be in the form of a number consuming as many
492 bytes as required to hold the vector's specified maximum (ceiling)
493 length. A variable length vector with an actual length field of zero
494 is referred to as an empty vector.
496 T T'<floor..ceiling>;
498 In the following example, mandatory is a vector that must contain
499 between 300 and 400 bytes of type opaque. It can never be empty. The
500 actual length field consumes two bytes, a uint16, sufficient to
501 represent the value 400 (see Section 4.4). On the other hand, longer
502 can represent up to 800 bytes of data, or 400 uint16 elements, and it
503 may be empty. Its encoding will include a two byte actual length
504 field prepended to the vector. The length of an encoded vector must
505 be an even multiple of the length of a single element (for example, a
506 17 byte vector of uint16 would be illegal).
508 opaque mandatory<300..400>;
509 /* length field is 2 bytes, cannot be empty */
510 uint16 longer<0..800>;
511 /* zero to 400 16-bit unsigned integers */
515 The basic numeric data type is an unsigned byte (uint8). All larger
516 numeric data types are formed from fixed length series of bytes
517 concatenated as described in Section 4.1 and are also unsigned. The
518 following numeric types are predefined.
525 All values, here and elsewhere in the specification, are stored in
526 "network" or "big-endian" order; the uint32 represented by the hex
527 bytes 01 02 03 04 is equivalent to the decimal value 16909060.
531 An additional sparse data type is available called enum. A field of
532 type enum can only assume the values declared in the definition.
533 Each definition is a different type. Only enumerateds of the same
534 type may be assigned or compared. Every element of an enumerated must
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542 be assigned a value, as demonstrated in the following example. Since
543 the elements of the enumerated are not ordered, they can be assigned
544 any unique value, in any order.
546 enum { e1(v1), e2(v2), ... , en(vn) [[, (n)]] } Te;
548 Enumerateds occupy as much space in the byte stream as would its
549 maximal defined ordinal value. The following definition would cause
550 one byte to be used to carry fields of type Color.
552 enum { red(3), blue(5), white(7) } Color;
554 One may optionally specify a value without its associated tag to
555 force the width definition without defining a superfluous element.
556 In the following example, Taste will consume two bytes in the data
557 stream but can only assume the values 1, 2 or 4.
559 enum { sweet(1), sour(2), bitter(4), (32000) } Taste;
561 The names of the elements of an enumeration are scoped within the
562 defined type. In the first example, a fully qualified reference to
563 the second element of the enumeration would be Color.blue. Such
564 qualification is not required if the target of the assignment is well
567 Color color = Color.blue; /* overspecified, legal */
568 Color color = blue; /* correct, type implicit */
570 For enumerateds that are never converted to external representation,
571 the numerical information may be omitted.
573 enum { low, medium, high } Amount;
575 4.6. Constructed types
577 Structure types may be constructed from primitive types for
578 convenience. Each specification declares a new, unique type. The
579 syntax for definition is much like that of C.
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596 The fields within a structure may be qualified using the type's name
597 using a syntax much like that available for enumerateds. For example,
598 T.f2 refers to the second field of the previous declaration.
599 Structure definitions may be embedded.
603 Defined structures may have variants based on some knowledge that is
604 available within the environment. The selector must be an enumerated
605 type that defines the possible variants the structure defines. There
606 must be a case arm for every element of the enumeration declared in
607 the select. The body of the variant structure may be given a label
608 for reference. The mechanism by which the variant is selected at
609 runtime is not prescribed by the presentation language.
626 enum { apple, orange } VariantTag;
629 opaque string<0..10>; /* variable length */
633 opaque string[10]; /* fixed length */
636 select (VariantTag) { /* value of selector is implicit */
637 case apple: V1; /* VariantBody, tag = apple */
638 case orange: V2; /* VariantBody, tag = orange */
639 } variant_body; /* optional label on variant */
642 Variant structures may be qualified (narrowed) by specifying a value
643 for the selector prior to the type. For example, a
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652 is a narrowed type of a VariantRecord containing a variant_body of
655 4.7. Cryptographic attributes
657 The four cryptographic operations digital signing, stream cipher
658 encryption, block cipher encryption, and public key encryption are
659 designated digitally-signed, stream-ciphered, block-ciphered, and
660 public-key-encrypted, respectively. A field's cryptographic
661 processing is specified by prepending an appropriate key word
662 designation before the field's type specification. Cryptographic keys
663 are implied by the current session state (see Section 6.1).
665 In digital signing, one-way hash functions are used as input for a
666 signing algorithm. A digitally-signed element is encoded as an opaque
667 vector <0..2^16-1>, where the length is specified by the signing
670 In RSA signing, a 36-byte structure of two hashes (one SHA and one
671 MD5) is signed (encrypted with the private key). It is encoded with
672 PKCS #1 block type 0 or type 1 as described in [PKCS1].
674 In DSS, the 20 bytes of the SHA hash are run directly through the
675 Digital Signing Algorithm with no additional hashing. This produces
676 two values, r and s. The DSS signature is an opaque vector, as above,
677 the contents of which are the DER encoding of:
679 Dss-Sig-Value ::= SEQUENCE {
684 In stream cipher encryption, the plaintext is exclusive-ORed with an
685 identical amount of output generated from a cryptographically-secure
686 keyed pseudorandom number generator.
688 In block cipher encryption, every block of plaintext encrypts to a
689 block of ciphertext. All block cipher encryption is done in CBC
690 (Cipher Block Chaining) mode, and all items which are block-ciphered
691 will be an exact multiple of the cipher block length.
693 In public key encryption, a public key algorithm is used to encrypt
694 data in such a way that it can be decrypted only with the matching
695 private key. A public-key-encrypted element is encoded as an opaque
696 vector <0..2^16-1>, where the length is specified by the signing
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704 An RSA encrypted value is encoded with PKCS #1 block type 2 as
705 described in [PKCS1].
707 In the following example:
709 stream-ciphered struct {
712 digitally-signed opaque hash[20];
715 The contents of hash are used as input for the signing algorithm,
716 then the entire structure is encrypted with a stream cipher. The
717 length of this structure, in bytes would be equal to 2 bytes for
718 field1 and field2, plus two bytes for the length of the signature,
719 plus the length of the output of the signing algorithm. This is known
720 due to the fact that the algorithm and key used for the signing are
721 known prior to encoding or decoding this structure.
725 Typed constants can be defined for purposes of specification by
726 declaring a symbol of the desired type and assigning values to it.
727 Under-specified types (opaque, variable length vectors, and
728 structures that contain opaque) cannot be assigned values. No fields
729 of a multi-element structure or vector may be elided.
738 Example1 ex1 = {1, 4}; /* assigns f1 = 1, f2 = 4 */
740 5. HMAC and the pseudorandom function
742 A number of operations in the TLS record and handshake layer required
743 a keyed MAC; this is a secure digest of some data protected by a
744 secret. Forging the MAC is infeasible without knowledge of the MAC
745 secret. The construction we use for this operation is known as HMAC,
748 HMAC can be used with a variety of different hash algorithms. TLS
749 uses it in the handshake with two different algorithms: MD5 and
750 SHA-1, denoting these as HMAC_MD5(secret, data) and HMAC_SHA(secret,
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758 data). Additional hash algorithms can be defined by cipher suites and
759 used to protect record data, but MD5 and SHA-1 are hard coded into
760 the description of the handshaking for this version of the protocol.
762 In addition, a construction is required to do expansion of secrets
763 into blocks of data for the purposes of key generation or validation.
764 This pseudo-random function (PRF) takes as input a secret, a seed,
765 and an identifying label and produces an output of arbitrary length.
767 In order to make the PRF as secure as possible, it uses two hash
768 algorithms in a way which should guarantee its security if either
769 algorithm remains secure.
771 First, we define a data expansion function, P_hash(secret, data)
772 which uses a single hash function to expand a secret and seed into an
773 arbitrary quantity of output:
775 P_hash(secret, seed) = HMAC_hash(secret, A(1) + seed) +
776 HMAC_hash(secret, A(2) + seed) +
777 HMAC_hash(secret, A(3) + seed) + ...
779 Where + indicates concatenation.
783 A(i) = HMAC_hash(secret, A(i-1))
785 P_hash can be iterated as many times as is necessary to produce the
786 required quantity of data. For example, if P_SHA-1 was being used to
787 create 64 bytes of data, it would have to be iterated 4 times
788 (through A(4)), creating 80 bytes of output data; the last 16 bytes
789 of the final iteration would then be discarded, leaving 64 bytes of
792 TLS's PRF is created by splitting the secret into two halves and
793 using one half to generate data with P_MD5 and the other half to
794 generate data with P_SHA-1, then exclusive-or'ing the outputs of
795 these two expansion functions together.
797 S1 and S2 are the two halves of the secret and each is the same
798 length. S1 is taken from the first half of the secret, S2 from the
799 second half. Their length is created by rounding up the length of the
800 overall secret divided by two; thus, if the original secret is an odd
801 number of bytes long, the last byte of S1 will be the same as the
804 L_S = length in bytes of secret;
805 L_S1 = L_S2 = ceil(L_S / 2);
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812 The secret is partitioned into two halves (with the possibility of
813 one shared byte) as described above, S1 taking the first L_S1 bytes
814 and S2 the last L_S2 bytes.
816 The PRF is then defined as the result of mixing the two pseudorandom
817 streams by exclusive-or'ing them together.
819 PRF(secret, label, seed) = P_MD5(S1, label + seed) XOR
820 P_SHA-1(S2, label + seed);
822 The label is an ASCII string. It should be included in the exact form
823 it is given without a length byte or trailing null character. For
824 example, the label "slithy toves" would be processed by hashing the
827 73 6C 69 74 68 79 20 74 6F 76 65 73
829 Note that because MD5 produces 16 byte outputs and SHA-1 produces 20
830 byte outputs, the boundaries of their internal iterations will not be
831 aligned; to generate a 80 byte output will involve P_MD5 being
832 iterated through A(5), while P_SHA-1 will only iterate through A(4).
834 6. The TLS Record Protocol
836 The TLS Record Protocol is a layered protocol. At each layer,
837 messages may include fields for length, description, and content.
838 The Record Protocol takes messages to be transmitted, fragments the
839 data into manageable blocks, optionally compresses the data, applies
840 a MAC, encrypts, and transmits the result. Received data is
841 decrypted, verified, decompressed, and reassembled, then delivered to
842 higher level clients.
844 Four record protocol clients are described in this document: the
845 handshake protocol, the alert protocol, the change cipher spec
846 protocol, and the application data protocol. In order to allow
847 extension of the TLS protocol, additional record types can be
848 supported by the record protocol. Any new record types SHOULD
849 allocate type values immediately beyond the ContentType values for
850 the four record types described here (see Appendix A.1). All such
851 values must be defined by RFC 2434 Standards Action. Section 11 for
852 IANA Considerations for ContentType values.
854 If a TLS implementation receives a record type it does not
855 understand, it SHOULD just ignore it. Any protocol designed for use
856 over TLS MUST be carefully designed to deal with all possible attacks
857 against it. Note that because the type and length of a record are
858 not protected by encryption, care SHOULD be taken to minimize the
859 value of traffic analysis of these values.
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866 6.1. Connection states
868 A TLS connection state is the operating environment of the TLS Record
869 Protocol. It specifies a compression algorithm, encryption algorithm,
870 and MAC algorithm. In addition, the parameters for these algorithms
871 are known: the MAC secret and the bulk encryption keys for the
872 connection in both the read and the write directions. Logically,
873 there are always four connection states outstanding: the current read
874 and write states, and the pending read and write states. All records
875 are processed under the current read and write states. The security
876 parameters for the pending states can be set by the TLS Handshake
877 Protocol, and the Change Cipher Spec can selectively make either of
878 the pending states current, in which case the appropriate current
879 state is disposed of and replaced with the pending state; the pending
880 state is then reinitialized to an empty state. It is illegal to make
881 a state which has not been initialized with security parameters a
882 current state. The initial current state always specifies that no
883 encryption, compression, or MAC will be used.
885 The security parameters for a TLS Connection read and write state are
886 set by providing the following values:
889 Whether this entity is considered the "client" or the "server" in
892 bulk encryption algorithm
893 An algorithm to be used for bulk encryption. This specification
894 includes the key size of this algorithm, how much of that key is
895 secret, whether it is a block or stream cipher, the block size of
896 the cipher (if appropriate).
899 An algorithm to be used for message authentication. This
900 specification includes the size of the hash which is returned by
903 compression algorithm
904 An algorithm to be used for data compression. This specification
905 must include all information the algorithm requires to do
909 A 48 byte secret shared between the two peers in the connection.
912 A 32 byte value provided by the client.
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921 A 32 byte value provided by the server.
923 These parameters are defined in the presentation language as:
925 enum { server, client } ConnectionEnd;
927 enum { null, rc4, rc2, des, 3des, des40 } BulkCipherAlgorithm;
929 enum { stream, block } CipherType;
931 enum { null, md5, sha } MACAlgorithm;
933 enum { null(0), (255) } CompressionMethod;
935 /* The algorithms specified in CompressionMethod,
936 BulkCipherAlgorithm, and MACAlgorithm may be added to. */
939 ConnectionEnd entity;
940 BulkCipherAlgorithm bulk_cipher_algorithm;
941 CipherType cipher_type;
943 uint8 key_material_length;
944 MACAlgorithm mac_algorithm;
946 CompressionMethod compression_algorithm;
947 opaque master_secret[48];
948 opaque client_random[32];
949 opaque server_random[32];
950 } SecurityParameters;
952 The record layer will use the security parameters to generate the
953 following four items:
955 client write MAC secret
956 server write MAC secret
960 The client write parameters are used by the server when receiving and
961 processing records and vice-versa. The algorithm used for generating
962 these items from the security parameters is described in section 6.3.
964 Once the security parameters have been set and the keys have been
965 generated, the connection states can be instantiated by making them
966 the current states. These current states MUST be updated for each
967 record processed. Each connection state includes the following
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977 The current state of the compression algorithm.
980 The current state of the encryption algorithm. This will consist
981 of the scheduled key for that connection. For stream ciphers,
982 this will also contain whatever the necessary state information
983 is to allow the stream to continue to encrypt or decrypt data.
986 The MAC secret for this connection as generated above.
989 Each connection state contains a sequence number, which is
990 maintained separately for read and write states. The sequence
991 number MUST be set to zero whenever a connection state is made
992 the active state. Sequence numbers are of type uint64 and may not
993 exceed 2^64-1. Sequence numbers do not wrap. If a TLS
994 implementation would need to wrap a sequence number it must
995 renegotiate instead. A sequence number is incremented after each
996 record: specifically, the first record which is transmitted under
997 a particular connection state MUST use sequence number 0.
1001 The TLS Record Layer receives uninterpreted data from higher layers
1002 in non-empty blocks of arbitrary size.
1004 6.2.1. Fragmentation
1006 The record layer fragments information blocks into TLSPlaintext
1007 records carrying data in chunks of 2^14 bytes or less. Client message
1008 boundaries are not preserved in the record layer (i.e., multiple
1009 client messages of the same ContentType MAY be coalesced into a
1010 single TLSPlaintext record, or a single message MAY be fragmented
1011 across several records).
1019 change_cipher_spec(20), alert(21), handshake(22),
1020 application_data(23), (255)
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1030 ProtocolVersion version;
1032 opaque fragment[TLSPlaintext.length];
1036 The higher level protocol used to process the enclosed fragment.
1039 The version of the protocol being employed. This document
1040 describes TLS Version 1.1, which uses the version { 3, 2 }. The
1041 version value 3.2 is historical: TLS version 1.1 is a minor
1042 modification to the TLS 1.0 protocol, which was itself a minor
1043 modification to the SSL 3.0 protocol, which bears the version
1044 value 3.0. (See Appendix A.1).
1047 The length (in bytes) of the following TLSPlaintext.fragment.
1048 The length should not exceed 2^14.
1051 The application data. This data is transparent and treated as an
1052 independent block to be dealt with by the higher level protocol
1053 specified by the type field.
1055 Note: Data of different TLS Record layer content types MAY be
1056 interleaved. Application data is generally of higher precedence
1057 for transmission than other content types and therefore handshake
1058 records may be held if application data is pending. However,
1059 records MUST be delivered to the network in the same order as
1060 they are protected by the record layer. Recipients MUST receive
1061 and process interleaved application layer traffic during
1062 handshakes subsequent to the first one on a connection.
1064 6.2.2. Record compression and decompression
1066 All records are compressed using the compression algorithm defined in
1067 the current session state. There is always an active compression
1068 algorithm; however, initially it is defined as
1069 CompressionMethod.null. The compression algorithm translates a
1070 TLSPlaintext structure into a TLSCompressed structure. Compression
1071 functions are initialized with default state information whenever a
1072 connection state is made active.
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1082 Compression must be lossless and may not increase the content length
1083 by more than 1024 bytes. If the decompression function encounters a
1084 TLSCompressed.fragment that would decompress to a length in excess of
1085 2^14 bytes, it should report a fatal decompression failure error.
1088 ContentType type; /* same as TLSPlaintext.type */
1089 ProtocolVersion version;/* same as TLSPlaintext.version */
1091 opaque fragment[TLSCompressed.length];
1095 The length (in bytes) of the following TLSCompressed.fragment.
1096 The length should not exceed 2^14 + 1024.
1099 The compressed form of TLSPlaintext.fragment.
1101 Note: A CompressionMethod.null operation is an identity operation; no
1104 Implementation note:
1105 Decompression functions are responsible for ensuring that
1106 messages cannot cause internal buffer overflows.
1108 6.2.3. Record payload protection
1110 The encryption and MAC functions translate a TLSCompressed structure
1111 into a TLSCiphertext. The decryption functions reverse the process.
1112 The MAC of the record also includes a sequence number so that
1113 missing, extra or repeated messages are detectable.
1117 ProtocolVersion version;
1119 select (CipherSpec.cipher_type) {
1120 case stream: GenericStreamCipher;
1121 case block: GenericBlockCipher;
1126 The type field is identical to TLSCompressed.type.
1129 The version field is identical to TLSCompressed.version.
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1137 The length (in bytes) of the following TLSCiphertext.fragment.
1138 The length may not exceed 2^14 + 2048.
1141 The encrypted form of TLSCompressed.fragment, with the MAC.
1143 6.2.3.1. Null or standard stream cipher
1145 Stream ciphers (including BulkCipherAlgorithm.null - see Appendix
1146 A.6) convert TLSCompressed.fragment structures to and from stream
1147 TLSCiphertext.fragment structures.
1149 stream-ciphered struct {
1150 opaque content[TLSCompressed.length];
1151 opaque MAC[CipherSpec.hash_size];
1152 } GenericStreamCipher;
1154 The MAC is generated as:
1156 HMAC_hash(MAC_write_secret, seq_num + TLSCompressed.type +
1157 TLSCompressed.version + TLSCompressed.length +
1158 TLSCompressed.fragment));
1160 where "+" denotes concatenation.
1163 The sequence number for this record.
1166 The hashing algorithm specified by
1167 SecurityParameters.mac_algorithm.
1169 Note that the MAC is computed before encryption. The stream cipher
1170 encrypts the entire block, including the MAC. For stream ciphers that
1171 do not use a synchronization vector (such as RC4), the stream cipher
1172 state from the end of one record is simply used on the subsequent
1173 packet. If the CipherSuite is TLS_NULL_WITH_NULL_NULL, encryption
1174 consists of the identity operation (i.e., the data is not encrypted
1175 and the MAC size is zero implying that no MAC is used).
1176 TLSCiphertext.length is TLSCompressed.length plus
1177 CipherSpec.hash_size.
1179 6.2.3.2. CBC block cipher
1181 For block ciphers (such as RC2 or DES), the encryption and MAC
1182 functions convert TLSCompressed.fragment structures to and from block
1183 TLSCiphertext.fragment structures.
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1190 block-ciphered struct {
1191 opaque IV[CipherSpec.block_length];
1192 opaque content[TLSCompressed.length];
1193 opaque MAC[CipherSpec.hash_size];
1194 uint8 padding[GenericBlockCipher.padding_length];
1195 uint8 padding_length;
1196 } GenericBlockCipher;
1198 The MAC is generated as described in Section 6.2.3.1.
1201 Unlike previous versions of SSL and TLS, TLS 1.1 uses an explicit
1202 IV in order to prevent the attacks described by [CBCATT].
1203 We recommend the following equivalently strong procedures.
1204 For clarity we use the following notation.
1206 IV -- the transmitted value of the IV field in the
1207 GenericBlockCipher structure.
1208 CBC residue -- the last ciphertext block of the previous record
1209 mask -- the actual value which the cipher XORs with the
1210 plaintext prior to encryption of the first cipher block
1213 In prior versions of TLS, there was no IV field and the CBC residue
1214 and mask were one and the same. See Sections 6.1, 6.2.3.2 and 6.3,
1215 of [TLS1.0] for details of TLS 1.0 IV handling.
1217 One of the following two algorithms SHOULD be used to generate the
1220 (1) Generate a cryptographically strong random string R of
1221 length CipherSpec.block_length. Place R
1222 in the IV field. Set the mask to R. Thus, the first
1223 cipher block will be encrypted as E(R XOR Data).
1225 (2) Generate a cryptographically strong random number R of
1226 length CipherSpec.block_length and prepend it to the plaintext
1227 prior to encryption. In
1230 (a) The cipher may use a fixed mask such as zero.
1231 (b) The CBC residue from the previous record may be used
1232 as the mask. This preserves maximum code compatibility
1233 with TLS 1.0 and SSL 3. It also has the advantage that
1234 it does not require the ability to quickly reset the IV,
1235 which is known to be a problem on some systems.
1237 In either case, the data (R || data) is fed into the
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1244 encryption process. The first cipher block (containing
1245 E(mask XOR R) is placed in the IV field. The first
1246 block of content contains E(IV XOR data)
1248 The following alternative procedure MAY be used: However, it has
1249 not been demonstrated to be equivalently cryptographically strong
1250 to the above procedures. The sender prepends a fixed block F to
1251 the plaintext (or alternatively a block generated with a weak
1252 PRNG). He then encrypts as in (2) above, using the CBC residue
1253 from the previous block as the mask for the prepended block. Note
1254 that in this case the mask for the first record transmitted by
1255 the application (the Finished) MUST be generated using a
1256 cryptographically strong PRNG.
1258 The decryption operation for all three alternatives is the same.
1259 The receiver decrypts the entire GenericBlockCipher structure and
1260 then discards the first cipher block, corresponding to the IV
1264 Padding that is added to force the length of the plaintext to be
1265 an integral multiple of the block cipher's block length. The
1266 padding MAY be any length up to 255 bytes long, as long as it
1267 results in the TLSCiphertext.length being an integral multiple of
1268 the block length. Lengths longer than necessary might be
1269 desirable to frustrate attacks on a protocol based on analysis of
1270 the lengths of exchanged messages. Each uint8 in the padding data
1271 vector MUST be filled with the padding length value. The receiver
1272 MUST check this padding and SHOULD use the bad_record_mac alert
1273 to indicate padding errors.
1276 The padding length MUST be such that the total size of the
1277 GenericBlockCipher structure is a multiple of the cipher's block
1278 length. Legal values range from zero to 255, inclusive. This
1279 length specifies the length of the padding field exclusive of the
1280 padding_length field itself.
1282 The encrypted data length (TLSCiphertext.length) is one more than the
1283 sum of TLSCompressed.length, CipherSpec.hash_size, and
1286 Example: If the block length is 8 bytes, the content length
1287 (TLSCompressed.length) is 61 bytes, and the MAC length is 20
1288 bytes, the length before padding is 82 bytes (this does not
1289 include the IV, which may or may not be encrypted, as
1290 discussed above). Thus, the padding length modulo 8 must be
1291 equal to 6 in order to make the total length an even multiple
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1298 of 8 bytes (the block length). The padding length can be 6,
1299 14, 22, and so on, through 254. If the padding length were the
1300 minimum necessary, 6, the padding would be 6 bytes, each
1301 containing the value 6. Thus, the last 8 octets of the
1302 GenericBlockCipher before block encryption would be xx 06 06
1303 06 06 06 06 06, where xx is the last octet of the MAC.
1305 Note: With block ciphers in CBC mode (Cipher Block Chaining),
1306 it is critical that the entire plaintext of the record be known
1307 before any ciphertext is transmitted. Otherwise it is possible
1308 for the attacker to mount the attack described in [CBCATT].
1310 Implementation Note: Canvel et. al. [CBCTIME] have demonstrated a
1311 timing attack on CBC padding based on the time required to
1312 compute the MAC. In order to defend against this attack,
1313 implementations MUST ensure that record processing time is
1314 essentially the same whether or not the padding is correct. In
1315 general, the best way to to do this is to compute the MAC even if
1316 the padding is incorrect, and only then reject the packet. For
1317 instance, if the pad appears to be incorrect the implementation
1318 might assume a zero-length pad and then compute the MAC. This
1319 leaves a small timing channel, since MAC performance depends to
1320 some extent on the size of the data fragment, but it is not
1321 believed to be large enough to be exploitable due to the large
1322 block size of existing MACs and the small size of the timing
1325 6.3. Key calculation
1327 The Record Protocol requires an algorithm to generate keys, and MAC
1328 secrets from the security parameters provided by the handshake
1331 The master secret is hashed into a sequence of secure bytes, which
1332 are assigned to the MAC secrets and keys required by the current
1333 connection state (see Appendix A.6). CipherSpecs require a client
1334 write MAC secret, a server write MAC secret, a client write key, and
1335 a server write key, which are generated from the master secret in
1336 that order. Unused values are empty.
1338 When generating keys and MAC secrets, the master secret is used as an
1341 To generate the key material, compute
1343 key_block = PRF(SecurityParameters.master_secret,
1345 SecurityParameters.server_random +
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1352 SecurityParameters.client_random);
1354 until enough output has been generated. Then the key_block is
1355 partitioned as follows:
1357 client_write_MAC_secret[SecurityParameters.hash_size]
1358 server_write_MAC_secret[SecurityParameters.hash_size]
1359 client_write_key[SecurityParameters.key_material_length]
1360 server_write_key[SecurityParameters.key_material_length]
1363 Implementation note:
1364 The currently defined which requires the most material is
1365 AES_256_CBC_SHA, defined in [TLSAES]. It requires 2 x 32 byte
1366 keys and 2 x 20 byte MAC secrets, for a total 104 bytes of key
1369 7. The TLS Handshaking Protocols
1371 TLS has three subprotocols which are used to allow peers to agree
1372 upon security parameters for the record layer, authenticate
1373 themselves, instantiate negotiated security parameters, and
1374 report error conditions to each other.
1376 The Handshake Protocol is responsible for negotiating a session,
1377 which consists of the following items:
1380 An arbitrary byte sequence chosen by the server to identify an
1381 active or resumable session state.
1384 X509v3 [X509] certificate of the peer. This element of the state
1388 The algorithm used to compress data prior to encryption.
1391 Specifies the bulk data encryption algorithm (such as null, DES,
1392 etc.) and a MAC algorithm (such as MD5 or SHA). It also defines
1393 cryptographic attributes such as the hash_size. (See Appendix A.6
1394 for formal definition)
1397 48-byte secret shared between the client and server.
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1406 A flag indicating whether the session can be used to initiate new
1409 These items are then used to create security parameters for use by
1410 the Record Layer when protecting application data. Many connections
1411 can be instantiated using the same session through the resumption
1412 feature of the TLS Handshake Protocol.
1414 7.1. Change cipher spec protocol
1416 The change cipher spec protocol exists to signal transitions in
1417 ciphering strategies. The protocol consists of a single message,
1418 which is encrypted and compressed under the current (not the pending)
1419 connection state. The message consists of a single byte of value 1.
1422 enum { change_cipher_spec(1), (255) } type;
1425 The change cipher spec message is sent by both the client and server
1426 to notify the receiving party that subsequent records will be
1427 protected under the newly negotiated CipherSpec and keys. Reception
1428 of this message causes the receiver to instruct the Record Layer to
1429 immediately copy the read pending state into the read current state.
1430 Immediately after sending this message, the sender MUST instruct the
1431 record layer to make the write pending state the write active state.
1432 (See section 6.1.) The change cipher spec message is sent during the
1433 handshake after the security parameters have been agreed upon, but
1434 before the verifying finished message is sent (see section 7.4.9).
1436 Note: if a rehandshake occurs while data is flowing on a connection,
1437 the communicating parties may continue to send data using the old
1438 CipherSpec However, once the ChangeCipherSpec has been sent, the new
1439 CipherSpec MUST be used. The first side to send the ChangeCipherSpec
1440 does not know that the other side has finished computing the new
1441 keying material (e.g. if it has to perform a time consuming public
1442 key operation). Thus, a small window of time during which the
1443 recipient must buffer the data MAY exist. In practice, with modern
1444 machines this interval is likely to be fairly short.
1448 One of the content types supported by the TLS Record layer is the
1449 alert type. Alert messages convey the severity of the message and a
1450 description of the alert. Alert messages with a level of fatal result
1451 in the immediate termination of the connection. In this case, other
1452 connections corresponding to the session may continue, but the
1453 session identifier MUST be invalidated, preventing the failed session
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1460 from being used to establish new connections. Like other messages,
1461 alert messages are encrypted and compressed, as specified by the
1462 current connection state.
1464 enum { warning(1), fatal(2), (255) } AlertLevel;
1468 unexpected_message(10),
1470 decryption_failed(21),
1471 record_overflow(22),
1472 decompression_failure(30),
1473 handshake_failure(40),
1474 no_certificate_RESERVED (41),
1475 bad_certificate(42),
1476 unsupported_certificate(43),
1477 certificate_revoked(44),
1478 certificate_expired(45),
1479 certificate_unknown(46),
1480 illegal_parameter(47),
1485 export_restriction_RESERVED(60),
1486 protocol_version(70),
1487 insufficient_security(71),
1490 no_renegotiation(100),
1496 AlertDescription description;
1499 7.2.1. Closure alerts
1501 The client and the server must share knowledge that the connection is
1502 ending in order to avoid a truncation attack. Either party may
1503 initiate the exchange of closing messages.
1506 This message notifies the recipient that the sender will not send
1507 any more messages on this connection. Note that as of TLS 1.1,
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1514 failure to properly close a connection no longer requires that a
1515 session not be resumed. This is a change from TLS 1.0 to conform
1516 with widespread implementation practice.
1518 Either party may initiate a close by sending a close_notify alert.
1519 Any data received after a closure alert is ignored.
1521 Unless some other fatal alert has been transmitted, each party is
1522 required to send a close_notify alert before closing the write side
1523 of the connection. The other party MUST respond with a close_notify
1524 alert of its own and close down the connection immediately,
1525 discarding any pending writes. It is not required for the initiator
1526 of the close to wait for the responding close_notify alert before
1527 closing the read side of the connection.
1529 If the application protocol using TLS provides that any data may be
1530 carried over the underlying transport after the TLS connection is
1531 closed, the TLS implementation must receive the responding
1532 close_notify alert before indicating to the application layer that
1533 the TLS connection has ended. If the application protocol will not
1534 transfer any additional data, but will only close the underlying
1535 transport connection, then the implementation MAY choose to close the
1536 transport without waiting for the responding close_notify. No part of
1537 this standard should be taken to dictate the manner in which a usage
1538 profile for TLS manages its data transport, including when
1539 connections are opened or closed.
1541 NB: It is assumed that closing a connection reliably delivers
1542 pending data before destroying the transport.
1546 Error handling in the TLS Handshake protocol is very simple. When an
1547 error is detected, the detecting party sends a message to the other
1548 party. Upon transmission or receipt of an fatal alert message, both
1549 parties immediately close the connection. Servers and clients MUST
1550 forget any session-identifiers, keys, and secrets associated with a
1551 failed connection. Thus, any connection terminated with a fatal alert
1552 MUST NOT be resumed. The following error alerts are defined:
1555 An inappropriate message was received. This alert is always fatal
1556 and should never be observed in communication between proper
1560 This alert is returned if a record is received with an incorrect
1561 MAC. This alert also MUST be returned if an alert is sent because
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1568 a TLSCiphertext decrypted in an invalid way: either it wasn't an
1569 even multiple of the block length, or its padding values, when
1570 checked, weren't correct. This message is always fatal.
1573 This alert MAY be returned if a TLSCiphertext decrypted in an
1574 invalid way: either it wasn't an even multiple of the block
1575 length, or its padding values, when checked, weren't correct.
1576 This message is always fatal.
1578 NB: Differentiating between bad_record_mac and decryption_failed
1579 alerts may permit certain attacks against CBC mode as used in TLS
1580 [CBCATT]. It is preferable to uniformly use the bad_record_mac
1581 alert to hide the specific type of the error.
1585 A TLSCiphertext record was received which had a length more than
1586 2^14+2048 bytes, or a record decrypted to a TLSCompressed record
1587 with more than 2^14+1024 bytes. This message is always fatal.
1589 decompression_failure
1590 The decompression function received improper input (e.g. data
1591 that would expand to excessive length). This message is always
1595 Reception of a handshake_failure alert message indicates that the
1596 sender was unable to negotiate an acceptable set of security
1597 parameters given the options available. This is a fatal error.
1599 no_certificate_RESERVED
1600 This alert was used in SSLv3 but not in TLS. It should not be
1601 sent by compliant implementations.
1604 A certificate was corrupt, contained signatures that did not
1605 verify correctly, etc.
1607 unsupported_certificate
1608 A certificate was of an unsupported type.
1611 A certificate was revoked by its signer.
1614 A certificate has expired or is not currently valid.
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1623 Some other (unspecified) issue arose in processing the
1624 certificate, rendering it unacceptable.
1627 A field in the handshake was out of range or inconsistent with
1628 other fields. This is always fatal.
1631 A valid certificate chain or partial chain was received, but the
1632 certificate was not accepted because the CA certificate could not
1633 be located or couldn't be matched with a known, trusted CA. This
1634 message is always fatal.
1637 A valid certificate was received, but when access control was
1638 applied, the sender decided not to proceed with negotiation.
1639 This message is always fatal.
1642 A message could not be decoded because some field was out of the
1643 specified range or the length of the message was incorrect. This
1644 message is always fatal.
1647 A handshake cryptographic operation failed, including being
1648 unable to correctly verify a signature, decrypt a key exchange,
1649 or validate a finished message.
1651 export_restriction_RESERVED
1652 This alert was used in TLS 1.0 but not TLS 1.1.
1655 The protocol version the client has attempted to negotiate is
1656 recognized, but not supported. (For example, old protocol
1657 versions might be avoided for security reasons). This message is
1660 insufficient_security
1661 Returned instead of handshake_failure when a negotiation has
1662 failed specifically because the server requires ciphers more
1663 secure than those supported by the client. This message is always
1667 An internal error unrelated to the peer or the correctness of the
1668 protocol makes it impossible to continue (such as a memory
1669 allocation failure). This message is always fatal.
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1677 This handshake is being canceled for some reason unrelated to a
1678 protocol failure. If the user cancels an operation after the
1679 handshake is complete, just closing the connection by sending a
1680 close_notify is more appropriate. This alert should be followed
1681 by a close_notify. This message is generally a warning.
1684 Sent by the client in response to a hello request or by the
1685 server in response to a client hello after initial handshaking.
1686 Either of these would normally lead to renegotiation; when that
1687 is not appropriate, the recipient should respond with this alert;
1688 at that point, the original requester can decide whether to
1689 proceed with the connection. One case where this would be
1690 appropriate would be where a server has spawned a process to
1691 satisfy a request; the process might receive security parameters
1692 (key length, authentication, etc.) at startup and it might be
1693 difficult to communicate changes to these parameters after that
1694 point. This message is always a warning.
1696 For all errors where an alert level is not explicitly specified, the
1697 sending party MAY determine at its discretion whether this is a fatal
1698 error or not; if an alert with a level of warning is received, the
1699 receiving party MAY decide at its discretion whether to treat this as
1700 a fatal error or not. However, all messages which are transmitted
1701 with a level of fatal MUST be treated as fatal messages.
1703 New alerts values MUST be defined by RFC 2434 Standards Action. See
1704 Section 11 for IANA Considerations for alert values.
1706 7.3. Handshake Protocol overview
1708 The cryptographic parameters of the session state are produced by the
1709 TLS Handshake Protocol, which operates on top of the TLS Record
1710 Layer. When a TLS client and server first start communicating, they
1711 agree on a protocol version, select cryptographic algorithms,
1712 optionally authenticate each other, and use public-key encryption
1713 techniques to generate shared secrets.
1715 The TLS Handshake Protocol involves the following steps:
1717 - Exchange hello messages to agree on algorithms, exchange random
1718 values, and check for session resumption.
1720 - Exchange the necessary cryptographic parameters to allow the
1721 client and server to agree on a premaster secret.
1723 - Exchange certificates and cryptographic information to allow the
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1730 client and server to authenticate themselves.
1732 - Generate a master secret from the premaster secret and exchanged
1735 - Provide security parameters to the record layer.
1737 - Allow the client and server to verify that their peer has
1738 calculated the same security parameters and that the handshake
1739 occurred without tampering by an attacker.
1741 Note that higher layers should not be overly reliant on TLS always
1742 negotiating the strongest possible connection between two peers:
1743 there are a number of ways a man in the middle attacker can attempt
1744 to make two entities drop down to the least secure method they
1745 support. The protocol has been designed to minimize this risk, but
1746 there are still attacks available: for example, an attacker could
1747 block access to the port a secure service runs on, or attempt to get
1748 the peers to negotiate an unauthenticated connection. The fundamental
1749 rule is that higher levels must be cognizant of what their security
1750 requirements are and never transmit information over a channel less
1751 secure than what they require. The TLS protocol is secure, in that
1752 any cipher suite offers its promised level of security: if you
1753 negotiate 3DES with a 1024 bit RSA key exchange with a host whose
1754 certificate you have verified, you can expect to be that secure.
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1784 However, you SHOULD never send data over a link encrypted with 40 bit
1785 security unless you feel that data is worth no more than the effort
1786 required to break that encryption.
1788 These goals are achieved by the handshake protocol, which can be
1789 summarized as follows: The client sends a client hello message to
1790 which the server must respond with a server hello message, or else a
1791 fatal error will occur and the connection will fail. The client hello
1792 and server hello are used to establish security enhancement
1793 capabilities between client and server. The client hello and server
1794 hello establish the following attributes: Protocol Version, Session
1795 ID, Cipher Suite, and Compression Method. Additionally, two random
1796 values are generated and exchanged: ClientHello.random and
1799 The actual key exchange uses up to four messages: the server
1800 certificate, the server key exchange, the client certificate, and the
1801 client key exchange. New key exchange methods can be created by
1802 specifying a format for these messages and defining the use of the
1803 messages to allow the client and server to agree upon a shared
1804 secret. This secret MUST be quite long; currently defined key
1805 exchange methods exchange secrets which range from 48 to 128 bytes in
1808 Following the hello messages, the server will send its certificate,
1809 if it is to be authenticated. Additionally, a server key exchange
1810 message may be sent, if it is required (e.g. if their server has no
1811 certificate, or if its certificate is for signing only). If the
1812 server is authenticated, it may request a certificate from the
1813 client, if that is appropriate to the cipher suite selected. Now the
1814 server will send the server hello done message, indicating that the
1815 hello-message phase of the handshake is complete. The server will
1816 then wait for a client response. If the server has sent a certificate
1817 request message, the client must send the certificate message. The
1818 client key exchange message is now sent, and the content of that
1819 message will depend on the public key algorithm selected between the
1820 client hello and the server hello. If the client has sent a
1821 certificate with signing ability, a digitally-signed certificate
1822 verify message is sent to explicitly verify the certificate.
1824 At this point, a change cipher spec message is sent by the client,
1825 and the client copies the pending Cipher Spec into the current Cipher
1826 Spec. The client then immediately sends the finished message under
1827 the new algorithms, keys, and secrets. In response, the server will
1828 send its own change cipher spec message, transfer the pending to the
1829 current Cipher Spec, and send its finished message under the new
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1838 Cipher Spec. At this point, the handshake is complete and the client
1839 and server may begin to exchange application layer data. (See flow
1840 chart below.) Application data MUST NOT be sent prior to the
1841 completion of the first handshake (before a cipher suite other
1842 TLS_NULL_WITH_NULL_NULL is established).
1845 ClientHello -------->
1850 <-------- ServerHelloDone
1858 Application Data <-------> Application Data
1860 Fig. 1 - Message flow for a full handshake
1862 * Indicates optional or situation-dependent messages that are not
1865 Note: To help avoid pipeline stalls, ChangeCipherSpec is an
1866 independent TLS Protocol content type, and is not actually a TLS
1869 When the client and server decide to resume a previous session or
1870 duplicate an existing session (instead of negotiating new security
1871 parameters) the message flow is as follows:
1873 The client sends a ClientHello using the Session ID of the session to
1874 be resumed. The server then checks its session cache for a match. If
1875 a match is found, and the server is willing to re-establish the
1876 connection under the specified session state, it will send a
1877 ServerHello with the same Session ID value. At this point, both
1878 client and server MUST send change cipher spec messages and proceed
1879 directly to finished messages. Once the re-establishment is complete,
1880 the client and server MAY begin to exchange application layer data.
1881 (See flow chart below.) If a Session ID match is not found, the
1882 server generates a new session ID and the TLS client and server
1883 perform a full handshake.
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1894 ClientHello -------->
1900 Application Data <-------> Application Data
1902 Fig. 2 - Message flow for an abbreviated handshake
1904 The contents and significance of each message will be presented in
1905 detail in the following sections.
1907 7.4. Handshake protocol
1909 The TLS Handshake Protocol is one of the defined higher level clients
1910 of the TLS Record Protocol. This protocol is used to negotiate the
1911 secure attributes of a session. Handshake messages are supplied to
1912 the TLS Record Layer, where they are encapsulated within one or more
1913 TLSPlaintext structures, which are processed and transmitted as
1914 specified by the current active session state.
1917 hello_request(0), client_hello(1), server_hello(2),
1918 certificate(11), server_key_exchange (12),
1919 certificate_request(13), server_hello_done(14),
1920 certificate_verify(15), client_key_exchange(16),
1925 HandshakeType msg_type; /* handshake type */
1926 uint24 length; /* bytes in message */
1927 select (HandshakeType) {
1928 case hello_request: HelloRequest;
1929 case client_hello: ClientHello;
1930 case server_hello: ServerHello;
1931 case certificate: Certificate;
1932 case server_key_exchange: ServerKeyExchange;
1933 case certificate_request: CertificateRequest;
1934 case server_hello_done: ServerHelloDone;
1935 case certificate_verify: CertificateVerify;
1936 case client_key_exchange: ClientKeyExchange;
1937 case finished: Finished;
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1946 The handshake protocol messages are presented below in the order they
1947 MUST be sent; sending handshake messages in an unexpected order
1948 results in a fatal error. Unneeded handshake messages can be omitted,
1949 however. Note one exception to the ordering: the Certificate message
1950 is used twice in the handshake (from server to client, then from
1951 client to server), but described only in its first position. The one
1952 message which is not bound by these ordering rules is the Hello
1953 Request message, which can be sent at any time, but which should be
1954 ignored by the client if it arrives in the middle of a handshake.
1956 New Handshake message type values MUST be defined via RFC 2434
1957 Standards Action. See Section 11 for IANA Considerations for these
1960 7.4.1. Hello messages
1962 The hello phase messages are used to exchange security enhancement
1963 capabilities between the client and server. When a new session
1964 begins, the Record Layer's connection state encryption, hash, and
1965 compression algorithms are initialized to null. The current
1966 connection state is used for renegotiation messages.
1968 7.4.1.1. Hello request
1970 When this message will be sent:
1971 The hello request message MAY be sent by the server at any time.
1973 Meaning of this message:
1974 Hello request is a simple notification that the client should
1975 begin the negotiation process anew by sending a client hello
1976 message when convenient. This message will be ignored by the
1977 client if the client is currently negotiating a session. This
1978 message may be ignored by the client if it does not wish to
1979 renegotiate a session, or the client may, if it wishes, respond
1980 with a no_renegotiation alert. Since handshake messages are
1981 intended to have transmission precedence over application data,
1982 it is expected that the negotiation will begin before no more
1983 than a few records are received from the client. If the server
1984 sends a hello request but does not receive a client hello in
1985 response, it may close the connection with a fatal alert.
1987 After sending a hello request, servers SHOULD not repeat the request
1988 until the subsequent handshake negotiation is complete.
1990 Structure of this message:
1991 struct { } HelloRequest;
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2000 Note: This message MUST NOT be included in the message hashes which are
2001 maintained throughout the handshake and used in the finished
2002 messages and the certificate verify message.
2004 7.4.1.2. Client hello
2006 When this message will be sent:
2007 When a client first connects to a server it is required to send
2008 the client hello as its first message. The client can also send a
2009 client hello in response to a hello request or on its own
2010 initiative in order to renegotiate the security parameters in an
2011 existing connection.
2013 Structure of this message:
2014 The client hello message includes a random structure, which is
2015 used later in the protocol.
2018 uint32 gmt_unix_time;
2019 opaque random_bytes[28];
2023 The current time and date in standard UNIX 32-bit format (seconds
2024 since the midnight starting Jan 1, 1970, GMT) according to the
2025 sender's internal clock. Clocks are not required to be set
2026 correctly by the basic TLS Protocol; higher level or application
2027 protocols may define additional requirements.
2030 28 bytes generated by a secure random number generator.
2032 The client hello message includes a variable length session
2033 identifier. If not empty, the value identifies a session between the
2034 same client and server whose security parameters the client wishes to
2035 reuse. The session identifier MAY be from an earlier connection, this
2036 connection, or another currently active connection. The second option
2037 is useful if the client only wishes to update the random structures
2038 and derived values of a connection, while the third option makes it
2039 possible to establish several independent secure connections without
2040 repeating the full handshake protocol. These independent connections
2041 may occur sequentially or simultaneously; a SessionID becomes valid
2042 when the handshake negotiating it completes with the exchange of
2043 Finished messages and persists until removed due to aging or because
2044 a fatal error was encountered on a connection associated with the
2045 session. The actual contents of the SessionID are defined by the
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2054 opaque SessionID<0..32>;
2057 Because the SessionID is transmitted without encryption or
2058 immediate MAC protection, servers MUST not place confidential
2059 information in session identifiers or let the contents of fake
2060 session identifiers cause any breach of security. (Note that the
2061 content of the handshake as a whole, including the SessionID, is
2062 protected by the Finished messages exchanged at the end of the
2065 The CipherSuite list, passed from the client to the server in the
2066 client hello message, contains the combinations of cryptographic
2067 algorithms supported by the client in order of the client's
2068 preference (favorite choice first). Each CipherSuite defines a key
2069 exchange algorithm, a bulk encryption algorithm (including secret key
2070 length) and a MAC algorithm. The server will select a cipher suite
2071 or, if no acceptable choices are presented, return a handshake
2072 failure alert and close the connection.
2074 uint8 CipherSuite[2]; /* Cryptographic suite selector */
2076 The client hello includes a list of compression algorithms supported
2077 by the client, ordered according to the client's preference.
2079 enum { null(0), (255) } CompressionMethod;
2082 ProtocolVersion client_version;
2084 SessionID session_id;
2085 CipherSuite cipher_suites<2..2^16-1>;
2086 CompressionMethod compression_methods<1..2^8-1>;
2090 The version of the TLS protocol by which the client wishes to
2091 communicate during this session. This SHOULD be the latest
2092 (highest valued) version supported by the client. For this
2093 version of the specification, the version will be 3.2 (See
2094 Appendix E for details about backward compatibility).
2097 A client-generated random structure.
2100 The ID of a session the client wishes to use for this connection.
2101 This field should be empty if no session_id is available or the
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2108 client wishes to generate new security parameters.
2111 This is a list of the cryptographic options supported by the
2112 client, with the client's first preference first. If the
2113 session_id field is not empty (implying a session resumption
2114 request) this vector MUST include at least the cipher_suite from
2115 that session. Values are defined in Appendix A.5.
2118 This is a list of the compression methods supported by the
2119 client, sorted by client preference. If the session_id field is
2120 not empty (implying a session resumption request) it must include
2121 the compression_method from that session. This vector must
2122 contain, and all implementations must support,
2123 CompressionMethod.null. Thus, a client and server will always be
2124 able to agree on a compression method.
2126 After sending the client hello message, the client waits for a server
2127 hello message. Any other handshake message returned by the server
2128 except for a hello request is treated as a fatal error.
2130 Forward compatibility note:
2131 In the interests of forward compatibility, it is permitted for a
2132 client hello message to include extra data after the compression
2133 methods. This data MUST be included in the handshake hashes, but
2134 must otherwise be ignored. This is the only handshake message for
2135 which this is legal; for all other messages, the amount of data
2136 in the message MUST match the description of the message
2139 Note: For the intended use of trailing data in the ClientHello, see RFC
2142 7.4.1.3. Server hello
2144 When this message will be sent:
2145 The server will send this message in response to a client hello
2146 message when it was able to find an acceptable set of algorithms.
2147 If it cannot find such a match, it will respond with a handshake
2150 Structure of this message:
2152 ProtocolVersion server_version;
2154 SessionID session_id;
2155 CipherSuite cipher_suite;
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2162 CompressionMethod compression_method;
2166 This field will contain the lower of that suggested by the client
2167 in the client hello and the highest supported by the server. For
2168 this version of the specification, the version is 3.2 (See
2169 Appendix E for details about backward compatibility).
2172 This structure is generated by the server and MUST be
2173 independently generated from the ClientHello.random.
2176 This is the identity of the session corresponding to this
2177 connection. If the ClientHello.session_id was non-empty, the
2178 server will look in its session cache for a match. If a match is
2179 found and the server is willing to establish the new connection
2180 using the specified session state, the server will respond with
2181 the same value as was supplied by the client. This indicates a
2182 resumed session and dictates that the parties must proceed
2183 directly to the finished messages. Otherwise this field will
2184 contain a different value identifying the new session. The server
2185 may return an empty session_id to indicate that the session will
2186 not be cached and therefore cannot be resumed. If a session is
2187 resumed, it must be resumed using the same cipher suite it was
2188 originally negotiated with.
2191 The single cipher suite selected by the server from the list in
2192 ClientHello.cipher_suites. For resumed sessions this field is the
2193 value from the state of the session being resumed.
2196 The single compression algorithm selected by the server from the
2197 list in ClientHello.compression_methods. For resumed sessions
2198 this field is the value from the resumed session state.
2200 7.4.2. Server certificate
2202 When this message will be sent:
2203 The server MUST send a certificate whenever the agreed-upon key
2204 exchange method is not an anonymous one. This message will always
2205 immediately follow the server hello message.
2207 Meaning of this message:
2208 The certificate type MUST be appropriate for the selected cipher
2209 suite's key exchange algorithm, and is generally an X.509v3
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2216 certificate. It MUST contain a key which matches the key exchange
2217 method, as follows. Unless otherwise specified, the signing
2218 algorithm for the certificate MUST be the same as the algorithm
2219 for the certificate key. Unless otherwise specified, the public
2220 key MAY be of any length.
2222 Key Exchange Algorithm Certificate Key Type
2224 RSA RSA public key; the certificate MUST
2225 allow the key to be used for encryption.
2227 DHE_DSS DSS public key.
2229 DHE_RSA RSA public key which can be used for
2232 DH_DSS Diffie-Hellman key. The algorithm used
2233 to sign the certificate MUST be DSS.
2235 DH_RSA Diffie-Hellman key. The algorithm used
2236 to sign the certificate MUST be RSA.
2238 All certificate profiles, key and cryptographic formats are defined
2239 by the IETF PKIX working group [PKIX]. When a key usage extension is
2240 present, the digitalSignature bit MUST be set for the key to be
2241 eligible for signing, as described above, and the keyEncipherment bit
2242 MUST be present to allow encryption, as described above. The
2243 keyAgreement bit must be set on Diffie-Hellman certificates.
2245 As CipherSuites which specify new key exchange methods are specified
2246 for the TLS Protocol, they will imply certificate format and the
2247 required encoded keying information.
2249 Structure of this message:
2250 opaque ASN.1Cert<1..2^24-1>;
2253 ASN.1Cert certificate_list<0..2^24-1>;
2257 This is a sequence (chain) of X.509v3 certificates. The sender's
2258 certificate must come first in the list. Each following
2259 certificate must directly certify the one preceding it. Because
2260 certificate validation requires that root keys be distributed
2261 independently, the self-signed certificate which specifies the
2262 root certificate authority may optionally be omitted from the
2263 chain, under the assumption that the remote end must already
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2270 possess it in order to validate it in any case.
2272 The same message type and structure will be used for the client's
2273 response to a certificate request message. Note that a client MAY
2274 send no certificates if it does not have an appropriate certificate
2275 to send in response to the server's authentication request.
2277 Note: PKCS #7 [PKCS7] is not used as the format for the certificate
2278 vector because PKCS #6 [PKCS6] extended certificates are not
2279 used. Also PKCS #7 defines a SET rather than a SEQUENCE, making
2280 the task of parsing the list more difficult.
2282 7.4.3. Server key exchange message
2284 When this message will be sent:
2285 This message will be sent immediately after the server
2286 certificate message (or the server hello message, if this is an
2287 anonymous negotiation).
2289 The server key exchange message is sent by the server only when
2290 the server certificate message (if sent) does not contain enough
2291 data to allow the client to exchange a premaster secret. This is
2292 true for the following key exchange methods:
2298 It is not legal to send the server key exchange message for the
2299 following key exchange methods:
2305 Meaning of this message:
2306 This message conveys cryptographic information to allow the
2307 client to communicate the premaster secret: either an RSA public
2308 key to encrypt the premaster secret with, or a Diffie-Hellman
2309 public key with which the client can complete a key exchange
2310 (with the result being the premaster secret.)
2312 As additional CipherSuites are defined for TLS which include new key
2313 exchange algorithms, the server key exchange message will be sent if
2314 and only if the certificate type associated with the key exchange
2315 algorithm does not provide enough information for the client to
2316 exchange a premaster secret.
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2324 Structure of this message:
2325 enum { rsa, diffie_hellman } KeyExchangeAlgorithm;
2328 opaque rsa_modulus<1..2^16-1>;
2329 opaque rsa_exponent<1..2^16-1>;
2333 The modulus of the server's temporary RSA key.
2336 The public exponent of the server's temporary RSA key.
2339 opaque dh_p<1..2^16-1>;
2340 opaque dh_g<1..2^16-1>;
2341 opaque dh_Ys<1..2^16-1>;
2342 } ServerDHParams; /* Ephemeral DH parameters */
2345 The prime modulus used for the Diffie-Hellman operation.
2348 The generator used for the Diffie-Hellman operation.
2351 The server's Diffie-Hellman public value (g^X mod p).
2354 select (KeyExchangeAlgorithm) {
2355 case diffie_hellman:
2356 ServerDHParams params;
2357 Signature signed_params;
2359 ServerRSAParams params;
2360 Signature signed_params;
2362 } ServerKeyExchange;
2365 select (KeyExchangeAlgorithm) {
2366 case diffie_hellman:
2367 ServerDHParams params;
2369 ServerRSAParams params;
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2379 The server's key exchange parameters.
2382 For non-anonymous key exchanges, a hash of the corresponding
2383 params value, with the signature appropriate to that hash
2387 MD5(ClientHello.random + ServerHello.random + ServerParams);
2390 SHA(ClientHello.random + ServerHello.random + ServerParams);
2392 enum { anonymous, rsa, dsa } SignatureAlgorithm;
2395 select (SignatureAlgorithm)
2397 case anonymous: struct { };
2399 digitally-signed struct {
2400 opaque md5_hash[16];
2401 opaque sha_hash[20];
2404 digitally-signed struct {
2405 opaque sha_hash[20];
2409 7.4.4. Certificate request
2411 When this message will be sent:
2412 A non-anonymous server can optionally request a certificate from
2413 the client, if appropriate for the selected cipher suite. This
2414 message, if sent, will immediately follow the Server Key Exchange
2415 message (if it is sent; otherwise, the Server Certificate
2418 Structure of this message:
2420 rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4),
2421 rsa_ephemeral_dh_RESERVED(5), dss_ephemeral_dh_RESERVED(6),
2422 fortezza_dms_RESERVED(20),
2424 } ClientCertificateType;
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2432 opaque DistinguishedName<1..2^16-1>;
2435 ClientCertificateType certificate_types<1..2^8-1>;
2436 DistinguishedName certificate_authorities<0..2^16-1>;
2437 } CertificateRequest;
2440 This field is a list of the types of certificates requested,
2441 sorted in order of the server's preference.
2443 certificate_authorities
2444 A list of the distinguished names of acceptable certificate
2445 authorities. These distinguished names may specify a desired
2446 distinguished name for a root CA or for a subordinate CA;
2447 thus, this message can be used both to describe known roots
2448 and a desired authorization space. If the
2449 certificate_authorities list is empty then the client MAY
2450 send any certificate of the appropriate
2451 ClientCertificateType, unless there is some external
2452 arrangement to the contrary.
2455 ClientCertificateType values are divided into three groups:
2457 1. Values from 0 (zero) through 63 decimal (0x3F) inclusive are
2458 reserved for IETF Standards Track protocols.
2460 2. Values from 64 decimal (0x40) through 223 decimal (0xDF) inclusive
2461 are reserved for assignment for non-Standards Track methods.
2463 3. Values from 224 decimal (0xE0) through 255 decimal (0xFF)
2464 inclusive are reserved for private use.
2466 Additional information describing the role of IANA in the
2467 allocation of ClientCertificateType code points is described
2470 Note: Values listed as RESERVED may not be used. They were used in SSLv3.
2472 Note: DistinguishedName is derived from [X509]. DistinguishedNames are
2473 represented in DER-encoded format.
2475 Note: It is a fatal handshake_failure alert for an anonymous server to
2476 request client authentication.
2478 7.4.5. Server hello done
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2486 When this message will be sent:
2487 The server hello done message is sent by the server to indicate
2488 the end of the server hello and associated messages. After
2489 sending this message the server will wait for a client response.
2491 Meaning of this message:
2492 This message means that the server is done sending messages to
2493 support the key exchange, and the client can proceed with its
2494 phase of the key exchange.
2496 Upon receipt of the server hello done message the client SHOULD
2497 verify that the server provided a valid certificate if required
2498 and check that the server hello parameters are acceptable.
2500 Structure of this message:
2501 struct { } ServerHelloDone;
2503 7.4.6. Client certificate
2505 When this message will be sent:
2506 This is the first message the client can send after receiving a
2507 server hello done message. This message is only sent if the
2508 server requests a certificate. If no suitable certificate is
2509 available, the client should send a certificate message
2510 containing no certificates: I.e. the certificate_list structure
2511 should have a length of zero. If client authentication is
2512 required by the server for the handshake to continue, it may
2513 respond with a fatal handshake failure alert. Client certificates
2514 are sent using the Certificate structure defined in Section
2518 Note: When using a static Diffie-Hellman based key exchange method
2519 (DH_DSS or DH_RSA), if client authentication is requested, the
2520 Diffie-Hellman group and generator encoded in the client's
2521 certificate must match the server specified Diffie-Hellman
2522 parameters if the client's parameters are to be used for the key
2525 7.4.7. Client key exchange message
2527 When this message will be sent:
2528 This message is always sent by the client. It will immediately
2529 follow the client certificate message, if it is sent. Otherwise
2530 it will be the first message sent by the client after it receives
2531 the server hello done message.
2533 Meaning of this message:
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2540 With this message, the premaster secret is set, either though
2541 direct transmission of the RSA-encrypted secret, or by the
2542 transmission of Diffie-Hellman parameters which will allow each
2543 side to agree upon the same premaster secret. When the key
2544 exchange method is DH_RSA or DH_DSS, client certification has
2545 been requested, and the client was able to respond with a
2546 certificate which contained a Diffie-Hellman public key whose
2547 parameters (group and generator) matched those specified by the
2548 server in its certificate, this message MUST not contain any
2551 Structure of this message:
2552 The choice of messages depends on which key exchange method has
2553 been selected. See Section 7.4.3 for the KeyExchangeAlgorithm
2557 select (KeyExchangeAlgorithm) {
2558 case rsa: EncryptedPreMasterSecret;
2559 case diffie_hellman: ClientDiffieHellmanPublic;
2561 } ClientKeyExchange;
2563 7.4.7.1. RSA encrypted premaster secret message
2565 Meaning of this message:
2566 If RSA is being used for key agreement and authentication, the
2567 client generates a 48-byte premaster secret, encrypts it using
2568 the public key from the server's certificate or the temporary RSA
2569 key provided in a server key exchange message, and sends the
2570 result in an encrypted premaster secret message. This structure
2571 is a variant of the client key exchange message, not a message in
2574 Structure of this message:
2576 ProtocolVersion client_version;
2581 The latest (newest) version supported by the client. This is
2582 used to detect version roll-back attacks. Upon receiving the
2583 premaster secret, the server SHOULD check that this value
2584 matches the value transmitted by the client in the client
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2594 46 securely-generated random bytes.
2597 public-key-encrypted PreMasterSecret pre_master_secret;
2598 } EncryptedPreMasterSecret;
2601 This random value is generated by the client and is used to
2602 generate the master secret, as specified in Section 8.1.
2604 Note: An attack discovered by Daniel Bleichenbacher [BLEI] can be used
2605 to attack a TLS server which is using PKCS#1 encoded RSA. The
2606 attack takes advantage of the fact that by failing in different
2607 ways, a TLS server can be coerced into revealing whether a
2608 particular message, when decrypted, is properly PKCS#1 formatted
2611 The best way to avoid vulnerability to this attack is to treat
2612 incorrectly formatted messages in a manner indistinguishable from
2613 correctly formatted RSA blocks. Thus, when it receives an
2614 incorrectly formatted RSA block, a server should generate a
2615 random 48-byte value and proceed using it as the premaster
2616 secret. Thus, the server will act identically whether the
2617 received RSA block is correctly encoded or not.
2619 Implementation Note: public-key-encrypted data is represented as an
2620 opaque vector <0..2^16-1> (see S. 4.7). Thus the RSA-encrypted
2621 PreMaster Secret in a ClientKeyExchange is preceded by two length
2622 bytes. These bytes are redundant in the case of RSA because the
2623 EncryptedPreMasterSecret is the only data in the
2624 ClientKeyExchange and its length can therefore be unambiguously
2625 determined. The SSLv3 specification was not clear about the
2626 encoding of public-key-encrypted data and therefore many SSLv3
2627 implementations do not include the the length bytes, encoding the
2628 RSA encrypted data directly in the ClientKeyExchange message.
2630 This specification requires correct encoding of the
2631 EncryptedPreMasterSecret complete with length bytes. The
2632 resulting PDU is incompatible with many SSLv3 implementations.
2633 Implementors upgrading from SSLv3 must modify their
2634 implementations to generate and accept the correct encoding.
2635 Implementors who wish to be compatible with both SSLv3 and TLS
2636 should make their implementation's behavior dependent on the
2639 Implementation Note: It is now known that remote timing-based attacks
2640 on SSL are possible, at least when the client and server are on
2641 the same LAN. Accordingly, implementations which use static RSA
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2648 keys SHOULD use RSA blinding or some other anti-timing technique,
2649 as described in [TIMING].
2651 Note: The version number in the PreMasterSecret is that offered by the
2652 client, NOT the version negotiated for the connection. This
2653 feature is designed to prevent rollback attacks. Unfortunately,
2654 many implementations use the negotiated version instead and
2655 therefore checking the version number may lead to failure to
2656 interoperate with such incorrect client implementations. Client
2657 implementations MUST and Server implementations MAY check the
2658 version number. In practice, since there are no significant known
2659 security differences between TLS and SSLv3, rollback to SSLv3 is
2660 not believed to be a serious security risk. Note that if servers
2661 choose to to check the version number, they should randomize the
2662 PreMasterSecret in case of error, rather than generate an alert,
2663 in order to avoid variants on the Bleichenbacher attack. [KPR03]
2665 7.4.7.2. Client Diffie-Hellman public value
2667 Meaning of this message:
2668 This structure conveys the client's Diffie-Hellman public value
2669 (Yc) if it was not already included in the client's certificate.
2670 The encoding used for Yc is determined by the enumerated
2671 PublicValueEncoding. This structure is a variant of the client
2672 key exchange message, not a message in itself.
2674 Structure of this message:
2675 enum { implicit, explicit } PublicValueEncoding;
2678 If the client certificate already contains a suitable Diffie-
2679 Hellman key, then Yc is implicit and does not need to be sent
2680 again. In this case, the Client Key Exchange message will be
2681 sent, but will be empty.
2684 Yc needs to be sent.
2687 select (PublicValueEncoding) {
2688 case implicit: struct { };
2689 case explicit: opaque dh_Yc<1..2^16-1>;
2691 } ClientDiffieHellmanPublic;
2694 The client's Diffie-Hellman public value (Yc).
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2702 7.4.8. Certificate verify
2704 When this message will be sent:
2705 This message is used to provide explicit verification of a client
2706 certificate. This message is only sent following a client
2707 certificate that has signing capability (i.e. all certificates
2708 except those containing fixed Diffie-Hellman parameters). When
2709 sent, it will immediately follow the client key exchange message.
2711 Structure of this message:
2713 Signature signature;
2714 } CertificateVerify;
2716 The Signature type is defined in 7.4.3.
2718 CertificateVerify.signature.md5_hash
2719 MD5(handshake_messages);
2721 CertificateVerify.signature.sha_hash
2722 SHA(handshake_messages);
2724 Here handshake_messages refers to all handshake messages sent or
2725 received starting at client hello up to but not including this
2726 message, including the type and length fields of the handshake
2727 messages. This is the concatenation of all the Handshake structures
2728 as defined in 7.4 exchanged thus far.
2732 When this message will be sent:
2733 A finished message is always sent immediately after a change
2734 cipher spec message to verify that the key exchange and
2735 authentication processes were successful. It is essential that a
2736 change cipher spec message be received between the other
2737 handshake messages and the Finished message.
2739 Meaning of this message:
2740 The finished message is the first protected with the just-
2741 negotiated algorithms, keys, and secrets. Recipients of finished
2742 messages MUST verify that the contents are correct. Once a side
2743 has sent its Finished message and received and validated the
2744 Finished message from its peer, it may begin to send and receive
2745 application data over the connection.
2748 opaque verify_data[12];
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2757 PRF(master_secret, finished_label, MD5(handshake_messages) +
2758 SHA-1(handshake_messages)) [0..11];
2761 For Finished messages sent by the client, the string "client
2762 finished". For Finished messages sent by the server, the
2763 string "server finished".
2766 All of the data from all messages in this handshake (not
2767 including any HelloRequest messages) up to but not including
2768 this message. This is only data visible at the handshake
2769 layer and does not include record layer headers. This is the
2770 concatenation of all the Handshake structures as defined in
2771 7.4 exchanged thus far.
2773 It is a fatal error if a finished message is not preceded by a change
2774 cipher spec message at the appropriate point in the handshake.
2776 The value handshake_messages includes all handshake messages starting
2777 at client hello up to, but not including, this finished message. This
2778 may be different from handshake_messages in Section 7.4.8 because it
2779 would include the certificate verify message (if sent). Also, the
2780 handshake_messages for the finished message sent by the client will
2781 be different from that for the finished message sent by the server,
2782 because the one which is sent second will include the prior one.
2784 Note: Change cipher spec messages, alerts and any other record types
2785 are not handshake messages and are not included in the hash
2786 computations. Also, Hello Request messages are omitted from
2789 8. Cryptographic computations
2791 In order to begin connection protection, the TLS Record Protocol
2792 requires specification of a suite of algorithms, a master secret, and
2793 the client and server random values. The authentication, encryption,
2794 and MAC algorithms are determined by the cipher_suite selected by the
2795 server and revealed in the server hello message. The compression
2796 algorithm is negotiated in the hello messages, and the random values
2797 are exchanged in the hello messages. All that remains is to calculate
2800 8.1. Computing the master secret
2802 For all key exchange methods, the same algorithm is used to convert
2803 the pre_master_secret into the master_secret. The pre_master_secret
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2810 should be deleted from memory once the master_secret has been
2813 master_secret = PRF(pre_master_secret, "master secret",
2814 ClientHello.random + ServerHello.random)
2817 The master secret is always exactly 48 bytes in length. The length of
2818 the premaster secret will vary depending on key exchange method.
2861 Dierks & Rescorla Standards Track [Page 53]
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2866 When RSA is used for server authentication and key exchange, a
2867 48-byte pre_master_secret is generated by the client, encrypted under
2868 the server's public key, and sent to the server. The server uses its
2869 private key to decrypt the pre_master_secret. Both parties then
2870 convert the pre_master_secret into the master_secret, as specified
2873 RSA digital signatures are performed using PKCS #1 [PKCS1] block type
2874 1. RSA public key encryption is performed using PKCS #1 block type 2.
2876 8.1.2. Diffie-Hellman
2878 A conventional Diffie-Hellman computation is performed. The
2879 negotiated key (Z) is used as the pre_master_secret, and is converted
2880 into the master_secret, as specified above. Leading 0 bytes of Z are
2881 stripped before it is used as the pre_master_secret.
2883 Note: Diffie-Hellman parameters are specified by the server, and may
2884 be either ephemeral or contained within the server's certificate.
2886 9. Mandatory Cipher Suites
2888 In the absence of an application profile standard specifying
2889 otherwise, a TLS compliant application MUST implement the cipher
2890 suite TLS_RSA_WITH_3DES_EDE_CBC_SHA.
2892 10. Application data protocol
2894 Application data messages are carried by the Record Layer and are
2895 fragmented, compressed and encrypted based on the current connection
2896 state. The messages are treated as transparent data to the record
2899 10. IANA Considerations
2901 Section 7.4.3 describes a TLS ClientCertificateType Registry to be
2902 maintained by the IANA, as defining a number of such code point
2903 identifiers. ClientCertificateType identifiers with values in the
2904 range 0-63 (decimal) inclusive are assigned via RFC 2434 Standards
2905 Action. Values from the range 64-223 (decimal) inclusive are assigned
2906 via [RFC 2434] Specification Required. Identifier values from
2907 224-255 (decimal) inclusive are reserved for RFC 2434 Private Use.
2908 The registry will be initially populated with the values in this
2911 Section A.5 describes a TLS Cipher Suite Registry to be maintained by
2915 Dierks & Rescorla Standards Track [Page 54]
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2918 the IANA, as well as defining a number of such cipher suite
2919 identifiers. Cipher suite values with the first byte in the range
2920 0-191 (decimal) inclusive are assigned via RFC 2434 Standards Action.
2921 Values with the first byte in the range 192-254 (decimal) are
2922 assigned via RFC 2434 Specification Required. Values with the first
2923 byte 255 (decimal) are reserved for RFC 2434 Private Use. The
2924 registry will be initially populated with the values from this
2925 document, [TLSAES], and [TLSKRB].
2927 Section 6 requires that all ContentType values be defined by RFC 2434
2928 Standards Action. IANA SHOULD create a TLS ContentType registry,
2929 initially populated with values from this document. Future values
2930 MUST be allocated via Standards Action as described in [RFC 2434].
2932 Section 7.2.2 requires that all Alert values be defined by RFC 2434
2933 Standards Action. IANA SHOULD create a TLS Alert registry, initially
2934 populated with values from this document and [TLSEXT]. Future values
2935 MUST be allocated via Standards Action as described in [RFC 2434].
2937 Section 7.4 requires that all HandshakeType values be defined by RFC
2938 2434 Standards Action. IANA SHOULD create a TLS HandshakeType
2939 registry, initially populated with values from this document,
2940 [TLSEXT], and [TLSKRB]. Future values MUST be allocated via
2941 Standards Action as described in [RFC 2434].
2969 Dierks & Rescorla Standards Track [Page 55]
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2972 A. Protocol constant values
2974 This section describes protocol types and constants.
2982 ProtocolVersion version = { 3, 2 }; /* TLS v1.1 */
2985 change_cipher_spec(20), alert(21), handshake(22),
2986 application_data(23), (255)
2991 ProtocolVersion version;
2993 opaque fragment[TLSPlaintext.length];
2998 ProtocolVersion version;
3000 opaque fragment[TLSCompressed.length];
3005 ProtocolVersion version;
3007 select (CipherSpec.cipher_type) {
3008 case stream: GenericStreamCipher;
3009 case block: GenericBlockCipher;
3013 stream-ciphered struct {
3014 opaque content[TLSCompressed.length];
3015 opaque MAC[CipherSpec.hash_size];
3016 } GenericStreamCipher;
3018 block-ciphered struct {
3019 opaque IV[CipherSpec.block_length];
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3026 opaque content[TLSCompressed.length];
3027 opaque MAC[CipherSpec.hash_size];
3028 uint8 padding[GenericBlockCipher.padding_length];
3029 uint8 padding_length;
3030 } GenericBlockCipher;
3032 A.2. Change cipher specs message
3035 enum { change_cipher_spec(1), (255) } type;
3040 enum { warning(1), fatal(2), (255) } AlertLevel;
3044 unexpected_message(10),
3046 decryption_failed(21),
3047 record_overflow(22),
3048 decompression_failure(30),
3049 handshake_failure(40),
3050 no_certificate_RESERVED (41),
3051 bad_certificate(42),
3052 unsupported_certificate(43),
3053 certificate_revoked(44),
3054 certificate_expired(45),
3055 certificate_unknown(46),
3056 illegal_parameter(47),
3061 export_restriction_RESERVED(60),
3062 protocol_version(70),
3063 insufficient_security(71),
3066 no_renegotiation(100),
3072 AlertDescription description;
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3080 A.4. Handshake protocol
3083 hello_request(0), client_hello(1), server_hello(2),
3084 certificate(11), server_key_exchange (12),
3085 certificate_request(13), server_hello_done(14),
3086 certificate_verify(15), client_key_exchange(16),
3091 HandshakeType msg_type;
3093 select (HandshakeType) {
3094 case hello_request: HelloRequest;
3095 case client_hello: ClientHello;
3096 case server_hello: ServerHello;
3097 case certificate: Certificate;
3098 case server_key_exchange: ServerKeyExchange;
3099 case certificate_request: CertificateRequest;
3100 case server_hello_done: ServerHelloDone;
3101 case certificate_verify: CertificateVerify;
3102 case client_key_exchange: ClientKeyExchange;
3103 case finished: Finished;
3107 A.4.1. Hello messages
3109 struct { } HelloRequest;
3112 uint32 gmt_unix_time;
3113 opaque random_bytes[28];
3116 opaque SessionID<0..32>;
3118 uint8 CipherSuite[2];
3120 enum { null(0), (255) } CompressionMethod;
3123 ProtocolVersion client_version;
3125 SessionID session_id;
3126 CipherSuite cipher_suites<2..2^16-1>;
3127 CompressionMethod compression_methods<1..2^8-1>;
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3137 ProtocolVersion server_version;
3139 SessionID session_id;
3140 CipherSuite cipher_suite;
3141 CompressionMethod compression_method;
3144 A.4.2. Server authentication and key exchange messages
3146 opaque ASN.1Cert<2^24-1>;
3149 ASN.1Cert certificate_list<0..2^24-1>;
3152 enum { rsa, diffie_hellman } KeyExchangeAlgorithm;
3155 opaque RSA_modulus<1..2^16-1>;
3156 opaque RSA_exponent<1..2^16-1>;
3160 opaque DH_p<1..2^16-1>;
3161 opaque DH_g<1..2^16-1>;
3162 opaque DH_Ys<1..2^16-1>;
3166 select (KeyExchangeAlgorithm) {
3167 case diffie_hellman:
3168 ServerDHParams params;
3169 Signature signed_params;
3171 ServerRSAParams params;
3172 Signature signed_params;
3174 } ServerKeyExchange;
3176 enum { anonymous, rsa, dsa } SignatureAlgorithm;
3179 select (KeyExchangeAlgorithm) {
3180 case diffie_hellman:
3181 ServerDHParams params;
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3189 ServerRSAParams params;
3193 select (SignatureAlgorithm)
3194 { case anonymous: struct { };
3196 digitally-signed struct {
3197 opaque md5_hash[16];
3198 opaque sha_hash[20];
3201 digitally-signed struct {
3202 opaque sha_hash[20];
3207 rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4),
3208 rsa_ephemeral_dh_RESERVED(5), dss_ephemeral_dh_RESERVED(6),
3209 fortezza_dms_RESERVED(20),
3211 } ClientCertificateType;
3213 opaque DistinguishedName<1..2^16-1>;
3216 ClientCertificateType certificate_types<1..2^8-1>;
3217 DistinguishedName certificate_authorities<0..2^16-1>;
3218 } CertificateRequest;
3220 struct { } ServerHelloDone;
3222 A.4.3. Client authentication and key exchange messages
3225 select (KeyExchangeAlgorithm) {
3226 case rsa: EncryptedPreMasterSecret;
3227 case diffie_hellman: DiffieHellmanClientPublicValue;
3229 } ClientKeyExchange;
3232 ProtocolVersion client_version;
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3243 public-key-encrypted PreMasterSecret pre_master_secret;
3244 } EncryptedPreMasterSecret;
3246 enum { implicit, explicit } PublicValueEncoding;
3249 select (PublicValueEncoding) {
3250 case implicit: struct {};
3251 case explicit: opaque DH_Yc<1..2^16-1>;
3253 } ClientDiffieHellmanPublic;
3256 Signature signature;
3257 } CertificateVerify;
3259 A.4.4. Handshake finalization message
3262 opaque verify_data[12];
3265 A.5. The CipherSuite
3267 The following values define the CipherSuite codes used in the client
3268 hello and server hello messages.
3270 A CipherSuite defines a cipher specification supported in TLS Version
3273 TLS_NULL_WITH_NULL_NULL is specified and is the initial state of a
3274 TLS connection during the first handshake on that channel, but must
3275 not be negotiated, as it provides no more protection than an
3276 unsecured connection.
3278 CipherSuite TLS_NULL_WITH_NULL_NULL = { 0x00,0x00 };
3280 The following CipherSuite definitions require that the server provide
3281 an RSA certificate that can be used for key exchange. The server may
3282 request either an RSA or a DSS signature-capable certificate in the
3283 certificate request message.
3285 CipherSuite TLS_RSA_WITH_NULL_MD5 = { 0x00,0x01 };
3286 CipherSuite TLS_RSA_WITH_NULL_SHA = { 0x00,0x02 };
3287 CipherSuite TLS_RSA_WITH_RC4_128_MD5 = { 0x00,0x04 };
3288 CipherSuite TLS_RSA_WITH_RC4_128_SHA = { 0x00,0x05 };
3289 CipherSuite TLS_RSA_WITH_IDEA_CBC_SHA = { 0x00,0x07 };
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3296 CipherSuite TLS_RSA_WITH_DES_CBC_SHA = { 0x00,0x09 };
3297 CipherSuite TLS_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x0A };
3299 The following CipherSuite definitions are used for server-
3300 authenticated (and optionally client-authenticated) Diffie-Hellman.
3301 DH denotes cipher suites in which the server's certificate contains
3302 the Diffie-Hellman parameters signed by the certificate authority
3303 (CA). DHE denotes ephemeral Diffie-Hellman, where the Diffie-Hellman
3304 parameters are signed by a DSS or RSA certificate, which has been
3305 signed by the CA. The signing algorithm used is specified after the
3306 DH or DHE parameter. The server can request an RSA or DSS signature-
3307 capable certificate from the client for client authentication or it
3308 may request a Diffie-Hellman certificate. Any Diffie-Hellman
3309 certificate provided by the client must use the parameters (group and
3310 generator) described by the server.
3312 CipherSuite TLS_DH_DSS_WITH_DES_CBC_SHA = { 0x00,0x0C };
3313 CipherSuite TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA = { 0x00,0x0D };
3314 CipherSuite TLS_DH_RSA_WITH_DES_CBC_SHA = { 0x00,0x0F };
3315 CipherSuite TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x10 };
3316 CipherSuite TLS_DHE_DSS_WITH_DES_CBC_SHA = { 0x00,0x12 };
3317 CipherSuite TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA = { 0x00,0x13 };
3318 CipherSuite TLS_DHE_RSA_WITH_DES_CBC_SHA = { 0x00,0x15 };
3319 CipherSuite TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x16 };
3321 The following cipher suites are used for completely anonymous Diffie-
3322 Hellman communications in which neither party is authenticated. Note
3323 that this mode is vulnerable to man-in-the-middle attacks and is
3324 therefore deprecated.
3326 CipherSuite TLS_DH_anon_WITH_RC4_128_MD5 = { 0x00,0x18 };
3327 CipherSuite TLS_DH_anon_WITH_DES_CBC_SHA = { 0x00,0x1A };
3328 CipherSuite TLS_DH_anon_WITH_3DES_EDE_CBC_SHA = { 0x00,0x1B };
3330 When SSLv3 and TLS 1.0 were designed, the United States restricted
3331 the export of cryptographic software containing certain strong
3332 encryption algorithms. A series of cipher suites were designed to
3333 operate at reduced key lengths in order to comply with those
3334 regulations. Due to advances in computer performance, these
3335 algorithms are now unacceptably weak and export restrictions have
3336 since been loosened. TLS 1.1 implementations MUST NOT negotiate these
3337 cipher suites in TLS 1.1 mode. However, for backward compatibility
3338 they may be offered in the ClientHello for use with TLS 1.0 or SSLv3
3339 only servers. TLS 1.1 clients MUST check that the server did not
3340 choose one of these cipher suites during the handshake. These
3341 ciphersuites are listed below for informational purposes and to
3342 reserve the numbers.
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3350 CipherSuite TLS_RSA_EXPORT_WITH_RC4_40_MD5 = { 0x00,0x03 };
3351 CipherSuite TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5 = { 0x00,0x06 };
3352 CipherSuite TLS_RSA_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x08 };
3353 CipherSuite TLS_DH_DSS_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x0B };
3354 CipherSuite TLS_DH_RSA_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x0E };
3355 CipherSuite TLS_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x11 };
3356 CipherSuite TLS_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x14 };
3357 CipherSuite TLS_DH_anon_EXPORT_WITH_RC4_40_MD5 = { 0x00,0x17 };
3358 CipherSuite TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x19 };
3360 The following cipher suites were defined in [TLSKRB] and are included
3361 here for completeness. See [TLSKRB] for details:
3363 CipherSuite TLS_KRB5_WITH_DES_CBC_SHA = { 0x00,0x1E };
3364 CipherSuite TLS_KRB5_WITH_3DES_EDE_CBC_SHA = { 0x00,0x1F };
3365 CipherSuite TLS_KRB5_WITH_RC4_128_SHA = { 0x00,0x20 };
3366 CipherSuite TLS_KRB5_WITH_IDEA_CBC_SHA = { 0x00,0x21 };
3367 CipherSuite TLS_KRB5_WITH_DES_CBC_MD5 = { 0x00,0x22 };
3368 CipherSuite TLS_KRB5_WITH_3DES_EDE_CBC_MD5 = { 0x00,0x23 };
3369 CipherSuite TLS_KRB5_WITH_RC4_128_MD5 = { 0x00,0x24 };
3370 CipherSuite TLS_KRB5_WITH_IDEA_CBC_MD5 = { 0x00,0x25 };
3372 The following exportable cipher suites were defined in [TLSKRB] and
3373 are included here for completeness. TLS 1.1 implementations MUST NOT
3374 negotiate these cipher suites.
3376 CipherSuite TLS_KRB5_EXPORT_WITH_DES_CBC_40_SHA = { 0x00,0x26
3378 CipherSuite TLS_KRB5_EXPORT_WITH_RC2_CBC_40_SHA = { 0x00,0x27
3380 CipherSuite TLS_KRB5_EXPORT_WITH_RC4_40_SHA = { 0x00,0x28
3382 CipherSuite TLS_KRB5_EXPORT_WITH_DES_CBC_40_MD5 = { 0x00,0x29
3384 CipherSuite TLS_KRB5_EXPORT_WITH_RC2_CBC_40_MD5 = { 0x00,0x2A
3386 CipherSuite TLS_KRB5_EXPORT_WITH_RC4_40_MD5 = { 0x00,0x2B
3389 The following cipher suites were defined in [TLSAES] and are included
3390 here for completeness. See [TLSAES] for details:
3392 CipherSuite TLS_RSA_WITH_AES_128_CBC_SHA = { 0x00, 0x2F };
3393 CipherSuite TLS_DH_DSS_WITH_AES_128_CBC_SHA = { 0x00, 0x30 };
3394 CipherSuite TLS_DH_RSA_WITH_AES_128_CBC_SHA = { 0x00, 0x31 };
3395 CipherSuite TLS_DHE_DSS_WITH_AES_128_CBC_SHA = { 0x00, 0x32 };
3396 CipherSuite TLS_DHE_RSA_WITH_AES_128_CBC_SHA = { 0x00, 0x33 };
3397 CipherSuite TLS_DH_anon_WITH_AES_128_CBC_SHA = { 0x00, 0x34 };
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3404 CipherSuite TLS_RSA_WITH_AES_256_CBC_SHA = { 0x00, 0x35 };
3405 CipherSuite TLS_DH_DSS_WITH_AES_256_CBC_SHA = { 0x00, 0x36 };
3406 CipherSuite TLS_DH_RSA_WITH_AES_256_CBC_SHA = { 0x00, 0x37 };
3407 CipherSuite TLS_DHE_DSS_WITH_AES_256_CBC_SHA = { 0x00, 0x38 };
3408 CipherSuite TLS_DHE_RSA_WITH_AES_256_CBC_SHA = { 0x00, 0x39 };
3409 CipherSuite TLS_DH_anon_WITH_AES_256_CBC_SHA = { 0x00, 0x3A };
3411 The cipher suite space is divided into three regions:
3413 1. Cipher suite values with first byte 0x00 (zero)
3414 through decimal 191 (0xBF) inclusive are reserved for the IETF
3415 Standards Track protocols.
3417 2. Cipher suite values with first byte decimal 192 (0xC0)
3418 through decimal 254 (0xFE) inclusive are reserved
3419 for assignment for non-Standards Track methods.
3421 3. Cipher suite values with first byte 0xFF are
3422 reserved for private use.
3423 Additional information describing the role of IANA in the allocation
3424 of cipher suite code points is described in Section 11.
3426 Note: The cipher suite values { 0x00, 0x1C } and { 0x00, 0x1D } are
3427 reserved to avoid collision with Fortezza-based cipher suites in SSL
3430 A.6. The Security Parameters
3432 These security parameters are determined by the TLS Handshake
3433 Protocol and provided as parameters to the TLS Record Layer in order
3434 to initialize a connection state. SecurityParameters includes:
3436 enum { null(0), (255) } CompressionMethod;
3438 enum { server, client } ConnectionEnd;
3440 enum { null, rc4, rc2, des, 3des, des40, idea }
3441 BulkCipherAlgorithm;
3443 enum { stream, block } CipherType;
3445 enum { null, md5, sha } MACAlgorithm;
3447 /* The algorithms specified in CompressionMethod,
3448 BulkCipherAlgorithm, and MACAlgorithm may be added to. */
3451 ConnectionEnd entity;
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3458 BulkCipherAlgorithm bulk_cipher_algorithm;
3459 CipherType cipher_type;
3461 uint8 key_material_length;
3462 MACAlgorithm mac_algorithm;
3464 CompressionMethod compression_algorithm;
3465 opaque master_secret[48];
3466 opaque client_random[32];
3467 opaque server_random[32];
3468 } SecurityParameters;
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3514 application protocol
3515 An application protocol is a protocol that normally layers
3516 directly on top of the transport layer (e.g., TCP/IP). Examples
3517 include HTTP, TELNET, FTP, and SMTP.
3520 See public key cryptography.
3523 Authentication is the ability of one entity to determine the
3524 identity of another entity.
3527 A block cipher is an algorithm that operates on plaintext in
3528 groups of bits, called blocks. 64 bits is a common block size.
3531 A symmetric encryption algorithm used to encrypt large quantities
3534 cipher block chaining (CBC)
3535 CBC is a mode in which every plaintext block encrypted with a
3536 block cipher is first exclusive-ORed with the previous ciphertext
3537 block (or, in the case of the first block, with the
3538 initialization vector). For decryption, every block is first
3539 decrypted, then exclusive-ORed with the previous ciphertext block
3543 As part of the X.509 protocol (a.k.a. ISO Authentication
3544 framework), certificates are assigned by a trusted Certificate
3545 Authority and provide a strong binding between a party's identity
3546 or some other attributes and its public key.
3549 The application entity that initiates a TLS connection to a
3550 server. This may or may not imply that the client initiated the
3551 underlying transport connection. The primary operational
3552 difference between the server and client is that the server is
3553 generally authenticated, while the client is only optionally
3557 The key used to encrypt data written by the client.
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3566 client write MAC secret
3567 The secret data used to authenticate data written by the client.
3570 A connection is a transport (in the OSI layering model
3571 definition) that provides a suitable type of service. For TLS,
3572 such connections are peer to peer relationships. The connections
3573 are transient. Every connection is associated with one session.
3575 Data Encryption Standard
3576 DES is a very widely used symmetric encryption algorithm. DES is
3577 a block cipher with a 56 bit key and an 8 byte block size. Note
3578 that in TLS, for key generation purposes, DES is treated as
3579 having an 8 byte key length (64 bits), but it still only provides
3580 56 bits of protection. (The low bit of each key byte is presumed
3581 to be set to produce odd parity in that key byte.) DES can also
3582 be operated in a mode where three independent keys and three
3583 encryptions are used for each block of data; this uses 168 bits
3584 of key (24 bytes in the TLS key generation method) and provides
3585 the equivalent of 112 bits of security. [DES], [3DES]
3587 Digital Signature Standard (DSS)
3588 A standard for digital signing, including the Digital Signing
3589 Algorithm, approved by the National Institute of Standards and
3590 Technology, defined in NIST FIPS PUB 186, "Digital Signature
3591 Standard," published May, 1994 by the U.S. Dept. of Commerce.
3595 Digital signatures utilize public key cryptography and one-way
3596 hash functions to produce a signature of the data that can be
3597 authenticated, and is difficult to forge or repudiate.
3600 An initial negotiation between client and server that establishes
3601 the parameters of their transactions.
3603 Initialization Vector (IV)
3604 When a block cipher is used in CBC mode, the initialization
3605 vector is exclusive-ORed with the first plaintext block prior to
3609 A 64-bit block cipher designed by Xuejia Lai and James Massey.
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3620 Message Authentication Code (MAC)
3621 A Message Authentication Code is a one-way hash computed from a
3622 message and some secret data. It is difficult to forge without
3623 knowing the secret data. Its purpose is to detect if the message
3627 Secure secret data used for generating encryption keys, MAC
3631 MD5 is a secure hashing function that converts an arbitrarily
3632 long data stream into a digest of fixed size (16 bytes). [MD5]
3634 public key cryptography
3635 A class of cryptographic techniques employing two-key ciphers.
3636 Messages encrypted with the public key can only be decrypted with
3637 the associated private key. Conversely, messages signed with the
3638 private key can be verified with the public key.
3640 one-way hash function
3641 A one-way transformation that converts an arbitrary amount of
3642 data into a fixed-length hash. It is computationally hard to
3643 reverse the transformation or to find collisions. MD5 and SHA are
3644 examples of one-way hash functions.
3647 A block cipher developed by Ron Rivest at RSA Data Security, Inc.
3648 [RSADSI] described in [RC2].
3651 A stream cipher invented by Ron Rivest. A compatible cipher is
3655 A very widely used public-key algorithm that can be used for
3656 either encryption or digital signing. [RSA]
3659 The server is the application entity that responds to requests
3660 for connections from clients. See also under client.
3663 A TLS session is an association between a client and a server.
3664 Sessions are created by the handshake protocol. Sessions define a
3665 set of cryptographic security parameters, which can be shared
3666 among multiple connections. Sessions are used to avoid the
3667 expensive negotiation of new security parameters for each
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3677 A session identifier is a value generated by a server that
3678 identifies a particular session.
3681 The key used to encrypt data written by the server.
3683 server write MAC secret
3684 The secret data used to authenticate data written by the server.
3687 The Secure Hash Algorithm is defined in FIPS PUB 180-2. It
3688 produces a 20-byte output. Note that all references to SHA
3689 actually use the modified SHA-1 algorithm. [SHA]
3692 Netscape's Secure Socket Layer protocol [SSL3]. TLS is based on
3696 An encryption algorithm that converts a key into a
3697 cryptographically-strong keystream, which is then exclusive-ORed
3703 Transport Layer Security (TLS)
3704 This protocol; also, the Transport Layer Security working group
3705 of the Internet Engineering Task Force (IETF). See "Comments" at
3706 the end of this document.
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3728 C. CipherSuite definitions
3730 CipherSuite Key Cipher Hash
3733 TLS_NULL_WITH_NULL_NULL NULL NULL NULL
3734 TLS_RSA_WITH_NULL_MD5 RSA NULL MD5
3735 TLS_RSA_WITH_NULL_SHA RSA NULL SHA
3736 TLS_RSA_WITH_RC4_128_MD5 RSA RC4_128 MD5
3737 TLS_RSA_WITH_RC4_128_SHA RSA RC4_128 SHA
3738 TLS_RSA_WITH_IDEA_CBC_SHA RSA IDEA_CBC SHA
3739 TLS_RSA_WITH_DES_CBC_SHA RSA DES_CBC SHA
3740 TLS_RSA_WITH_3DES_EDE_CBC_SHA RSA 3DES_EDE_CBC SHA
3741 TLS_DH_DSS_WITH_DES_CBC_SHA DH_DSS DES_CBC SHA
3742 TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA DH_DSS 3DES_EDE_CBC SHA
3743 TLS_DH_RSA_WITH_DES_CBC_SHA DH_RSA DES_CBC SHA
3744 TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA DH_RSA 3DES_EDE_CBC SHA
3745 TLS_DHE_DSS_WITH_DES_CBC_SHA DHE_DSS DES_CBC SHA
3746 TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA DHE_DSS 3DES_EDE_CBC SHA
3747 TLS_DHE_RSA_WITH_DES_CBC_SHA DHE_RSA DES_CBC SHA
3748 TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA DHE_RSA 3DES_EDE_CBC SHA
3749 TLS_DH_anon_WITH_RC4_128_MD5 DH_anon RC4_128 MD5
3750 TLS_DH_anon_WITH_DES_CBC_SHA DH_anon DES_CBC SHA
3751 TLS_DH_anon_WITH_3DES_EDE_CBC_SHA DH_anon 3DES_EDE_CBC SHA
3755 Algorithm Description Key size limit
3757 DHE_DSS Ephemeral DH with DSS signatures None
3758 DHE_RSA Ephemeral DH with RSA signatures None
3759 DH_anon Anonymous DH, no signatures None
3760 DH_DSS DH with DSS-based certificates None
3761 DH_RSA DH with RSA-based certificates None
3763 NULL No key exchange N/A
3764 RSA RSA key exchange None
3766 Key Expanded IV Block
3767 Cipher Type Material Key Material Size Size
3769 NULL Stream 0 0 0 N/A
3770 IDEA_CBC Block 16 16 8 8
3771 RC2_CBC_40 Block 5 16 8 8
3772 RC4_40 Stream 5 16 0 N/A
3773 RC4_128 Stream 16 16 0 N/A
3774 DES40_CBC Block 5 8 8 8
3775 DES_CBC Block 8 8 8 8
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3782 3DES_EDE_CBC Block 24 24 8 8
3785 Indicates whether this is a stream cipher or a block cipher
3786 running in CBC mode.
3789 The number of bytes from the key_block that are used for
3790 generating the write keys.
3792 Expanded Key Material
3793 The number of bytes actually fed into the encryption algorithm
3796 How much data needs to be generated for the initialization
3797 vector. Zero for stream ciphers; equal to the block size for
3801 The amount of data a block cipher enciphers in one chunk; a
3802 block cipher running in CBC mode can only encrypt an even
3803 multiple of its block size.
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3836 D. Implementation Notes
3838 The TLS protocol cannot prevent many common security mistakes. This
3839 section provides several recommendations to assist implementors.
3841 D.1 Random Number Generation and Seeding
3843 TLS requires a cryptographically-secure pseudorandom number generator
3844 (PRNG). Care must be taken in designing and seeding PRNGs. PRNGs
3845 based on secure hash operations, most notably MD5 and/or SHA, are
3846 acceptable, but cannot provide more security than the size of the
3847 random number generator state. (For example, MD5-based PRNGs usually
3848 provide 128 bits of state.)
3850 To estimate the amount of seed material being produced, add the
3851 number of bits of unpredictable information in each seed byte. For
3852 example, keystroke timing values taken from a PC compatible's 18.2 Hz
3853 timer provide 1 or 2 secure bits each, even though the total size of
3854 the counter value is 16 bits or more. To seed a 128-bit PRNG, one
3855 would thus require approximately 100 such timer values.
3857 [RANDOM] provides guidance on the generation of random values.
3859 D.2 Certificates and authentication
3861 Implementations are responsible for verifying the integrity of
3862 certificates and should generally support certificate revocation
3863 messages. Certificates should always be verified to ensure proper
3864 signing by a trusted Certificate Authority (CA). The selection and
3865 addition of trusted CAs should be done very carefully. Users should
3866 be able to view information about the certificate and root CA.
3870 TLS supports a range of key sizes and security levels, including some
3871 which provide no or minimal security. A proper implementation will
3872 probably not support many cipher suites. For example, 40-bit
3873 encryption is easily broken, so implementations requiring strong
3874 security should not allow 40-bit keys. Similarly, anonymous Diffie-
3875 Hellman is strongly discouraged because it cannot prevent man-in-the-
3876 middle attacks. Applications should also enforce minimum and maximum
3877 key sizes. For example, certificate chains containing 512-bit RSA
3878 keys or signatures are not appropriate for high-security
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3890 E. Backward Compatibility With SSL
3892 For historical reasons and in order to avoid a profligate consumption
3893 of reserved port numbers, application protocols which are secured by
3894 TLS 1.1, TLS 1.0, SSL 3.0, and SSL 2.0 all frequently share the same
3895 connection port: for example, the https protocol (HTTP secured by SSL
3896 or TLS) uses port 443 regardless of which security protocol it is
3897 using. Thus, some mechanism must be determined to distinguish and
3898 negotiate among the various protocols.
3900 TLS versions 1.1, 1.0, and SSL 3.0 are very similar; thus, supporting
3901 both is easy. TLS clients who wish to negotiate with such older
3902 servers SHOULD send client hello messages using the SSL 3.0 record
3903 format and client hello structure, sending {3, 2} for the version
3904 field to note that they support TLS 1.1. If the server supports only
3905 TLS 1.0 or SSL 3.0, it will respond with a downrev 3.0 server hello;
3906 if it supports TLS 1.1 it will respond with a TLS 1.1 server hello.
3907 The negotiation then proceeds as appropriate for the negotiated
3910 Similarly, a TLS 1.1 server which wishes to interoperate with TLS
3911 1.0 or SSL 3.0 clients SHOULD accept SSL 3.0 client hello messages
3912 and respond with a SSL 3.0 server hello if an SSL 3.0 client hello
3913 with a version field of {3, 0} is received, denoting that this client
3914 does not support TLS. Similarly, if a SSL 3.0 or TLS 1.0 hello with a
3915 version field of {3, 1} is received, the server SHOULD respond with a
3916 TLS 1.0 hello with a version field of {3, 1}.
3918 Whenever a client already knows the highest protocol known to a
3919 server (for example, when resuming a session), it SHOULD initiate the
3920 connection in that native protocol.
3922 TLS 1.1 clients that support SSL Version 2.0 servers MUST send SSL
3923 Version 2.0 client hello messages [SSL2]. TLS servers SHOULD accept
3924 either client hello format if they wish to support SSL 2.0 clients on
3925 the same connection port. The only deviations from the Version 2.0
3926 specification are the ability to specify a version with a value of
3927 three and the support for more ciphering types in the CipherSpec.
3929 Warning: The ability to send Version 2.0 client hello messages will be
3930 phased out with all due haste. Implementors SHOULD make every
3931 effort to move forward as quickly as possible. Version 3.0
3932 provides better mechanisms for moving to newer versions.
3934 The following cipher specifications are carryovers from SSL Version
3935 2.0. These are assumed to use RSA for key exchange and
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3944 V2CipherSpec TLS_RC4_128_WITH_MD5 = { 0x01,0x00,0x80 };
3945 V2CipherSpec TLS_RC4_128_EXPORT40_WITH_MD5 = { 0x02,0x00,0x80 };
3946 V2CipherSpec TLS_RC2_CBC_128_CBC_WITH_MD5 = { 0x03,0x00,0x80 };
3947 V2CipherSpec TLS_RC2_CBC_128_CBC_EXPORT40_WITH_MD5
3948 = { 0x04,0x00,0x80 };
3949 V2CipherSpec TLS_IDEA_128_CBC_WITH_MD5 = { 0x05,0x00,0x80 };
3950 V2CipherSpec TLS_DES_64_CBC_WITH_MD5 = { 0x06,0x00,0x40 };
3951 V2CipherSpec TLS_DES_192_EDE3_CBC_WITH_MD5 = { 0x07,0x00,0xC0 };
3953 Cipher specifications native to TLS can be included in Version 2.0
3954 client hello messages using the syntax below. Any V2CipherSpec
3955 element with its first byte equal to zero will be ignored by Version
3956 2.0 servers. Clients sending any of the above V2CipherSpecs SHOULD
3957 also include the TLS equivalent (see Appendix A.5):
3959 V2CipherSpec (see TLS name) = { 0x00, CipherSuite };
3961 Note: TLS 1.1 clients may generate the SSLv2 EXPORT cipher suites in
3962 handshakes for backward compatibility but MUST NOT negotiate them in
3965 E.1. Version 2 client hello
3967 The Version 2.0 client hello message is presented below using this
3968 document's presentation model. The true definition is still assumed
3969 to be the SSL Version 2.0 specification. Note that this message MUST
3970 be sent directly on the wire, not wrapped as an SSLv3 record
3972 uint8 V2CipherSpec[3];
3978 uint16 cipher_spec_length;
3979 uint16 session_id_length;
3980 uint16 challenge_length;
3981 V2CipherSpec cipher_specs[V2ClientHello.cipher_spec_length];
3982 opaque session_id[V2ClientHello.session_id_length];
3983 opaque challenge[V2ClientHello.challenge_length;
3987 This field is the length of the following data in bytes. The high
3988 bit MUST be 1 and is not part of the length.
3991 This field, in conjunction with the version field, identifies a
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3998 version 2 client hello message. The value SHOULD be one (1).
4001 The highest version of the protocol supported by the client
4002 (equals ProtocolVersion.version, see Appendix A.1).
4005 This field is the total length of the field cipher_specs. It
4006 cannot be zero and MUST be a multiple of the V2CipherSpec length
4010 This field MUST have a value of zero.
4013 The length in bytes of the client's challenge to the server to
4014 authenticate itself. When using the SSLv2 backward compatible
4015 handshake the client MUST use a 32-byte challenge.
4018 This is a list of all CipherSpecs the client is willing and able
4019 to use. There MUST be at least one CipherSpec acceptable to the
4023 This field MUST be empty.
4026 The client challenge to the server for the server to identify
4027 itself is a (nearly) arbitrary length random. The TLS server will
4028 right justify the challenge data to become the ClientHello.random
4029 data (padded with leading zeroes, if necessary), as specified in
4030 this protocol specification. If the length of the challenge is
4031 greater than 32 bytes, only the last 32 bytes are used. It is
4032 legitimate (but not necessary) for a V3 server to reject a V2
4033 ClientHello that has fewer than 16 bytes of challenge data.
4035 Note: Requests to resume a TLS session MUST use a TLS client hello.
4037 E.2. Avoiding man-in-the-middle version rollback
4039 When TLS clients fall back to Version 2.0 compatibility mode, they
4040 SHOULD use special PKCS #1 block formatting. This is done so that TLS
4041 servers will reject Version 2.0 sessions with TLS-capable clients.
4043 When TLS clients are in Version 2.0 compatibility mode, they set the
4044 right-hand (least-significant) 8 random bytes of the PKCS padding
4045 (not including the terminal null of the padding) for the RSA
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4052 encryption of the ENCRYPTED-KEY-DATA field of the CLIENT-MASTER-KEY
4053 to 0x03 (the other padding bytes are random). After decrypting the
4054 ENCRYPTED-KEY-DATA field, servers that support TLS SHOULD issue an
4055 error if these eight padding bytes are 0x03. Version 2.0 servers
4056 receiving blocks padded in this manner will proceed normally.
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4106 F. Security analysis
4108 The TLS protocol is designed to establish a secure connection between
4109 a client and a server communicating over an insecure channel. This
4110 document makes several traditional assumptions, including that
4111 attackers have substantial computational resources and cannot obtain
4112 secret information from sources outside the protocol. Attackers are
4113 assumed to have the ability to capture, modify, delete, replay, and
4114 otherwise tamper with messages sent over the communication channel.
4115 This appendix outlines how TLS has been designed to resist a variety
4118 F.1. Handshake protocol
4120 The handshake protocol is responsible for selecting a CipherSpec and
4121 generating a Master Secret, which together comprise the primary
4122 cryptographic parameters associated with a secure session. The
4123 handshake protocol can also optionally authenticate parties who have
4124 certificates signed by a trusted certificate authority.
4126 F.1.1. Authentication and key exchange
4128 TLS supports three authentication modes: authentication of both
4129 parties, server authentication with an unauthenticated client, and
4130 total anonymity. Whenever the server is authenticated, the channel is
4131 secure against man-in-the-middle attacks, but completely anonymous
4132 sessions are inherently vulnerable to such attacks. Anonymous
4133 servers cannot authenticate clients. If the server is authenticated,
4134 its certificate message must provide a valid certificate chain
4135 leading to an acceptable certificate authority. Similarly,
4136 authenticated clients must supply an acceptable certificate to the
4137 server. Each party is responsible for verifying that the other's
4138 certificate is valid and has not expired or been revoked.
4140 The general goal of the key exchange process is to create a
4141 pre_master_secret known to the communicating parties and not to
4142 attackers. The pre_master_secret will be used to generate the
4143 master_secret (see Section 8.1). The master_secret is required to
4144 generate the finished messages, encryption keys, and MAC secrets (see
4145 Sections 7.4.8, 7.4.9 and 6.3). By sending a correct finished
4146 message, parties thus prove that they know the correct
4149 F.1.1.1. Anonymous key exchange
4151 Completely anonymous sessions can be established using RSA or Diffie-
4152 Hellman for key exchange. With anonymous RSA, the client encrypts a
4153 pre_master_secret with the server's uncertified public key extracted
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4160 from the server key exchange message. The result is sent in a client
4161 key exchange message. Since eavesdroppers do not know the server's
4162 private key, it will be infeasible for them to decode the
4165 Note: No anonymous RSA Cipher Suites are defined in this document.
4167 With Diffie-Hellman, the server's public parameters are contained in
4168 the server key exchange message and the client's are sent in the
4169 client key exchange message. Eavesdroppers who do not know the
4170 private values should not be able to find the Diffie-Hellman result
4171 (i.e. the pre_master_secret).
4173 Warning: Completely anonymous connections only provide protection
4174 against passive eavesdropping. Unless an independent tamper-
4175 proof channel is used to verify that the finished messages
4176 were not replaced by an attacker, server authentication is
4177 required in environments where active man-in-the-middle
4178 attacks are a concern.
4180 F.1.1.2. RSA key exchange and authentication
4182 With RSA, key exchange and server authentication are combined. The
4183 public key may be either contained in the server's certificate or may
4184 be a temporary RSA key sent in a server key exchange message. When
4185 temporary RSA keys are used, they are signed by the server's RSA
4186 certificate. The signature includes the current ClientHello.random,
4187 so old signatures and temporary keys cannot be replayed. Servers may
4188 use a single temporary RSA key for multiple negotiation sessions.
4190 Note: The temporary RSA key option is useful if servers need large
4191 certificates but must comply with government-imposed size limits
4192 on keys used for key exchange.
4194 Note that if ephemeral RSA is not used, compromise of the server's
4195 static RSA key results in a loss of confidentiality for all sessions
4196 protected under that static key. TLS users desiring Perfect Forward
4197 Secrecy should use DHE cipher suites. The damage done by exposure of
4198 a private key can be limited by changing one's private key (and
4199 certificate) frequently.
4201 After verifying the server's certificate, the client encrypts a
4202 pre_master_secret with the server's public key. By successfully
4203 decoding the pre_master_secret and producing a correct finished
4204 message, the server demonstrates that it knows the private key
4205 corresponding to the server certificate.
4207 When RSA is used for key exchange, clients are authenticated using
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4214 the certificate verify message (see Section 7.4.8). The client signs
4215 a value derived from the master_secret and all preceding handshake
4216 messages. These handshake messages include the server certificate,
4217 which binds the signature to the server, and ServerHello.random,
4218 which binds the signature to the current handshake process.
4220 F.1.1.3. Diffie-Hellman key exchange with authentication
4222 When Diffie-Hellman key exchange is used, the server can either
4223 supply a certificate containing fixed Diffie-Hellman parameters or
4224 can use the server key exchange message to send a set of temporary
4225 Diffie-Hellman parameters signed with a DSS or RSA certificate.
4226 Temporary parameters are hashed with the hello.random values before
4227 signing to ensure that attackers do not replay old parameters. In
4228 either case, the client can verify the certificate or signature to
4229 ensure that the parameters belong to the server.
4231 If the client has a certificate containing fixed Diffie-Hellman
4232 parameters, its certificate contains the information required to
4233 complete the key exchange. Note that in this case the client and
4234 server will generate the same Diffie-Hellman result (i.e.,
4235 pre_master_secret) every time they communicate. To prevent the
4236 pre_master_secret from staying in memory any longer than necessary,
4237 it should be converted into the master_secret as soon as possible.
4238 Client Diffie-Hellman parameters must be compatible with those
4239 supplied by the server for the key exchange to work.
4241 If the client has a standard DSS or RSA certificate or is
4242 unauthenticated, it sends a set of temporary parameters to the server
4243 in the client key exchange message, then optionally uses a
4244 certificate verify message to authenticate itself.
4246 If the same DH keypair is to be used for multiple handshakes, either
4247 because the client or server has a certificate containing a fixed DH
4248 keypair or because the server is reusing DH keys, care must be taken
4249 to prevent small subgroup attacks. Implementations SHOULD follow the
4250 guidelines found in [SUBGROUP].
4252 Small subgroup attacks are most easily avoided by using one of the
4253 DHE ciphersuites and generating a fresh DH private key (X) for each
4254 handshake. If a suitable base (such as 2) is chosen, g^X mod p can be
4255 computed very quickly so the performance cost is minimized.
4256 Additionally, using a fresh key for each handshake provides Perfect
4257 Forward Secrecy. Implementations SHOULD generate a new X for each
4258 handshake when using DHE ciphersuites.
4260 F.1.2. Version rollback attacks
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4268 Because TLS includes substantial improvements over SSL Version 2.0,
4269 attackers may try to make TLS-capable clients and servers fall back
4270 to Version 2.0. This attack can occur if (and only if) two TLS-
4271 capable parties use an SSL 2.0 handshake.
4273 Although the solution using non-random PKCS #1 block type 2 message
4274 padding is inelegant, it provides a reasonably secure way for Version
4275 3.0 servers to detect the attack. This solution is not secure against
4276 attackers who can brute force the key and substitute a new ENCRYPTED-
4277 KEY-DATA message containing the same key (but with normal padding)
4278 before the application specified wait threshold has expired. Parties
4279 concerned about attacks of this scale should not be using 40-bit
4280 encryption keys anyway. Altering the padding of the least-significant
4281 8 bytes of the PKCS padding does not impact security for the size of
4282 the signed hashes and RSA key lengths used in the protocol, since
4283 this is essentially equivalent to increasing the input block size by
4286 F.1.3. Detecting attacks against the handshake protocol
4288 An attacker might try to influence the handshake exchange to make the
4289 parties select different encryption algorithms than they would
4292 For this attack, an attacker must actively change one or more
4293 handshake messages. If this occurs, the client and server will
4294 compute different values for the handshake message hashes. As a
4295 result, the parties will not accept each others' finished messages.
4296 Without the master_secret, the attacker cannot repair the finished
4297 messages, so the attack will be discovered.
4299 F.1.4. Resuming sessions
4301 When a connection is established by resuming a session, new
4302 ClientHello.random and ServerHello.random values are hashed with the
4303 session's master_secret. Provided that the master_secret has not been
4304 compromised and that the secure hash operations used to produce the
4305 encryption keys and MAC secrets are secure, the connection should be
4306 secure and effectively independent from previous connections.
4307 Attackers cannot use known encryption keys or MAC secrets to
4308 compromise the master_secret without breaking the secure hash
4309 operations (which use both SHA and MD5).
4311 Sessions cannot be resumed unless both the client and server agree.
4312 If either party suspects that the session may have been compromised,
4313 or that certificates may have expired or been revoked, it should
4314 force a full handshake. An upper limit of 24 hours is suggested for
4315 session ID lifetimes, since an attacker who obtains a master_secret
4319 Dierks & Rescorla Standards Track [Page 80]
\fdraft-ietf-tls-rfc2246-bis-10.txt TLS April 2005
4322 may be able to impersonate the compromised party until the
4323 corresponding session ID is retired. Applications that may be run in
4324 relatively insecure environments should not write session IDs to
4329 TLS uses hash functions very conservatively. Where possible, both MD5
4330 and SHA are used in tandem to ensure that non-catastrophic flaws in
4331 one algorithm will not break the overall protocol.
4333 F.2. Protecting application data
4335 The master_secret is hashed with the ClientHello.random and
4336 ServerHello.random to produce unique data encryption keys and MAC
4337 secrets for each connection.
4339 Outgoing data is protected with a MAC before transmission. To prevent
4340 message replay or modification attacks, the MAC is computed from the
4341 MAC secret, the sequence number, the message length, the message
4342 contents, and two fixed character strings. The message type field is
4343 necessary to ensure that messages intended for one TLS Record Layer
4344 client are not redirected to another. The sequence number ensures
4345 that attempts to delete or reorder messages will be detected. Since
4346 sequence numbers are 64-bits long, they should never overflow.
4347 Messages from one party cannot be inserted into the other's output,
4348 since they use independent MAC secrets. Similarly, the server-write
4349 and client-write keys are independent so stream cipher keys are used
4352 If an attacker does break an encryption key, all messages encrypted
4353 with it can be read. Similarly, compromise of a MAC key can make
4354 message modification attacks possible. Because MACs are also
4355 encrypted, message-alteration attacks generally require breaking the
4356 encryption algorithm as well as the MAC.
4358 Note: MAC secrets may be larger than encryption keys, so messages can
4359 remain tamper resistant even if encryption keys are broken.
4363 [CBCATT] describes a chosen plaintext attack on TLS that depends
4364 on knowing the IV for a record. Previous versions of TLS [TLS1.0]
4365 used the CBC residue of the previous record as the IV and
4366 therefore enabled this attack. This version uses an explicit IV
4367 in order to protect against this attack.
4373 Dierks & Rescorla Standards Track [Page 81]
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4376 F.4 Security of Composite Cipher Modes
4378 TLS secures transmitted application data via the use of symmetric
4379 encryption and authentication functions defined in the negotiated
4380 ciphersuite. The objective is to protect both the integrity and
4381 confidentiality of the transmitted data from malicious actions by
4382 active attackers in the network. It turns out that the order in
4383 which encryption and authentication functions are applied to the
4384 data plays an important role for achieving this goal [ENCAUTH].
4386 The most robust method, called encrypt-then-authenticate, first
4387 applies encryption to the data and then applies a MAC to the
4388 ciphertext. This method ensures that the integrity and
4389 confidentiality goals are obtained with ANY pair of encryption
4390 and MAC functions provided that the former is secure against
4391 chosen plaintext attacks and the MAC is secure against chosen-
4392 message attacks. TLS uses another method, called authenticate-
4393 then-encrypt, in which first a MAC is computed on the plaintext
4394 and then the concatenation of plaintext and MAC is encrypted.
4395 This method has been proven secure for CERTAIN combinations of
4396 encryption functions and MAC functions, but is not guaranteed to
4397 be secure in general. In particular, it has been shown that there
4398 exist perfectly secure encryption functions (secure even in the
4399 information theoretic sense) that combined with any secure MAC
4400 function fail to provide the confidentiality goal against an
4401 active attack. Therefore, new ciphersuites and operation modes
4402 adopted into TLS need to be analyzed under the authenticate-then-
4403 encrypt method to verify that they achieve the stated integrity
4404 and confidentiality goals.
4406 Currently, the security of the authenticate-then-encrypt method
4407 has been proven for some important cases. One is the case of
4408 stream ciphers in which a computationally unpredictable pad of
4409 the length of the message plus the length of the MAC tag is
4410 produced using a pseudo-random generator and this pad is xor-ed
4411 with the concatenation of plaintext and MAC tag. The other is
4412 the case of CBC mode using a secure block cipher. In this case,
4413 security can be shown if one applies one CBC encryption pass to
4414 the concatenation of plaintext and MAC and uses a new,
4415 independent and unpredictable, IV for each new pair of plaintext
4416 and MAC. In previous versions of SSL, CBC mode was used properly
4417 EXCEPT that it used a predictable IV in the form of the last
4418 block of the previous ciphertext. This made TLS open to chosen
4419 plaintext attacks. This verson of the protocol is immune to
4420 those attacks. For exact details in the encryption modes proven
4421 secure see [ENCAUTH].
4427 Dierks & Rescorla Standards Track [Page 82]
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4430 F.5 Denial of Service
4432 TLS is susceptible to a number of denial of service (DoS)
4433 attacks. In particular, an attacker who initiates a large number
4434 of TCP connections can cause a server to consume large amounts of
4435 CPU doing RSA decryption. However, because TLS is generally used
4436 over TCP, it is difficult for the attacker to hide his point of
4437 origin if proper TCP SYN randomization is used [SEQNUM] by the
4440 Because TLS runs over TCP, it is also susceptible to a number of
4441 denial of service attacks on individual connections. In
4442 particular, attackers can forge RSTs, terminating connections, or
4443 forge partial TLS records, causing the connection to stall.
4444 These attacks cannot in general be defended against by a TCP-
4445 using protocol. Implementors or users who are concerned with this
4446 class of attack should use IPsec AH [AH] or ESP [ESP].
4450 For TLS to be able to provide a secure connection, both the client
4451 and server systems, keys, and applications must be secure. In
4452 addition, the implementation must be free of security errors.
4454 The system is only as strong as the weakest key exchange and
4455 authentication algorithm supported, and only trustworthy
4456 cryptographic functions should be used. Short public keys, 40-bit
4457 bulk encryption keys, and anonymous servers should be used with great
4458 caution. Implementations and users must be careful when deciding
4459 which certificates and certificate authorities are acceptable; a
4460 dishonest certificate authority can do tremendous damage.
4481 Dierks & Rescorla Standards Track [Page 83]
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4484 Security Considerations
4486 Security issues are discussed throughout this memo, especially in
4487 Appendices D, E, and F.
4489 Normative References
4491 [3DES] W. Tuchman, "Hellman Presents No Shortcut Solutions To DES,"
4492 IEEE Spectrum, v. 16, n. 7, July 1979, pp40-41.
4494 [DES] ANSI X3.106, "American National Standard for Information
4495 Systems-Data Link Encryption," American National Standards
4498 [DSS] NIST FIPS PUB 186-2, "Digital Signature Standard," National
4499 Institute of Standards and Technology, U.S. Department of
4502 [HMAC] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
4503 Hashing for Message Authentication," RFC 2104, February
4506 [IDEA] X. Lai, "On the Design and Security of Block Ciphers," ETH
4507 Series in Information Processing, v. 1, Konstanz: Hartung-
4510 [MD2] Kaliski, B., "The MD2 Message Digest Algorithm", RFC 1319,
4513 [MD5] Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321,
4516 [PKCS1] J. Jonsson, B. Kaliski, "3447 Public-Key Cryptography
4517 Standards (PKCS) #1: RSA Cryptography Specifications Version
4518 2.1", RFC 3447, February 2003"
4520 [PKIX] Housley, R., Ford, W., Polk, W. and D. Solo, "Internet
4521 Public Key Infrastructure: Part I: X.509 Certificate and CRL
4522 Profile", RFC 3280, April 2002.
4524 [RC2] Rivest, R., "A Description of the RC2(r) Encryption
4525 Algorithm", RFC 2268, January 1998.
4527 [SCH] B. Schneier. "Applied Cryptography: Protocols, Algorithms,
4528 and Source Code in C, 2ed", Published by John Wiley & Sons,
4535 Dierks & Rescorla Standards Track [Page 84]
\fdraft-ietf-tls-rfc2246-bis-10.txt TLS April 2005
4538 [SHA] NIST FIPS PUB 180-2, "Secure Hash Standard," National
4539 Institute of Standards and Technology, U.S. Department of
4540 Commerce., August 2001.
4542 [REQ] Bradner, S., "Key words for use in RFCs to Indicate
4543 Requirement Levels", BCP 14, RFC 2119, March 1997.
4545 [RFC2434] T. Narten, H. Alvestrand, "Guidelines for Writing an IANA
4546 Considerations Section in RFCs", RFC 3434, October 1998.
4548 [TLSAES] Chown, P. "Advanced Encryption Standard (AES) Ciphersuites
4549 for Transport Layer Security (TLS)", RFC 3268, Junr 2002.
4551 [TLSEXT] Blake-Wilson, S., Nystrom, M, Hopwood, D., Mikkelsen, J.,
4552 Wright, T., "Transport Layer Security (TLS) Extensions", RFC
4553 3546, June 2003. [TLSKRB] A. Medvinsky, M. Hur,
4554 "Addition of Kerberos Cipher Suites to Transport Layer
4555 Security (TLS)", RFC 2712, October 1999.
4558 Informative References
4560 [AH] Kent, S., and Atkinson, R., "IP Authentication Header", RFC
4561 2402, November 1998.
4563 [BLEI] Bleichenbacher D., "Chosen Ciphertext Attacks against
4564 Protocols Based on RSA Encryption Standard PKCS #1" in
4565 Advances in Cryptology -- CRYPTO'98, LNCS vol. 1462, pages:
4568 [CBCATT] Moeller, B., "Security of CBC Ciphersuites in SSL/TLS:
4569 Problems and Countermeasures",
4570 http://www.openssl.org/~bodo/tls-cbc.txt.
4572 [CBCTIME] Canvel, B., "Password Interception in a SSL/TLS Channel",
4573 http://lasecwww.epfl.ch/memo_ssl.shtml, 2003.
4575 [ENCAUTH] Krawczyk, H., "The Order of Encryption and Authentication
4576 for Protecting Communications (Or: How Secure is SSL?)",
4579 [ESP] Kent, S., and Atkinson, R., "IP Encapsulating Security
4580 Payload (ESP)", RFC 2406, November 1998.
4582 [FTP] Postel J., and J. Reynolds, "File Transfer Protocol", STD 9,
4583 RFC 959, October 1985.
4585 [HTTP] Berners-Lee, T., Fielding, R., and H. Frystyk, "Hypertext
4589 Dierks & Rescorla Standards Track [Page 85]
\fdraft-ietf-tls-rfc2246-bis-10.txt TLS April 2005
4592 Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996.
4594 [KPR03] Klima, V., Pokorny, O., Rosa, T., "Attacking RSA-based
4595 Sessions in SSL/TLS", http://eprint.iacr.org/2003/052/,
4598 [PKCS6] RSA Laboratories, "PKCS #6: RSA Extended Certificate Syntax
4599 Standard," version 1.5, November 1993.
4601 [PKCS7] RSA Laboratories, "PKCS #7: RSA Cryptographic Message Syntax
4602 Standard," version 1.5, November 1993.
4604 [RANDOM] D. Eastlake 3rd, S. Crocker, J. Schiller.
4605 "Randomness Recommendations for Security", RFC 1750,
4608 [RSA] R. Rivest, A. Shamir, and L. M. Adleman, "A Method for
4609 Obtaining Digital Signatures and Public-Key Cryptosystems,"
4610 Communications of the ACM, v. 21, n. 2, Feb 1978, pp.
4613 [SEQNUM] Bellovin. S., "Defending Against Sequence Number Attacks",
4616 [SSL2] Hickman, Kipp, "The SSL Protocol", Netscape Communications
4619 [SSL3] A. Frier, P. Karlton, and P. Kocher, "The SSL 3.0 Protocol",
4620 Netscape Communications Corp., Nov 18, 1996.
4622 [SUBGROUP] R. Zuccherato, "Methods for Avoiding the Small-Subgroup
4623 Attacks on the Diffie-Hellman Key Agreement Method for
4624 S/MIME", RFC 2785, March 2000.
4626 [TCP] Postel, J., "Transmission Control Protocol," STD 7, RFC 793,
4629 [TIMING] Boneh, D., Brumley, D., "Remote timing attacks are
4630 practical", USENIX Security Symposium 2003.
4632 [TLS1.0] Dierks, T., and Allen, C., "The TLS Protocol, Version 1.0",
4633 RFC 2246, January 1999.
4635 [X509] CCITT. Recommendation X.509: "The Directory - Authentication
4638 [XDR] R. Srinivansan, Sun Microsystems, RFC-1832: XDR: External
4639 Data Representation Standard, August 1995.
4643 Dierks & Rescorla Standards Track [Page 86]
\fdraft-ietf-tls-rfc2246-bis-10.txt TLS April 2005
4648 Working Group Chairs
4650 EMail: treese@acm.org
4658 Tim Dierks Eric Rescorla
4659 Independent RTFM, Inc.
4661 EMail: tim@dierks.org EMail: ekr@rtfm.com
4667 Christopher Allen (co-editor of TLS 1.0)
4669 ChristopherA@AlacrityManagement.com
4672 University of California, Santa Cruz
4677 canetti@watson.ibm.com
4688 Phil Karlton (co-author of SSLv3)
4690 Paul Kocher (co-author of SSLv3)
4691 Cryptography Research
4692 paul@cryptography.com
4697 Dierks & Rescorla Standards Track [Page 87]
\fdraft-ietf-tls-rfc2246-bis-10.txt TLS April 2005
4701 Technion Israel Institute of Technology
4702 hugo@ee.technion.ac.il
4705 Netscape Communications
4709 Netscape Communications
4716 dansimon@microsoft.com
4722 The discussion list for the IETF TLS working group is located at the
4723 e-mail address <ietf-tls@lists.consensus.com>. Information on the
4724 group and information on how to subscribe to the list is at
4725 <http://lists.consensus.com/>.
4727 Archives of the list can be found at:
4728 <http://www.imc.org/ietf-tls/mail-archive/>
4751 Dierks & Rescorla Standards Track [Page 88]
\fdraft-ietf-tls-rfc2246-bis-10.txt TLS April 2005
4754 Full Copyright Statement
4756 The IETF takes no position regarding the validity or scope of any
4757 Intellectual Property Rights or other rights that might be claimed to
4758 pertain to the implementation or use of the technology described in
4759 this document or the extent to which any license under such rights
4760 might or might not be available; nor does it represent that it has
4761 made any independent effort to identify any such rights. Information
4762 on the procedures with respect to rights in RFC documents can be
4763 found in BCP 78 and BCP 79.
4765 Copies of IPR disclosures made to the IETF Secretariat and any
4766 assurances of licenses to be made available, or the result of an
4767 attempt made to obtain a general license or permission for the use of
4768 such proprietary rights by implementers or users of this
4769 specification can be obtained from the IETF on-line IPR repository at
4770 http://www.ietf.org/ipr.
4772 The IETF invites any interested party to bring to its attention any
4773 copyrights, patents or patent applications, or other proprietary
4774 rights that may cover technology that may be required to implement
4775 this standard. Please address the information to the IETF at ietf-
4779 Copyright (C) The Internet Society (2003). This document is subject
4780 to the rights, licenses and restrictions contained in BCP 78, and
4781 except as set forth therein, the authors retain all their rights.
4783 This document and the information contained herein are provided on an
4784 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
4785 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
4786 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
4787 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
4788 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
4789 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
4805 Dierks & Rescorla Standards Track [Page 89]
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