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Melnikov 3 Internet-Draft Isode 4 Expires: December 11, 2006 June 9, 2006 6 The Kerberos V5 ("GSSAPI") SASL mechanism 7 draft-ietf-sasl-gssapi-06 9 Status of this Memo 11 By submitting this Internet-Draft, each author represents that any 12 applicable patent or other IPR claims of which he or she is aware 13 have been or will be disclosed, and any of which he or she becomes 14 aware will be disclosed, in accordance with Section 6 of BCP 79. 16 Internet-Drafts are working documents of the Internet Engineering 17 Task Force (IETF), its areas, and its working groups. Note that 18 other groups may also distribute working documents as Internet- 19 Drafts. 21 Internet-Drafts are draft documents valid for a maximum of six months 22 and may be updated, replaced, or obsoleted by other documents at any 23 time. It is inappropriate to use Internet-Drafts as reference 24 material or to cite them other than as "work in progress." 26 The list of current Internet-Drafts can be accessed at 27 http://www.ietf.org/ietf/1id-abstracts.txt. 29 The list of Internet-Draft Shadow Directories can be accessed at 30 http://www.ietf.org/shadow.html. 32 This Internet-Draft will expire on December 11, 2006. 34 Copyright Notice 36 Copyright (C) The Internet Society (2006). 38 Abstract 40 The Simple Authentication and Security Layer (SASL, RFC 4422) is a 41 framework for adding authentication support to connection-based 42 protocols. This document describes the method for using the Generic 43 Security Service Application Program Interface (GSS-API) Kerberos V5 44 in the SASL. 46 This document replaces section 7.2 of RFC 2222, the definition of the 47 "GSSAPI" SASL mechanism. 49 Table of Contents 51 1. Conventions Used in this Document . . . . . . . . . . . . . . 3 52 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 53 2.1. Relationship to Other Documents . . . . . . . . . . . . . 3 54 3. Kerberos V5 GSS-API mechanism . . . . . . . . . . . . . . . . 3 55 3.1. Client side of authentication protocol exchange . . . . . 3 56 3.2. Server side of authentication protocol exchange . . . . . 4 57 3.3. Security layer . . . . . . . . . . . . . . . . . . . . . . 6 58 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 59 5. Security Considerations . . . . . . . . . . . . . . . . . . . 7 60 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8 61 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 62 7.1. Normative References . . . . . . . . . . . . . . . . . . . 8 63 7.2. Informative References . . . . . . . . . . . . . . . . . . 8 64 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 9 65 Intellectual Property and Copyright Statements . . . . . . . . . . 10 67 1. Conventions Used in this Document 69 The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY" 70 in this document are to be interpreted as defined in "Key words for 71 use in RFCs to Indicate Requirement Levels" [KEYWORDS]. 73 2. Introduction 75 This specification documents currently deployed Simple Authentication 76 and Security Layer (SASL [SASL]) mechanism supporting the Kerberos V5 77 [KERBEROS] Generic Security Service Application Program Interface 78 ([GSS-API]) mechanism [RFC4121]. The authentication sequence is 79 described in Section 3. Note that the described authentication 80 sequence has known limitations in particular it lacks channel 81 bindings and the number of round trips required to complete 82 authentication exchange is not minimal. SASL WG is working on a 83 separate document that should address these limitations. 85 2.1. Relationship to Other Documents 87 This document, together with RFC 4422, obsoletes RFC 2222 in its 88 entirety. This document replaces Section 7.2 of RFC 2222. The 89 remainder is obsoleted as detailed in Section 1.2 of RFC 4422. 91 3. Kerberos V5 GSS-API mechanism 93 The SASL mechanism name for the Kerberos V5 GSS-API mechanism 94 [RFC4121] is "GSSAPI". Though known as the SASL GSSAPI mechanism, 95 the mechanism is specifically tied to Kerberos V5 and GSS-API's 96 Kerberos V5 mechanism. 98 The GSSAPI SASL mechanism is a "client goes first" SASL mechanism, 99 i.e. it starts with the client sending a "response" created as 100 described in the following section. 102 The implementation MAY set any GSS-API flags or arguments not 103 mentioned in this specification as is necessary for the 104 implementation to enforce its security policy. 106 3.1. Client side of authentication protocol exchange 108 The client calls GSS_Init_sec_context, passing in 109 input_context_handle of 0 (initially), mech_type of the Kerberos V5 110 GSS-API mechanism [KRB5GSS], chan_binding of NULL, and targ_name 111 equal to output_name from GSS_Import_Name called with input_name_type 112 of GSS_C_NT_HOSTBASED_SERVICE and input_name_string of 113 "service@hostname" where "service" is the service name specified in 114 the protocol's profile, and "hostname" is the fully qualified host 115 name of the server. If the client will be requesting a security 116 layer, it MUST also supply to the GSS_Init_sec_context a 117 mutual_req_flag of TRUE, a sequence_req_flag of TRUE, and an 118 integ_req_flag of TRUE. If the client will be requesting a security 119 layer providing confidentiality protection, it MUST also supply to 120 the GSS_Init_sec_context a conf_req_flag of TRUE. The client then 121 responds with the resulting output_token. If GSS_Init_sec_context 122 returns GSS_S_CONTINUE_NEEDED, then the client should expect the 123 server to issue a token in a subsequent challenge. The client must 124 pass the token to another call to GSS_Init_sec_context, repeating the 125 actions in this paragraph. 127 When GSS_Init_sec_context returns GSS_S_COMPLETE, the client examines 128 the context to ensure that it provides a level of protection 129 permitted by the client's security policy. If the context is 130 acceptable, the client takes the following actions: If the last call 131 to GSS_Init_sec_context returned an output_token, then the client 132 responds with the output_token, otherwise the client responds with no 133 data. The client should then expect the server to issue a token in a 134 subsequent challenge. The client passes this token to GSS_Unwrap and 135 interprets the first octet of resulting cleartext as a bit-mask 136 specifying the security layers supported by the server and the second 137 through fourth octets as the maximum size output_message the server 138 is able to receive (in network byte order). If the resulting 139 cleartext is not 4 octets long, the client fails the negotiation. 140 The client verifies that the server maximum buffer is 0 if the server 141 doesn't advertise support for any security layer. The client then 142 constructs data, with the first octet containing the bit-mask 143 specifying the selected security layer, the second through fourth 144 octets containing in network byte order the maximum size 145 output_message the client is able to receive (which MUST be 0 if the 146 client doesn't support any security layer), and the remaining octets 147 containing the UTF-8 [UTF8] encoded authorization identity. 148 (Implementation note: the authorization identity is not terminated 149 with the zero-valued (%x00) octet (e.g., the UTF-8 encoding of the 150 NUL (U+0000) character)). The client passes the data to GSS_Wrap 151 with conf_flag set to FALSE, and responds with the generated 152 output_message. The client can then consider the server 153 authenticated. 155 3.2. Server side of authentication protocol exchange 157 A server MUST NOT advertise support for the "GSSAPI" SASL mechanism 158 described in this document unless it has acceptor credential for the 159 Kerberos V GSS-API Mechanism [KRB5GSS]. 161 The server passes the initial client response to 162 GSS_Accept_sec_context as input_token, setting input_context_handle 163 to 0 (initially), chan_binding of NULL, and a suitable 164 acceptor_cred_handle (see below). If GSS_Accept_sec_context returns 165 GSS_S_CONTINUE_NEEDED, the server returns the generated output_token 166 to the client in challenge and passes the resulting response to 167 another call to GSS_Accept_sec_context, repeating the actions in this 168 paragraph. 170 Servers SHOULD use a credential obtained by calling GSS_Acquire_cred 171 or GSS_Add_cred for the GSS_C_NO_NAME desired_name and the OID of the 172 Kerberos V5 GSS-API mechanism [KRB5GSS](*). Servers MAY use 173 GSS_C_NO_CREDENTIAL as an acceptor credential handle. Servers MAY 174 use a credential obtained by calling GSS_Acquire_cred or GSS_Add_cred 175 for the server's principal name(s) (**) and the Kerberos V5 GSS-API 176 mechanism [KRB5GSS]. 178 (*) - Unlike GSS_Add_cred the GSS_Acquire_cred uses an OID set of 179 GSS-API mechanism as an input parameter. The OID set can be created 180 by using GSS_Create_empty_OID_set and GSS_Add_OID_set_member. It can 181 be freed by calling the GSS_Release_oid_set. 183 (**) - Use of server's principal names having 184 GSS_C_NT_HOSTBASED_SERVICE name type and "service@hostname" format, 185 where "service" is the service name specified in the protocol's 186 profile, is RECOMMENDED. 188 Upon successful establishment of the security context (i.e. 189 GSS_Accept_sec_context returns GSS_S_COMPLETE) the server SHOULD 190 verify that the negotiated GSS-API mechanism is indeed Kerberos V5 191 [KRB5GSS]. This is done by examining the value of the mech_type 192 parameter returned from the GSS_Accept_sec_context call. If the 193 value differ SASL authentication MUST be aborted. 195 Upon successful establishment of the security context the server 196 SHOULD also check using the GSS_Inquire_context that the target_name 197 used by the client matches either: 199 - the GSS_C_NT_HOSTBASED_SERVICE "service@hostname" name syntax, 200 where "service" is the service name specified in the application 201 protocol's profile, 203 or that 205 - the GSS_KRB5_NT_PRINCIPAL_NAME [KRB5GSS] name syntax for a two- 206 component principal where the first component matches the service 207 name specified in the application protocol's profile. 209 When GSS_Accept_sec_context returns GSS_S_COMPLETE, the server 210 examines the context to ensure that it provides a level of protection 211 permitted by the server's security policy. If the context is 212 acceptable, the server takes the following actions: If the last call 213 to GSS_Accept_sec_context returned an output_token, the server 214 returns it to the client in a challenge and expects a reply from the 215 client with no data. Whether or not an output_token was returned 216 (and after receipt of any response from the client to such an 217 output_token), the server then constructs 4 octets of data, with the 218 first octet containing a bit-mask specifying the security layers 219 supported by the server and the second through fourth octets 220 containing in network byte order the maximum size output_token the 221 server is able to receive (which MUST be 0 if the server doesn't 222 support any security layer). The server must then pass the plaintext 223 to GSS_Wrap with conf_flag set to FALSE and issue the generated 224 output_message to the client in a challenge. The server must then 225 pass the resulting response to GSS_Unwrap and interpret the first 226 octet of resulting cleartext as the bit-mask for the selected 227 security layer, the second through fourth octets as the maximum size 228 output_message the client is able to receive (in network byte order), 229 and the remaining octets as the authorization identity. The server 230 verifies that the client has selected a security layer that was 231 offered, and that the client maximum buffer is 0 if no security layer 232 was chosen. The server must verify that the src_name is authorized 233 to act as the authorization identity. After these verifications, the 234 authentication process is complete. 236 3.3. Security layer 238 The security layers and their corresponding bit-masks are as follows: 240 1 No security layer 241 2 Integrity protection. 242 Sender calls GSS_Wrap with conf_flag set to FALSE 243 4 Confidentiality protection. 244 Sender calls GSS_Wrap with conf_flag set to TRUE 246 Other bit-masks may be defined in the future; bits which are not 247 understood must be negotiated off. 249 Note that SASL negotiates the maximum size of the output_message to 250 send. Implementations can use the GSS_Wrap_size_limit call to 251 determine the corresponding maximum size input_message. 253 4. IANA Considerations 255 The IANA is directed to modify the existing registration for "GSSAPI" 256 as follows: 258 Family of SASL mechanisms: NO 260 SASL mechanism name: GSSAPI 262 Security considerations: See Section 5 of RFC [THIS-DOC] 264 Published Specification: RFC [THIS-DOC] 266 Person & email address to contact for further information: Alexey 267 Melnikov 269 Intended usage: COMMON 271 Owner/Change controller: iesg@ietf.org 273 Additional Information: This mechanism is for the Kerberos V5 274 mechanism of GSS-API. 276 5. Security Considerations 278 Security issues are discussed throughout this memo. 280 The integrity protection provided by the GSSAPI security layer is 281 useless to the client unless the client also requests mutual 282 authentication. Therefore, a client wishing to benefit from the 283 integrity protection of a security layer MUST pass to the 284 GSS_Init_sec_context call a mutual_req_flag of TRUE. 286 When constructing the input_name_string, the client SHOULD NOT 287 canonicalize the server's fully qualified domain name using an 288 insecure or untrusted directory service. 290 For compatibility with deployed software this document requires that 291 the chan_binding (channel bindings) parameter to GSS_Init_sec_context 292 and GSS_Accept_sec_context be NULL, hence disallowing use of GSS-API 293 support for channel bindings. GSS-API channel bindings in SASL is 294 expected to be supported via a new GSS-API family of SASL mechanisms 295 (to be introduced in a future document). 297 Additional security considerations are in the [SASL] and [GSS-API] 298 specifications. Additional security considerations for the GSS-API 299 mechanism can be found in [KRB5GSS] and [KERBEROS]. 301 6. Acknowledgements 303 This document replaces section 7.2 of RFC 2222 [RFC2222] by John G. 304 Myers. He also contributed significantly to this revision. 306 Lawrence Greenfield converted text of this draft to the XML format. 308 Contributions of many members of the SASL mailing list are gratefully 309 acknowledged, in particular comments from Chris Newman, Nicolas 310 Williams and Jeffrey Hutzelman. 312 7. References 314 7.1. Normative References 316 [GSS-API] Linn, J., "Generic Security Service Application Program 317 Interface Version 2, Update 1", RFC 2743, January 2000. 319 [KERBEROS] 320 Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The 321 Kerberos Network Authentication Service (V5)", RFC 4120, 322 July 2005. 324 [KEYWORDS] 325 Bradner, S., "Key words for use in RFCs to Indicate 326 Requirement Levels", BCP 14, RFC 2119, March 1997. 328 [KRB5GSS] Linn, J., "The Kerberos Version 5 GSS-API Mechanism", 329 RFC 1964, June 1996. 331 [RFC4121] Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos 332 Version 5 Generic Security Service Application Program 333 Interface (GSS-API) Mechanism: Version 2", RFC 4121, 334 July 2005. 336 [SASL] Melnikov, A. and K. Zeilenga, "Simple Authentication and 337 Security Layer (SASL)", RFC 4422, June 2006. 339 [UTF8] Yergeau, F., "UTF-8, a transformation format of ISO 340 10646", RFC 3629, November 2003. 342 7.2. Informative References 344 [RFC2222] Myers, J., "Simple Authentication and Security Layer 345 (SASL)", RFC 2222, October 1997. 347 Author's Address 349 Alexey Melnikov (Ed.) 350 Isode Limited 351 5 Castle Business Village 352 36 Station Road 353 Hampton, Middlesex TW12 2BX 354 UK 356 Email: Alexey.Melnikov@isode.com 357 URI: http://www.melnikov.ca/ 359 Intellectual Property Statement 361 The IETF takes no position regarding the validity or scope of any 362 Intellectual Property Rights or other rights that might be claimed to 363 pertain to the implementation or use of the technology described in 364 this document or the extent to which any license under such rights 365 might or might not be available; nor does it represent that it has 366 made any independent effort to identify any such rights. 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