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Shekh-Yusef 3 Internet-Draft Avaya 4 Updates: 3261 (if approved) May 26, 2019 5 Intended status: Standards Track 6 Expires: November 27, 2019 8 The Session Initiation Protocol (SIP) Digest Authentication Scheme 9 draft-ietf-sipcore-digest-scheme-03 11 Abstract 13 This document updates the Digest Access Authentication scheme used by 14 the Session Initiation Protocol (SIP) to add support for more secure 15 digest algorithms, e.g. SHA-256 and SHA-512-256, to replace the 16 broken MD5 algorithm. 18 Status of This Memo 20 This Internet-Draft is submitted in full conformance with the 21 provisions of BCP 78 and BCP 79. 23 Internet-Drafts are working documents of the Internet Engineering 24 Task Force (IETF). Note that other groups may also distribute 25 working documents as Internet-Drafts. The list of current Internet- 26 Drafts is at https://datatracker.ietf.org/drafts/current/. 28 Internet-Drafts are draft documents valid for a maximum of six months 29 and may be updated, replaced, or obsoleted by other documents at any 30 time. It is inappropriate to use Internet-Drafts as reference 31 material or to cite them other than as "work in progress." 33 This Internet-Draft will expire on November 27, 2019. 35 Copyright Notice 37 Copyright (c) 2019 IETF Trust and the persons identified as the 38 document authors. All rights reserved. 40 This document is subject to BCP 78 and the IETF Trust's Legal 41 Provisions Relating to IETF Documents 42 (https://trustee.ietf.org/license-info) in effect on the date of 43 publication of this document. Please review these documents 44 carefully, as they describe your rights and restrictions with respect 45 to this document. Code Components extracted from this document must 46 include Simplified BSD License text as described in Section 4.e of 47 the Trust Legal Provisions and are provided without warranty as 48 described in the Simplified BSD License. 50 This document may contain material from IETF Documents or IETF 51 Contributions published or made publicly available before November 52 10, 2008. The person(s) controlling the copyright in some of this 53 material may not have granted the IETF Trust the right to allow 54 modifications of such material outside the IETF Standards Process. 55 Without obtaining an adequate license from the person(s) controlling 56 the copyright in such materials, this document may not be modified 57 outside the IETF Standards Process, and derivative works of it may 58 not be created outside the IETF Standards Process, except to format 59 it for publication as an RFC or to translate it into languages other 60 than English. 62 Table of Contents 64 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 65 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 66 2. SIP Digest Authentication Scheme Updates . . . . . . . . . . 3 67 2.1. Hash Algorithms . . . . . . . . . . . . . . . . . . . . . 3 68 2.2. Representation of Digest Values . . . . . . . . . . . . . 4 69 2.3. UAS Behavior . . . . . . . . . . . . . . . . . . . . . . 4 70 2.4. UAC Behavior . . . . . . . . . . . . . . . . . . . . . . 4 71 2.5. Forking . . . . . . . . . . . . . . . . . . . . . . . . . 5 72 2.6. HTTP Modifications . . . . . . . . . . . . . . . . . . . 5 73 2.7. Augmented BNF for the SIP Protocol . . . . . . . . . . . 7 74 3. Security Considerations . . . . . . . . . . . . . . . . . . . 7 75 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 76 5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8 77 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 78 6.1. Normative References . . . . . . . . . . . . . . . . . . 8 79 6.2. Informative References . . . . . . . . . . . . . . . . . 8 80 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8 82 1. Introduction 84 The SIP protocol [RFC3261] uses the same mechanism used by the HTTP 85 protocol for authenticating users, which is a simple challenge- 86 response authentication mechanism that allows a server to challenge a 87 client request and allows a client to provide authentication 88 information in response to that challenge. 90 The SIP protocol uses the Digest Authentication scheme that is used 91 with the HTTP authentication mechanism, which uses MD5 as the default 92 algorithm. 94 The HTTP Digest Access Authentication [RFC7616] document defines the 95 Digest Authentication scheme and defines a few algorithms that could 96 be used with the Digest Authentication scheme, and establishes a 97 registry for these algorithms to allow for additional algorithms to 98 be added in the future. 100 This document updates the Digest Access Authentication scheme used by 101 SIP to support the algorithms defined in the "Hash Algorithms for 102 HTTP Digest Authentication" registry defined by [RFC7616]. 104 1.1. Terminology 106 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 107 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 108 document are to be interpreted as described in [RFC2119]. 110 2. SIP Digest Authentication Scheme Updates 112 This section describes the modifications to the operation of the 113 Digest mechanism as specified in [RFC3261] in order to support the 114 algorithms defined in the "Hash Algorithms for HTTP Digest 115 Authentication" registry described in [RFC7616]. 117 It replaces the reference to [RFC2617] with a reference to [RFC7616] 118 in [RFC3261], and describes the modifications to the usage of the 119 Digest mechanism in [RFC3261] resulting from that reference update. 120 It adds support for the SHA-256 and SHA-512/256 algorithms. It adds 121 required support for the "qop" option. It provides additional UAC 122 and UAS procedures regarding usage of multiple SIP Authorization, 123 WWW-Authenticate and Proxy-Authenticate header fields, including in 124 which order to insert and process them. It provides guidance 125 regarding forking. Finally, it updates the SIP protocol BNF as 126 required by the updates. 128 2.1. Hash Algorithms 130 The Digest scheme has an 'algorithm' parameter that specifies the 131 algorithm to be used to compute the digest of the response. The IANA 132 registry named "HTTP Digest Hash Algorithms" specifies the algorithms 133 that correspond to 'algorithm' values, and specifies a priority for 134 each algorithm. 136 [RFC3261] specifies only one algorithm, MD5, which is used by 137 default. This document extends [RFC3261] to allow use of any 138 registered algorithm. 140 A UAS prioritizes which algorithm to use based on the ordering of the 141 challenge header fields in the response it is processing. That 142 process is specified in section 2.3 and parallels the process used in 143 HTTP specified by [RFC7616]. 145 2.2. Representation of Digest Values 147 The size of the digest depends on the algorithm used. The bits in 148 the digest are converted from the most significant to the least 149 significant bit, four bits at a time to the ASCII representation as 150 follows. Each four bits is represented by its familiar hexadecimal 151 notation from the characters 0123456789abcdef, that is binary 0000 is 152 represented by the character '0', 0001 by '1' and so on up to the 153 representation of 1111 as 'f'. If the MD5 algorithm is used to 154 calculate the digest, then the digest will be represented as 32 155 hexadecimal characters, SHA-256 and SHA-512/256 by 64 hexadecimal 156 characters. 158 2.3. UAS Behavior 160 When a UAS receives a request from a UAC, and an acceptable 161 Authorization header field is not sent, the UAS can challenge the 162 originator to provide credentials by rejecting the request with a 163 401/407 status code with the WWW-Authenticate/Proxy-Authenticate 164 header field respectively. The UAS MAY include multiple WWW- 165 Authenticate/Proxy-Authenticate header fields to allow the UAS to 166 utilize the best available algorithm supported by the client. 168 If the UAS challenges with multiple WWW-Authenticate/Proxy- 169 Authenticate header fields with the same realm, then each one of 170 these header fields MUST use a different digest algorithm. The UAS 171 MUST add these header fields to the response in the order that it 172 would prefer to see them used, starting with the most preferred 173 algorithm at the top, followed by the less preferred algorithms. The 174 UAS cannot assume that the client will use the algorithm specified at 175 the topmost header field. 177 2.4. UAC Behavior 179 When the UAC receives a response with multiple header fields with the 180 same realm it SHOULD use the topmost header field that it supports, 181 unless a local policy dictates otherwise. The client MUST ignore any 182 challenge it does not understand. 184 When the UAC receives a 401 response with multiple WWW-Authenticate 185 header fields with different realms it SHOULD retry and include an 186 Authorization header field containing credentials that match the 187 topmost header field of any one of the realms. 189 If the UAC cannot respond to any of the challenges in the response, 190 then it should abandon attempts to send the request; e.g., if the UAC 191 does not have credentials for any of the realms. 193 2.5. Forking 195 Section 22.3 of [RFC3261] discusses the operation of the proxy-to- 196 user authentication, which describes the operation of the proxy when 197 it forks a request. This section introduces some clarification to 198 that operation. 200 If a request is forked, various proxy servers and/or UAs may wish to 201 challenge the UAC. In this case, the forking proxy server is 202 responsible for aggregating these challenges into a single response. 203 Each WWW-Authenticate and Proxy-Authenticate value received in 204 responses to the forked request MUST be placed into the single 205 response that is sent by the forking proxy to the UA. 207 When the forking proxy places multiple WWW-Authenticate and Proxy- 208 Authenticate header fields from one received response into the single 209 response it MUST maintain the order of these header fields. The 210 ordering of the header field values from the various proxies is not 211 significant. 213 2.6. HTTP Modifications 215 This section describes the modifications and clarifications required 216 to apply the HTTP Digest authentication scheme to SIP. The SIP 217 scheme usage is similar to that for HTTP. For completeness, the 218 bullets specified below are mostly copied from section 22.4 of 219 [RFC3261]; the only semantic changes are specified in bullets 7 and 8 220 below. 222 SIP clients and servers MUST NOT accept or request Basic 223 authentication. 225 The rules for Digest authentication follow those defined in HTTP, 226 with "HTTP/1.1" replaced by "SIP/2.0" in addition to the following 227 differences: 229 1. The URI included in the challenge has the following BNF: 231 URI = Request-URI ; as defined in [RFC3261], Section 25 233 2. The 'uri' parameter of the Authorization header field MUST be 234 enclosed in quotation marks. 236 3. The BNF for digest-uri-value is: 238 digest-uri-value = Request-URI 240 4. The example procedure for choosing a nonce based on Etag does not 241 work for SIP. 243 5. The text in [RFC7234] regarding cache operation does not apply to 244 SIP. 246 6. [RFC7616] requires that a server check that the URI in the 247 request line and the URI included in the Authorization header field 248 point to the same resource. In a SIP context, these two URIs may 249 refer to different users, due to forwarding at some proxy. 250 Therefore, in SIP, a server MAY check that the Request-URI in the 251 Authorization header field value corresponds to a user for whom the 252 server is willing to accept forwarded or direct requests, but it is 253 not necessarily a failure if the two fields are not equivalent. 255 7. As a clarification to the calculation of the A2 value for message 256 integrity assurance in the Digest authentication scheme, implementers 257 should assume, when the entity-body is empty (that is, when SIP 258 messages have no body) that the hash of the entity-body resolves to 259 the hash of an empty string: 261 H(entity-body) = ("") 263 For example, when the chosen algorithm is SHA-256, then: 265 H(entity-body) = SHA-256("") = 266 "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855" 268 8. Servers MUST be able to properly handle "qop" parameter received 269 in an authorization header field, and clients MUST be able to 270 properly handle "qop" parameter received in WWW-Authenticate and 271 Proxy-Authenticate header fields. Servers MUST always send a "qop" 272 parameter in WWW-Authenticate and Proxy-Authenticate header field 273 values, and clients MUST send the "qop" parameter in any resulting 274 authorization header field. 276 The usage of the Authentication-Info header field continue to be 277 allowed, since it provides integrity checks over the bodies and 278 provides mutual authentication. 280 2.7. Augmented BNF for the SIP Protocol 282 This document updates the Augmented BNF for the SIP Protocol as 283 follows. 285 It extends the request-digest as follows to allow for different 286 digest sizes: 288 request-digest = LDQUOT *LHEX RDQUOT 290 The number of hex digits is implied by the length of the value of the 291 algorithm used. 293 It extends the algorithm parameter as follows to allow for any 294 algorithm in the registry to be used: 296 algorithm = "algorithm" EQUAL ( "MD5" / "SHA-512-256" / "SHA-256" 297 / token ) 299 3. Security Considerations 301 This specification adds new secure algorithms to be used to with the 302 Digest mechanism to authenticate users, but leaves the broken MD5 303 algorithm for backward compatibility. 305 This opens the system to the potential of a downgrade attack by man- 306 in-the-middle. The most effective way of dealing with this type of 307 attack is to either validate the client and challenge it accordingly, 308 or remove the support for backward compatibility by not supporting 309 MD5. 311 See section 5 of [RFC7616] for a detailed security discussion of the 312 Digest scheme. 314 4. IANA Considerations 316 [RFC7616] defines an IANA registry named "Hash Algorithms for HTTP 317 Digest Authentication" to simplify the introduction of new algorithms 318 in the future. This document specifies that algorithms defined in 319 that registry may be used in SIP digest authentication. 321 5. Acknowledgments 323 The author would like to thank the following individuals for their 324 careful reviews, comments, and suggestions: Paul Kyzivat, Olle 325 Johansson, Dale Worley, Michael Procter, Inaki Baz Castillo, Tolga 326 Asveren, Christer Holmberg, and Brian Rosen. 328 6. References 330 6.1. Normative References 332 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 333 Requirement Levels", BCP 14, RFC 2119, March 1997. 335 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, H., Johnston, 336 A., Peterson, J., Sparks, R., Handley, M., and E. 337 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 338 June 2002. 340 [RFC7234] Fielding, R., Nottingham, M., and J. Reschke, "Hypertext 341 Transfer Protocol (HTTP/1.1): Caching", RFC 7234, June 342 2014. 344 [RFC7616] Shekh-Yusef, R., Ahrens, D., and S. Bremer, "HTTP Digest 345 Access Authentication", RFC 7616, September 2015. 347 6.2. Informative References 349 [RFC2617] Franks, J., M. Hallam-Baker, P., L. Hostetler, J., D. 350 Lawrence, S., J. Leach, P., Luotonen, A., and L. C. 351 Stewart, "HTTP Authentication: Basic and Digest Access 352 Authentication", RFC 2617, June 1999. 354 Author's Address 356 Rifaat Shekh-Yusef 357 Avaya 358 425 Legget Dr. 359 Ottawa, Ontario 360 Canada 362 Phone: +1-613-595-9106 363 EMail: rifaat.ietf@gmail.com