1 INTERNET-DRAFT Brian Tung
2 draft-ietf-cat-kerberos-pk-init-05.txt Clifford Neuman
4 expires May 26, 1998 John Wray
5 Digital Equipment Corporation
13 Public Key Cryptography for Initial Authentication in Kerberos
16 0. Status Of This Memo
18 This document is an Internet-Draft. Internet-Drafts are working
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33 munnari.oz.au (Pacific Rim).
35 The distribution of this memo is unlimited. It is filed as
36 draft-ietf-cat-kerberos-pk-init-05.txt, and expires May 26, 1998.
37 Please send comments to the authors.
42 This document defines extensions (PKINIT) to the Kerberos protocol
43 specification (RFC 1510 [1]) to provide a method for using public
44 key cryptography during initial authentication. The methods
45 defined specify the ways in which preauthentication data fields and
46 error data fields in Kerberos messages are to be used to transport
52 The popularity of public key cryptography has produced a desire for
53 its support in Kerberos [2]. The advantages provided by public key
54 cryptography include simplified key management (from the Kerberos
55 perspective) and the ability to leverage existing and developing
56 public key certification infrastructures.
58 Public key cryptography can be integrated into Kerberos in a number
59 of ways. One is to associate a key pair with each realm, which can
60 then be used to facilitate cross-realm authentication; this is the
61 topic of another draft proposal. Another way is to allow users with
62 public key certificates to use them in initial authentication. This
63 is the concern of the current document.
65 One of the guiding principles in the design of PKINIT is that
66 changes should be as minimal as possible. As a result, the basic
67 mechanism of PKINIT is as follows: The user sends a request to the
68 KDC as before, except that if that user is to use public key
69 cryptography in the initial authentication step, his certificate
70 accompanies the initial request, in the preauthentication fields.
72 Upon receipt of this request, the KDC verifies the certificate and
73 issues a ticket granting ticket (TGT) as before, except that
74 the encPart from the AS-REP message carrying the TGT is now
75 encrypted in a randomly-generated key, instead of the user's
76 long-term key (which is derived from a password). This
77 random key is in turn encrypted using the public key from the
78 certificate that came with the request and signed using the KDC's
79 private key, and accompanies the reply, in the preauthentication
82 PKINIT also allows for users with only digital signature keys to
83 authenticate using those keys, and for users to store and retrieve
84 private keys on the KDC.
86 The PKINIT specification may also be used for direct peer to peer
87 authentication without contacting a central KDC. This application
88 of PKINIT is described in PKTAPP [4] and is based on concepts
89 introduced in [5, 6]. For direct client-to-server authentication,
90 the client uses PKINIT to authenticate to the end server (instead
91 of a central KDC), which then issues a ticket for itself. This
92 approach has an advantage over SSL [7] in that the server does not
93 need to save state (cache session keys). Furthermore, an
94 additional benefit is that Kerberos tickets can facilitate
98 3. Proposed Extensions
100 This section describes extensions to RFC 1510 for supporting the
101 use of public key cryptography in the initial request for a ticket
102 granting ticket (TGT).
104 In summary, the following changes to RFC 1510 are proposed:
106 * Users may authenticate using either a public key pair or a
107 conventional (symmetric) key. If public key cryptography is
108 used, public key data is transported in preauthentication
109 data fields to help establish identity.
110 * Users may store private keys on the KDC for retrieval during
111 Kerberos initial authentication.
113 This proposal addresses two ways that users may use public key
114 cryptography for initial authentication. Users may present public
115 key certificates, or they may generate their own session key,
116 signed by their digital signature key. In either case, the end
117 result is that the user obtains an ordinary TGT that may be used for
118 subsequent authentication, with such authentication using only
119 conventional cryptography.
121 Section 3.1 provides definitions to help specify message formats.
122 Section 3.2 and 3.3 describe the extensions for the two initial
123 authentication methods. Section 3.4 describes a way for the user to
124 store and retrieve his private key on the KDC, as an adjunct to the
125 initial authentication.
130 The extensions involve new encryption methods; we propose the
131 addition of the following types:
139 The proposal of these encryption types notwithstanding, we do not
140 mandate the use of any particular public key encryption method.
142 The extensions involve new preauthentication fields; we propose the
143 addition of the following types:
151 The extensions also involve new error types; we propose the addition
152 of the following types:
154 KDC_ERR_CLIENT_NOT_TRUSTED 62
155 KDC_ERR_KDC_NOT_TRUSTED 63
156 KDC_ERR_INVALID_SIG 64
157 KDC_ERR_KEY_TOO_WEAK 65
158 KDC_ERR_CERTIFICATE_MISMATCH 66
160 In many cases, PKINIT requires the encoding of an X.500 name as a
161 Realm. In these cases, the realm will be represented using a
162 different style, specified in RFC 1510 with the following example:
164 NAMETYPE:rest/of.name=without-restrictions
166 For a realm derived from an X.500 name, NAMETYPE will have the value
167 X500-RFC1779. The full realm name will appear as follows:
169 X500-RFC1779:RFC1779Encode(DistinguishedName)
171 where DistinguishedName is an X.500 name, and RFC1779Encode is a
172 readable ASCII encoding of an X.500 name, as defined by RFC 1779.
173 To ensure that this encoding is unique, we add the following rules
174 to those specified by RFC 1779:
176 * The optional spaces specified in RFC 1779 are not allowed.
177 * The character that separates relative distinguished names
178 must be ',' (i.e., it must never be ';').
179 * Attribute values must not be enclosed in double quotes.
180 * Attribute values must not be specified as hexadecimal
182 * When an attribute name is specified in the form of an OID,
183 it must start with the 'OID.' prefix, and not the 'oid.'
185 * The order in which the attributes appear in the RFC 1779
186 encoding must be the reverse of the order in the ASN.1
187 encoding of the X.500 name that appears in the public key
188 certificate, because RFC 1779 requires that the least
189 significant relative distinguished name appear first. The
190 order of the relative distinguished names, as well as the
191 order of the attributes within each relative distinguished
192 name, will be reversed.
194 Similarly, PKINIT may require the encoding of an X.500 name as a
195 PrincipalName. In these cases, the name-type of the principal name
196 shall be set to NT-X500-PRINCIPAL. This new name type is defined
198 #define CSFC5c_NT_X500_PRINCIPAL 6
200 The name-string shall be set as follows:
202 RFC1779Encode(DistinguishedName)
207 3.1.1. Encryption and Key Formats
209 In the exposition below, we use the terms public key and private
210 key generically. It should be understood that the term "public
211 key" may be used to refer to either a public encryption key or a
212 signature verification key, and that the term "private key" may be
213 used to refer to either a private decryption key or a signature
214 generation key. The fact that these are logically distinct does
215 not preclude the assignment of bitwise identical keys.
217 All additional symmetric keys specified in this draft shall use the
218 same encryption type as the session key in the response from the
219 KDC. These include the temporary keys used to encrypt the signed
220 random key encrypting the response, as well as the key derived from
221 Diffie-Hellman agreement. In the case of Diffie-Hellman, the key
222 shall be produced from the agreed bit string as follows:
224 * Truncate the bit string to the appropriate length.
225 * Rectify parity in each byte (if necessary) to obtain the key.
227 For instance, in the case of a DES key, we take the first eight
228 bytes of the bit stream, and then adjust the least significant bit
229 of each byte to ensure that each byte has odd parity.
231 RFC 1510, Section 6.1, defines EncryptedData as follows:
233 EncryptedData ::= SEQUENCE {
235 kvno [1] INTEGER OPTIONAL,
236 cipher [2] OCTET STRING
240 RFC 1510 also defines how CipherText is to be composed. It is not
241 an ASN.1 data structure, but rather an octet string which is the
242 encryption of a plaintext string. This plaintext string is in turn
243 a concatenation of the following octet strings: a confounder, a
244 checksum, the message, and padding. Details of how these components
245 are arranged can be found in RFC 1510.
247 The PKINIT protocol introduces several new types of encryption.
248 Data that is encrypted with a public key will allow only the check
249 optional field, as it is defined above. This type of the checksum
250 will be specified in the etype field, together with the encryption
253 In order to identify the checksum type, etype will have the
259 In the case that etype is set to rsa-pub, the optional 'check'
260 field will not be provided.
262 Data that is encrypted with a private key will not use any optional
263 fields. PKINIT uses private key encryption only for signatures,
264 which are encrypted checksums. Therefore, the optional check field
268 3.2. Standard Public Key Authentication
270 Implementation of the changes in this section is REQUIRED for
271 compliance with PKINIT.
273 It is assumed that all public keys are signed by some certification
274 authority (CA). The initial authentication request is sent as per
275 RFC 1510, except that a preauthentication field containing data
276 signed by the user's private key accompanies the request:
278 PA-PK-AS-REQ ::= SEQUENCE {
280 signedAuthPack [0] SignedAuthPack
281 userCert [1] SEQUENCE OF Certificate OPTIONAL,
282 -- the user's certificate chain
283 trustedCertifiers [2] SEQUENCE OF PrincipalName OPTIONAL,
284 -- CAs that the client trusts
285 serialNumber [3] CertificateSerialNumber OPTIONAL
286 -- specifying a particular
287 -- certificate if the client
289 -- must be accompanied by
290 -- a single trustedCertifier
293 CertificateSerialNumber ::= INTEGER
294 -- as specified by PKCS 6
296 SignedAuthPack ::= SEQUENCE {
297 authPack [0] AuthPack,
298 authPackSig [1] Signature,
300 -- using user's private key
303 AuthPack ::= SEQUENCE {
304 pkAuthenticator [0] PKAuthenticator,
305 clientPublicValue [1] SubjectPublicKeyInfo OPTIONAL
306 -- if client is using Diffie-Hellman
309 PKAuthenticator ::= SEQUENCE {
310 kdcName [0] PrincipalName,
313 -- for replay prevention
314 ctime [3] KerberosTime,
315 -- for replay prevention
319 Signature ::= SEQUENCE {
320 signedHash [0] EncryptedData
324 Checksum ::= SEQUENCE {
325 cksumtype [0] INTEGER,
326 checksum [1] OCTET STRING
327 } -- as specified by RFC 1510
329 SubjectPublicKeyInfo ::= SEQUENCE {
330 algorithm [0] AlgorithmIdentifier,
331 subjectPublicKey [1] BIT STRING
333 -- public exponent (INTEGER encoded
334 -- as payload of BIT STRING)
335 } -- as specified by the X.509 recommendation [9]
337 AlgorithmIdentifier ::= SEQUENCE {
338 algorithm [0] ALGORITHM.&id,
341 -- ({iso(1) member-body(2) US(840)
342 -- rsadsi(113549) pkcs(1) pkcs-3(3)
344 parameters [1] ALGORITHM.&type
345 -- for DH, is DHParameter
346 } -- as specified by the X.509 recommendation [9]
348 DHParameter ::= SEQUENCE {
353 privateValueLength [2] INTEGER OPTIONAL
356 Certificate ::= SEQUENCE {
357 certType [0] INTEGER,
358 -- type of certificate
359 -- 1 = X.509v3 (DER encoding)
360 -- 2 = PGP (per PGP specification)
361 certData [1] OCTET STRING
362 -- actual certificate
363 -- type determined by certType
366 If the client passes a certificate serial number in the request,
367 the KDC is requested to use the referred-to certificate. If none
368 exists, then the KDC returns an error of type
369 KDC_ERR_CERTIFICATE_MISMATCH. It also returns this error if, on the
370 other hand, the client does not pass any trustedCertifiers,
371 believing that it has the KDC's certificate, but the KDC has more
372 than one certificate.
374 The PKAuthenticator carries information to foil replay attacks,
375 to bind the request and response, and to optionally pass the
376 client's Diffie-Hellman public value (i.e. for using DSA in
377 combination with Diffie-Hellman). The PKAuthenticator is signed
378 with the private key corresponding to the public key in the
379 certificate found in userCert (or cached by the KDC).
381 In the PKAuthenticator, the client may specify the KDC name in one
384 * The Kerberos principal name krbtgt/<realm_name>@<realm_name>,
385 where <realm_name> is replaced by the applicable realm name.
386 * The name in the KDC's certificate (e.g., an X.500 name, or a
389 Note that the first case requires that the certificate name and the
390 Kerberos principal name be bound together (e.g., via an X.509v3
393 The userCert field is a sequence of certificates, the first of which
394 must be the user's public key certificate. Any subsequent
395 certificates will be certificates of the certifiers of the user's
396 certificate. These cerificates may be used by the KDC to verify the
397 user's public key. This field may be left empty if the KDC already
398 has the user's certificate.
400 The trustedCertifiers field contains a list of certification
401 authorities trusted by the client, in the case that the client does
402 not possess the KDC's public key certificate. If the KDC has no
403 certificate signed by any of the trustedCertifiers, then it returns
404 an error of type KDC_ERR_CERTIFICATE_MISMATCH.
406 Upon receipt of the AS_REQ with PA-PK-AS-REQ pre-authentication
407 type, the KDC attempts to verify the user's certificate chain
408 (userCert), if one is provided in the request. This is done by
409 verifying the certification path against the KDC's policy of
410 legitimate certifiers. This may be based on a certification
411 hierarchy, or it may be simply a list of recognized certifiers in a
414 If verification of the user's certificate fails, the KDC sends back
415 an error message of type KDC_ERR_CLIENT_NOT_TRUSTED. The e-data
416 field contains additional information pertaining to this error, and
417 is formatted as follows:
419 METHOD-DATA ::= SEQUENCE {
420 method-type [0] INTEGER,
421 -- 1 = cannot verify public key
422 -- 2 = invalid certificate
423 -- 3 = revoked certificate
424 -- 4 = invalid KDC name
425 -- 5 = client name mismatch
426 method-data [1] OCTET STRING OPTIONAL
427 } -- syntax as for KRB_AP_ERR_METHOD (RFC 1510)
429 The values for the method-type and method-data fields are described
432 If trustedCertifiers is provided in the PA-PK-AS-REQ, the KDC
433 verifies that it has a certificate issued by one of the certifiers
434 trusted by the client. If it does not have a suitable certificate,
435 the KDC returns an error message of type KDC_ERR_KDC_NOT_TRUSTED to
438 If a trust relationship exists, the KDC then verifies the client's
439 signature on AuthPack. If that fails, the KDC returns an error
440 message of type KDC_ERR_INVALID_SIG. Otherwise, the KDC uses the
441 timestamp in the PKAuthenticator to assure that the request is not a
442 replay. The KDC also verifies that its name is specified in the
445 If the clientPublicValue field is filled in, indicating that the
446 client wishes to use Diffie-Hellman key agreement, then the KDC
447 checks to see that the parameters satisfy its policy. If they do
448 not (e.g., the prime size is insufficient for the expected
449 encryption type), then the KDC sends back an error message of type
450 KDC_ERR_KEY_TOO_WEAK. Otherwise, it generates its own public and
451 private values for the response.
453 The KDC also checks that the timestamp in the PKAuthenticator is
454 within the allowable window. If the local (server) time and the
455 client time in the authenticator differ by more than the allowable
456 clock skew, then the KDC returns an error message of type
459 Assuming no errors, the KDC replies as per RFC 1510, except as
460 follows: The user's name in the ticket is as represented in the
461 certificate, unless a Kerberos principal name is bound to the name
462 in the certificate (e.g., via an X.509v3 extension). The user's
463 realm in the ticket shall be the name of the Certification
464 Authority that issued the user's public key certificate.
466 The KDC encrypts the reply not with the user's long-term key, but
467 with a random key generated only for this particular response. This
468 random key is sealed in the preauthentication field:
470 PA-PK-AS-REP ::= SEQUENCE {
472 encSignedReplyKeyPack [0] EncryptedData,
473 -- of type SignedReplyKeyPack
474 -- using the temporary key
476 encTmpKeyPack [1] EncryptedData,
477 -- of type TmpKeyPack
478 -- using either the client public
479 -- key or the Diffie-Hellman key
480 -- specified by SignedDHPublicValue
481 signedKDCPublicValue [2] SignedKDCPublicValue OPTIONAL
482 -- if one was passed in the request
483 kdcCert [3] SEQUENCE OF Certificate OPTIONAL,
484 -- the KDC's certificate chain
487 SignedReplyKeyPack ::= SEQUENCE {
488 replyKeyPack [0] ReplyKeyPack,
489 replyKeyPackSig [1] Signature,
490 -- of replyEncKeyPack
491 -- using KDC's private key
494 ReplyKeyPack ::= SEQUENCE {
495 replyKey [0] EncryptionKey,
496 -- used to encrypt main reply
497 -- of same ENCTYPE as session key
499 -- binds response to the request
500 -- must be same as the nonce
501 -- passed in the PKAuthenticator
504 TmpKeyPack ::= SEQUENCE {
505 tmpKey [0] EncryptionKey,
506 -- used to encrypt the
507 -- SignedReplyKeyPack
508 -- of same ENCTYPE as session key
511 SignedKDCPublicValue ::= SEQUENCE {
512 kdcPublicValue [0] SubjectPublicKeyInfo,
513 -- as described above
514 kdcPublicValueSig [1] Signature
516 -- using KDC's private key
519 The kdcCert field is a sequence of certificates, the first of which
520 must be the KDC's public key certificate. Any subsequent
521 certificates will be certificates of the certifiers of the KDC's
522 certificate. The last of these must have as its certifier one of
523 the certifiers sent to the KDC in the PA-PK-AS-REQ. These
524 cerificates may be used by the client to verify the KDC's public
525 key. This field is empty if the client did not send to the KDC a
526 list of trusted certifiers (the trustedCertifiers field was empty).
528 Since each certifier in the certification path of a user's
529 certificate is essentially a separate realm, the name of each
530 certifier shall be added to the transited field of the ticket. The
531 format of these realm names is defined in Section 3.1 of this
532 document. If applicable, the transit-policy-checked flag should be
533 set in the issued ticket.
535 The KDC's certificate must bind the public key to a name derivable
536 from the name of the realm for that KDC. For an X.509 certificate,
537 this is done as follows. The name of the KDC will be represented
538 as an OtherName, encoded as a GeneralString:
540 GeneralName ::= CHOICE {
541 otherName [0] KDCPrincipalName,
545 KDCPrincipalNameTypes OTHER-NAME ::= {
546 { PrincipalNameSrvInst IDENTIFIED BY principalNameSrvInst }
549 KDCPrincipalName ::= SEQUENCE {
550 nameType OTHER-NAME.&id ( { KDCPrincipalNameTypes } ),
551 name OTHER-NAME.&type ( { KDCPrincipalNameTypes }
555 PrincipalNameSrvInst ::= GeneralString
557 where (from the Kerberos specification) we have
559 krb5 OBJECT IDENTIFIER ::= { iso (1)
566 principalName OBJECT IDENTIFIER ::= { krb5 2 }
568 principalNameSrvInst OBJECT IDENTIFIER ::= { principalName 2 }
570 The client then extracts the random key used to encrypt the main
571 reply. This random key (in encPaReply) is encrypted with either the
572 client's public key or with a key derived from the DH values
573 exchanged between the client and the KDC.
576 3.2.1. Additional Information for Errors
578 This section describes the interpretation of the method-type and
579 method-data fields of the KDC_ERR_CLIENT_NOT_TRUSTED error.
581 If method-type=1, the client's public key certificate chain does not
582 contain a certificate that is signed by a certification authority
583 trusted by the KDC. The format of the method-data field will be an
584 ASN.1 encoding of a list of trusted certifiers, as defined above:
586 TrustedCertifiers ::= SEQUENCE OF PrincipalName
588 If method-type=2, the signature on one of the certificates in the
589 chain cannot be verified. The format of the method-data field will
590 be an ASN.1 encoding of the integer index of the certificate in
593 CertificateIndex ::= INTEGER
594 -- 0 = 1st certificate,
595 -- 1 = 2nd certificate, etc
597 If method-type=3, one of the certificates in the chain has been
598 revoked. The format of the method-data field will be an ASN.1
599 encoding of the integer index of the certificate in question:
601 CertificateIndex ::= INTEGER
602 -- 0 = 1st certificate,
603 -- 1 = 2nd certificate, etc
605 If method-type=4, the KDC name or realm in the PKAuthenticator does
606 not match the principal name of the KDC. There is no method-data
609 If method-type=5, the client name or realm in the certificate does
610 not match the principal name of the client. There is no
611 method-data field in this case.
614 3.3. Digital Signature
616 Implementation of the changes in this section are OPTIONAL for
617 compliance with PKINIT.
619 We offer this option with the warning that it requires the client to
620 generate a random key; the client may not be able to guarantee the
621 same level of randomness as the KDC.
623 If the user registered, or presents a certificate for, a digital
624 signature key with the KDC instead of an encryption key, then a
625 separate exchange must be used. The client sends a request for a
626 TGT as usual, except that it (rather than the KDC) generates the
627 random key that will be used to encrypt the KDC response. This key
628 is sent to the KDC along with the request in a preauthentication
629 field, encrypted with the KDC's public key:
631 PA-PK-AS-SIGN ::= SEQUENCE {
633 encSignedRandomKeyPack [0] EncryptedData,
634 -- of type SignedRandomKeyPack
635 -- using the key in encTmpKeyPack
636 encTmpKeyPack [1] EncryptedData,
637 -- of type TmpKeyPack
638 -- using the KDC's public key
639 userCert [2] SEQUENCE OF Certificate OPTIONAL
640 -- the user's certificate chain
643 SignedRandomKeyPack ::= SEQUENCE {
644 randomkeyPack [0] RandomKeyPack,
645 randomkeyPackSig [1] Signature
647 -- using user's private key
650 RandomKeyPack ::= SEQUENCE {
651 randomKey [0] EncryptionKey,
652 -- will be used to encrypt reply
653 randomKeyAuth [1] PKAuthenticator
654 -- nonce copied from AS-REQ
657 If the KDC does not accept client-generated random keys as a matter
658 of policy, then it sends back an error message of type
659 KDC_ERR_KEY_TOO_WEAK. Otherwise, it extracts the random key as
662 Upon receipt of the PA-PK-AS-SIGN, the KDC decrypts then verifies
663 the randomKey. It then replies as per RFC 1510, except that the
664 reply is encrypted not with a password-derived user key, but with
665 the randomKey sent in the request. Since the client already knows
666 this key, there is no need to accompany the reply with an extra
667 preauthentication field. The transited field of the ticket should
668 specify the certification path as described in Section 3.2.
671 3.4. Retrieving the User's Private Key from the KDC
673 Implementation of the changes described in this section are OPTIONAL
674 for compliance with PKINIT.
676 When the user's private key is not stored local to the user, he may
677 choose to store the private key (normally encrypted using a
678 password-derived key) on the KDC. In this case, the client makes a
679 request as described above, except that instead of preauthenticating
680 with his private key, he uses a symmetric key shared with the KDC.
682 For simplicity's sake, this shared key is derived from the password-
683 derived key used to encrypt the private key, in such a way that the
684 KDC can authenticate the user with the shared key without being able
685 to extract the private key.
687 We provide this option to present the user with an alternative to
688 storing the private key on local disk at each machine where he
689 expects to authenticate himself using PKINIT. It should be noted
690 that it replaces the added risk of long-term storage of the private
691 key on possibly many workstations with the added risk of storing the
692 private key on the KDC in a form vulnerable to brute-force attack.
694 Denote by K1 the symmetric key used to encrypt the private key.
695 Then construct symmetric key K2 as follows:
697 * Perform a hash on K1.
698 * Truncate the digest to Length(K1) bytes.
699 * Rectify parity in each byte (if necessary) to obtain K2.
701 The KDC stores K2, the public key, and the encrypted private key.
702 This key pair is designated as the "primary" key pair for that user.
703 This primary key pair is the one used to perform initial
704 authentication using the PA-PK-AS-REP preauthentication field. If
705 he desires, he may also store additional key pairs on the KDC; these
706 may be requested in addition to the primary. When the client
707 requests initial authentication using public key cryptography, it
708 must then include in its request, instead of a PA-PK-AS-REQ, the
709 following preauthentication sequence:
711 PA-PK-KEY-REQ ::= SEQUENCE {
713 signedPKAuth [0] SignedPKAuth,
714 trustedCertifiers [1] SEQUENCE OF PrincipalName OPTIONAL,
715 -- CAs that the client trusts
716 keyIDList [2] SEQUENCE OF Checksum OPTIONAL
717 -- payload is hash of public key
718 -- corresponding to desired
720 -- if absent, KDC will return all
721 -- stored private keys
724 SignedPKAuth ::= SEQUENCE {
725 pkAuth [0] PKAuthenticator,
726 pkAuthSig [1] Signature
728 -- using the symmetric key K2
731 If a keyIDList is present, the first identifier should indicate
732 the primary private key. No public key certificate is required,
733 since the KDC stores the public key along with the private key.
734 If there is no keyIDList, all the user's private keys are returned.
736 Upon receipt, the KDC verifies the signature using K2. If the
737 verification fails, the KDC sends back an error of type
738 KDC_ERR_INVALID_SIG. If the signature verifies, but the requested
739 keys are not found on the KDC, then the KDC sends back an error of
740 type KDC_ERR_PREAUTH_FAILED. If all checks out, the KDC responds as
741 described in Section 3.2, except that in addition, the KDC appends
742 the following preauthentication sequence:
744 PA-PK-KEY-REP ::= SEQUENCE {
746 encKeyRep [0] EncryptedData
747 -- of type EncKeyReply
748 -- using the symmetric key K2
751 EncKeyReply ::= SEQUENCE {
752 keyPackList [0] SEQUENCE OF KeyPack,
753 -- the first KeyPair is
754 -- the primary key pair
756 -- binds reply to request
757 -- must be identical to the nonce
758 -- sent in the SignedAuthPack
761 KeyPack ::= SEQUENCE {
763 encPrivKey [1] OCTET STRING
766 Upon receipt of the reply, the client extracts the encrypted private
767 keys (and may store them, at the client's option). The primary
768 private key, which must be the first private key in the keyPack
769 SEQUENCE, is used to decrypt the random key in the PA-PK-AS-REP;
770 this key in turn is used to decrypt the main reply as described in
774 4. Logistics and Policy
776 This section describes a way to define the policy on the use of
777 PKINIT for each principal and request.
779 The KDC is not required to contain a database record for users
780 that use either the Standard Public Key Authentication or Public Key
781 Authentication with a Digital Signature. However, if these users
782 are registered with the KDC, it is recommended that the database
783 record for these users be modified to include three additional flags
784 in the attributes field.
786 The first flag, use_standard_pk_init, indicates that the user should
787 authenticate using standard PKINIT as described in Section 3.2. The
788 second flag, use_digital_signature, indicates that the user should
789 authenticate using digital signature PKINIT as described in Section
790 3.3. The third flag, store_private_key, indicates that the user
791 has stored his private key on the KDC and should retrieve it using
792 the exchange described in Section 3.4.
794 If one of the preauthentication fields defined above is included in
795 the request, then the KDC shall respond as described in Sections 3.2
796 through 3.4, ignoring the aforementioned database flags. If more
797 than one of the preauthentication fields is present, the KDC shall
798 respond with an error of type KDC_ERR_PREAUTH_FAILED.
800 In the event that none of the preauthentication fields defined above
801 are included in the request, the KDC checks to see if any of the
802 above flags are set. If the first flag is set, then it sends back
803 an error of type KDC_ERR_PREAUTH_REQUIRED indicating that a
804 preauthentication field of type PA-PK-AS-REQ must be included in the
807 Otherwise, if the first flag is clear, but the second flag is set,
808 then the KDC sends back an error of type KDC_ERR_PREAUTH_REQUIRED
809 indicating that a preauthentication field of type PA-PK-AS-SIGN must
810 be included in the request.
812 Lastly, if the first two flags are clear, but the third flag is set,
813 then the KDC sends back an error of type KDC_ERR_PREAUTH_REQUIRED
814 indicating that a preauthentication field of type PA-PK-KEY-REQ must
815 be included in the request.
820 Certificate chains can potentially grow quite large and span several
821 UDP packets; this in turn increases the probability that a Kerberos
822 message involving PKINIT extensions will be broken in transit. In
823 light of the possibility that the Kerberos specification will
824 require KDCs to accept requests using TCP as a transport mechanism,
825 we make the same recommendation with respect to the PKINIT
831 [1] J. Kohl, C. Neuman. The Kerberos Network Authentication Service
832 (V5). Request for Comments 1510.
834 [2] B.C. Neuman, Theodore Ts'o. Kerberos: An Authentication Service
835 for Computer Networks, IEEE Communications, 32(9):33-38. September
838 [3] A. Medvinsky, M. Hur. Addition of Kerberos Cipher Suites to
839 Transport Layer Security (TLS).
840 draft-ietf-tls-kerb-cipher-suites-00.txt
842 [4] A. Medvinsky, M. Hur, B. Clifford Neuman. Public Key Utilizing
843 Tickets for Application Servers (PKTAPP).
844 draft-ietf-cat-pktapp-00.txt
846 [5] M. Sirbu, J. Chuang. Distributed Authentication in Kerberos
847 Using Public Key Cryptography. Symposium On Network and Distributed
848 System Security, 1997.
850 [6] B. Cox, J.D. Tygar, M. Sirbu. NetBill Security and Transaction
851 Protocol. In Proceedings of the USENIX Workshop on Electronic
854 [7] Alan O. Freier, Philip Karlton and Paul C. Kocher. The SSL
855 Protocol, Version 3.0 - IETF Draft.
857 [8] B.C. Neuman, Proxy-Based Authorization and Accounting for
858 Distributed Systems. In Proceedings of the 13th International
859 Conference on Distributed Computing Systems, May 1993.
861 [9] ITU-T (formerly CCITT) Information technology - Open Systems
862 Interconnection - The Directory: Authentication Framework
863 Recommendation X.509 ISO/IEC 9594-8
868 Sasha Medvinsky contributed several ideas to the protocol changes
869 and specifications in this document. His additions have been most
872 Some of the ideas on which this proposal is based arose during
873 discussions over several years between members of the SAAG, the IETF
874 CAT working group, and the PSRG, regarding integration of Kerberos
875 and SPX. Some ideas have also been drawn from the DASS system.
876 These changes are by no means endorsed by these groups. This is an
877 attempt to revive some of the goals of those groups, and this
878 proposal approaches those goals primarily from the Kerberos
879 perspective. Lastly, comments from groups working on similar ideas
880 in DCE have been invaluable.
885 This draft expires May 26, 1998.
892 USC Information Sciences Institute
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