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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 INTERNET-DRAFT Brian Tung 2 draft-ietf-cat-kerberos-pk-init-11.txt Clifford Neuman 3 Updates: RFC 1510 USC/ISI 4 expires September 15, 2000 Matthew Hur 5 CyberSafe Corporation 6 Ari Medvinsky 7 Keen.com, Inc. 8 Sasha Medvinsky 9 Motorola 10 John Wray 11 Iris Associates, Inc. 12 Jonathan Trostle 13 Cisco 15 Public Key Cryptography for Initial Authentication in Kerberos 17 0. Status Of This Memo 19 This document is an Internet-Draft and is in full conformance with 20 all provisions of Section 10 of RFC 2026. Internet-Drafts are 21 working documents of the Internet Engineering Task Force (IETF), 22 its areas, and its working groups. Note that other groups may also 23 distribute working documents as Internet-Drafts. 25 Internet-Drafts are draft documents valid for a maximum of six 26 months and may be updated, replaced, or obsoleted by other 27 documents at any time. It is inappropriate to use Internet-Drafts 28 as reference material or to cite them other than as "work in 29 progress." 31 The list of current Internet-Drafts can be accessed at 32 http://www.ietf.org/ietf/1id-abstracts.txt 34 The list of Internet-Draft Shadow Directories can be accessed at 35 http://www.ietf.org/shadow.html. 37 To learn the current status of any Internet-Draft, please check 38 the "1id-abstracts.txt" listing contained in the Internet-Drafts 39 Shadow Directories on ftp.ietf.org (US East Coast), 40 nic.nordu.net (Europe), ftp.isi.edu (US West Coast), or 41 munnari.oz.au (Pacific Rim). 43 The distribution of this memo is unlimited. It is filed as 44 draft-ietf-cat-kerberos-pk-init-11.txt, and expires September 15, 45 2000. Please send comments to the authors. 47 1. Abstract 49 This document defines extensions (PKINIT) to the Kerberos protocol 50 specification (RFC 1510 [1]) to provide a method for using public 51 key cryptography during initial authentication. The methods 52 defined specify the ways in which preauthentication data fields and 53 error data fields in Kerberos messages are to be used to transport 54 public key data. 56 2. Introduction 58 The popularity of public key cryptography has produced a desire for 59 its support in Kerberos [2]. The advantages provided by public key 60 cryptography include simplified key management (from the Kerberos 61 perspective) and the ability to leverage existing and developing 62 public key certification infrastructures. 64 Public key cryptography can be integrated into Kerberos in a number 65 of ways. One is to associate a key pair with each realm, which can 66 then be used to facilitate cross-realm authentication; this is the 67 topic of another draft proposal. Another way is to allow users with 68 public key certificates to use them in initial authentication. This 69 is the concern of the current document. 71 PKINIT utilizes ephemeral-ephemeral Diffie-Hellman keys in 72 combination with digital signature keys as the primary, required 73 mechanism. It also allows for the use of RSA keys and/or (static) 74 Diffie-Hellman certificates. Note in particular that PKINIT supports 75 the use of separate signature and encryption keys. 77 PKINIT enables access to Kerberos-secured services based on initial 78 authentication utilizing public key cryptography. PKINIT utilizes 79 standard public key signature and encryption data formats within the 80 standard Kerberos messages. The basic mechanism is as follows: The 81 user sends an AS-REQ message to the KDC as before, except that if that 82 user is to use public key cryptography in the initial authentication 83 step, his certificate and a signature accompany the initial request 84 in the preauthentication fields. Upon receipt of this request, the 85 KDC verifies the certificate and issues a ticket granting ticket 86 (TGT) as before, except that the encPart from the AS-REP message 87 carrying the TGT is now encrypted utilizing either a Diffie-Hellman 88 derived key or the user's public key. This message is authenticated 89 utilizing the public key signature of the KDC. 91 Note that PKINIT does not require the use of certificates. A KDC 92 may store the public key of a principal as part of that principal's 93 record. In this scenario, the KDC is the trusted party that vouches 94 for the principal (as in a standard, non-cross realm, Kerberos 95 environment). Thus, for any principal, the KDC may maintain a 96 secret key, a public key, or both. 98 The PKINIT specification may also be used as a building block for 99 other specifications. PKCROSS [3] utilizes PKINIT for establishing 100 the inter-realm key and associated inter-realm policy to be applied 101 in issuing cross realm service tickets. As specified in [4], 102 anonymous Kerberos tickets can be issued by applying a NULL 103 signature in combination with Diffie-Hellman in the PKINIT exchange. 104 Additionally, the PKINIT specification may be used for direct peer 105 to peer authentication without contacting a central KDC. This 106 application of PKINIT is described in PKTAPP [5] and is based on 107 concepts introduced in [6, 7]. For direct client-to-server 108 authentication, the client uses PKINIT to authenticate to the end 109 server (instead of a central KDC), which then issues a ticket for 110 itself. This approach has an advantage over TLS [8] in that the 111 server does not need to save state (cache session keys). 112 Furthermore, an additional benefit is that Kerberos tickets can 113 facilitate delegation (see [9]). 115 3. Proposed Extensions 117 This section describes extensions to RFC 1510 for supporting the 118 use of public key cryptography in the initial request for a ticket 119 granting ticket (TGT). 121 In summary, the following change to RFC 1510 is proposed: 123 * Users may authenticate using either a public key pair or a 124 conventional (symmetric) key. If public key cryptography is 125 used, public key data is transported in preauthentication 126 data fields to help establish identity. The user presents 127 a public key certificate and obtains an ordinary TGT that may 128 be used for subsequent authentication, with such 129 authentication using only conventional cryptography. 131 Section 3.1 provides definitions to help specify message formats. 132 Section 3.2 describes the extensions for the initial authentication 133 method. 135 3.1. Definitions 137 The extensions involve new preauthentication fields; we introduce 138 the following preauthentication types: 140 PA-PK-AS-REQ 14 141 PA-PK-AS-REP 15 143 The extensions also involve new error types; we introduce the 144 following types: 146 KDC_ERR_CLIENT_NOT_TRUSTED 62 147 KDC_ERR_KDC_NOT_TRUSTED 63 148 KDC_ERR_INVALID_SIG 64 149 KDC_ERR_KEY_TOO_WEAK 65 150 KDC_ERR_CERTIFICATE_MISMATCH 66 151 KDC_ERR_CANT_VERIFY_CERTIFICATE 70 152 KDC_ERR_INVALID_CERTIFICATE 71 153 KDC_ERR_REVOKED_CERTIFICATE 72 154 KDC_ERR_REVOCATION_STATUS_UNKNOWN 73 155 KDC_ERR_REVOCATION_STATUS_UNAVAILABLE 74 156 KDC_ERR_CLIENT_NAME_MISMATCH 75 157 KDC_ERR_KDC_NAME_MISMATCH 76 159 We utilize the following typed data for errors: 161 TD-PKINIT-CMS-CERTIFICATES 101 162 TD-KRB-PRINCIPAL 102 163 TD-KRB-REALM 103 164 TD-TRUSTED-CERTIFIERS 104 165 TD-CERTIFICATE-INDEX 105 167 We utilize the following encryption types (which map directly to 168 OIDs): 170 dsaWithSHA1-CmsOID 9 171 md5WithRSAEncryption-CmsOID 10 172 sha1WithRSAEncryption-CmsOID 11 173 rc2CBC-EnvOID 12 174 rsaEncryption-EnvOID (PKCS#1 v1.5) 13 175 rsaES-OAEP-ENV-OID (PKCS#1 v2.0) 14 176 des-ede3-cbc-Env-OID 15 178 These mappings are provided so that a client may send the 179 appropriate enctypes in the AS-REQ message in order to indicate 180 support for the corresponding OIDs (for performing PKINIT). 182 In many cases, PKINIT requires the encoding of the X.500 name of a 183 certificate authority as a Realm. When such a name appears as 184 a realm it will be represented using the "other" form of the realm 185 name as specified in the naming constraints section of RFC1510. 186 For a realm derived from an X.500 name, NAMETYPE will have the value 187 X500-RFC2253. The full realm name will appear as follows: 189 + ":" + 191 where nametype is "X500-RFC2253" and string is the result of doing 192 an RFC2253 encoding of the distinguished name, i.e. 194 "X500-RFC2253:" + RFC2253Encode(DistinguishedName) 196 where DistinguishedName is an X.500 name, and RFC2253Encode is a 197 function returing a readable UTF encoding of an X.500 name, as 198 defined by RFC 2253 [14] (part of LDAPv3 [18]). 200 To ensure that this encoding is unique, we add the following rule 201 to those specified by RFC 2253: 203 The order in which the attributes appear in the RFC 2253 204 encoding must be the reverse of the order in the ASN.1 205 encoding of the X.500 name that appears in the public key 206 certificate. The order of the relative distinguished names 207 (RDNs), as well as the order of the AttributeTypeAndValues 208 within each RDN, will be reversed. (This is despite the fact 209 that an RDN is defined as a SET of AttributeTypeAndValues, where 210 an order is normally not important.) 212 Similarly, in cases where the KDC does not provide a specific 213 policy based mapping from the X.500 name or X.509 Version 3 214 SubjectAltName extension in the user's certificate to a Kerberos 215 principal name, PKINIT requires the direct encoding of the X.500 216 name as a PrincipalName. In this case, the name-type of the 217 principal name shall be set to KRB_NT-X500-PRINCIPAL. This new 218 name type is defined in RFC 1510 as: 220 KRB_NT_X500_PRINCIPAL 6 222 The name-string shall be set as follows: 224 RFC2253Encode(DistinguishedName) 226 as described above. When this name type is used, the principal's 227 realm shall be set to the certificate authority's distinguished 228 name using the X500-RFC2253 realm name format described earlier in 229 this section 231 RFC 1510 specifies the ASN.1 structure for PrincipalName as follows: 233 PrincipalName ::= SEQUENCE { 234 name-type[0] INTEGER, 235 name-string[1] SEQUENCE OF GeneralString 236 } 238 For the purposes of encoding an X.500 name as a Kerberos name for 239 use in Kerberos structures, the name-string shall be encoded as a 240 single GeneralString. The name-type should be KRB_NT_X500_PRINCIPAL, 241 as noted above. All Kerberos names must conform to validity 242 requirements as given in RFC 1510. Note that name mapping may be 243 required or optional, based on policy. 245 We also define the following similar ASN.1 structure: 247 CertPrincipalName ::= SEQUENCE { 248 name-type[0] INTEGER, 249 name-string[1] SEQUENCE OF UTF8String 250 } 252 When a Kerberos PrincipalName is to be placed within an X.509 data 253 structure, the CertPrincipalName structure is to be used, with the 254 name-string encoded as a single UTF8String. The name-type should be 255 as identified in the original PrincipalName structure. The mapping 256 between the GeneralString and UTF8String formats can be found in 257 [19]. 259 The following rules relate to the the matching of PrincipalNames (or 260 corresponding CertPrincipalNames) with regard to the PKI name 261 constraints for CAs as laid out in RFC 2459 [15]. In order to be 262 regarded as a match (for permitted and excluded name trees), the 263 following must be satisfied. 265 1. If the constraint is given as a user plus realm name, or 266 as a user plus instance plus realm name (as specified in 267 RFC 1510), the realm name must be valid (see 2.a-d below) 268 and the match must be exact, byte for byte. 270 2. If the constraint is given only as a realm name, matching 271 depends on the type of the realm: 273 a. If the realm contains a colon (':') before any equal 274 sign ('='), it is treated as a realm of type Other, 275 and must match exactly, byte for byte. 277 b. Otherwise, if the realm contains an equal sign, it 278 is treated as an X.500 name. In order to match, every 279 component in the constraint MUST be in the principal 280 name, and have the same value. For example, 'C=US' 281 matches 'C=US/O=ISI' but not 'C=UK'. 283 c. Otherwise, if the realm name conforms to rules regarding 284 the format of DNS names, it is considered a realm name of 285 type Domain. The constraint may be given as a realm 286 name 'FOO.BAR', which matches any PrincipalName within 287 the realm 'FOO.BAR' but not those in subrealms such as 288 'CAR.FOO.BAR'. A constraint of the form '.FOO.BAR' 289 matches PrincipalNames in subrealms of the form 290 'CAR.FOO.BAR' but not the realm 'FOO.BAR' itself. 292 d. Otherwise, the realm name is invalid and does not match 293 under any conditions. 295 3.1.1. Encryption and Key Formats 297 In the exposition below, we use the terms public key and private 298 key generically. It should be understood that the term "public 299 key" may be used to refer to either a public encryption key or a 300 signature verification key, and that the term "private key" may be 301 used to refer to either a private decryption key or a signature 302 generation key. The fact that these are logically distinct does 303 not preclude the assignment of bitwise identical keys for RSA 304 keys. 306 In the case of Diffie-Hellman, the key shall be produced from the 307 agreed bit string as follows: 309 * Truncate the bit string to the appropriate length. 310 * Rectify parity in each byte (if necessary) to obtain the key. 312 For instance, in the case of a DES key, we take the first eight 313 bytes of the bit stream, and then adjust the least significant bit 314 of each byte to ensure that each byte has odd parity. 316 3.1.2. Algorithm Identifiers 318 PKINIT does not define, but does permit, the algorithm identifiers 319 listed below. 321 3.1.2.1. Signature Algorithm Identifiers 323 The following signature algorithm identifiers specified in [11] and 324 in [15] shall be used with PKINIT: 326 id-dsa-with-sha1 (DSA with SHA1) 327 md5WithRSAEncryption (RSA with MD5) 328 sha-1WithRSAEncryption (RSA with SHA1) 330 3.1.2.2 Diffie-Hellman Key Agreement Algorithm Identifier 332 The following algorithm identifier shall be used within the 333 SubjectPublicKeyInfo data structure: dhpublicnumber 335 This identifier and the associated algorithm parameters are 336 specified in RFC 2459 [15]. 338 3.1.2.3. Algorithm Identifiers for RSA Encryption 340 These algorithm identifiers are used inside the EnvelopedData data 341 structure, for encrypting the temporary key with a public key: 343 rsaEncryption (RSA encryption, PKCS#1 v1.5) 344 id-RSAES-OAEP (RSA encryption, PKCS#1 v2.0) 346 Both of the above RSA encryption schemes are specified in [16]. 347 Currently, only PKCS#1 v1.5 is specified by CMS [11], although the 348 CMS specification says that it will likely include PKCS#1 v2.0 in 349 the future. (PKCS#1 v2.0 addresses adaptive chosen ciphertext 350 vulnerability discovered in PKCS#1 v1.5.) 352 3.1.2.4. Algorithm Identifiers for Encryption with Secret Keys 354 These algorithm identifiers are used inside the EnvelopedData data 355 structure in the PKINIT Reply, for encrypting the reply key with the 356 temporary key: 357 des-ede3-cbc (3-key 3-DES, CBC mode) 358 rc2-cbc (RC2, CBC mode) 360 The full definition of the above algorithm identifiers and their 361 corresponding parameters (an IV for block chaining) is provided in 362 the CMS specification [11]. 364 3.2. Public Key Authentication 366 Implementation of the changes in this section is REQUIRED for 367 compliance with PKINIT. 369 3.2.1. Client Request 371 Public keys may be signed by some certification authority (CA), or 372 they may be maintained by the KDC in which case the KDC is the 373 trusted authority. Note that the latter mode does not require the 374 use of certificates. 376 The initial authentication request is sent as per RFC 1510, except 377 that a preauthentication field containing data signed by the user's 378 private key accompanies the request: 380 PA-PK-AS-REQ ::= SEQUENCE { 381 -- PA TYPE 14 382 signedAuthPack [0] SignedData 383 -- Defined in CMS [11]; 384 -- AuthPack (below) defines the 385 -- data that is signed. 386 trustedCertifiers [1] SEQUENCE OF TrustedCas OPTIONAL, 387 -- This is a list of CAs that the 388 -- client trusts and that certify 389 -- KDCs. 390 kdcCert [2] IssuerAndSerialNumber OPTIONAL 391 -- As defined in CMS [11]; 392 -- specifies a particular KDC 393 -- certificate if the client 394 -- already has it. 395 encryptionCert [3] IssuerAndSerialNumber OPTIONAL 396 -- For example, this may be the 397 -- client's Diffie-Hellman 398 -- certificate, or it may be the 399 -- client's RSA encryption 400 -- certificate. 401 } 403 TrustedCas ::= CHOICE { 404 principalName [0] KerberosName, 405 -- as defined below 406 caName [1] Name 407 -- fully qualified X.500 name 408 -- as defined by X.509 409 issuerAndSerial [2] IssuerAndSerialNumber 410 -- Since a CA may have a number of 411 -- certificates, only one of which 412 -- a client trusts 413 } 415 Usage of SignedData: 417 The SignedData data type is specified in the Cryptographic 418 Message Syntax, a product of the S/MIME working group of the 419 IETF. The following describes how to fill in the fields of 420 this data: 422 1. The encapContentInfo field must contain the PKAuthenticator 423 and, optionally, the client's Diffie Hellman public value. 425 a. The eContentType field shall contain the OID value for 426 pkdata: iso (1) org (3) dod (6) internet (1) security (5) 427 kerberosv5 (2) pkinit (3) pkdata (1) 429 b. The eContent field is data of the type AuthPack (below). 431 2. The signerInfos field contains the signature of AuthPack. 433 3. The Certificates field, when non-empty, contains the client's 434 certificate chain. If present, the KDC uses the public key 435 from the client's certificate to verify the signature in the 436 request. Note that the client may pass different certificate 437 chains that are used for signing or for encrypting. Thus, 438 the KDC may utilize a different client certificate for 439 signature verification than the one it uses to encrypt the 440 reply to the client. For example, the client may place a 441 Diffie-Hellman certificate in this field in order to convey 442 its static Diffie Hellman certificate to the KDC to enable 443 static-ephemeral Diffie-Hellman mode for the reply; in this 444 case, the client does NOT place its public value in the 445 AuthPack (defined below). As another example, the client may 446 place an RSA encryption certificate in this field. However, 447 there must always be (at least) a signature certificate. 449 AuthPack ::= SEQUENCE { 450 pkAuthenticator [0] PKAuthenticator, 451 clientPublicValue [1] SubjectPublicKeyInfo OPTIONAL 452 -- if client is using Diffie-Hellman 453 -- (ephemeral-ephemeral only) 454 } 456 PKAuthenticator ::= SEQUENCE { 457 kdcName [0] PrincipalName, 458 kdcRealm [1] Realm, 459 cusec [2] INTEGER, 460 -- for replay prevention as in RFC1510 461 ctime [3] KerberosTime, 462 -- for replay prevention as in RFC1510 463 nonce [4] INTEGER 464 } 466 SubjectPublicKeyInfo ::= SEQUENCE { 467 algorithm AlgorithmIdentifier, 468 -- dhKeyAgreement 469 subjectPublicKey BIT STRING 470 -- for DH, equals 471 -- public exponent (INTEGER encoded 472 -- as payload of BIT STRING) 473 } -- as specified by the X.509 recommendation [10] 475 AlgorithmIdentifier ::= SEQUENCE { 476 algorithm ALGORITHM.&id, 477 parameters ALGORITHM.&type 478 } -- as specified by the X.509 recommendation [10] 480 If the client passes an issuer and serial number in the request, 481 the KDC is requested to use the referred-to certificate. If none 482 exists, then the KDC returns an error of type 483 KDC_ERR_CERTIFICATE_MISMATCH. It also returns this error if, on the 484 other hand, the client does not pass any trustedCertifiers, 485 believing that it has the KDC's certificate, but the KDC has more 486 than one certificate. The KDC should include information in the 487 KRB-ERROR message that indicates the KDC certificate(s) that a 488 client may utilize. This data is specified in the e-data, which 489 is defined in RFC 1510 revisions as a SEQUENCE of TypedData: 491 TypedData ::= SEQUENCE { 492 data-type [0] INTEGER, 493 data-value [1] OCTET STRING, 494 } -- per Kerberos RFC 1510 revisions 496 where: 497 data-type = TD-PKINIT-CMS-CERTIFICATES = 101 498 data-value = CertificateSet // as specified by CMS [11] 500 The PKAuthenticator carries information to foil replay attacks, and 501 to bind the request and response. The PKAuthenticator is signed 502 with the client's signature key. 504 3.2.2. KDC Response 506 Upon receipt of the AS_REQ with PA-PK-AS-REQ pre-authentication 507 type, the KDC attempts to verify the user's certificate chain 508 (userCert), if one is provided in the request. This is done by 509 verifying the certification path against the KDC's policy of 510 legitimate certifiers. This may be based on a certification 511 hierarchy, or it may be simply a list of recognized certifiers in a 512 system like PGP. 514 If the client's certificate chain contains no certificate signed by 515 a CA trusted by the KDC, then the KDC sends back an error message 516 of type KDC_ERR_CANT_VERIFY_CERTIFICATE. The accompanying e-data 517 is a SEQUENCE of one TypedData (with type TD-TRUSTED-CERTIFIERS=104) 518 whose data-value is an OCTET STRING which is the DER encoding of 520 TrustedCertifiers ::= SEQUENCE OF PrincipalName 521 -- X.500 name encoded as a principal name 522 -- see Section 3.1 524 If while verifying a certificate chain the KDC determines that the 525 signature on one of the certificates in the CertificateSet from 526 the signedAuthPack fails verification, then the KDC returns an 527 error of type KDC_ERR_INVALID_CERTIFICATE. The accompanying 528 e-data is a SEQUENCE of one TypedData (with type 529 TD-CERTIFICATE-INDEX=105) whose data-value is an OCTET STRING 530 which is the DER encoding of the index into the CertificateSet 531 ordered as sent by the client. 533 CertificateIndex ::= INTEGER 534 -- 0 = 1st certificate, 535 -- (in order of encoding) 536 -- 1 = 2nd certificate, etc 538 The KDC may also check whether any of the certificates in the 539 client's chain has been revoked. If one of the certificates has 540 been revoked, then the KDC returns an error of type 541 KDC_ERR_REVOKED_CERTIFICATE; if such a query reveals that 542 the certificate's revocation status is unknown or not 543 available, then if required by policy, the KDC returns the 544 appropriate error of type KDC_ERR_REVOCATION_STATUS_UNKNOWN or 545 KDC_ERR_REVOCATION_STATUS_UNAVAILABLE. In any of these three 546 cases, the affected certificate is identified by the accompanying 547 e-data, which contains a CertificateIndex as described for 548 KDC_ERR_INVALID_CERTIFICATE. 550 If the certificate chain can be verified, but the name of the 551 client in the certificate does not match the client's name in the 552 request, then the KDC returns an error of type 553 KDC_ERR_CLIENT_NAME_MISMATCH. There is no accompanying e-data 554 field in this case. 556 Finally, if the certificate chain is verified, but the KDC's name 557 or realm as given in the PKAuthenticator does not match the KDC's 558 actual principal name, then the KDC returns an error of type 559 KDC_ERR_KDC_NAME_MISMATCH. The accompanying e-data field is again 560 a SEQUENCE of one TypedData (with type TD-KRB-PRINCIPAL=102 or 561 TD-KRB-REALM=103 as appropriate) whose data-value is an OCTET 562 STRING whose data-value is the DER encoding of a PrincipalName or 563 Realm as defined in RFC 1510 revisions. 565 Even if all succeeds, the KDC may--for policy reasons--decide not 566 to trust the client. In this case, the KDC returns an error message 567 of type KDC_ERR_CLIENT_NOT_TRUSTED. One specific case of this is 568 the presence or absence of an Enhanced Key Usage (EKU) OID within 569 the certificate extensions. The rules regarding acceptability of 570 an EKU sequence (or the absence of any sequence) are a matter of 571 local policy. For the benefit of implementers, we define a PKINIT 572 EKU OID as the following: iso (1) org (3) dod (6) internet (1) 573 security (5) kerberosv5 (2) pkinit (3) pkekuoid (2). 575 If a trust relationship exists, the KDC then verifies the client's 576 signature on AuthPack. If that fails, the KDC returns an error 577 message of type KDC_ERR_INVALID_SIG. Otherwise, the KDC uses the 578 timestamp (ctime and cusec) in the PKAuthenticator to assure that 579 the request is not a replay. The KDC also verifies that its name 580 is specified in the PKAuthenticator. 582 If the clientPublicValue field is filled in, indicating that the 583 client wishes to use Diffie-Hellman key agreement, then the KDC 584 checks to see that the parameters satisfy its policy. If they do 585 not (e.g., the prime size is insufficient for the expected 586 encryption type), then the KDC sends back an error message of type 587 KDC_ERR_KEY_TOO_WEAK. Otherwise, it generates its own public and 588 private values for the response. 590 The KDC also checks that the timestamp in the PKAuthenticator is 591 within the allowable window and that the principal name and realm 592 are correct. If the local (server) time and the client time in the 593 authenticator differ by more than the allowable clock skew, then the 594 KDC returns an error message of type KRB_AP_ERR_SKEW as defined in 1510. 596 Assuming no errors, the KDC replies as per RFC 1510, except as 597 follows. The user's name in the ticket is determined by the 598 following decision algorithm: 600 1. If the KDC has a mapping from the name in the certificate 601 to a Kerberos name, then use that name. 602 Else 603 2. If the certificate contains the SubjectAltName extention 604 and the local KDC policy defines a mapping from the 605 SubjectAltName to a Kerberos name, then use that name. 606 Else 607 3. Use the name as represented in the certificate, mapping 608 mapping as necessary (e.g., as per RFC 2253 for X.500 609 names). In this case the realm in the ticket shall be the 610 name of the certifier that issued the user's certificate. 612 Note that a principal name may be carried in the subject alt name 613 field of a certificate. This name may be mapped to a principal 614 record in a security database based on local policy, for example 615 the subject alt name may be kerberos/principal@realm format. In 616 this case the realm name is not that of the CA but that of the 617 local realm doing the mapping (or some realm name chosen by that 618 realm). 620 If a non-KDC X.509 certificate contains the principal name within 621 the subjectAltName version 3 extension , that name may utilize 622 KerberosName as defined below, or, in the case of an S/MIME 623 certificate [17], may utilize the email address. If the KDC 624 is presented with an S/MIME certificate, then the email address 625 within subjectAltName will be interpreted as a principal and realm 626 separated by the "@" sign, or as a name that needs to be 627 canonicalized. If the resulting name does not correspond to a 628 registered principal name, then the principal name is formed as 629 defined in section 3.1. 631 The trustedCertifiers field contains a list of certification 632 authorities trusted by the client, in the case that the client does 633 not possess the KDC's public key certificate. If the KDC has no 634 certificate signed by any of the trustedCertifiers, then it returns 635 an error of type KDC_ERR_KDC_NOT_TRUSTED. 637 KDCs should try to (in order of preference): 638 1. Use the KDC certificate identified by the serialNumber included 639 in the client's request. 640 2. Use a certificate issued to the KDC by the client's CA (if in the 641 middle of a CA key roll-over, use the KDC cert issued under same 642 CA key as user cert used to verify request). 643 3. Use a certificate issued to the KDC by one of the client's 644 trustedCertifier(s); 645 If the KDC is unable to comply with any of these options, then the 646 KDC returns an error message of type KDC_ERR_KDC_NOT_TRUSTED to the 647 client. 649 The KDC encrypts the reply not with the user's long-term key, but 650 with the Diffie Hellman derived key or a random key generated 651 for this particular response which is carried in the padata field of 652 the TGS-REP message. 654 PA-PK-AS-REP ::= CHOICE { 655 -- PA TYPE 15 656 dhSignedData [0] SignedData, 657 -- Defined in CMS and used only with 658 -- Diffie-Hellman key exchange (if the 659 -- client public value was present in the 660 -- request). 661 -- This choice MUST be supported 662 -- by compliant implementations. 663 encKeyPack [1] EnvelopedData, 664 -- Defined in CMS 665 -- The temporary key is encrypted 666 -- using the client public key 667 -- key 668 -- SignedReplyKeyPack, encrypted 669 -- with the temporary key, is also 670 -- included. 671 } 673 Usage of SignedData: 675 When the Diffie-Hellman option is used, dhSignedData in 676 PA-PK-AS-REP provides authenticated Diffie-Hellman parameters 677 of the KDC. The reply key used to encrypt part of the KDC reply 678 message is derived from the Diffie-Hellman exchange: 680 1. Both the KDC and the client calculate a secret value 681 (g^ab mod p), where a is the client's private exponent and 682 b is the KDC's private exponent. 684 2. Both the KDC and the client take the first N bits of this 685 secret value and convert it into a reply key. N depends on 686 the reply key type. 688 3. If the reply key is DES, N=64 bits, where some of the bits 689 are replaced with parity bits, according to FIPS PUB 74. 691 4. If the reply key is (3-key) 3-DES, N=192 bits, where some 692 of the bits are replaced with parity bits, according to 693 FIPS PUB 74. 695 5. The encapContentInfo field must contain the KdcDHKeyInfo as 696 defined below. 698 a. The eContentType field shall contain the OID value for 699 pkdata: iso (1) org (3) dod (6) internet (1) security (5) 700 kerberosv5 (2) pkinit (3) pkdata (1) 702 b. The eContent field is data of the type KdcDHKeyInfo 703 (below). 705 6. The certificates field must contain the certificates 706 necessary for the client to establish trust in the KDC's 707 certificate based on the list of trusted certifiers sent by 708 the client in the PA-PK-AS-REQ. This field may be empty if 709 the client did not send to the KDC a list of trusted 710 certifiers (the trustedCertifiers field was empty, meaning 711 that the client already possesses the KDC's certificate). 713 7. The signerInfos field is a SET that must contain at least 714 one member, since it contains the actual signature. 716 KdcDHKeyInfo ::= SEQUENCE { 717 -- used only when utilizing Diffie-Hellman 718 nonce [0] INTEGER, 719 -- binds responce to the request 720 subjectPublicKey [2] BIT STRING 721 -- Equals public exponent (g^a mod p) 722 -- INTEGER encoded as payload of 723 -- BIT STRING 724 } 726 Usage of EnvelopedData: 728 The EnvelopedData data type is specified in the Cryptographic 729 Message Syntax, a product of the S/MIME working group of the 730 IETF. It contains a temporary key encrypted with the PKINIT 731 client's public key. It also contains a signed and encrypted 732 reply key. 734 1. The originatorInfo field is not required, since that 735 information may be presented in the signedData structure 736 that is encrypted within the encryptedContentInfo field. 738 2. The optional unprotectedAttrs field is not required for 739 PKINIT. 741 3. The recipientInfos field is a SET which must contain exactly 742 one member of the KeyTransRecipientInfo type for encryption 743 with an RSA public key. 745 a. The encryptedKey field (in KeyTransRecipientInfo) 746 contains the temporary key which is encrypted with the 747 PKINIT client's public key. 749 4. The encryptedContentInfo field contains the signed and 750 encrypted reply key. 752 a. The contentType field shall contain the OID value for 753 id-signedData: iso (1) member-body (2) us (840) 754 rsadsi (113549) pkcs (1) pkcs7 (7) signedData (2) 756 b. The encryptedContent field is encrypted data of the CMS 757 type signedData as specified below. 759 i. The encapContentInfo field must contains the 760 ReplyKeyPack. 762 * The eContentType field shall contain the OID value 763 for pkdata: iso (1) org (3) dod (6) internet (1) 764 security (5) kerberosv5 (2) pkinit (3) pkdata (1) 766 * The eContent field is data of the type ReplyKeyPack 767 (below). 769 ii. The certificates field must contain the certificates 770 necessary for the client to establish trust in the 771 KDC's certificate based on the list of trusted 772 certifiers sent by the client in the PA-PK-AS-REQ. 773 This field may be empty if the client did not send 774 to the KDC a list of trusted certifiers (the 775 trustedCertifiers field was empty, meaning that the 776 client already possesses the KDC's certificate). 778 iii. The signerInfos field is a SET that must contain at 779 least one member, since it contains the actual 780 signature. 782 ReplyKeyPack ::= SEQUENCE { 783 -- not used for Diffie-Hellman 784 replyKey [0] EncryptionKey, 785 -- used to encrypt main reply 786 -- ENCTYPE is at least as strong as 787 -- ENCTYPE of session key 788 nonce [1] INTEGER, 789 -- binds response to the request 790 -- must be same as the nonce 791 -- passed in the PKAuthenticator 792 } 794 Since each certifier in the certification path of a user's 795 certificate is equivalent to a separate Kerberos realm, the name 796 of each certifier in the certificate chain must be added to the 797 transited field of the ticket. The format of these realm names is 798 defined in Section 3.1 of this document. If applicable, the 799 transit-policy-checked flag should be set in the issued ticket. 801 The KDC's certificate(s) must bind the public key(s) of the KDC to 802 a name derivable from the name of the realm for that KDC. X.509 803 certificates shall contain the principal name of the KDC 804 (defined in section 8.2 of RFC 1510) as the SubjectAltName version 805 3 extension. Below is the definition of this version 3 extension, 806 as specified by the X.509 standard: 808 subjectAltName EXTENSION ::= { 809 SYNTAX GeneralNames 810 IDENTIFIED BY id-ce-subjectAltName 811 } 813 GeneralNames ::= SEQUENCE SIZE(1..MAX) OF GeneralName 815 GeneralName ::= CHOICE { 816 otherName [0] OtherName, 817 ... 818 } 820 OtherName ::= SEQUENCE { 821 type-id OBJECT IDENTIFIER, 822 value [0] EXPLICIT ANY DEFINED BY type-id 823 } 825 For the purpose of specifying a Kerberos principal name, the value 826 in OtherName shall be a KerberosName as defined in RFC 1510, but with 827 the PrincipalName replaced by CertPrincipalName as mentioned in 828 Section 3.1: 830 KerberosName ::= SEQUENCE { 831 realm [0] Realm, 832 principalName [1] CertPrincipalName -- defined above 833 } 835 This specific syntax is identified within subjectAltName by setting 836 the type-id in OtherName to krb5PrincipalName, where (from the 837 Kerberos specification) we have 839 krb5 OBJECT IDENTIFIER ::= { iso (1) 840 org (3) 841 dod (6) 842 internet (1) 843 security (5) 844 kerberosv5 (2) } 846 krb5PrincipalName OBJECT IDENTIFIER ::= { krb5 2 } 848 (This specification may also be used to specify a Kerberos name 849 within the user's certificate.) The KDC's certificate may be signed 850 directly by a CA, or there may be intermediaries if the server resides 851 within a large organization, or it may be unsigned if the client 852 indicates possession (and trust) of the KDC's certificate. 854 The client then extracts the random key used to encrypt the main 855 reply. This random key (in encPaReply) is encrypted with either the 856 client's public key or with a key derived from the DH values 857 exchanged between the client and the KDC. The client uses this 858 random key to decrypt the main reply, and subsequently proceeds as 859 described in RFC 1510. 861 3.2.3. Required Algorithms 863 Not all of the algorithms in the PKINIT protocol specification have 864 to be implemented in order to comply with the proposed standard. 865 Below is a list of the required algorithms: 867 * Diffie-Hellman public/private key pairs 868 * utilizing Diffie-Hellman ephemeral-ephemeral mode 869 * SHA1 digest and DSA for signatures 870 * 3-key triple DES keys derived from the Diffie-Hellman Exchange 871 * 3-key triple DES Temporary and Reply keys 873 4. Logistics and Policy 875 This section describes a way to define the policy on the use of 876 PKINIT for each principal and request. 878 The KDC is not required to contain a database record for users 879 who use public key authentication. However, if these users are 880 registered with the KDC, it is recommended that the database record 881 for these users be modified to an additional flag in the attributes 882 field to indicate that the user should authenticate using PKINIT. 883 If this flag is set and a request message does not contain the 884 PKINIT preauthentication field, then the KDC sends back as error of 885 type KDC_ERR_PREAUTH_REQUIRED indicating that a preauthentication 886 field of type PA-PK-AS-REQ must be included in the request. 888 5. Security Considerations 890 PKINIT raises a few security considerations, which we will address 891 in this section. 893 First of all, PKINIT introduces a new trust model, where KDCs do not 894 (necessarily) certify the identity of those for whom they issue 895 tickets. PKINIT does allow KDCs to act as their own CAs, in the 896 limited capacity of self-signing their certificates, but one of the 897 additional benefits is to align Kerberos authentication with a global 898 public key infrastructure. Anyone using PKINIT in this way must be 899 aware of how the certification infrastructure they are linking to 900 works. 902 Secondly, PKINIT also introduces the possibility of interactions 903 between different cryptosystems, which may be of widely varying 904 strengths. Many systems, for instance, allow the use of 512-bit 905 public keys. Using such keys to wrap data encrypted under strong 906 conventional cryptosystems, such as triple-DES, is inappropriate; 907 it adds a weak link to a strong one at extra cost. Implementors 908 and administrators should take care to avoid such wasteful and 909 deceptive interactions. 911 Lastly, PKINIT calls for randomly generated keys for conventional 912 cryptosystems. Many such systems contain systematically "weak" 913 keys. PKINIT implementations MUST avoid use of these keys, either 914 by discarding those keys when they are generated, or by fixing them 915 in some way (e.g., by XORing them with a given mask). These 916 precautions vary from system to system; it is not our intention to 917 give an explicit recipe for them here. 919 6. Transport Issues 921 Certificate chains can potentially grow quite large and span several 922 UDP packets; this in turn increases the probability that a Kerberos 923 message involving PKINIT extensions will be broken in transit. In 924 light of the possibility that the Kerberos specification will 925 require KDCs to accept requests using TCP as a transport mechanism, 926 we make the same recommendation with respect to the PKINIT 927 extensions as well. 929 7. Bibliography 931 [1] J. Kohl, C. Neuman. The Kerberos Network Authentication Service 932 (V5). Request for Comments 1510. 934 [2] B.C. Neuman, Theodore Ts'o. Kerberos: An Authentication Service 935 for Computer Networks, IEEE Communications, 32(9):33-38. September 936 1994. 938 [3] B. Tung, T. Ryutov, C. Neuman, G. Tsudik, B. Sommerfeld, 939 A. Medvinsky, M. Hur. Public Key Cryptography for Cross-Realm 940 Authentication in Kerberos. draft-ietf-cat-kerberos-pk-cross-04.txt 942 [4] A. Medvinsky, J. Cargille, M. Hur. Anonymous Credentials in 943 Kerberos. draft-ietf-cat-kerberos-anoncred-00.txt 945 [5] Ari Medvinsky, M. Hur, Alexander Medvinsky, B. Clifford Neuman. 946 Public Key Utilizing Tickets for Application Servers (PKTAPP). 947 draft-ietf-cat-pktapp-02.txt 949 [6] M. Sirbu, J. Chuang. Distributed Authentication in Kerberos 950 Using Public Key Cryptography. Symposium On Network and Distributed 951 System Security, 1997. 953 [7] B. Cox, J.D. Tygar, M. Sirbu. NetBill Security and Transaction 954 Protocol. In Proceedings of the USENIX Workshop on Electronic 955 Commerce, July 1995. 957 [8] T. Dierks, C. Allen. The TLS Protocol, Version 1.0 958 Request for Comments 2246, January 1999. 960 [9] B.C. Neuman, Proxy-Based Authorization and Accounting for 961 Distributed Systems. In Proceedings of the 13th International 962 Conference on Distributed Computing Systems, May 1993. 964 [10] ITU-T (formerly CCITT) Information technology - Open Systems 965 Interconnection - The Directory: Authentication Framework 966 Recommendation X.509 ISO/IEC 9594-8 968 [11] R. Housley. Cryptographic Message Syntax. 969 draft-ietf-smime-cms-13.txt, April 1999, approved for publication 970 as RFC. 972 [12] PKCS #7: Cryptographic Message Syntax Standard, 973 An RSA Laboratories Technical Note Version 1.5 974 Revised November 1, 1993 976 [13] R. Rivest, MIT Laboratory for Computer Science and RSA Data 977 Security, Inc. A Description of the RC2(r) Encryption Algorithm 978 March 1998. 979 Request for Comments 2268. 981 [14] M. Wahl, S. Kille, T. Howes. Lightweight Directory Access 982 Protocol (v3): UTF-8 String Representation of Distinguished Names. 983 Request for Comments 2253. 985 [15] R. Housley, W. Ford, W. Polk, D. Solo. Internet X.509 Public 986 Key Infrastructure, Certificate and CRL Profile, January 1999. 987 Request for Comments 2459. 989 [16] B. Kaliski, J. Staddon. PKCS #1: RSA Cryptography 990 Specifications, October 1998. Request for Comments 2437. 992 [17] S. Dusse, P. Hoffman, B. Ramsdell, J. Weinstein. S/MIME 993 Version 2 Certificate Handling, March 1998. Request for 994 Comments 2312. 996 [18] M. Wahl, T. Howes, S. Kille. Lightweight Directory Access 997 Protocol (v3), December 1997. Request for Comments 2251. 999 [19] ITU-T (formerly CCITT) Information Processing Systems - Open 1000 Systems Interconnection - Specification of Abstract Syntax Notation 1001 One (ASN.1) Rec. X.680 ISO/IEC 8824-1 1003 8. Acknowledgements 1005 Some of the ideas on which this proposal is based arose during 1006 discussions over several years between members of the SAAG, the IETF 1007 CAT working group, and the PSRG, regarding integration of Kerberos 1008 and SPX. Some ideas have also been drawn from the DASS system. 1009 These changes are by no means endorsed by these groups. This is an 1010 attempt to revive some of the goals of those groups, and this 1011 proposal approaches those goals primarily from the Kerberos 1012 perspective. Lastly, comments from groups working on similar ideas 1013 in DCE have been invaluable. 1015 9. Expiration Date 1017 This draft expires September 15, 2000. 1019 10. Authors 1021 Brian Tung 1022 Clifford Neuman 1023 USC Information Sciences Institute 1024 4676 Admiralty Way Suite 1001 1025 Marina del Rey CA 90292-6695 1026 Phone: +1 310 822 1511 1027 E-mail: {brian, bcn}@isi.edu 1029 Matthew Hur 1030 CyberSafe Corporation 1031 1605 NW Sammamish Road 1032 Issaquah WA 98027-5378 1033 Phone: +1 425 391 6000 1034 E-mail: matt.hur@cybersafe.com 1036 Ari Medvinsky 1037 Keen.com, Inc. 1038 150 Independence Drive 1039 Menlo Park CA 94025 1040 Phone: +1 650 289 3134 1041 E-mail: ari@keen.com 1043 Sasha Medvinsky 1044 Motorola 1045 6450 Sequence Drive 1046 San Diego, CA 92121 1047 Phone +1 619 404 2825 1048 E-mail: smedvinsky@gi.com 1050 John Wray 1051 Iris Associates, Inc. 1052 5 Technology Park Dr. 1053 Westford, MA 01886 1054 E-mail: John_Wray@iris.com 1056 Jonathan Trostle 1057 170 W. Tasman Dr. 1058 San Jose, CA 95134 1059 E-mail: jtrostle@cisco.com