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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Internet-Draft M. Brown 3 Updates: 4346 (once approved) RedPhone Security 4 June 2006 R. Housley 5 Expires: December 2006 Vigil Security 7 Transport Layer Security (TLS) Authorization Extensions 8 10 Status of this Memo 12 By submitting this Internet-Draft, each author represents that any 13 applicable patent or other IPR claims of which he or she is aware 14 have been or will be disclosed, and any of which he or she becomes 15 aware will be disclosed, in accordance with Section 6 of BCP 79. 17 Internet-Drafts are working documents of the Internet Engineering 18 Task Force (IETF), its areas, and its working groups. Note that 19 other groups may also distribute working documents as Internet- 20 Drafts. 22 Internet-Drafts are draft documents valid for a maximum of six months 23 and may be updated, replaced, or obsoleted by other documents at any 24 time. It is inappropriate to use Internet-Drafts as reference 25 material or to cite them other than as "work in progress." 27 The list of current Internet-Drafts can be accessed at 28 http://www.ietf.org/ietf/1id-abstracts.txt. 30 The list of Internet-Draft Shadow Directories can be accessed at 31 http://www.ietf.org/shadow.html. 33 Copyright Notice 35 Copyright (C) The Internet Society (2006). All Rights Reserved. 37 Abstract 39 This document specifies authorization extensions to the Transport 40 Layer Security (TLS) Handshake Protocol. Extensions carried in the 41 client and server hello messages to confirm that both parties support 42 the desired authorization data types. Then, if supported by both the 43 client and the server, authorization information is exchanged in the 44 supplemental data handshake message. 46 1. Introduction 48 Transport Layer Security (TLS) protocol [TLS1.0][TLS1.1] is being 49 used in an increasing variety of operational environments, including 50 ones that were not envisioned at the time of the original design for 51 TLS. The extensions introduced in this document are designed to 52 enable TLS to operate in environments where authorization information 53 needs to be exchanged between the client and the server before any 54 protected data is exchanged. 56 The use of these TLS authorization extensions is especially 57 attractive when more than one application protocol can make use of 58 the same authorization information. Straightforward binding of 59 identification, authentication, and authorization information is 60 possible when all of these are handled within TLS. If each 61 application requires unique authorization information, then it might 62 best be carried within the TLS-protected application protocol. 63 However, care must be taken to ensure appropriate bindings when 64 identification, authentication, and authorization information are 65 handled at different protocol layers. 67 This document describes authorization extensions for the TLS 68 Handshake Protocol in both TLS 1.0 and TLS 1.1. These extensions 69 observe the conventions defined for TLS Extensions [TLSEXT] that make 70 use of the general extension mechanisms for the client hello message 71 and the server hello message. The extensions described in this 72 document confirm that both the client and the server support the 73 desired authorization data types. Then, if supported, authorization 74 information is exchanged in the supplemental data handshake message 75 [TLSSUPP]. 77 The authorization extensions may be used in conjunction with TLS 1.0 78 and TLS 1.1. The extensions are designed to be backwards compatible, 79 meaning that the Handshake Protocol Supplemental Data messages will 80 only contain authorization information of a particular type if the 81 client indicates support for them in the client hello message and the 82 server indicates support for them in the server hello message. 84 Clients typically know the context of the TLS session that is being 85 setup, thus the client can use the authorization extensions when they 86 are needed. Servers must accept extended client hello messages, even 87 if the server does not "understand" the all of the listed extensions. 88 However, the server will not indicate support for these "not 89 understood" extensions. Then, clients may reject communications with 90 servers that do not support the authorization extensions. 92 1.1. Conventions 94 The syntax for the authorization messages is defined using the TLS 95 Presentation Language, which is specified in Section 4 of [TLS1.0]. 97 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 98 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 99 document are to be interpreted as described in RFC 2119 [STDWORDS]. 101 1.2. Overview 103 Figure 1 illustrates the placement of the authorization extensions 104 and supplemental data messages in the full TLS handshake. 106 Client Server 108 ClientHello (w/ extensions) --------> 110 ServerHello (w/ extensions) 111 SupplementalData* 112 Certificate* 113 ServerKeyExchange* 114 CertificateRequest* 115 <-------- ServerHelloDone 116 SupplementalData* 117 Certificate* 118 ClientKeyExchange 119 CertificateVerify* 120 [ChangeCipherSpec] 121 Finished --------> 122 [ChangeCipherSpec] 123 <-------- Finished 124 Application Data <-------> Application Data 126 * Indicates optional or situation-dependent messages that 127 are not always sent. 129 [] Indicates that ChangeCipherSpec is an independent TLS 130 Protocol content type; it is not actually a TLS 131 handshake message. 133 Figure 1. Authorization data exchange in full TLS handshake 135 The ClientHello message includes an indication of the client 136 authorization data formats that are supported and an indication of 137 the server authorization data formats that are supported. The 138 ServerHello message contains similar indications, but any 139 authorization data formats that are not supported by the server are 140 not included. Both the client and the server MUST indicate support 141 for the authorization data types. If the list of mutually supported 142 authorization data formats is empty, then the ServerHello message 143 MUST NOT carry the affected extension at all. 145 2. Authorization Extension Types 147 The general extension mechanisms enable clients and servers to 148 negotiate whether to use specific extensions, and how to use specific 149 extensions. As specified in [TLSEXT], the extension format used in 150 the extended client hello message and extended server hello message 151 is repeated here for convenience: 153 struct { 154 ExtensionType extension_type; 155 opaque extension_data<0..2^16-1>; 156 } Extension; 158 The extension_type identifies a particular extension type, and the 159 extension_data contains information specific to the particular 160 extension type. 162 As specified in [TLSEXT], for all extension types, the extension type 163 MUST NOT appear in the extended server hello message unless the same 164 extension type appeared in the corresponding client hello message. 165 Clients MUST abort the handshake if they receive an extension type in 166 the extended server hello message that they did not request in the 167 associated extended client hello message. 169 When multiple extensions of different types are present in the 170 extended client hello message or the extended server hello message, 171 the extensions can appear in any order, but there MUST NOT be more 172 than one extension of the same type. 174 This document specifies the use of two new extension types: 175 client_authz and server_authz. These extension types are described 176 in Section 2.1 and Section 2.2, respectively. This specification 177 adds two new types to ExtensionType: 179 enum { 180 client_authz(TBD), server_authz(TBD), (65535) 181 } ExtensionType; 183 The authorization extensions are relevant when a session is initiated 184 and any subsequent session resumption. However, a client that 185 requests resumption of a session does not know whether the server 186 will have all of the context necessary to accept this request, and 187 therefore the client SHOULD send an extended client hello message 188 that includes the extension types associated with the authorization 189 extensions. This way, if the resumption request is denied, then the 190 authorization extensions will be negotiated as normal. 192 2.1. The client_authz Extension Type 194 Clients MUST include the client_authz extension type in the extended 195 client hello message to indicate their desire to send authorization 196 data to the server. The extension_data field indicates the format of 197 the authorization data that will be sent in the supplemental data 198 handshake message. The syntax of the client_authz extension_data 199 field is described in Section 2.3. 201 Servers that receive an extended client hello message containing the 202 client_authz extension MUST respond with the same client_authz 203 extension in the extended server hello message if the server is 204 willing to receive authorization data in the indicated format. Any 205 unacceptable formats must be removed from the list provided by the 206 client. The client_authz extension MUST be omitted from the extended 207 server hello message if the server is not willing to receive 208 authorization data in any of the indicated formats. 210 2.2. The server_authz Extension Type 212 Clients MUST include the server_authz extension type in the extended 213 client hello message to indicate their desire to receive 214 authorization data from the server. The extension_data field 215 indicates the format of the authorization data that will be sent in 216 the supplemental data handshake message. The syntax of the 217 server_authz extension_data field as described in Section 2.3. 219 Servers that receive an extended client hello message containing the 220 server_authz extension MUST respond with the same server_authz 221 extension in the extended server hello message if the server is 222 willing to provide authorization data in the requested format. Any 223 unacceptable formats must be removed from the list provided by the 224 client. The server_authz extension MUST be omitted from the extended 225 server hello message if the server is not able to provide 226 authorization data in any of the indicated formats. 228 2.3. AuthzDataFormat Type 230 The AuthzDataFormat type is used in both the client_authz and the 231 server_authz extensions. It indicates the format of the 232 authorization data that will be transferred. The AuthzDataFormats 233 type definition is: 235 enum { 236 x509_attr_cert(0), saml_assertion(1), x509_attr_cert_url(2), 237 saml_assertion_url(3), (255) 238 } AuthzDataFormat; 240 AuthzDataFormats authz_format_list<1..2^8-1>; 242 When the x509_attr_cert value is present, the authorization data is 243 an X.509 Attribute Certificate (AC) that conforms to the profile in 244 RFC 3281 [ATTRCERT]. 246 When the saml_assertion value is present, the authorization data is 247 an assertion composed using the Security Assertion Markup Language 248 (SAML) [SAML1.1][SAML2.0]. 250 When the x509_attr_cert_url value is present, the authorization data 251 is an X.509 AC that conforms to the profile in RFC 3281 [ATTRCERT]; 252 however, the AC is fetched with the supplied URL. A one-way hash 253 value is provided to ensure that the intended AC is obtained. 255 When the saml_assertion_url value is present, the authorization data 256 is a SAML Assertion; however, the SAML Assertion is fetched with the 257 supplied URL. A one-way hash value is provided to ensure that the 258 intended SAML Assertion is obtained. 260 3. Supplemental Data Handshake Message Usage 262 As shown in Figure 1, supplemental data can be exchanges in two 263 places in the handshake protocol. The client_authz extension 264 determines what authorization data formats are acceptable for 265 transfer from the client to the server, and the server_authz 266 extension determines what authorization data formats are acceptable 267 for transfer from the server to the client. In both cases, the 268 syntax specified in [TLSSUPP] is used along with the authz_data type 269 defined in this document. 271 enum { 272 authz_data(TBD), (65535) 273 } SupplementalDataType; 275 struct { 276 SupplementalDataType supplemental_data_type; 277 select(SupplementalDataType) { 278 case authz_data: AuthorizationData; 279 } 280 } SupplementalData; 282 3.1. Client Authorization Data 284 The SupplementalData message sent from the client to the server 285 contains authorization data associated with the TLS client. 286 Following the principle of least privilege, the client ought to send 287 the minimal set of authorization information necessary to accomplish 288 the task at hand. That is, only those authorizations that are 289 expected to be required by the server in order to gain access to the 290 needed server resources ought to be included. The format of the 291 authorization data depends on the format negotiated in the 292 client_authz hello message extension. The AuthorizationData 293 structure is described in Section 3.3. 295 In some systems, clients present authorization information to the 296 server, and then the server provides new authorization information. 297 This type of transaction is not supported by SupplementalData 298 messages. In cases where the client intends to request the TLS 299 server to perform authorization translation or expansion services, 300 such translation services ought to occur within the ApplicationData 301 messages, not within the TLS Handshake protocol. 303 3.2. Server Authorization Data 305 The SupplementalData message sent from the server to the client 306 contains authorization data associated with the TLS server. This 307 authorization information is expected to include statements about the 308 server's qualifications, reputation, accreditation, and so on. 309 Wherever possible, authorizations that can be misappropriated for 310 fraudulent use ought to be avoided. The format of the authorization 311 data depends on the format negotiated in the server_authz hello 312 message extensions. The AuthorizationData structure is described in 313 Section 3.3. 315 3.3. AuthorizationData Type 317 The AuthorizationData structure carried authorization information for 318 either the client or the server. The AuthzDataFormat specified in 319 Section 2.3 for use in the hello extensions is also used in this 320 structure. 322 All of the entries in the authz_data_list MUST employ authorization 323 data formats that were negotiated in the relevant hello message 324 extension. 326 struct{ 327 AuthorizationDataEntry authz_data_list<1..2^16-1>; 328 } AuthorizationData; 330 struct { 331 AuthzDataFormat authz_format; 332 select (AuthzDataFormat) { 333 case x509_attr_cert: X509AttrCert; 334 case saml_assertion: SAMLAssertion; 335 case x509_attr_cert_url: URLandHash; 336 case saml_assertion_url: URLandHash; 337 } 338 } AuthorizationDataEntry; 340 enum { 341 x509_attr_cert(0), saml_assertion(1), x509_attr_cert_url(2), 342 saml_assertion_url(3), (255) 343 } AuthzDataFormat; 345 opaque X509AttrCert<1..2^16-1>; 347 opaque SAMLAssertion<1..2^16-1>; 349 struct { 350 opaque url<1..2^16-1>; 351 HashType hash_type; 352 select (hash_type) { 353 case sha1: SHA1Hash; 354 case sha256: SHA256Hash; 355 } hash; 356 } URLandHash; 357 enum { 358 sha1(0), sha256(1), (255) 359 } HashType; 361 opaque SHA1Hash[20]; 363 opaque SHA256Hash[32]; 365 3.3.1. X.509 Attribute Certificate 367 When X509AttrCert is used, the field contains an ASN.1 DER-encoded 368 X.509 Attribute Certificate (AC) that follows the profile in RFC 3281 369 [ATTRCERT]. An AC is a structure similar to a public key certificate 370 (PKC) [PKIX1]; the main difference being that the AC contains no 371 public key. An AC may contain attributes that specify group 372 membership, role, security clearance, or other authorization 373 information associated with the AC holder. 375 When making an authorization decision based on an AC, proper linkage 376 between the AC holder and the public key certificate that is 377 transferred in the TLS Certificate message is needed. The AC holder 378 field provides this linkage. The holder field is a SEQUENCE allowing 379 three different (optional) syntaxes: baseCertificateID, entityName 380 and objectDigestInfo. In the TLS authorization context, the holder 381 field MUST use the either baseCertificateID or entityName. In the 382 baseCertificateID case, the baseCertificateID field MUST match the 383 issuer and serialNumber fields in the certificate. In the entityName 384 case, the entityName MUST be the same as the subject field in the 385 certificate or one of the subjectAltName extension values in the 386 certificate. Note that [PKIX1] mandates that the subjectAltName 387 extension be present if the subject field contains an empty 388 distinguished name. 390 3.3.2. SAML Assertion 392 When SAMLAssertion is used, the field contains an XML-encoded 393 element using the AssertionType complex type as defined 394 in [SAML1.1][SAML2.0]. SAML is an XML-based framework for exchanging 395 security information. This security information is expressed in the 396 form of assertions about subjects, where a subject is either human or 397 computer with an identity. In this context, the SAML assertions are 398 most likely to convey authentication or attribute statements to be 399 used as input to authorization policy governing whether subjects are 400 allowed to access certain resources. Assertions are issued by SAML 401 authorities. 403 When making an authorization decision based on a SAML assertion, 404 proper linkage between the SAML assertion and the public key 405 certificate that is transferred in the TLS Certificate message may be 406 needed. A "Holder of Key" subject confirmation method in the SAML 407 assertion can provide this linkage. In other scenarios, it may be 408 acceptable to use alternate confirmation methods that do not provide 409 a strong binding, such as a bearer mechanism. SAML assertion 410 recipients MUST decide which subject confirmation methods are 411 acceptable; such decisions MAY be specific to the SAML assertion 412 contents and the TLS session context. 414 There is no general requirement that the subject of the SAML 415 assertion correspond directly to the subject of the certificate. 416 They may represent the same or different entities. When they are 417 different, SAML also provides a mechanism by which the certificate 418 subject can be identified separately from the subject in the SAML 419 assertion subject confirmation method. 421 Since the SAML assertion is being provided at a part of the TLS 422 Handshake that is unencrypted, an eavesdropper could replay the same 423 SAML assertion when they establish their own TLS session. This is 424 especially important when a bearer mechanism is employed, the 425 recipient of the SAML assertion assumes that the sender is an 426 acceptable attesting entity for the SAML assertion. Some constraints 427 may be included to limit the context where the bearer mechanism will 428 be accepted. For example, the period of time that the SAML assertion 429 can be short-lived (often minutes), the source address can be 430 constrained, or the destination endpoint can be identified. Also, 431 bearer assertions are often checked against a cache of SAML assertion 432 unique identifiers that were recently received in order to detect 433 replay. This is an appropriate countermeasure if the bearer 434 assertion is intended to be used just once. Section 5 provides a way 435 to protect authorization information when necessary. 437 3.3.3. URL and Hash 439 Since the X.509 AC and SAML assertion can be large, alternatives 440 provide a URL to obtain the ASN.1 DER-encoded X.509 AC or SAML 441 Assertion. To ensure that the intended object is obtained, a one-way 442 hash value of the object is also included. Integrity of this one-way 443 hash value is provided by the TLS Finished message. 445 Implementations that support either x509_attr_cert_url or 446 saml_assertion_url MUST support URLs that employ the http scheme. 447 Other schemes may also be supported; however, to avoid circular 448 dependencies, supported schemes SHOULD NOT themselves make use of 449 TLS, such as the https scheme. 451 Implementations that support either x509_attr_cert_url or 452 saml_assertion_url MUST support both SHA-1 [SHA1] and SHA-256 [SHA2] 453 as one-way hash functions. Other one-way hash functions may also be 454 supported. Additional one-way hash functions can be registered in 455 the future using the procedures in section 3. 457 4. IANA Considerations 459 This document defines a two TLS extensions: client_authz(TBD) and 460 server_authz(TBD). These extension type values are assigned from the 461 TLS Extension Type registry defined in [TLSEXT]. 463 This document defines one TLS supplemental data type: 464 authz_data(TBD). This supplemental data type is assigned from the 465 TLS Supplemental Data Type registry defined in [TLSSUPP]. 467 This document establishes a new registry, to be maintained by IANA, 468 for TLS Authorization Data Formats. The first four entries in the 469 registry are x509_attr_cert(0), saml_assertion(1), 470 x509_attr_cert_url(2), and saml_assertion_url(3). TLS Authorization 471 Data Format identifiers with values in the inclusive range 0-63 472 (decimal) are assigned via RFC 2434 [IANA] Standards Action. Values 473 from the inclusive range 64-223 (decimal) are assigned via RFC 2434 474 Specification Required. Values from the inclusive range 224-255 475 (decimal) are reserved for RFC 2434 Private Use. 477 This document establishes a new registry, to be maintained by IANA, 478 for TLS Hash Types. The first two entries in the registry are 479 sha1(0) and sha256(1). TLS Hash Type identifiers with values in the 480 inclusive range 0-158 (decimal) are assigned via RFC 2434 [IANA] 481 Standards Action. Values from the inclusive range 159-223 (decimal) 482 are assigned via RFC 2434 Specification Required. Values from the 483 inclusive range 224-255 (decimal) are reserved for RFC 2434 Private 484 Use. 486 5. Security Considerations 488 A TLS server can support more than one application, and each 489 application may include several features, each of which requires 490 separate authorization checks. This is the reason that more than one 491 piece of authorization information can be provided. 493 A TLS server that requires different authorization information for 494 different applications or different application features may find 495 that a client has provided sufficient authorization information to 496 grant access to a subset of these offerings. In this situation the 497 TLS Handshake protocol will complete successfully; however, the 498 server must ensure that the client will only be able to use the 499 appropriate applications and application features. That is, the TLS 500 server must deny access to the applications and application features 501 for which authorization has not been confirmed. 503 In many cases, the authorization information is itself sensitive. 504 The double handshake technique can be used to provide protection for 505 the authorization information. Figure 2 illustrates the double 506 handshake, where the initial handshake does not include any 507 authorization extensions, but it does result in protected 508 communications. Then, a second handshake that includes the 509 authorization information is performed using the protected 510 communications. In Figure 2, the number on the right side indicates 511 the amount of protection for the TLS message on that line. A zero 512 (0) indicates that there is no communication protection; a one (1) 513 indicates that protection is provided by the first TLS session; and a 514 two (2) indicates that protection is provided by both TLS sessions. 516 The placement of the SupplementalData message in the TLS Handshake 517 results in the server providing its authorization information before 518 the client is authenticated. In many situations, servers will not 519 want to provide authorization information until the client is 520 authenticated. The double handshake illustrated in Figure 2 provides 521 a technique to ensure that the parties are mutually authenticated 522 before either party provides authorization information. 524 The use of bearer SAML assertions allows an eavesdropper or a man-in- 525 the-middle to capture the SAML assertion and try to reuse it in 526 another context. The constraints discussed in Section 3.3.2 might be 527 effective against an eavesdropper, but they are less likely to be 528 effective against a man-in-the-middle. Authentication of both 529 parties in the TLS session, which involves the use of client 530 authentication, will prevent an undetected man-in the-middle, and the 531 use of the double handshake illustrated in Figure 2 will prevent the 532 disclosure of the bearer SAML assertion to any party other than the 533 TLS peer. 535 6. Acknowledgement 537 The authors thank Scott Cantor for his assistance with the SAML 538 Assertion portion of the document. 540 Client Server 542 ClientHello (no extensions) --------> |0 543 ServerHello (no extensions) |0 544 Certificate* |0 545 ServerKeyExchange* |0 546 CertificateRequest* |0 547 <-------- ServerHelloDone |0 548 Certificate* |0 549 ClientKeyExchange |0 550 CertificateVerify* |0 551 [ChangeCipherSpec] |0 552 Finished --------> |1 553 [ChangeCipherSpec] |0 554 <-------- Finished |1 555 ClientHello (w/ extensions) --------> |1 556 ServerHello (w/ extensions) |1 557 SupplementalData (w/ authz data)* |1 558 Certificate* |1 559 ServerKeyExchange* |1 560 CertificateRequest* |1 561 <-------- ServerHelloDone |1 562 SupplementalData (w/ authz data)* |1 563 Certificate* |1 564 ClientKeyExchange |1 565 CertificateVerify* |1 566 [ChangeCipherSpec] |1 567 Finished --------> |2 568 [ChangeCipherSpec] |1 569 <-------- Finished |2 570 Application Data <-------> Application Data |2 572 Figure 2. Double Handshake to Protect Authorization Data 574 7. Normative References 576 [ATTRCERT] Farrell, S., and R. Housley, "An Internet Attribute 577 Certificate Profile for Authorization", RFC 3281, 578 April 2002. 580 [IANA] Narten, T., and H. Alvestrand, "Guidelines for Writing 581 an IANA Considerations Section in RFCs", RFC 3434, 582 October 1998. 584 [PKIX1] Housley, R., Polk, W., Ford, W. and D. Solo, "Internet 585 X.509 Public Key Infrastructure Certificate and 586 Certificate Revocation List (CRL) Profile", RFC 3280, 587 April 2002. 589 [TLS1.0] Dierks, T., and C. Allen, "The TLS Protocol, Version 1.0", 590 RFC 2246, January 1999. 592 [TLS1.1] Dierks, T., and E. Rescorla, "The Transport Layer Security 593 (TLS) Protocol, Version 1.1", RFC 4346, February 2006. 595 [TLSEXT] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., 596 and T. Wright, "Transport Layer Security (TLS) Extensions", 597 RFC 3546, June 2003. 599 [TLSSUPP] Santesson, S., " TLS Handshake Message for Supplemental 600 Data", work in progress: draft-santesson-tls-supp, 601 March 2006. 603 [SAML1.1] OASIS Security Services Technical Committee, "Security 604 Assertion Markup Language (SAML) Version 1.1 605 Specification Set", September 2003. 607 [SAML2.0] OASIS Security Services Technical Committee, "Security 608 Assertion Markup Language (SAML) Version 2.0 609 Specification Set", March2005. 611 [SHA1] National Institute of Standards and Technology (NIST), 612 FIPS PUB 180-1, Secure Hash Standard, 17 April 1995. 614 [SHA2] National Institute of Standards and Technology (NIST), 615 FIPS PUB 180-2: Secure Hash Standard, 1 August 2002. 617 [STDWORDS] Bradner, S., "Key words for use in RFCs to Indicate 618 Requirement Levels", BCP 14, RFC 2119, March 1997. 620 Author's Address 622 Mark Brown 623 RedPhone Security 624 2019 Palace Avenue 625 Saint Paul, MN 55105 626 USA 627 mark redphonesecurity com 629 Russell Housley 630 Vigil Security, LLC 631 918 Spring Knoll Drive 632 Herndon, VA 20170 633 USA 634 housley vigilsec com 636 Full Copyright Statement 638 Copyright (C) The Internet Society (2006). 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