7 Network Working Group L. Zhu
8 Request for Comments: 4121 K. Jaganathan
9 Updates: 1964 Microsoft
10 Category: Standards Track S. Hartman
15 The Kerberos Version 5
16 Generic Security Service Application Program Interface (GSS-API)
21 This document specifies an Internet standards track protocol for the
22 Internet community, and requests discussion and suggestions for
23 improvements. Please refer to the current edition of the "Internet
24 Official Protocol Standards" (STD 1) for the standardization state
25 and status of this protocol. Distribution of this memo is unlimited.
29 Copyright (C) The Internet Society (2005).
33 This document defines protocols, procedures, and conventions to be
34 employed by peers implementing the Generic Security Service
35 Application Program Interface (GSS-API) when using the Kerberos
38 RFC 1964 is updated and incremental changes are proposed in response
39 to recent developments such as the introduction of Kerberos
40 cryptosystem framework. These changes support the inclusion of new
41 cryptosystems, by defining new per-message tokens along with their
42 encryption and checksum algorithms based on the cryptosystem
58 Zhu, et al. Standards Track [Page 1]
60 RFC 4121 Kerberos Version 5 GSS-API July 2005
65 1. Introduction ....................................................2
66 2. Key Derivation for Per-Message Tokens ...........................4
67 3. Quality of Protection ...........................................4
68 4. Definitions and Token Formats ...................................5
69 4.1. Context Establishment Tokens ...............................5
70 4.1.1. Authenticator Checksum ..............................6
71 4.2. Per-Message Tokens .........................................9
72 4.2.1. Sequence Number .....................................9
73 4.2.2. Flags Field .........................................9
74 4.2.3. EC Field ...........................................10
75 4.2.4. Encryption and Checksum Operations .................10
76 4.2.5. RRC Field ..........................................11
77 4.2.6. Message Layouts ....................................12
78 4.3. Context Deletion Tokens ...................................13
79 4.4. Token Identifier Assignment Considerations ................13
80 5. Parameter Definitions ..........................................14
81 5.1. Minor Status Codes ........................................14
82 5.1.1. Non-Kerberos-specific Codes ........................14
83 5.1.2. Kerberos-specific Codes ............................15
84 5.2. Buffer Sizes ..............................................15
85 6. Backwards Compatibility Considerations .........................15
86 7. Security Considerations ........................................16
87 8. Acknowledgements................................................17
88 9. References .....................................................18
89 9.1. Normative References ......................................18
90 9.2. Informative References ....................................18
94 [RFC3961] defines a generic framework for describing encryption and
95 checksum types to be used with the Kerberos protocol and associated
98 [RFC1964] describes the GSS-API mechanism for Kerberos Version 5. It
99 defines the format of context establishment, per-message and context
100 deletion tokens, and uses algorithm identifiers for each cryptosystem
101 in per-message and context deletion tokens.
103 The approach taken in this document obviates the need for algorithm
104 identifiers. This is accomplished by using the same encryption
105 algorithm, specified by the crypto profile [RFC3961] for the session
106 key or subkey that is created during context negotiation, and its
107 required checksum algorithm. Message layouts of the per-message
108 tokens are therefore revised to remove algorithm indicators and to
109 add extra information to support the generic crypto framework
114 Zhu, et al. Standards Track [Page 2]
116 RFC 4121 Kerberos Version 5 GSS-API July 2005
119 Tokens transferred between GSS-API peers for security context
120 establishment are also described in this document. The data elements
121 exchanged between a GSS-API endpoint implementation and the Kerberos
122 Key Distribution Center (KDC) [RFC4120] are not specific to GSS-API
123 usage and are therefore defined within [RFC4120] rather than this
126 The new token formats specified in this document MUST be used with
127 all "newer" encryption types [RFC4120] and MAY be used with
128 encryption types that are not "newer", provided that the initiator
129 and acceptor know from the context establishment that they can both
130 process these new token formats.
132 "Newer" encryption types are those which have been specified along
133 with or since the new Kerberos cryptosystem specification [RFC3961],
134 as defined in section 3.1.3 of [RFC4120]. The list of not-newer
135 encryption types is as follows [RFC3961]:
137 Encryption Type Assigned Number
138 ----------------------------------------------
145 md5WithRSAEncryption-CmsOID 10
146 sha1WithRSAEncryption-CmsOID 11
148 rsaEncryption-EnvOID 13
149 rsaES-OAEP-ENV-OID 14
150 des-ede3-cbc-Env-OID 15
154 Conventions used in this document
156 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
157 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
158 document are to be interpreted as described in [RFC2119].
160 The term "little-endian order" is used for brevity to refer to the
161 least-significant-octet-first encoding, while the term "big-endian
162 order" is for the most-significant-octet-first encoding.
170 Zhu, et al. Standards Track [Page 3]
172 RFC 4121 Kerberos Version 5 GSS-API July 2005
175 2. Key Derivation for Per-Message Tokens
177 To limit the exposure of a given key, [RFC3961] adopted "one-way"
178 "entropy-preserving" derived keys, from a base key or protocol key,
179 for different purposes or key usages.
181 This document defines four key usage values below that are used to
182 derive a specific key for signing and sealing messages from the
183 session key or subkey [RFC4120] created during the context
187 -------------------------------------
188 KG-USAGE-ACCEPTOR-SEAL 22
189 KG-USAGE-ACCEPTOR-SIGN 23
190 KG-USAGE-INITIATOR-SEAL 24
191 KG-USAGE-INITIATOR-SIGN 25
193 When the sender is the context acceptor, KG-USAGE-ACCEPTOR-SIGN is
194 used as the usage number in the key derivation function for deriving
195 keys to be used in MIC tokens (as defined in section 4.2.6.1).
196 KG-USAGE-ACCEPTOR-SEAL is used for Wrap tokens (as defined in section
197 4.2.6.2). Similarly, when the sender is the context initiator,
198 KG-USAGE-INITIATOR-SIGN is used as the usage number in the key
199 derivation function for MIC tokens, while KG-USAGE-INITIATOR-SEAL is
200 used for Wrap tokens. Even if the Wrap token does not provide for
201 confidentiality, the same usage values specified above are used.
203 During the context initiation and acceptance sequence, the acceptor
204 MAY assert a subkey in the AP-REP message. If the acceptor asserts a
205 subkey, the base key is the acceptor-asserted subkey and subsequent
206 per-message tokens MUST be flagged with "AcceptorSubkey", as
207 described in section 4.2.2. Otherwise, if the initiator asserts a
208 subkey in the AP-REQ message, the base key is this subkey; if the
209 initiator does not assert a subkey, the base key is the session key
210 in the service ticket.
212 3. Quality of Protection
214 The GSS-API specification [RFC2743] provides Quality of Protection
215 (QOP) values that can be used by applications to request a certain
216 type of encryption or signing. A zero QOP value is used to indicate
217 the "default" protection; applications that do not use the default
218 QOP are not guaranteed to be portable across implementations, or even
219 to inter-operate with different deployment configurations of the same
220 implementation. Using a different algorithm than the one for which
221 the key is defined may not be appropriate. Therefore, when the new
222 method in this document is used, the QOP value is ignored.
226 Zhu, et al. Standards Track [Page 4]
228 RFC 4121 Kerberos Version 5 GSS-API July 2005
231 The encryption and checksum algorithms in per-message tokens are now
232 implicitly defined by the algorithms associated with the session key
233 or subkey. Therefore, algorithm identifiers as described in
234 [RFC1964] are no longer needed and are removed from the new token
237 4. Definitions and Token Formats
239 This section provides terms and definitions, as well as descriptions
240 for tokens specific to the Kerberos Version 5 GSS-API mechanism.
242 4.1. Context Establishment Tokens
244 All context establishment tokens emitted by the Kerberos Version 5
245 GSS-API mechanism SHALL have the framing described in section 3.1 of
246 [RFC2743], as illustrated by the following pseudo-ASN.1 structures:
248 GSS-API DEFINITIONS ::=
252 MechType ::= OBJECT IDENTIFIER
253 -- representing Kerberos V5 mechanism
256 -- option indication (delegation, etc.) indicated within
257 -- mechanism-specific token
258 [APPLICATION 0] IMPLICIT SEQUENCE {
260 innerToken ANY DEFINED BY thisMech
261 -- contents mechanism-specific
262 -- ASN.1 structure not required
267 The innerToken field starts with a two-octet token-identifier
268 (TOK_ID) expressed in big-endian order, followed by a Kerberos
271 Following are the TOK_ID values used in the context establishment
274 Token TOK_ID Value in Hex
275 -----------------------------------------
282 Zhu, et al. Standards Track [Page 5]
284 RFC 4121 Kerberos Version 5 GSS-API July 2005
287 Where Kerberos message KRB_AP_REQUEST, KRB_AP_REPLY, and KRB_ERROR
288 are defined in [RFC4120].
290 If an unknown token identifier (TOK_ID) is received in the initial
291 context establishment token, the receiver MUST return
292 GSS_S_CONTINUE_NEEDED major status, and the returned output token
293 MUST contain a KRB_ERROR message with the error code
294 KRB_AP_ERR_MSG_TYPE [RFC4120].
296 4.1.1. Authenticator Checksum
298 The authenticator in the KRB_AP_REQ message MUST include the optional
299 sequence number and the checksum field. The checksum field is used
300 to convey service flags, channel bindings, and optional delegation
303 The checksum type MUST be 0x8003. When delegation is used, a
304 ticket-granting ticket will be transferred in a KRB_CRED message.
305 This ticket SHOULD have its forwardable flag set. The EncryptedData
306 field of the KRB_CRED message [RFC4120] MUST be encrypted in the
307 session key of the ticket used to authenticate the context.
309 The authenticator checksum field SHALL have the following format:
311 Octet Name Description
312 -----------------------------------------------------------------
313 0..3 Lgth Number of octets in Bnd field; Represented
314 in little-endian order; Currently contains
315 hex value 10 00 00 00 (16).
316 4..19 Bnd Channel binding information, as described in
318 20..23 Flags Four-octet context-establishment flags in
319 little-endian order as described in section
321 24..25 DlgOpt The delegation option identifier (=1) in
322 little-endian order [optional]. This field
323 and the next two fields are present if and
324 only if GSS_C_DELEG_FLAG is set as described
326 26..27 Dlgth The length of the Deleg field in
327 little-endian order [optional].
328 28..(n-1) Deleg A KRB_CRED message (n = Dlgth + 28)
330 n..last Exts Extensions [optional].
332 The length of the checksum field MUST be at least 24 octets when
333 GSS_C_DELEG_FLAG is not set (as described in section 4.1.1.1), and at
334 least 28 octets plus Dlgth octets when GSS_C_DELEG_FLAG is set. When
338 Zhu, et al. Standards Track [Page 6]
340 RFC 4121 Kerberos Version 5 GSS-API July 2005
343 GSS_C_DELEG_FLAG is set, the DlgOpt, Dlgth, and Deleg fields of the
344 checksum data MUST immediately follow the Flags field. The optional
345 trailing octets (namely the "Exts" field) facilitate future
346 extensions to this mechanism. When delegation is not used, but the
347 Exts field is present, the Exts field starts at octet 24 (DlgOpt,
348 Dlgth and Deleg are absent).
350 Initiators that do not support the extensions MUST NOT include more
351 than 24 octets in the checksum field (when GSS_C_DELEG_FLAG is not
352 set) or more than 28 octets plus the KRB_CRED in the Deleg field
353 (when GSS_C_DELEG_FLAG is set). Acceptors that do not understand the
355 Extensions MUST ignore any octets past the Deleg field of the
356 checksum data (when GSS_C_DELEG_FLAG is set) or past the Flags field
357 of the checksum data (when GSS_C_DELEG_FLAG is not set).
359 4.1.1.1. Checksum Flags Field
361 The checksum "Flags" field is used to convey service options or
362 extension negotiation information.
364 The following context establishment flags are defined in [RFC2744].
367 ---------------------------------
371 GSS_C_SEQUENCE_FLAG 8
375 Context establishment flags are exposed to the calling application.
376 If the calling application desires a particular service option, then
377 it requests that option via GSS_Init_sec_context() [RFC2743]. If the
378 corresponding return state values [RFC2743] indicate that any of the
379 above optional context level services will be active on the context,
380 the corresponding flag values in the table above MUST be set in the
381 checksum Flags field.
383 Flag values 4096..524288 (2^12, 2^13, ..., 2^19) are reserved for use
384 with legacy vendor-specific extensions to this mechanism.
394 Zhu, et al. Standards Track [Page 7]
396 RFC 4121 Kerberos Version 5 GSS-API July 2005
399 All other flag values not specified herein are reserved for future
400 use. Future revisions of this mechanism may use these reserved flags
401 and may rely on implementations of this version to not use such flags
402 in order to properly negotiate mechanism versions. Undefined flag
403 values MUST be cleared by the sender, and unknown flags MUST be
404 ignored by the receiver.
406 4.1.1.2. Channel Binding Information
408 These tags are intended to be used to identify the particular
409 communications channel for which the GSS-API security context
410 establishment tokens are intended, thus limiting the scope within
411 which an intercepted context establishment token can be reused by an
412 attacker (see [RFC2743], section 1.1.6).
414 When using C language bindings, channel bindings are communicated to
415 the GSS-API using the following structure [RFC2744]:
417 typedef struct gss_channel_bindings_struct {
418 OM_uint32 initiator_addrtype;
419 gss_buffer_desc initiator_address;
420 OM_uint32 acceptor_addrtype;
421 gss_buffer_desc acceptor_address;
422 gss_buffer_desc application_data;
423 } *gss_channel_bindings_t;
425 The member fields and constants used for different address types are
426 defined in [RFC2744].
428 The "Bnd" field contains the MD5 hash of channel bindings, taken over
429 all non-null components of bindings, in order of declaration.
430 Integer fields within channel bindings are represented in little-
431 endian order for the purposes of the MD5 calculation.
433 In computing the contents of the Bnd field, the following detailed
436 (1) For purposes of MD5 hash computation, each integer field and
437 input length field SHALL be formatted into four octets, using
438 little-endian octet ordering.
440 (2) All input length fields within gss_buffer_desc elements of a
441 gss_channel_bindings_struct even those which are zero-valued,
442 SHALL be included in the hash calculation. The value elements of
443 gss_buffer_desc elements SHALL be dereferenced, and the resulting
444 data SHALL be included within the hash computation, only for the
445 case of gss_buffer_desc elements having non-zero length
450 Zhu, et al. Standards Track [Page 8]
452 RFC 4121 Kerberos Version 5 GSS-API July 2005
455 (3) If the caller passes the value GSS_C_NO_BINDINGS instead of a
456 valid channel binding structure, the Bnd field SHALL be set to 16
459 If the caller to GSS_Accept_sec_context [RFC2743] passes in
460 GSS_C_NO_CHANNEL_BINDINGS [RFC2744] as the channel bindings, then the
461 acceptor MAY ignore any channel bindings supplied by the initiator,
462 returning success even if the initiator did pass in channel bindings.
464 If the application supplies, in the channel bindings, a buffer with a
465 length field larger than 4294967295 (2^32 - 1), the implementation of
466 this mechanism MAY choose to reject the channel bindings altogether,
467 using major status GSS_S_BAD_BINDINGS [RFC2743]. In any case, the
468 size of channel-binding data buffers that can be used (interoperable,
469 without extensions) with this specification is limited to 4294967295
472 4.2. Per-Message Tokens
474 Two classes of tokens are defined in this section: (1) "MIC" tokens,
475 emitted by calls to GSS_GetMIC() and consumed by calls to
476 GSS_VerifyMIC(), and (2) "Wrap" tokens, emitted by calls to
477 GSS_Wrap() and consumed by calls to GSS_Unwrap().
479 These new per-message tokens do not include the generic GSS-API token
480 framing used by the context establishment tokens. These new tokens
481 are designed to be used with newer crypto systems that can have
482 variable-size checksums.
484 4.2.1. Sequence Number
486 To distinguish intentionally-repeated messages from maliciously-
487 replayed ones, per-message tokens contain a sequence number field,
488 which is a 64 bit integer expressed in big-endian order. After
489 sending a GSS_GetMIC() or GSS_Wrap() token, the sender's sequence
490 numbers SHALL be incremented by one.
494 The "Flags" field is a one-octet integer used to indicate a set of
495 attributes for the protected message. For example, one flag is
496 allocated as the direction-indicator, thus preventing the acceptance
497 of the same message sent back in the reverse direction by an
506 Zhu, et al. Standards Track [Page 9]
508 RFC 4121 Kerberos Version 5 GSS-API July 2005
511 The meanings of bits in this field (the least significant bit is bit
515 --------------------------------------------------------------
516 0 SentByAcceptor When set, this flag indicates the sender
517 is the context acceptor. When not set,
518 it indicates the sender is the context
520 1 Sealed When set in Wrap tokens, this flag
521 indicates confidentiality is provided
522 for. It SHALL NOT be set in MIC tokens.
523 2 AcceptorSubkey A subkey asserted by the context acceptor
524 is used to protect the message.
526 The rest of available bits are reserved for future use and MUST be
527 cleared. The receiver MUST ignore unknown flags.
531 The "EC" (Extra Count) field is a two-octet integer field expressed
534 In Wrap tokens with confidentiality, the EC field SHALL be used to
535 encode the number of octets in the filler, as described in section
538 In Wrap tokens without confidentiality, the EC field SHALL be used to
539 encode the number of octets in the trailing checksum, as described in
542 4.2.4. Encryption and Checksum Operations
544 The encryption algorithms defined by the crypto profiles provide for
545 integrity protection [RFC3961]. Therefore, no separate checksum is
548 The result of decryption can be longer than the original plaintext
549 [RFC3961] and the extra trailing octets are called "crypto-system
550 residue" in this document. However, given the size of any plaintext
551 data, one can always find a (possibly larger) size, such that when
552 padding the to-be-encrypted text to that size, there will be no
553 crypto-system residue added [RFC3961].
555 In Wrap tokens that provide for confidentiality, the first 16 octets
556 of the Wrap token (the "header", as defined in section 4.2.6), SHALL
557 be appended to the plaintext data before encryption. Filler octets
558 MAY be inserted between the plaintext data and the "header." The
562 Zhu, et al. Standards Track [Page 10]
564 RFC 4121 Kerberos Version 5 GSS-API July 2005
567 values and size of the filler octets are chosen by implementations,
568 such that there SHALL be no crypto-system residue present after the
569 decryption. The resulting Wrap token is {"header" |
570 encrypt(plaintext-data | filler | "header")}, where encrypt() is the
571 encryption operation (which provides for integrity protection)
572 defined in the crypto profile [RFC3961], and the RRC field (as
573 defined in section 4.2.5) in the to-be-encrypted header contains the
576 In Wrap tokens that do not provide for confidentiality, the checksum
577 SHALL be calculated first over the to-be-signed plaintext data, and
578 then over the first 16 octets of the Wrap token (the "header", as
579 defined in section 4.2.6). Both the EC field and the RRC field in
580 the token header SHALL be filled with zeroes for the purpose of
581 calculating the checksum. The resulting Wrap token is {"header" |
582 plaintext-data | get_mic(plaintext-data | "header")}, where get_mic()
583 is the checksum operation for the required checksum mechanism of the
584 chosen encryption mechanism defined in the crypto profile [RFC3961].
586 The parameters for the key and the cipher-state in the encrypt() and
587 get_mic() operations have been omitted for brevity.
589 For MIC tokens, the checksum SHALL be calculated as follows: the
590 checksum operation is calculated first over the to-be-signed
591 plaintext data, and then over the first 16 octets of the MIC token,
592 where the checksum mechanism is the required checksum mechanism of
593 the chosen encryption mechanism defined in the crypto profile
596 The resulting Wrap and MIC tokens bind the data to the token header,
597 including the sequence number and the direction indicator.
601 The "RRC" (Right Rotation Count) field in Wrap tokens is added to
602 allow the data to be encrypted in-place by existing SSPI (Security
603 Service Provider Interface) [SSPI] applications that do not provide
604 an additional buffer for the trailer (the cipher text after the in-
605 place-encrypted data) in addition to the buffer for the header (the
606 cipher text before the in-place-encrypted data). Excluding the first
607 16 octets of the token header, the resulting Wrap token in the
608 previous section is rotated to the right by "RRC" octets. The net
609 result is that "RRC" octets of trailing octets are moved toward the
612 Consider the following as an example of this rotation operation:
613 Assume that the RRC value is 3 and the token before the rotation is
614 {"header" | aa | bb | cc | dd | ee | ff | gg | hh}. The token after
618 Zhu, et al. Standards Track [Page 11]
620 RFC 4121 Kerberos Version 5 GSS-API July 2005
623 rotation would be {"header" | ff | gg | hh | aa | bb | cc | dd | ee
624 }, where {aa | bb | cc |...| hh} would be used to indicate the octet
627 The RRC field is expressed as a two-octet integer in big-endian
630 The rotation count value is chosen by the sender based on
631 implementation details. The receiver MUST be able to interpret all
632 possible rotation count values, including rotation counts greater
633 than the length of the token.
635 4.2.6. Message Layouts
637 Per-message tokens start with a two-octet token identifier (TOK_ID)
638 field, expressed in big-endian order. These tokens are defined
639 separately in the following sub-sections.
643 Use of the GSS_GetMIC() call yields a token (referred as the MIC
644 token in this document), separate from the user data being protected,
645 which can be used to verify the integrity of that data as received.
646 The token has the following format:
648 Octet no Name Description
649 --------------------------------------------------------------
650 0..1 TOK_ID Identification field. Tokens emitted by
651 GSS_GetMIC() contain the hex value 04 04
652 expressed in big-endian order in this
654 2 Flags Attributes field, as described in section
656 3..7 Filler Contains five octets of hex value FF.
657 8..15 SND_SEQ Sequence number field in clear text,
658 expressed in big-endian order.
659 16..last SGN_CKSUM Checksum of the "to-be-signed" data and
660 octet 0..15, as described in section 4.2.4.
662 The Filler field is included in the checksum calculation for
674 Zhu, et al. Standards Track [Page 12]
676 RFC 4121 Kerberos Version 5 GSS-API July 2005
681 Use of the GSS_Wrap() call yields a token (referred as the Wrap token
682 in this document), which consists of a descriptive header, followed
683 by a body portion that contains either the input user data in
684 plaintext concatenated with the checksum, or the input user data
685 encrypted. The GSS_Wrap() token SHALL have the following format:
687 Octet no Name Description
688 --------------------------------------------------------------
689 0..1 TOK_ID Identification field. Tokens emitted by
690 GSS_Wrap() contain the hex value 05 04
691 expressed in big-endian order in this
693 2 Flags Attributes field, as described in section
695 3 Filler Contains the hex value FF.
696 4..5 EC Contains the "extra count" field, in big-
697 endian order as described in section 4.2.3.
698 6..7 RRC Contains the "right rotation count" in big-
699 endian order, as described in section
701 8..15 SND_SEQ Sequence number field in clear text,
702 expressed in big-endian order.
703 16..last Data Encrypted data for Wrap tokens with
704 confidentiality, or plaintext data followed
705 by the checksum for Wrap tokens without
706 confidentiality, as described in section
709 4.3. Context Deletion Tokens
711 Context deletion tokens are empty in this mechanism. Both peers to a
712 security context invoke GSS_Delete_sec_context() [RFC2743]
713 independently, passing a null output_context_token buffer to indicate
714 that no context_token is required. Implementations of
715 GSS_Delete_sec_context() should delete relevant locally-stored
718 4.4. Token Identifier Assignment Considerations
720 Token identifiers (TOK_ID) from 0x60 0x00 through 0x60 0xFF inclusive
721 are reserved and SHALL NOT be assigned. Thus, by examining the first
722 two octets of a token, one can tell unambiguously if it is wrapped
723 with the generic GSS-API token framing.
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732 RFC 4121 Kerberos Version 5 GSS-API July 2005
735 5. Parameter Definitions
737 This section defines parameter values used by the Kerberos V5 GSS-API
738 mechanism. It defines interface elements that support portability,
739 and assumes use of C language bindings per [RFC2744].
741 5.1. Minor Status Codes
743 This section recommends common symbolic names for minor_status values
744 to be returned by the Kerberos V5 GSS-API mechanism. Use of these
745 definitions will enable independent implementers to enhance
746 application portability across different implementations of the
747 mechanism defined in this specification. (In all cases,
748 implementations of GSS_Display_status() will enable callers to
749 convert minor_status indicators to text representations.) Each
750 implementation should make available, through include files or other
751 means, a facility to translate these symbolic names into the concrete
752 values that a particular GSS-API implementation uses to represent the
753 minor_status values specified in this section.
755 This list may grow over time and the need for additional minor_status
756 codes, specific to particular implementations, may arise. However,
757 it is recommended that implementations should return a minor_status
758 value as defined on a mechanism-wide basis within this section when
759 that code accurately represents reportable status rather than using a
760 separate, implementation-defined code.
762 5.1.1. Non-Kerberos-specific Codes
764 GSS_KRB5_S_G_BAD_SERVICE_NAME
765 /* "No @ in SERVICE-NAME name string" */
766 GSS_KRB5_S_G_BAD_STRING_UID
767 /* "STRING-UID-NAME contains nondigits" */
769 /* "UID does not resolve to username" */
770 GSS_KRB5_S_G_VALIDATE_FAILED
771 /* "Validation error" */
772 GSS_KRB5_S_G_BUFFER_ALLOC
773 /* "Couldn't allocate gss_buffer_t data" */
774 GSS_KRB5_S_G_BAD_MSG_CTX
775 /* "Message context invalid" */
776 GSS_KRB5_S_G_WRONG_SIZE
777 /* "Buffer is the wrong size" */
778 GSS_KRB5_S_G_BAD_USAGE
779 /* "Credential usage type is unknown" */
780 GSS_KRB5_S_G_UNKNOWN_QOP
781 /* "Unknown quality of protection specified" */
786 Zhu, et al. Standards Track [Page 14]
788 RFC 4121 Kerberos Version 5 GSS-API July 2005
791 5.1.2. Kerberos-specific Codes
793 GSS_KRB5_S_KG_CCACHE_NOMATCH
794 /* "Client principal in credentials does not match
796 GSS_KRB5_S_KG_KEYTAB_NOMATCH
797 /* "No key available for specified service
799 GSS_KRB5_S_KG_TGT_MISSING
800 /* "No Kerberos ticket-granting ticket available" */
801 GSS_KRB5_S_KG_NO_SUBKEY
802 /* "Authenticator has no subkey" */
803 GSS_KRB5_S_KG_CONTEXT_ESTABLISHED
804 /* "Context is already fully established" */
805 GSS_KRB5_S_KG_BAD_SIGN_TYPE
806 /* "Unknown signature type in token" */
807 GSS_KRB5_S_KG_BAD_LENGTH
808 /* "Invalid field length in token" */
809 GSS_KRB5_S_KG_CTX_INCOMPLETE
810 /* "Attempt to use incomplete security context" */
814 All implementations of this specification MUST be capable of
815 accepting buffers of at least 16K octets as input to GSS_GetMIC(),
816 GSS_VerifyMIC(), and GSS_Wrap(). They MUST also be capable of
817 accepting the output_token generated by GSS_Wrap() for a 16K octet
818 input buffer as input to GSS_Unwrap(). Implementations SHOULD
819 support 64K octet input buffers, and MAY support even larger input
822 6. Backwards Compatibility Considerations
824 The new token formats defined in this document will only be
825 recognized by new implementations. To address this, implementations
826 can always use the explicit sign or seal algorithm in [RFC1964] when
827 the key type corresponds to not "newer" enctypes. As an alternative,
828 one might retry sending the message with the sign or seal algorithm
829 explicitly defined as in [RFC1964]. However, this would require
830 either the use of a mechanism such as [RFC2478] to securely negotiate
831 the method, or the use of an out-of-band mechanism to choose the
832 appropriate mechanism. For this reason, it is RECOMMENDED that the
833 new token formats defined in this document SHOULD be used only if
834 both peers are known to support the new mechanism during context
835 negotiation because of, for example, the use of "new" enctypes.
842 Zhu, et al. Standards Track [Page 15]
844 RFC 4121 Kerberos Version 5 GSS-API July 2005
847 GSS_Unwrap() or GSS_VerifyMIC() can process a message token as
848 follows: it can look at the first octet of the token header, and if
849 it is 0x60, then the token must carry the generic GSS-API pseudo
850 ASN.1 framing. Otherwise, the first two octets of the token contain
851 the TOK_ID that uniquely identify the token message format.
853 7. Security Considerations
855 Channel bindings are validated by the acceptor. The acceptor can
856 ignore the channel bindings restriction supplied by the initiator and
857 carried in the authenticator checksum, if (1) channel bindings are
858 not used by GSS_Accept_sec_context [RFC2743], and (2) the acceptor
859 does not prove to the initiator that it has the same channel bindings
860 as the initiator (even if the client requested mutual
861 authentication). This limitation should be considered by designers
862 of applications that would use channel bindings, whether to limit the
863 use of GSS-API contexts to nodes with specific network addresses, to
864 authenticate other established, secure channels using Kerberos
865 Version 5, or for any other purpose.
867 Session key types are selected by the KDC. Under the current
868 mechanism, no negotiation of algorithm types occurs, so server-side
869 (acceptor) implementations cannot request that clients not use
870 algorithm types not understood by the server. However,
871 administrators can control what enctypes can be used for session keys
872 for this mechanism by controlling the set of the ticket session key
873 enctypes which the KDC is willing to use in tickets for a given
874 acceptor principal. Therefore, the KDC could be given the task of
875 limiting session keys for a given service to types actually supported
876 by the Kerberos and GSSAPI software on the server. This has a
877 drawback for cases in which a service principal name is used for both
878 GSSAPI-based and non-GSSAPI-based communication (most notably the
879 "host" service key), if the GSSAPI implementation does not understand
880 (for example) AES [RFC3962], but the Kerberos implementation does.
881 This means that AES session keys cannot be issued for that service
882 principal, which keeps the protection of non-GSSAPI services weaker
883 than necessary. KDC administrators desiring to limit the session key
884 types to support interoperability with such GSSAPI implementations
885 should carefully weigh the reduction in protection offered by such
886 mechanisms against the benefits of interoperability.
898 Zhu, et al. Standards Track [Page 16]
900 RFC 4121 Kerberos Version 5 GSS-API July 2005
905 Ken Raeburn and Nicolas Williams corrected many of our errors in the
906 use of generic profiles and were instrumental in the creation of this
909 The text for security considerations was contributed by Nicolas
910 Williams and Ken Raeburn.
912 Sam Hartman and Ken Raeburn suggested the "floating trailer" idea,
913 namely the encoding of the RRC field.
915 Sam Hartman and Nicolas Williams recommended the replacing our
916 earlier key derivation function for directional keys with different
917 key usage numbers for each direction as well as retaining the
918 directional bit for maximum compatibility.
920 Paul Leach provided numerous suggestions and comments.
922 Scott Field, Richard Ward, Dan Simon, Kevin Damour, and Simon
923 Josefsson also provided valuable inputs on this document.
925 Jeffrey Hutzelman provided comments and clarifications for the text
926 related to the channel bindings.
928 Jeffrey Hutzelman and Russ Housley suggested many editorial changes.
930 Luke Howard provided implementations of this document for the Heimdal
931 code base, and helped inter-operability testing with the Microsoft
932 code base, together with Love Hornquist Astrand. These experiments
933 formed the basis of this document.
935 Martin Rex provided suggestions of TOK_ID assignment recommendations,
936 thus the token tagging in this document is unambiguous if the token
937 is wrapped with the pseudo ASN.1 header.
939 John Linn wrote the original Kerberos Version 5 mechanism
940 specification [RFC1964], of which some text has been retained.
954 Zhu, et al. Standards Track [Page 17]
956 RFC 4121 Kerberos Version 5 GSS-API July 2005
961 9.1. Normative References
963 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
964 Requirement Levels", BCP 14, RFC 2119, March 1997.
966 [RFC2743] Linn, J., "Generic Security Service Application Program
967 Interface Version 2, Update 1", RFC 2743, January 2000.
969 [RFC2744] Wray, J., "Generic Security Service API Version 2:
970 C-bindings", RFC 2744, January 2000.
972 [RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism", RFC
975 [RFC3961] Raeburn, K., "Encryption and Checksum Specifications for
976 Kerberos 5", RFC 3961, February 2005.
978 [RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
979 Kerberos Network Authentication Service (V5)", RFC 4120,
982 9.2. Informative References
984 [SSPI] Leach, P., "Security Service Provider Interface",
985 Microsoft Developer Network (MSDN), April 2003.
987 [RFC3962] Raeburn, K., "Advanced Encryption Standard (AES)
988 Encryption for Kerberos 5", RFC 3962, February 2005.
990 [RFC2478] Baize, E. and D. Pinkas, "The Simple and Protected GSS-API
991 Negotiation Mechanism", RFC 2478, December 1998.
1010 Zhu, et al. Standards Track [Page 18]
1012 RFC 4121 Kerberos Version 5 GSS-API July 2005
1019 Redmond, WA 98052 - USA
1021 EMail: LZhu@microsoft.com
1026 Redmond, WA 98052 - USA
1028 EMail: karthikj@microsoft.com
1032 Massachusetts Institute of Technology
1033 77 Massachusetts Avenue
1034 Cambridge, MA 02139 - USA
1036 EMail: hartmans-ietf@mit.edu
1066 Zhu, et al. Standards Track [Page 19]
1068 RFC 4121 Kerberos Version 5 GSS-API July 2005
1071 Full Copyright Statement
1073 Copyright (C) The Internet Society (2005).
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1122 Zhu, et al. Standards Track [Page 20]