idnits 2.17.1 draft-ietf-stir-rfc4474bis-08.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- ** There are 3 instances of too long lines in the document, the longest one being 26 characters in excess of 72. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year == The document seems to lack the recommended RFC 2119 boilerplate, even if it appears to use RFC 2119 keywords. (The document does seem to have the reference to RFC 2119 which the ID-Checklist requires). -- The exact meaning of the all-uppercase expression 'MAY NOT' is not defined in RFC 2119. If it is intended as a requirements expression, it should be rewritten using one of the combinations defined in RFC 2119; otherwise it should not be all-uppercase. == The expression 'MAY NOT', while looking like RFC 2119 requirements text, is not defined in RFC 2119, and should not be used. Consider using 'MUST NOT' instead (if that is what you mean). Found 'MAY NOT' in this paragraph: In a departure from JWT practice, the SIP usage of PASSporT MAY NOT include the base64 encoded version of the JSON objects in the Identity header: only the signature component of the PASSporT is REQUIRED. Optionally, as a debugging measure or optimization, the base64 encoded concatenation of the JSON header and claims may be included as the value of a "canon" parameter of the Identity header. Note that this may be lengthy string. -- The document date (March 21, 2016) is 2950 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Unused Reference: 'RFC3280' is defined on line 1539, but no explicit reference was found in the text == Unused Reference: 'RFC3370' is defined on line 1545, but no explicit reference was found in the text == Unused Reference: 'RFC3548' is defined on line 1602, but no explicit reference was found in the text == Outdated reference: A later version (-11) exists of draft-ietf-stir-passport-00 ** Obsolete normative reference: RFC 2818 (Obsoleted by RFC 9110) ** Obsolete normative reference: RFC 3280 (Obsoleted by RFC 5280) ** Downref: Normative reference to an Experimental RFC: RFC 6919 == Outdated reference: A later version (-18) exists of draft-ietf-stir-certificates-02 -- Obsolete informational reference (is this intentional?): RFC 3548 (Obsoleted by RFC 4648) -- Obsolete informational reference (is this intentional?): RFC 4234 (Obsoleted by RFC 5234) -- Obsolete informational reference (is this intentional?): RFC 4474 (Obsoleted by RFC 8224) -- Obsolete informational reference (is this intentional?): RFC 7159 (Obsoleted by RFC 8259) Summary: 4 errors (**), 0 flaws (~~), 8 warnings (==), 6 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group J. Peterson 3 Internet-Draft NeuStar 4 Intended status: Standards Track C. Jennings 5 Expires: September 22, 2016 Cisco 6 E. Rescorla 7 RTFM, Inc. 8 C. Wendt 9 Comcast 10 March 21, 2016 12 Authenticated Identity Management in the Session Initiation Protocol 13 (SIP) 14 draft-ietf-stir-rfc4474bis-08.txt 16 Abstract 18 The baseline security mechanisms in the Session Initiation Protocol 19 (SIP) are inadequate for cryptographically assuring the identity of 20 the end users that originate SIP requests, especially in an 21 interdomain context. This document defines a mechanism for securely 22 identifying originators of SIP requests. It does so by defining a 23 SIP header field for conveying a signature used for validating the 24 identity, and for conveying a reference to the credentials of the 25 signer. 27 Status of This Memo 29 This Internet-Draft is submitted in full conformance with the 30 provisions of BCP 78 and BCP 79. 32 Internet-Drafts are working documents of the Internet Engineering 33 Task Force (IETF). Note that other groups may also distribute 34 working documents as Internet-Drafts. The list of current Internet- 35 Drafts is at http://datatracker.ietf.org/drafts/current/. 37 Internet-Drafts are draft documents valid for a maximum of six months 38 and may be updated, replaced, or obsoleted by other documents at any 39 time. It is inappropriate to use Internet-Drafts as reference 40 material or to cite them other than as "work in progress." 42 This Internet-Draft will expire on September 22, 2016. 44 Copyright Notice 46 Copyright (c) 2016 IETF Trust and the persons identified as the 47 document authors. All rights reserved. 49 This document is subject to BCP 78 and the IETF Trust's Legal 50 Provisions Relating to IETF Documents 51 (http://trustee.ietf.org/license-info) in effect on the date of 52 publication of this document. Please review these documents 53 carefully, as they describe your rights and restrictions with respect 54 to this document. Code Components extracted from this document must 55 include Simplified BSD License text as described in Section 4.e of 56 the Trust Legal Provisions and are provided without warranty as 57 described in the Simplified BSD License. 59 Table of Contents 61 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 62 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 63 3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4 64 4. Overview of Operations . . . . . . . . . . . . . . . . . . . 6 65 5. Signature Generation and Validation . . . . . . . . . . . . . 7 66 5.1. Authentication Service Behavior . . . . . . . . . . . . . 7 67 5.2. Verifier Behavior . . . . . . . . . . . . . . . . . . . . 10 68 6. Credentials . . . . . . . . . . . . . . . . . . . . . . . . . 11 69 6.1. Credential Use by the Authentication Service . . . . . . 11 70 6.2. Credential Use by the Verification Service . . . . . . . 12 71 6.3. Handling 'info' parameter URIs . . . . . . . . . . . . . 13 72 6.4. Credential System Requirements . . . . . . . . . . . . . 14 73 7. Identity Types . . . . . . . . . . . . . . . . . . . . . . . 15 74 7.1. Telephone Numbers . . . . . . . . . . . . . . . . . . . . 15 75 7.1.1. Canonicalization Procedures . . . . . . . . . . . . . 16 76 7.2. Domain Names . . . . . . . . . . . . . . . . . . . . . . 18 77 8. Header Syntax . . . . . . . . . . . . . . . . . . . . . . . . 19 78 9. Extensibility . . . . . . . . . . . . . . . . . . . . . . . . 22 79 10. Gatewaying to PASSporT for non-SIP Transit . . . . . . . . . 22 80 11. Privacy Considerations . . . . . . . . . . . . . . . . . . . 23 81 12. Security Considerations . . . . . . . . . . . . . . . . . . . 25 82 12.1. Protected Request Fields . . . . . . . . . . . . . . . . 25 83 12.1.1. Protection of the To Header and Retargeting . . . . 27 84 12.2. Unprotected Request Fields . . . . . . . . . . . . . . . 27 85 12.3. Malicious Removal of Identity Headers . . . . . . . . . 28 86 12.4. Securing the Connection to the Authentication Service . 28 87 12.5. Authorization and Transitional Strategies . . . . . . . 29 88 12.6. Display-Names and Identity . . . . . . . . . . . . . . . 30 89 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31 90 13.1. Identity-Info Parameters . . . . . . . . . . . . . . . . 31 91 13.2. Identity-Info Algorithm Parameter Values . . . . . . . . 31 92 14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 32 93 15. Changes from RFC4474 . . . . . . . . . . . . . . . . . . . . 32 94 16. References . . . . . . . . . . . . . . . . . . . . . . . . . 32 95 16.1. Normative References . . . . . . . . . . . . . . . . . . 32 96 16.2. Informative References . . . . . . . . . . . . . . . . . 33 98 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 35 100 1. Introduction 102 This document provides enhancements to the existing mechanisms for 103 authenticated identity management in the Session Initiation Protocol 104 (SIP, [RFC3261]). An identity, for the purposes of this document, is 105 defined as either a SIP URI, commonly a canonical address-of-record 106 (AoR) employed to reach a user (such as 107 'sip:alice@atlanta.example.com'), or a telephone number, which can be 108 represented as either a TEL URI [RFC3966] or as the user portion of a 109 SIP URI. 111 [RFC3261] stipulates several places within a SIP request where users 112 can express an identity for themselves, primarily the user-populated 113 From header field. However, the recipient of a SIP request has no 114 way to verify that the From header field has been populated 115 appropriately, in the absence of some sort of cryptographic 116 authentication mechanism. This leaves SIP vulnerable to a category 117 of abuses, including impersonation attacks that enable robocalling 118 and related problems as described in [RFC7340]. Ideally, a 119 cryptographic approach to identity can provide a much stronger and 120 less spoofable assurance of identity than the Caller ID services that 121 the telephone network provides today. 123 [RFC3261] specifies a number of security mechanisms that can be 124 employed by SIP user agents (UAs), including Digest authentication, 125 Transport Layer Security (TLS), and S/MIME (implementations may 126 support other security schemes as well). However, few SIP user 127 agents today support the end-user certificates necessary to 128 authenticate themselves (via S/MIME, for example), and furthermore 129 Digest authentication is limited by the fact that the originator and 130 destination must share a prearranged secret. It is desirable for SIP 131 user agents to be able to send requests to destinations with which 132 they have no previous association. 134 [RFC4474] previously specified a means of signing portions of SIP 135 requests in order to provide an identity assurance. However, RFC 136 4474 was in several ways misaligned with deployment realities (see 137 [I-D.rosenberg-sip-rfc4474-concerns]). Most significantly, RFC 4474 138 did not deal well with telephone numbers as identifiers, despite 139 their enduring use in SIP deployments. RFC 4474 also provided a 140 signature over material that intermediaries in the field commonly 141 altered. This specification therefore revises RFC 4474 in light of 142 recent reconsideration of the problem space to align with the threat 143 model in [RFC7375]. 145 2. Terminology 147 In this document, the key words "MUST", "MUST NOT", "REQUIRED", 148 "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT 149 RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as 150 described in RFC 2119 [RFC2119] and RFC 6919 [RFC6919]. 152 3. Background 154 Per [RFC7340], problems such as robocalling, voicemail hacking, and 155 swatting are enabled by an attacker's ability to impersonate someone 156 else. The secure operation of most SIP applications and services 157 depends on authorizing the source of communications as it is 158 represented in a SIP request. Such authorization policies can be 159 automated or be a part of human operation of SIP devices. An example 160 of the former would be a voicemail service that compares the identity 161 of the caller to a whitelist before determining whether it should 162 allow the caller access to recorded messages. An example of the 163 latter would be an Internet telephone application that displays the 164 calling party number (and/or Caller-ID) of a caller, which a human 165 may review to make a policy decision before answering a call. In 166 both of these cases, attackers might attempt to circumvent these 167 authorization policies through impersonation. Since the primary 168 identifier of the sender of a SIP request, the From header field, can 169 be populated arbitrarily by the controller of a user agent, 170 impersonation is very simple today in many environments. The 171 mechanism described in this document provides a strong identity 172 system for detecting attempted impersonation in SIP requests. 174 This identity architecture for SIP depends on a logical 175 "authentication service" which validates outgoing requests; the 176 authentication service may be implemented either as part of a user 177 agent or as a proxy server. Once the sender of the message has been 178 authenticated, the authentication service then computes and adds 179 cryptographic information (including a digital signature over some 180 components of messages) to requests to communicate to other SIP 181 entities that the sending user has been authenticated and its claim 182 of a particular identity has been authorized. A "verification 183 service" on the receiving end then validates this signature and 184 enables policy decisions to be made based on the results of the 185 verification. 187 Identities are issued to users by authorities. When a new user 188 becomes associated with example.com, the administrator of the SIP 189 service for that domain can issue them an identity in that namespace, 190 such as alice@example.com. Alice may then send REGISTER requests to 191 example.com that make her user agents eligible to receive requests 192 for sip:alice@example.com. In some cases, Alice may be the owner of 193 the domain herself, and may issue herself identities as she chooses. 194 But ultimately, it is the controller of the SIP service at 195 example.com that must be responsible for authorizing the use of names 196 in the example.com domain. Therefore, for the purposes of baseline 197 SIP, the credentials needed to prove a user is authorized to use a 198 particular From header field must ultimately derive from the domain 199 owner: either a user agent gives requests to the domain name owner in 200 order for them to be signed by the domain owner's credentials, or the 201 user agent must possess credentials that prove in some fashion that 202 the domain owner has given the user agent the right to a name. 204 The situation is however more complicated for telephone numbers, 205 however. Authority over telephone numbers does not correspond 206 directly to Internet domains. While a user could register at a SIP 207 domain with a username that corresponds to a telephone number, any 208 connection between the administrator of that domain and the 209 assignment of telephone numbers is not currently reflected on the 210 Internet. Telephone numbers do not share the domain-scope property 211 described above, as they are dialed without any domain component. 212 This document thus assumes the existence of a separate means of 213 establishing authority over telephone numbers, for cases where the 214 telephone number is the identity of the user. As with SIP URIs, the 215 necessary credentials to prove authority for a name might reside 216 either in the endpoint or at some intermediary. 218 This document specifies a means of sharing a cryptographic assurance 219 of end-user SIP identity in an interdomain or intradomain context. 220 It relies on the authentication service constructing tokens based on 221 the PASSporT [I-D.ietf-stir-passport] format, a JSON [RFC7159] object 222 comprising values copied from certain header field values in the SIP 223 request. The authentication service then computes a signature over 224 those JSON object in a manner following PASSporT. That signature is 225 then placed in a SIP Identity header. In order to assist in the 226 validation of the Identity header, this specification also describes 227 some metadata fields associated with the header that can be used by 228 the recipient of a request to recover the credentials of the signer. 229 Note that the scope of this document is limited to providing this 230 identity assurance for SIP requests; solving this problem for SIP 231 responses is outside the scope of this work (see [RFC4916]). Future 232 work might specify ways that a SIP implementation could gateway 233 PASSporT objects to other protocols. 235 This specification allows either a user agent or a proxy server to 236 provide the authentication service function and/or the verification 237 service function. To maximize end-to-end security, it is obviously 238 preferable for end-users to acquire their own credentials; if they 239 do, their user agents can act as authentication services. However, 240 for some deployments, end-user credentials may be neither practical 241 nor affordable, given the potentially large number of SIP user agents 242 (phones, PCs, laptops, PDAs, gaming devices) that may be employed by 243 a single user. In such environments, synchronizing keying material 244 across multiple devices may be prohibitively complex and require 245 quite a good deal of additional endpoint behavior. Managing several 246 credentials for the various devices could also be burdensome. In 247 these cases, implementation the authentication service at an 248 intermediary may be more practical. This trade-off needs to be 249 understood by implementers of this specification. 251 4. Overview of Operations 253 This section provides an informative (non-normative) high-level 254 overview of the mechanisms described in this document. 256 Imagine a case where Alice, who has the home proxy of example.com and 257 the address-of-record sip:alice@example.com, wants to communicate 258 with Bob at sip:bob@example.org. They have no prior relationship, 259 and Bob implements best practices to prevent impersonation attacks. 261 Alice generates an INVITE and places her identity, in this case her 262 address-of-record, in the From header field of the request. She then 263 sends an INVITE over TLS to an authentication service proxy for the 264 example.com domain. 266 The authentication service authenticates Alice (possibly by sending a 267 Digest authentication challenge) and validates that she is authorized 268 to assert the identity that she populated in the From header field. 269 This value is Alice's AoR, but in other cases it could be some 270 different value that the proxy server has authority over, such as a 271 telephone number. The authentication service then constructs a JSON 272 PASSporT object that mirrors particular SIP headers and fields, 273 including part of the From header field of the message, and generates 274 a hash of the object. This hash is then signed with the appropriate 275 credential for the identity (example.com, in the 276 sip:alice@example.com case) and the signature is inserted by the 277 proxy server into the Identity header field value of the request. 279 The proxy, as the holder of the private key for the example.com 280 domain, is asserting that the originator of this request has been 281 authenticated and that she is authorized to claim the identity that 282 appears in the From header field. The proxy inserts an "info" 283 parameter into the Identity header that tells Bob how to acquire 284 keying material necessary to validate its credentials (a public key), 285 in case he doesn't already have it. 287 When Bob's domain receives the request, it verifies the signature 288 provided in the Identity header, and thus can validate that the 289 authority over the identity in the From header field authenticated 290 the user, and permitted the user to assert that From header field 291 value. This same validation operation may be performed by Bob's user 292 agent server (UAS). As the request has been validated, it is 293 rendered to Bob. If the validation was unsuccessful, some other 294 treatment would be applied by the receiving domain. 296 5. Signature Generation and Validation 298 5.1. Authentication Service Behavior 300 This document specifies a role for SIP entities called an 301 authentication service. The authentication service role can be 302 instantiated, for example, by an intermediary such as a proxy server 303 or by a user agent. Any entity that instantiates the authentication 304 service role MUST possess the private key of one or more credentials 305 that can be used to sign for a domain or a telephone number (see 306 Section 6.1). Intermediaries that instantiate this role MUST be 307 capable of authenticating one or more SIP users who can register for 308 that identity. Commonly, this role will be instantiated by a proxy 309 server, since these entities are more likely to have a static 310 hostname, hold corresponding credentials, and have access to SIP 311 registrar capabilities that allow them to authenticate users. It is 312 also possible that the authentication service role might be 313 instantiated by an entity that acts as a redirect server, but that is 314 left as a topic for future work. 316 An authentication service adds the Identity header to SIP requests. 317 The procedures below define the steps that must be taken when each an 318 header is added. More than one may appear in a single request, and 319 an authentication service may add an Identity header to a request 320 that already contains one or more Identity headers. If the Identity 321 header added follows extended signing procedures beyond the baseline 322 given in Section 8, then it differentiates the header with a "ppt" 323 parameter per the fourth step below. 325 Entities instantiating the authentication service role perform the 326 following steps, in order, to generate an Identity header for a SIP 327 request: 329 Step 1: 331 First, the authentication service must determine whether it is 332 authoritative for the identity of the sender of the request. In 333 ordinary operations, the authentication service decides this by 334 inspecting the URI value from the addr-spec component of From header 335 field; this URI will be referred to here as the 'identity field'. If 336 the identity field contains a SIP or SIP Secure (SIPS) URI, and the 337 user portion is not a telephone number, the authentication service 338 MUST extract the hostname portion of the identity field and compare 339 it to the domain(s) for which it is responsible (following the 340 procedures in RFC 3261 [RFC3261], Section 16.4). If the identity 341 field uses the TEL URI scheme [RFC3966], or the identity field is a 342 SIP or SIPS URI with a telephone number in the user portion, the 343 authentication service determines whether or not it is responsible 344 for this telephone number; see Section 7.1 for more information. An 345 authentication service proceeding with a signature over a telephone 346 number MUST then follow the canonicalization procedures described in 347 Section 7.1.1. If the authentication service is not authoritative 348 for the identity in question, it SHOULD process and forward the 349 request normally unless the local policy is to block such requests. 350 The authentication service MUST NOT add an Identity header if the 351 authentication service does not hav ethe authority to make the claim 352 it asserts. 354 Step 2: 356 The authentication service MUST then determine whether or not the 357 sender of the request is authorized to claim the identity given in 358 the identity field. In order to do so, the authentication service 359 MUST authenticate the sender of the message. Some possible ways in 360 which this authentication might be performed include: 362 If the authentication service is instantiated by a SIP 363 intermediary (proxy server), it may authenticate the request with 364 the authentication scheme used for registration in its domain 365 (e.g., Digest authentication). 367 If the authentication service is instantiated by a SIP user agent, 368 a user agent may authenticate its own user through any system- 369 specific means, perhaps simply by virtue of having physical access 370 to the user agent. 372 Authorization of the use of a particular username or telephone number 373 in the user part of the From header field is a matter of local policy 374 for the authentication service; see Section 6.1 for more information. 376 Note that this check is performed only on the addr-spec in the 377 identity field (e.g., the URI of the sender, like 378 'sip:alice@atlanta.example.com'); it does not convert the display- 379 name portion of the From header field (e.g., 'Alice Atlanta'). For 380 more information, see Section 12.6. 382 Step 3: 384 An authentication service MUST add a Date header field to SIP 385 requests that do not have one. The authentication service MUST 386 ensure that any preexisting Date header in the request is accurate. 387 Local policy can dictate precisely how accurate the Date must be; a 388 RECOMMENDED maximum discrepancy of sixty seconds will ensure that the 389 request is unlikely to upset any verifiers. If the Date header 390 contains a time different by more than one minute from the current 391 time noted by the authentication service, the authentication service 392 SHOULD reject the request. This behavior is not mandatory because a 393 user agent client (UAC) could only exploit the Date header in order 394 to cause a request to fail verification; the Identity header is not 395 intended to provide a source of non-repudiation or a perfect record 396 of when messages are processed. Finally, the authentication service 397 MUST verify that both the Date header and the current time fall 398 within the validity period of its credential. 400 See Section 12 for information on how the Date header field assists 401 verifiers. 403 Step 4: 405 Subsequently, the authentication service MUST form a PASSporT object 406 and add a corresponding an Identity header to the request containing 407 this signature. For baseline PASSporT objects headers (without an 408 Identity header "ppt" parameter), this follows the procedures in 409 Section 8; if the authentication service is using an alternative 410 "ppt", it MUST add an appropriate "ppt" parameter and follow the 411 procedures associated with it (see Section 9). After the Identity 412 header has been added to the request, the authentication service MUST 413 also add a "info" parameter to the Identity header. The "info" 414 parameter contains a URI from which the authentication service's 415 credential can be acquired; see Section 6.3 for more on credential 416 acquisition. 418 Finally, the authentication service MUST forward the message 419 normally. 421 In some cases, a request sent through an authentication service will 422 be rejected by the verification service, and the authentication 423 service will receive a 4xx status code (such as 438) in the backwards 424 direction. If the authentication service did not originally send the 425 request with the "canon" parameter, it MAY retry a request once after 426 receiving a 438 response, this time including the "canon". The 427 information in "canon" is useful for debugging errors, and there are 428 some known causes of verification failures (such as the Date header 429 changing in transit, see Section 12.1 for more information) that can 430 be resolved by the inclusion of "canon". 432 5.2. Verifier Behavior 434 This document specifies a logical role for SIP entities called a 435 verification service, or verifier. When a verifier receives a SIP 436 message containing one or more Identity headers, it inspects the 437 signature to verify the identity of the sender of the message. The 438 results of a verification are provided as input to an authorization 439 process that is outside the scope of this document. 441 A SIP request may contain zero, one, or more Identity headers. A 442 verification service performs the procedures below on each Identity 443 header that appears in a request. If the verifier does not support 444 an Identity header present in a request due to the presence of an 445 unsupported "ppt" parameter, or if no Identity header is present, and 446 the presence of an Identity header is required by local policy (for 447 example, based on a per-sending-domain policy, or a per-sending-user 448 policy), then a 428 'Use Identity Header' response MUST be sent in 449 the backwards direction. 451 In order to verify the identity of the sender of a message, an entity 452 acting as a verifier MUST perform the following steps, in the order 453 here specified. 455 Step 1: 457 The verifier MUST inspect any optional "ppt" parameter appearing the 458 Identity request. If no "ppt" parameter is present, then the 459 verifier proceeds normally below. If a "ppt" parameter value is 460 present, and the verifier does not support it, it MUST ignore the 461 Identity header. If a supported "ppt" parameter value is present, 462 the verifier follows the procedures below, including the variations 463 described in Step 5. 465 Step 2: 467 In order to determine whether the signature for the identity field 468 should be over the entire identity field URI or just a canonicalized 469 telephone number, the verification service MUST follow the 470 canonicalization process described in Section 7.1.1. That section 471 also describes the procedures the verification service MUST follow to 472 determine if the signer is authoritative for a telephone number. For 473 domains, the verifier MUST follow the process described in 474 Section 7.2 to determine if the signer is authoritative for the 475 identity field. 477 Step 3: 479 The verifier must first ensure that it possesses the proper keying 480 material to validate the signature in the Identity header field, 481 which usually involves dereferencing a URI in the "info" parameter of 482 the Identity header. See Section 6.2 for more information on these 483 procedures. If the verifier does not suport the credential described 484 in the "info" parameter, it MUST return a 437 "Unsupported 485 Certificate" response. 487 Step 4: 489 The verifier MUST furthermore ensure that the value of the Date 490 header meets local policy for freshness (usually, within sixty 491 seconds) and that it falls within the validity period of the 492 credential used to sign the Identity header. For more on the attacks 493 this prevents, see Section 12.1. 495 Step 5: 497 The verifier MUST validate the signature in the Identity header field 498 over the PASSporT object. For baseline PASSporT objects (with no 499 Identity header "ppt" parameter) the verifier MUST follow the 500 procedures for generating the signature over a PASSporT object 501 described in Section 8. If a "ppt" parameter is present, the 502 verifier follows the procedures for that "ppt" (see Section 9). If a 503 verifier determines that the signature on the message does not 504 correspond to the reconstructed signed-identity-digest, then a 438 505 'Invalid Identity Header' response MUST be returned. 507 The handling of the message after the verification process depends on 508 how the implementation service is implemented and on local policy. 509 This specification does not propose any authorization policy for user 510 agents or proxy servers to follow based on the presence of a valid 511 Identity header, the presence of an invalid Identity header, or the 512 absence of an Identity header, but it is anticipated that local 513 policies could involve making different forwarding decisions in 514 intermediary implementations, or changing how the user is alerted, or 515 how identity is rendered, in user agent implementations. 517 6. Credentials 519 6.1. Credential Use by the Authentication Service 521 In order to act as an authentication service, a SIP entity must have 522 access to the private keying material of one or more credentials that 523 cover domain names or telephone numbers. These credentials may 524 represent authority over an entire domain (such as example.com) or 525 potentially a set of domains enumerated by the credential. 526 Similarly, a credential may represent authority over a single 527 telephone number or a range of telephone numbers. The way that the 528 scope of a credential is expressed is specific to the credential 529 mechanism. 531 Authorization of the use of a particular username or telephone number 532 in the identity field is a matter of local policy for the 533 authentication service, one that depends greatly on the manner in 534 which authentication is performed. For non-telephone number user 535 parts, one policy might be as follows: the username given in the 536 'username' parameter of the Proxy-Authorization header MUST 537 correspond exactly to the username in the From header field of the 538 SIP message. However, there are many cases in which this is too 539 limiting or inappropriate; a realm might use 'username' parameters in 540 Proxy-Authorization that do not correspond to the user-portion of SIP 541 From headers, or a user might manage multiple accounts in the same 542 administrative domain. In this latter case, a domain might maintain 543 a mapping between the values in the 'username' parameter of Proxy- 544 Authorization and a set of one or more SIP URIs that might 545 legitimately be asserted for that 'username'. For example, the 546 username can correspond to the 'private identity' as defined in Third 547 Generation Partnership Project (3GPP), in which case the From header 548 field can contain any one of the public identities associated with 549 this private identity. In this instance, another policy might be as 550 follows: the URI in the From header field MUST correspond exactly to 551 one of the mapped URIs associated with the 'username' given in the 552 Proxy-Authorization header. This is a suitable approach for 553 telephone numbers in particular. 555 This specification could also be used with credentials that cover a 556 single name or URI, such as alice@example.com or 557 sip:alice@example.com. This would require a modification to 558 authentication service behavior to operate on a whole URI rather than 559 a domain name. Because this is not believed to be a pressing use 560 case, this is deferred to future work, but implementors should note 561 this as a possible future direction. 563 Exceptions to such authentication service policies arise for cases 564 like anonymity; if the AoR asserted in the From header field uses a 565 form like 'sip:anonymous@example.com' (see [RFC3323]), then the 566 'example.com' proxy might authenticate only that the user is a valid 567 user in the domain and insert the signature over the From header 568 field as usual. 570 6.2. Credential Use by the Verification Service 572 In order to act as a verification service, a SIP entity must have a 573 way to acquire and retain credentials for authorities over particular 574 domain names and/or telephone numbers or number ranges. 576 Dereferencing the URI found in the "info" parameter of the Identity 577 header (as described in the next section) MUST be supported by all 578 verification service implementations to create a baseline means of 579 credential acquisition. Provided that the credential used to sign a 580 message is not previously known to the verifier, SIP entities SHOULD 581 discover this credential by dereferencing the "info" parameter, 582 unless they have some more other implementation-specific way of 583 acquiring the needed keying material, such as an offline store of 584 periodically-updated credentials. If the URI in the "info" parameter 585 cannot be dereferenced, then a 436 'Bad Identity-Info' response MUST 586 be returned. 588 This specification does not propose any particular policy for a 589 verification service to determine whether or not the holder of a 590 credential is the appropriate party to sign for a given SIP identity. 591 Guidance on this is deferred to the credential mechanism 592 specifications, which must meet the requirements in Section 6.4. 594 Verification service implementations supporting this specification 595 may wish to have some means of retaining credentials (in accordance 596 with normal practices for credential lifetimes and revocation) in 597 order to prevent themselves from needlessly downloading the same 598 credential every time a request from the same identity is received. 599 Credentials cached in this manner may be indexed in accordance with 600 local policy: for example, by their scope, or the URI given in the 601 "info" parameter value. Further consideration of how to cache 602 credentials is deferred to the credential mechanism specifications. 604 6.3. Handling 'info' parameter URIs 606 An "info" parameter MUST contain a URI which dereferences to a 607 resource that contains the public key components of the credential 608 used by the authentication service to sign a request. It is 609 essential that a URI in the "info parameter" be dereferencable by any 610 entity that could plausibly receive the request. For common cases, 611 this means that the URI must be dereferencable by any entity on the 612 public Internet. In constrained deployment environments, a service 613 private to the environment might be used instead. 615 Beyond providing a means of accessing credentials for an identity, 616 the "info" parameter further serves as a means of differentiating 617 which particular credential was used to sign a request, when there 618 are potentially multiple authorities eligible to sign. For example, 619 imagine a case where a domain implements the authentication service 620 role for a range of telephone and a user agent belonging to Alice has 621 acquired a credential for a single telephone number within that 622 range. Either would be eligible to sign a SIP request for the number 623 in question. Verification services however need a means to 624 differentiate which one performed the signature. The "info" 625 parameter performs that function. 627 If the optional "canon" parameter is present, it contains the bae64 628 encoded result of JSON object construction process performed by the 629 authentication service (see Section 7.1.1), including the 630 canonicalization processes applied to the identity in the identity 631 fields of the sender and intended recipient. The "canon" is provided 632 purely as an optimization for the verification service. The 633 verification service MAY compute its own canonicalization of the 634 numbers and compare them to the values in the "canon" parameter 635 before performing any cryptographic functions in order to ascertain 636 whether or not the two ends agree on the canonical number form. 638 6.4. Credential System Requirements 640 This document makes no recommendation for the use of any specific 641 credential system. Today, there are two primary credential systems 642 in place for proving ownership of domain names: certificates (e.g., 643 X.509 v3, see [RFC5280]) and the domain name system itself (e.g., 644 DANE, see [RFC6698]). It is envisioned that either could be used in 645 the SIP identity context: an "info" parameter could for example give 646 an HTTP URL of the form 'application/pkix-cert' pointing to a 647 certificate (following the conventions of [RFC2585]). The "info" 648 parameter may use the DNS URL scheme (see [RFC4501]) to designate 649 keys in the DNS. 651 While no comparable public credentials exist for telephone numbers, 652 either approach could be applied to telephone numbers. A credential 653 system based on certificates is given in 654 [I-D.ietf-stir-certificates]. One based on the domain name system is 655 given in [I-D.kaplan-stir-cider]. 657 In order for a credential system to work with this mechanism, its 658 specification must detail: 660 which URIs schemes the credential will use in the "info" 661 parameter, and any special procedures required to dereference the 662 URIs 664 how the verifier can learn the scope of the credential 666 any special procedures required to extract keying material from 667 the resources designated by the URI 669 any algorithms that would appear in the Identity-Info "alg" 670 parameter other than 'RS256.' Note that the policy for adding 671 algorithms to this registry requires Standards Action 673 SIP entities cannot reliably predict where SIP requests will 674 terminate. When choosing a credential scheme for deployments of this 675 specification, it is therefore essential that the trust anchor(s) for 676 credentials be widely trusted, or that deployments restrict the use 677 of this mechanism to environments where the reliance on particular 678 trust anchors is assured by business arrangements or similar 679 constraints. 681 Note that credential systems must address key lifecycle management 682 concerns: were a domain to change the credential available at the 683 Identity-Info URI before a verifier evaluates a request signed by an 684 authentication service, this would cause obvious verifier failures. 685 When a rollover occurs, authentication services SHOULD thus provide 686 new Identity-Info URIs for each new credential, and SHOULD continue 687 to make older key acquisition URIs available for a duration longer 688 than the plausible lifetime of a SIP transaction (a minute would most 689 likely suffice). 691 7. Identity Types 693 7.1. Telephone Numbers 695 Since many SIP applications provide a Voice over IP (VoIP) service, 696 telephone numbers are commonly used as identities in SIP deployments. 697 In order for telephone numbers to be used with the mechanism 698 described in this document, authentication services must enroll with 699 an authority that issues credentials for telephone numbers or 700 telephone number ranges, and verification services must trust the 701 authority employed by the authentication service that signs a 702 request. Enrollment procedures and credential management are outside 703 the scope of this document. 705 In the longer term, it is possible that some directory or other 706 discovery mechanism may provide a way to determine which 707 administrative domain is responsible for a telephone number, and this 708 may aid in the signing and verification of SIP identities that 709 contain telephone numbers. This is a subject for future work. 711 In order to work with any such authorities, authentication and 712 verification services must be able to identify when a request should 713 be signed by an authority for a telephone number, and when it should 714 be signed by an authority for a domain. Telephone numbers most 715 commonly appear in SIP header field values in the username portion of 716 a SIP URI (e.g., 'sip:+17005551008@chicago.example.com;user=phone'). 717 The user part of that URI conforms to the syntax of the TEL URI 718 scheme (RFC 3966 [RFC3966]). It is also possible for a TEL URI to 719 appear in the SIP To or From header field outside the context of a 720 SIP or SIPS URI (e.g., 'tel:+17005551008'). In both of these cases, 721 it's clear that the signer must have authority over the telephone 722 number, not the domain name of the SIP URI. It is also possible, 723 however, for requests to contain a URI like 724 'sip:7005551000@chicago.example.com'. It may be non-trivial for a 725 service to ascertain in this case whether the URI contains a 726 telephone number or not. 728 7.1.1. Canonicalization Procedures 730 In order to determine whether or not the user portion of a SIP URI is 731 a telephone number, authentication services and verification services 732 must perform the following canonicalization procedure on any SIP URI 733 they inspect which contains a wholly numeric user part. Note that 734 the same procedures are followed for creating the canonical form of 735 URIs found in both the From and To header field values; this section 736 also describes procedures for extracting the URI containing the 737 telephone number from the P-Asserted-Identity header field value for 738 environments where that is applicable. 740 In some networks, the P-Asserted-Identity header field value is used 741 in lieu of the From header field to convey the telephone number of 742 the sender of a request; while it is not envisioned that most of 743 those networks would or should make use of the Identity mechanism 744 described in this specification, where they do, local policy might 745 therefore dictate that the canonical string derive from the P- 746 Asserted-Identity header field rather than the From. In any case 747 where local policy canonicalizes the number into a form different 748 from how it appears in the From header field, the use of the "canon" 749 parameter by authentication services is RECOMMENDED, but because 750 "canon" itself could then divulge information about users or 751 networks, implementers should be mindful of the guidelines in 752 Section 11. 754 First, implementations must assess if the user-portion of the URI 755 constitutes a telephone number. In some environments, numbers 756 will be explicitly labeled by the use of TEL URIs or the 757 'user=phone' parameter, or implicitly by the presence of the '+' 758 indicator at the start of the user-portion. Absent these 759 indications, if there are numbers present in the user-portion, 760 implementations may also detect that the user-portion of the URI 761 contains a telephone number by determining whether or not those 762 numbers would be dialable or routable in the local environment -- 763 bearing in mind that the telephone number may be a valid E.164 764 number, a nationally-specific number, or even a private branch 765 exchange number. 767 Once an implementation has identified a telephone number, it must 768 construct a number string. Implementations MUST drop any leading 769 +'s, any internal dashes, parentheses or other non-numeric 770 characters, excepting only the leading "#" or "*" keys used in 771 some special service numbers (typically, these will appear only in 772 the To header field value). This MUST result in an ASCII string 773 limited to "#", "*" and digits without whitespace or visual 774 separators. 776 Next, an implementation must assess if the number string is a 777 valid, globally-routable number with a leading country code. If 778 not, implementations SHOULD convert the number into E.164 format, 779 adding a country code if necessary; this may involve transforming 780 the number from a dial string (see [RFC3966]), removing any 781 national or international dialing prefixes or performing similar 782 procedures. It is only in the case that an implementation cannot 783 determine how to convert the number to a globally-routable format 784 that this step may be skipped. This will be the case, for 785 example, for nationally-specific service numbers (e.g. 911, 112); 786 however, the routing procedures associated with those numbers will 787 likely make sure that the verification service understands the 788 context of their use. 790 Oher transformations during canonicalization MAY be made in 791 accordance with specific policies used within a local domain. For 792 example, one domain may only use local number formatting and need 793 to convert all To/From user portions to E.164 by prepending 794 country-code and region code digits; another domain might prefix 795 usernames with trunk-routing codes and need to remove the prefix. 796 This specification cannot anticipate all of the potential 797 transformations that might be useful. 799 The resulting canonical number string will be used as input to the 800 hash calculation during signing and verifying processes. 802 The ABNF of this number string is: 804 tn-spec = [ "#" / "*" ] 1*DIGIT 806 If the result of this procedure forms a complete telephone number, 807 that number is used for the purpose of creating and signing the 808 signed-identity-string by both the authentication service and 809 verification service. Practically, entities that perform the 810 authentication service role will sometimes alter the telephone 811 numbers that appear in the To and From header field values, 812 converting them to this format (though note this is not a function 813 that [RFC3261] permits proxy servers to perform). The result of the 814 canonicalization process of the From header field value may also be 815 recorded through the use of the "canon" parameter of the Identity(see 816 Section 8). If the result of the canonicalization of the From header 817 field value does not form a complete telephone number, the 818 authentication service and verification service should treat the 819 entire URI as a SIP URI, and apply a domain signature per the 820 procedures in Section 7.2. 822 7.2. Domain Names 824 When a verifier processes a request containing an Identity-Info 825 header with a domain signature, it must compare the domain portion of 826 the URI in the From header field of the request with the domain name 827 that is the subject of the credential acquired from the "info" 828 parameter. While it might seem that this should be a straightforward 829 process, it is complicated by two deployment realities. In the first 830 place, credentials have varying ways of describing their subjects, 831 and may indeed have multiple subjects, especially in 'virtual 832 hosting' cases where multiple domains are managed by a single 833 application. Secondly, some SIP services may delegate SIP functions 834 to a subordinate domain and utilize the procedures in RFC 3263 835 [RFC3263] that allow requests for, say, 'example.com' to be routed to 836 'sip.example.com'. As a result, a user with the AoR 837 'sip:jon@example.com' may process requests through a host like 838 'sip.example.com', and it may be that latter host that acts as an 839 authentication service. 841 To meet the second of these problems, a domain that deploys an 842 authentication service on a subordinate host MUST be willing to 843 supply that host with the private keying material associated with a 844 credential whose subject is a domain name that corresponds to the 845 domain portion of the AoRs that the domain distributes to users. 846 Note that this corresponds to the comparable case of routing inbound 847 SIP requests to a domain. When the NAPTR and SRV procedures of RFC 848 3263 are used to direct requests to a domain name other than the 849 domain in the original Request-URI (e.g., for 'sip:jon@example.com', 850 the corresponding SRV records point to the service 851 'sip1.example.org'), the client expects that the certificate passed 852 back in any TLS exchange with that host will correspond exactly with 853 the domain of the original Request-URI, not the domain name of the 854 host. Consequently, in order to make inbound routing to such SIP 855 services work, a domain administrator must similarly be willing to 856 share the domain's private key with the service. This design 857 decision was made to compensate for the insecurity of the DNS, and it 858 makes certain potential approaches to DNS-based 'virtual hosting' 859 unsecurable for SIP in environments where domain administrators are 860 unwilling to share keys with hosting services. 862 A verifier MUST evaluate the correspondence between the user's 863 identity and the signing credential by following the procedures 864 defined in RFC 2818 [RFC2818], Section 3.1. While RFC 2818 [RFC2818] 865 deals with the use of HTTP in TLS and is specific to certificates, 866 the procedures described are applicable to verifying identity if one 867 substitutes the "hostname of the server" in HTTP for the domain 868 portion of the user's identity in the From header field of a SIP 869 request with an Identity header. 871 8. Header Syntax 873 The Identity and Identity-Info headers that were previously defined 874 in RFC4474 are deprecated. This revised specification collapses the 875 grammar of Identity-Info into the Identity header via the "info" 876 parameter. Note that unlike the prior specification in RFC4474, the 877 Identity header is now allowed to appear more than one time in a SIP 878 request. The revised grammar for the Identity header is (following 879 the ABNF [RFC4234] in RFC 3261 [RFC3261]): 881 Identity = "Identity" HCOLON signed-identity-digest SEMI ident-info *( SEMI ident-info-params ) 882 signed-identity-digest = LDQUOT *base64-char RDQUOT 883 ident-info = "info" EQUAL ident-info-uri 884 ident-info-uri = LAQUOT absoluteURI RAQUOT 885 ident-info-params = ident-info-alg / ident-type / canonical-str / ident-info-extension 886 ident-info-alg = "alg" EQUAL token 887 ident-type = "ppt" EQUAL token 888 canonical-str = "canon" EQUAL *base64-char 889 ident-info-extension = generic-param 891 base64-char = ALPHA / DIGIT / "/" / "+" 893 In addition to "info" parameter, and the "alg" parameter previously 894 defined in RFC4474, this specification includes the optional "canon" 895 and "ppt" parameters. Note that in RFC4474, the signed-identity- 896 digest (see ABNF above) was given as quoted 32LHEX, whereas here it 897 is given as a quoted sequence of base64-char. 899 The 'absoluteURI' portion of ident-info-uri MUST contain a URI; see 900 Section 6.3 for more on choosing how to advertise credentials through 901 this parameter. 903 The signed-identity-digest is the signed hash component of a PASSporT 904 object [I-D.ietf-stir-passport], a signature which PASSporT generates 905 over a pair of JSON objects. The first PASSporT object contains 906 header information, and the second contains claims, following the 907 conventions of JWT [RFC7519]; some header and claim values will 908 mirror elements of the SIP request. Once these two JSON objects have 909 been generated, they will be encoded, then hashed with a SHA-256 910 hash. Those two hashes are then concatenated (header then claims) 911 into a string separated by a single "." per baseline PASSporT. 913 Finally, that string is signed to generate the signed-identity-digest 914 value of the Identity header. 916 For SIP implementations to populate the PASSporT header object from a 917 SIP request, the following elements message MUST be placed as the 918 values corresponding to the designated JSON keys: 920 First, per baseline [I-D.ietf-stir-passport], the JSON key "typ" 921 key MUST have the value "passport". 923 Second, the JSON key "alg" MUST mirror the value of the optional 924 "alg" parameter in the SIP Identity header. Note if the "alg" 925 parameter is absent, the default value is "RS256". 927 Third, the JSON key "x5u" MUST have a value equivalent to the 928 quoted URI in the "info" parameter. 930 Fourth, the optional JSON key "ppt", if present, MUST have a value 931 equivalent to the quoted value of the "ppt" parameter of the 932 Identity header. If the "ppt" parameter is absent from the 933 header, the "ppt" key MUST NOT not appear in the JSON heaer 934 object. 936 For example: 938 { "typ":"passport", 939 "alg":"RS256", 940 "x5u":"https://www.example.com/cert.pkx" } 942 To populate the PASSporT claims JSON object, the following elements 943 MUST be placed as values corresponding to the designated JSON keys: 945 First, if the originating identity is a telephone number, the JSON 946 key "otn" MUST be used, set to the value of the quoted originating 947 identity, a canonicalized telephone number (see Section 7.1.1). 948 Otherwise, the JSON key "ouri" MUST be used, set to the value of 949 the AoR of the UA sending the message as taken from addr-spec of 950 the From header field. 952 Second, if the destination identity is a telephone number, the 953 JSON key "dtn" MUST be used, set to the value of the quoted 954 destination identity, a canonicalized telephone number (see 955 Section 7.1.1). Otherwise, the JSON key "duri" MUST be used, set 956 to the value of the addr-spec component of the To header field, 957 which is the AoR to which the request is being sent. 959 Third, the JSON key "iat" MUST appear, set to the value of a 960 quoted encoding of the value of the SIP Date header field as a 961 JSON NumericDate (as UNIX time, per [RFC7519] Section 2). 963 Fourth, if the request contains an SDP message body, and if that 964 SDP contains one or more "a=fingerprint" attributes, then the JSON 965 key "mky" MUST appear with the quoted value(s) of the fingerprint 966 attributes (if they differ). Each attribute value consists of all 967 characters following the colon after "a=fingerprint" including the 968 algorithm description and hexadecimal key representation, after 969 removing any whitespace, carriage returns, and "/" line break 970 indicators. If multiple non-identical "a=fingerprint" attributes 971 appear in an SDP body, then all non-identical attributes values 972 MUST be concatenated, with no separating character, after sorting 973 the values in alphanumeric order. If the SDP body contains no 974 "a=fingerprint" attribute, then no JSON "mky" key is added to the 975 object. 977 For example: 979 { "otn":"12155551212", 980 "dtn":"12155551213", 981 "iat":"1443208345" } 983 For more information on the security properties of these SIP message 984 elements, and why their inclusion mitigates replay attacks, see 985 Section 12 and [RFC3893]. 987 After these two JSON objects, the header and the claims, have been 988 constructed, they must each be hashed per [I-D.ietf-stir-passport] 989 Section 3.3. The signed value of those concatenated hashes then 990 becomes the signed-identity-string of the Identity header. The 991 hashing and signing algorithm is specified by the 'alg' parameter of 992 the Identity header and the mirrored "alg" parameter of PASSporT. 993 This specification defines only one value for the 'alg' parameter: 994 'RS256', as defined in [RFC7519], which connotes a SHA-256 hash 995 followed by a RSASSA-PKCS1-v1_5 signature. All implementations of 996 this specification MUST support 'RS256'. Any further 'alg' values 997 MUST be defined in a Standards Track RFC, see Section 13.2 for more 998 information. 1000 The complete form of the Identity header will therefore look like the 1001 following example: 1003 Identity: "sv5CTo05KqpSmtHt3dcEiO/1CWTSZtnG3iV+1nmurLXV/HmtyNS7Ltrg9dlxkWzo 1004 eU7d7OV8HweTTDobV3itTmgPwCFjaEmMyEI3d7SyN21yNDo2ER/Ovgtw0Lu5csIp 1005 pPqOg1uXndzHbG7mR6Rl9BnUhHufVRbp51Mn3w0gfUs="; \ 1006 info=;alg=RS256 1008 In a departure from JWT practice, the SIP usage of PASSporT MAY NOT 1009 include the base64 encoded version of the JSON objects in the 1010 Identity header: only the signature component of the PASSporT is 1011 REQUIRED. Optionally, as a debugging measure or optimization, the 1012 base64 encoded concatenation of the JSON header and claims may be 1013 included as the value of a "canon" parameter of the Identity header. 1014 Note that this may be lengthy string. 1016 9. Extensibility 1018 As future requirements may warrant increasing the scope of the 1019 Identity mechanism, this specification defines an optional "ppt" 1020 parameter of the Identity header, which mirrors the "ppt" header key 1021 in PASSporT. The "ppt" parameter value MUST consist of a token 1022 containing an extension specification, which denotes an extended set 1023 of one or more signed claims per the type extensibility mechanism 1024 specified in [I-D.ietf-stir-passport]. 1026 An authentication service cannot assume that verifiers will 1027 understand any given extension. Verifiers that do support an 1028 extension may then trigger appropriate application-level behavior in 1029 the presence of an extension; authors of extensions should provide 1030 appropriate extension-specific guidance to application developers on 1031 this point. 1033 If any claim in an extension contains a JSON value that does not 1034 correspond to any field of the SIP request, but then the optional 1035 "canon" parameter MUST be used for the Identity header containing 1036 that extension. 1038 10. Gatewaying to PASSporT for non-SIP Transit 1040 As defined in this specification, the signature in the Identity 1041 header is equivalent to the signature that would appear in a PASSporT 1042 token. This is so that a valid PASSporT can be generated based on a 1043 SIP request containing an Identity header. This PASSporT could then 1044 be transported in alternate protocols, stored in a repository and 1045 later accessed, or similarly used outside the context of establishing 1046 an end-to-end SIP session. Third-party services could also generate 1047 PASSporT tokens which could be transformed into Identity headers and 1048 added to SIP requests in transit by authentication services. 1050 Because the base64 encoding of JSON objects containing headers and 1051 claims can be quite long, and because the information baseline 1052 PASSporT contains is necessarily redundant with information in the 1053 header field values of the SIP request itself, SIP does not require 1054 implementations to carry the base64 encodings of those objects. The 1055 optional "canon" parameter of the Identity-Info, if present, contains 1056 the encoded objects used to generate the hash and signature (see 1057 Section 8), but if the "canon" parameter is not present, the contents 1058 of the objects can be regenerated by constructing the object anew 1059 from the SIP header fields received. 1061 Alternative transports for PASSporT and their requirements are left 1062 to future specifications. 1064 11. Privacy Considerations 1066 The purpose of this mechanism is to provide a strong identification 1067 of the originator of a SIP request, specifically a cryptographic 1068 assurance that an authority asserts the orginator can claim the URI 1069 given in the From header field. This URI may contain a variety of 1070 personally identifying information, including the name of a human 1071 being, their place of work or service provider, and possibly further 1072 details. The intrinsic privacy risks associated with that URI are, 1073 however, no different from those of baseline SIP. Per the guidance 1074 in [RFC6973], implementors should make users aware of the privacy 1075 trade-off of providing secure identity. 1077 The identity mechanism presented in this document is compatible with 1078 the standard SIP practices for privacy described in [RFC3323]. A SIP 1079 proxy server can act both as a privacy service and as an 1080 authentication service. Since a user agent can provide any From 1081 header field value that the authentication service is willing to 1082 authorize, there is no reason why private SIP URIs that contain 1083 legitimate domains (e.g., sip:anonymous@example.com) cannot be signed 1084 by an authentication service. The construction of the Identity 1085 header is the same for private URIs as it is for any other sort of 1086 URIs. Similar practices could be used to support opportunistic 1087 signing of SIP requests for UA-integrated authentications services 1088 with self-signed certificates, though that is outside the scope of 1089 this specification and is left as a matter for future investigation. 1091 Note, however, that even when using anonymous SIP URIs, an 1092 authentication service must possess a certificate corresponding to 1093 the host portion of the addr-spec of the From header field of the 1094 request; accordingly, using domains like 'anonymous.invalid' will not 1095 be possible for privacy services that also act as authentication 1096 services. The assurance offered by the usage of anonymous URIs with 1097 a valid domain portion is "this is a known user in my domain that I 1098 have authenticated, but I am keeping its identity private". 1100 It is worth noting two features of this more anonymous form of 1101 identity. One can eliminate any identifying information in a domain 1102 through the use of the domain 'anonymous.invalid," but we must then 1103 acknowledge that it is difficult for a domain to be both anonymous 1104 and authenticated. The use of the "anonymous.invalid" domain entails 1105 that no corresponding authority for the domain can exist, and as a 1106 consequence, authentication service functions for that domain are 1107 meaningless. The second feature is more germane to the threats this 1108 document mitigates [RFC7375]. None of the relevant attacks, all of 1109 which rely on the attacker taking on the identity of a victim or 1110 hiding their identity using someone else's identity, are enabled by 1111 an anonymous identity. As such, the inability to assert an authority 1112 over an anonymous domain is irrelevant to our threat model. 1114 [RFC3325] defines the "id" priv-value token, which is specific to the 1115 P-Asserted-Identity header. The sort of assertion provided by the P- 1116 Asserted-Identity header is very different from the Identity header 1117 presented in this document. It contains additional information about 1118 the sender of a message that may go beyond what appears in the From 1119 header field; P-Asserted-Identity holds a definitive identity for the 1120 sender that is somehow known to a closed network of intermediaries. 1121 Presumably, that network will use this identity for billing or 1122 security purposes. The danger of this network-specific information 1123 leaking outside of the closed network motivated the "id" priv-value 1124 token. The "id" priv-value token has no implications for the 1125 Identity header, and privacy services MUST NOT remove the Identity 1126 header when a priv-value of "id" appears in a Privacy header. 1128 The optional "canon" parameter of the Identity header specified in 1129 this document provides the complete JSON objects used to generate the 1130 signed-identity-digest of the Identity header, including the 1131 canonicalized form of the telephone number of the originator of a 1132 call, if the signature is over a telephone number. In some contexts, 1133 local policy may require a canonicalization which differs 1134 substantially from the original From header field. Depending on 1135 those policies, potentially the "canon" parameter might divulge 1136 information about the originating network or user that might not 1137 appear elsewhere in the SIP request. Were it to be used to reflect 1138 the contents of the P-Asserted-Identity header field, for example, 1139 then "canon" would need to be removed when the P-Asserted-Identity 1140 header is removed to avoid any such leakage outside of a trust 1141 domain. Since, in those contexts, the canonical form of the sender's 1142 identity could not be reassembled by a verifier, and thus the 1143 Identity signature validation process would fail, using P-Asserted- 1144 Identity with the Identity "canon" parameter in this fashion is NOT 1145 RECOMMENDED outside of environments where SIP requests will never 1146 leave the trust domain. As a side note, history shows that closed 1147 networks never stay closed and one should design their implementation 1148 assuming connectivity to the broader Internet. 1150 Finally, note that unlike [RFC3325], the mechanism described in this 1151 specification adds no information to SIP requests that has privacy 1152 implications. 1154 12. Security Considerations 1156 This document describes a mechanism that provides a signature over 1157 the Date header field of SIP requests, parts of the To and From 1158 header fields, and when present any media keying material in the 1159 message body. In general, the considerations related to the security 1160 of these headers are the same as those given in [RFC3261] for 1161 including headers in tunneled 'message/sip' MIME bodies (see 1162 Section 23 of RFC3261 in particular). The following section details 1163 the individual security properties obtained by including each of 1164 these header fields within the signature; collectively, this set of 1165 header fields provides the necessary properties to prevent 1166 impersonation. It addresses the solution-specific attacks against 1167 in-band solutions enumerated in [RFC7375] Section 4.1. 1169 12.1. Protected Request Fields 1171 The From header field value (in ordinary operations) indicates the 1172 identity of the sender of the message. The SIP address-of-record 1173 URI, or an embedded telephone number, in the From header field is the 1174 identity of a SIP user, for the purposes of this document. Note that 1175 in some deployments the identity of the sender may reside in P- 1176 Asserted-Id instead. The sender's identity is the key piece of 1177 information that this mechanism secures; the remainder of the signed 1178 parts of a SIP request are present to provide reference integrity and 1179 to prevent certain types of cut-and-paste attacks. 1181 The Date header field value protects against cut-and-paste attacks, 1182 as described in [RFC3261], Section 23.4.2. Implementations of this 1183 specification MUST NOT deem valid a request with an outdated Date 1184 header field (the RECOMMENDED interval is that the Date header must 1185 indicate a time within 60 seconds of the receipt of a message). Note 1186 that per baseline [RFC3261] behavior, servers keep state of recently 1187 received requests, and thus if an Identity header is replayed by an 1188 attacker within the Date interval, verifiers can detect that it is 1189 spoofed because a message with an identical Date from the same source 1190 had recently been received. 1192 It has been observed in the wild that some networks change the Date 1193 header field value of SIP requests in transit, and that alternative 1194 behavior might be necessary to accomodate that use case. 1195 Verification services that observe a signature validation failure MAY 1196 therefore reconstruct the Date header field component of the 1197 signature from the "iat" carried in PASSporT via the "canon" 1198 parameter: provided that time recorded by "iat" falls within the 1199 local policy for freshness that would ordinarily apply to the Date 1200 header, the verification service MAY treat the signature as valid, 1201 provided it keeps adequate state to detect recent replays. Note that 1202 this will require the inclusion of the "canon" parameter by 1203 authentication services in networks where such failures are observed. 1205 The To header field value provides the identity of the SIP user that 1206 this request originally targeted. Providing the To header field in 1207 the Identity signature serves two purposes. First, it prevents cut- 1208 and-paste attacks in which an Identity header from legitimate request 1209 for one user is cut-and-pasted into a request for a different user. 1210 Second, it preserves the starting URI scheme of the request, which 1211 helps prevent downgrade attacks against the use of SIPS. The To 1212 offers additional protection against cut-and-paste attacks beyond the 1213 Date header field. For example, without a signature over the To, an 1214 attacker who receives a call from a target could immediately forward 1215 the INVITE to the target's voicemail service within the Date 1216 interval, and the voicemail service would have no way knowing that 1217 the Identity header it received had been originally signed for a call 1218 intended for a different number. However, note the caveats below in 1219 Section 12.1.1. 1221 When signing a request that contains a fingerprint of keying material 1222 in SDP for DTLS-SRTP [RFC5763], this mechanism always provides a 1223 signature over that fingerprint. This signature prevents certain 1224 classes of impersonation attacks in which an attacker forwards or 1225 cut-and-pastes a legitimate request. Although the target of the 1226 attack may accept the request, the attacker will be unable to 1227 exchange media with the target as they will not possess a key 1228 corresponding to the fingerprint. For example, there are some 1229 baiting attacks, launched with the REFER method or through social 1230 engineering, where the attacker receives a request from the target 1231 and reoriginates it to a third party. These might not be prevented 1232 by only a signature over the From, To and Date, but could be 1233 prevented by securing a fingerprint for DTLS-SRTP. While this is a 1234 different form of impersonation than is commonly used for 1235 robocalling, ultimately there is little purpose in establishing the 1236 identity of the user that originated a SIP request if this assurance 1237 is not coupled with a comparable assurance over the contents of the 1238 subsequent media communication. This signature also, per [RFC7258], 1239 reduces the potential for passive monitoring attacks against the SIP 1240 media. In environments where DTLS-SRTP is unsupported, however, no 1241 field is signed and no protections are provided. 1243 12.1.1. Protection of the To Header and Retargeting 1245 The mechanism in this document provides a signature over the identity 1246 information in the To header field value of requests. This provides 1247 a means for verifiers to detect replay attacks where a signed request 1248 originally sent to one target is modified and then forwarded by an 1249 attacker to another, unrelated target. Armed with the original value 1250 of the To header field, the recipient of a request may compare it to 1251 their own identity in order to determine whether or not the identity 1252 information in this call might have been replayed. However, any 1253 request may be legitimately retargeted as well, and as a result 1254 legitimate requests may reach a SIP endpoint whose user is not 1255 identified by the URI designated in the To header field value. It is 1256 therefore difficult for any verifier to decide whether or not some 1257 prior retargeting was "legitimate." Retargeting can also cause 1258 confusion when identity information is provided for requests sent in 1259 the backwards direction in a dialog, as the dialog identifiers may 1260 not match credentials held by the ultimate target of the dialog. For 1261 further information on the problems of response identity see 1262 [I-D.peterson-sipping-retarget]. 1264 Any means for authentication services or verifiers to anticipate 1265 retargeting is outside the scope of this document, and likely to have 1266 equal applicability to response identity as it does to requests in 1267 the backwards direction within a dialog. Consequently, no special 1268 guidance is given for implementers here regarding the 'connected 1269 party' problem (see [RFC4916]); authentication service behavior is 1270 unchanged if retargeting has occurred for a dialog-forming request. 1271 Ultimately, the authentication service provides an Identity header 1272 for requests in the backwards dialog when the user is authorized to 1273 assert the identity given in the From header field, and if they are 1274 not, an Identity header is not provided. And per the threat model of 1275 [RFC7375], resolving problems with 'connected' identity has little 1276 bearing on detecting robocalling or related impersonation attacks. 1278 12.2. Unprotected Request Fields 1280 RFC4474 originally had protections for the Contact, Call-ID and CSeq. 1281 These are removed from RFC4474bis. The absence of these header 1282 values creates some opportunities for determined attackers to 1283 impersonate based on cut-and-paste attacks; however, the absence of 1284 these headers does not seem impactful to preventing the simple 1285 unauthorized claiming of an identity for the purposes of robocalling, 1286 voicemail hacking, or swatting, which is the primary scope of the 1287 current document. 1289 It might seem attractive to provide a signature over some of the 1290 information present in the Via header field value(s). For example, 1291 without a signature over the sent-by field of the topmost Via header, 1292 an attacker could remove that Via header and insert its own in a cut- 1293 and-paste attack, which would cause all responses to the request to 1294 be routed to a host of the attacker's choosing. However, a signature 1295 over the topmost Via header does not prevent attacks of this nature, 1296 since the attacker could leave the topmost Via intact and merely 1297 insert a new Via header field directly after it, which would cause 1298 responses to be routed to the attacker's host "on their way" to the 1299 valid host, which has exactly the same end result. Although it is 1300 possible that an intermediary-based authentication service could 1301 guarantee that no Via hops are inserted between the sending user 1302 agent and the authentication service, it could not prevent an 1303 attacker from adding a Via hop after the authentication service, and 1304 thereby preempting responses. It is necessary for the proper 1305 operation of SIP for subsequent intermediaries to be capable of 1306 inserting such Via header fields, and thus it cannot be prevented. 1307 As such, though it is desirable, securing Via is not possible through 1308 the sort of identity mechanism described in this document; the best 1309 known practice for securing Via is the use of SIPS. 1311 12.3. Malicious Removal of Identity Headers 1313 In the end analysis, the Identity header cannot protect itself. Any 1314 attacker could remove the header from a SIP request, and modify the 1315 request arbitrarily afterwards. However, this mechanism is not 1316 intended to protect requests from men-in-the-middle who interfere 1317 with SIP messages; it is intended only to provide a way that the 1318 originators of SIP requests can prove that they are who they claim to 1319 be. At best, by stripping identity information from a request, a 1320 man-in-the-middle could make it impossible to distinguish any 1321 illegitimate messages he would like to send from those messages sent 1322 by an authorized user. However, it requires a considerably greater 1323 amount of energy to mount such an attack than it does to mount 1324 trivial impersonations by just copying someone else's From header 1325 field. This mechanism provides a way that an authorized user can 1326 provide a definitive assurance of his identity that an unauthorized 1327 user, an impersonator, cannot. 1329 12.4. Securing the Connection to the Authentication Service 1331 In the absence of user agent-based authentication services, the 1332 assurance provided by this mechanism is strongest when a user agent 1333 forms a direct connection, preferably one secured by TLS, to an 1334 intermediary-based authentication service. The reasons for this are 1335 twofold: 1337 If a user does not receive a certificate from the authentication 1338 service over the TLS connection that corresponds to the expected 1339 domain (especially when the user receives a challenge via a 1340 mechanism such as Digest), then it is possible that a rogue server 1341 is attempting to pose as an authentication service for a domain 1342 that it does not control, possibly in an attempt to collect shared 1343 secrets for that domain. A similar practice could be used for 1344 telephone numbers, though the application of certificates for 1345 telephone numbers to TLS is left as a matter for future study. 1347 Without TLS, the various header field values and the body of the 1348 request will not have integrity protection when the request 1349 arrives at an authentication service. Accordingly, a prior 1350 legitimate or illegitimate intermediary could modify the message 1351 arbitrarily. 1353 Of these two concerns, the first is most material to the intended 1354 scope of this mechanism. This mechanism is intended to prevent 1355 impersonation attacks, not man-in-the-middle attacks; integrity over 1356 the header and bodies is provided by this mechanism only to prevent 1357 replay attacks. However, it is possible that applications relying on 1358 the presence of the Identity header could leverage this integrity 1359 protection for services other than replay protection. 1361 Accordingly, direct TLS connections SHOULD be used between the UAC 1362 and the authentication service whenever possible. The opportunistic 1363 nature of this mechanism, however, makes it very difficult to 1364 constrain UAC behavior, and moreover there will be some deployment 1365 architectures where a direct connection is simply infeasible and the 1366 UAC cannot act as an authentication service itself. Accordingly, 1367 when a direct connection and TLS are not possible, a UAC should use 1368 the SIPS mechanism, Digest 'auth-int' for body integrity, or both 1369 when it can. The ultimate decision to add an Identity header to a 1370 request lies with the authentication service, of course; domain 1371 policy must identify those cases where the UAC's security association 1372 with the authentication service is too weak. 1374 12.5. Authorization and Transitional Strategies 1376 Ultimately, the worth of an assurance provided by an Identity header 1377 is limited by the security practices of the authentication service 1378 that issues the assurance. Relying on an Identity header generated 1379 by a remote administrative domain assumes that the issuing domain 1380 uses recommended administrative practices to authenticate its users. 1381 However, it is possible that some authentication services will 1382 implement policies that effectively make users unaccountable (e.g., 1383 ones that accept unauthenticated registrations from arbitrary users). 1384 The value of an Identity header from such authentication services is 1385 questionable. While there is no magic way for a verifier to 1386 distinguish "good" from "bad" signers by inspecting a SIP request, it 1387 is expected that further work in authorization practices could be 1388 built on top of this identity solution; without such an identity 1389 solution, many promising approaches to authorization policy are 1390 impossible. That much said, it is RECOMMENDED that authentication 1391 services based on proxy servers employ strong authentication 1392 practices. 1394 One cannot expect the Identity header to be supported by every SIP 1395 entity overnight. This leaves the verifier in a compromising 1396 position; when it receives a request from a given SIP user, how can 1397 it know whether or not the sender's domain supports Identity? In the 1398 absence of ubiquitous support for identity, some transitional 1399 strategies are necessary. 1401 A verifier could remember when it receives a request from a domain 1402 or telephone number that uses Identity, and in the future, view 1403 messages received from that sources without Identity headers with 1404 skepticism. 1406 A verifier could consult some sort of directory that indications 1407 whether a given caller should have a signed identity. There are a 1408 number of potential ways in which this could be implemented. This 1409 is left as a subject for future work. 1411 In the long term, some sort of identity mechanism, either the one 1412 documented in this specification or a successor, must become 1413 mandatory-to-use for the SIP protocol; that is the only way to 1414 guarantee that this protection can always be expected by verifiers. 1416 Finally, it is worth noting that the presence or absence of the 1417 Identity headers cannot be the sole factor in making an authorization 1418 decision. Permissions might be granted to a message on the basis of 1419 the specific verified Identity or really on any other aspect of a SIP 1420 request. Authorization policies are outside the scope of this 1421 specification, but this specification advises any future 1422 authorization work not to assume that messages with valid Identity 1423 headers are always good. 1425 12.6. Display-Names and Identity 1427 As a matter of interface design, SIP user agents might render the 1428 display-name portion of the From header field of a caller as the 1429 identity of the caller; there is a significant precedent in email 1430 user interfaces for this practice. Securing the display-name 1431 component of the From header field value is outside the scope of this 1432 document, but may be the subject of future work, such as through the 1433 "ppt" name mechanism. 1435 In the absence of signing the display-name, authentication services 1436 might check and validate it, and compare it to a list of acceptable 1437 display-names that may be used by the sender; if the display-name 1438 does not meet policy constraints, the authentication service could 1439 return a 403 response code. In this case, the reason phrase should 1440 indicate the nature of the problem; for example, "Inappropriate 1441 Display Name". However, the display-name is not always present, and 1442 in many environments the requisite operational procedures for 1443 display-name validation may not exist, so no normative guidance is 1444 given here. 1446 13. IANA Considerations 1448 This document relies on the headers and response codes defined in RFC 1449 4474. It also retains the requirements for the specification of new 1450 algorithms or headers related to the mechanisms described in that 1451 document. 1453 13.1. Identity-Info Parameters 1455 The IANA has already created a registry for Identity-Info parameters. 1456 This specification defines a new value called "canon" as defined in 1457 Section 6.3. Note however that unlike in RFC4474, Identity-Info 1458 parameters now appear in the Identity header. 1460 13.2. Identity-Info Algorithm Parameter Values 1462 The IANA has already created a registry for Identity-Info "alg" 1463 parameter values. This registry is to be populated with a value for 1464 'RS256', which describes the algorithm used to create the signature 1465 that appears in the Identity header. Registry entries must contain 1466 the name of the 'alg' parameter value and the specification in which 1467 the value is described. New values for the 'alg' parameter may be 1468 defined only in Standards Track RFCs. 1470 RFC4474 defined the 'rsa-sha1' value for this registry. That value 1471 is hereby deprecated, and should be treated as such. It is not 1472 believed that any implementations are making use of this value. 1474 Future specifications may consider elliptical curves for smaller key 1475 sizes. 1477 Note that the Identity-Info header is also deprecated by this 1478 specification, and thus the "alg" parameter is now a value of the 1479 Identity header, not Identity-Info. 1481 14. Acknowledgments 1483 The authors would like to thank Stephen Kent, Brian Rosen, Alex 1484 Bobotek, Paul Kyzviat, Jonathan Lennox, Richard Shockey, Martin 1485 Dolly, Andrew Allen, Hadriel Kaplan, Sanjay Mishra, Anton Baskov, 1486 Pierce Gorman, David Schwartz, Philippe Fouquart, Michael Hamer, 1487 Henning Schulzrinne, and Richard Barnes for their comments. 1489 15. Changes from RFC4474 1491 The following are salient changes from the original RFC 4474: 1493 Generalized the credential mechanism; credential enrollment, 1494 acquisition and trust is now outside the scope of this document 1496 Reduced the scope of the Identity signature to remove CSeq, Call- 1497 ID, Contact, and the message body 1499 Removed the Identity-Info header and relocated its components into 1500 parameters of the Identity header 1502 Added any DTLS-SRTP fingerprint in SDP as a mandatory element of 1503 the PASSporT 1505 Deprecated 'rsa-sha1' in favor of new baseline signing algorithm 1507 Changed the signed-identity-digest format for compatibility with 1508 PASSporT 1510 16. References 1512 16.1. Normative References 1514 [I-D.ietf-stir-passport] 1515 Wendt, C. and J. Peterson, "Persona Assertion Token", 1516 draft-ietf-stir-passport-00 (work in progress), February 1517 2016. 1519 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1520 Requirement Levels", BCP 14, RFC 2119, 1521 DOI 10.17487/RFC2119, March 1997, 1522 . 1524 [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, 1525 DOI 10.17487/RFC2818, May 2000, 1526 . 1528 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 1529 A., Peterson, J., Sparks, R., Handley, M., and E. 1530 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 1531 DOI 10.17487/RFC3261, June 2002, 1532 . 1534 [RFC3263] Rosenberg, J. and H. Schulzrinne, "Session Initiation 1535 Protocol (SIP): Locating SIP Servers", RFC 3263, 1536 DOI 10.17487/RFC3263, June 2002, 1537 . 1539 [RFC3280] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet 1540 X.509 Public Key Infrastructure Certificate and 1541 Certificate Revocation List (CRL) Profile", RFC 3280, 1542 DOI 10.17487/RFC3280, April 2002, 1543 . 1545 [RFC3370] Housley, R., "Cryptographic Message Syntax (CMS) 1546 Algorithms", RFC 3370, DOI 10.17487/RFC3370, August 2002, 1547 . 1549 [RFC3966] Schulzrinne, H., "The tel URI for Telephone Numbers", 1550 RFC 3966, DOI 10.17487/RFC3966, December 2004, 1551 . 1553 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 1554 Housley, R., and W. Polk, "Internet X.509 Public Key 1555 Infrastructure Certificate and Certificate Revocation List 1556 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, 1557 . 1559 [RFC6919] Barnes, R., Kent, S., and E. Rescorla, "Further Key Words 1560 for Use in RFCs to Indicate Requirement Levels", RFC 6919, 1561 DOI 10.17487/RFC6919, April 2013, 1562 . 1564 16.2. Informative References 1566 [I-D.ietf-stir-certificates] 1567 Peterson, J., "Secure Telephone Identity Credentials: 1568 Certificates", draft-ietf-stir-certificates-02 (work in 1569 progress), July 2015. 1571 [I-D.kaplan-stir-cider] 1572 Kaplan, H., "A proposal for Caller Identity in a DNS-based 1573 Entrusted Registry (CIDER)", draft-kaplan-stir-cider-00 1574 (work in progress), July 2013. 1576 [I-D.peterson-sipping-retarget] 1577 Peterson, J., "Retargeting and Security in SIP: A 1578 Framework and Requirements", draft-peterson-sipping- 1579 retarget-00 (work in progress), February 2005. 1581 [I-D.rosenberg-sip-rfc4474-concerns] 1582 Rosenberg, J., "Concerns around the Applicability of RFC 1583 4474", draft-rosenberg-sip-rfc4474-concerns-00 (work in 1584 progress), February 2008. 1586 [RFC2585] Housley, R. and P. Hoffman, "Internet X.509 Public Key 1587 Infrastructure Operational Protocols: FTP and HTTP", 1588 RFC 2585, DOI 10.17487/RFC2585, May 1999, 1589 . 1591 [RFC3323] Peterson, J., "A Privacy Mechanism for the Session 1592 Initiation Protocol (SIP)", RFC 3323, 1593 DOI 10.17487/RFC3323, November 2002, 1594 . 1596 [RFC3325] Jennings, C., Peterson, J., and M. Watson, "Private 1597 Extensions to the Session Initiation Protocol (SIP) for 1598 Asserted Identity within Trusted Networks", RFC 3325, 1599 DOI 10.17487/RFC3325, November 2002, 1600 . 1602 [RFC3548] Josefsson, S., Ed., "The Base16, Base32, and Base64 Data 1603 Encodings", RFC 3548, DOI 10.17487/RFC3548, July 2003, 1604 . 1606 [RFC3893] Peterson, J., "Session Initiation Protocol (SIP) 1607 Authenticated Identity Body (AIB) Format", RFC 3893, 1608 DOI 10.17487/RFC3893, September 2004, 1609 . 1611 [RFC4234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 1612 Specifications: ABNF", RFC 4234, DOI 10.17487/RFC4234, 1613 October 2005, . 1615 [RFC4474] Peterson, J. and C. Jennings, "Enhancements for 1616 Authenticated Identity Management in the Session 1617 Initiation Protocol (SIP)", RFC 4474, 1618 DOI 10.17487/RFC4474, August 2006, 1619 . 1621 [RFC4501] Josefsson, S., "Domain Name System Uniform Resource 1622 Identifiers", RFC 4501, DOI 10.17487/RFC4501, May 2006, 1623 . 1625 [RFC4916] Elwell, J., "Connected Identity in the Session Initiation 1626 Protocol (SIP)", RFC 4916, DOI 10.17487/RFC4916, June 1627 2007, . 1629 [RFC5763] Fischl, J., Tschofenig, H., and E. Rescorla, "Framework 1630 for Establishing a Secure Real-time Transport Protocol 1631 (SRTP) Security Context Using Datagram Transport Layer 1632 Security (DTLS)", RFC 5763, DOI 10.17487/RFC5763, May 1633 2010, . 1635 [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication 1636 of Named Entities (DANE) Transport Layer Security (TLS) 1637 Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August 1638 2012, . 1640 [RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., 1641 Morris, J., Hansen, M., and R. Smith, "Privacy 1642 Considerations for Internet Protocols", RFC 6973, 1643 DOI 10.17487/RFC6973, July 2013, 1644 . 1646 [RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data 1647 Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March 1648 2014, . 1650 [RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an 1651 Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May 1652 2014, . 1654 [RFC7340] Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure 1655 Telephone Identity Problem Statement and Requirements", 1656 RFC 7340, DOI 10.17487/RFC7340, September 2014, 1657 . 1659 [RFC7375] Peterson, J., "Secure Telephone Identity Threat Model", 1660 RFC 7375, DOI 10.17487/RFC7375, October 2014, 1661 . 1663 [RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token 1664 (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015, 1665 . 1667 Authors' Addresses 1668 Jon Peterson 1669 Neustar, Inc. 1670 1800 Sutter St Suite 570 1671 Concord, CA 94520 1672 US 1674 Email: jon.peterson@neustar.biz 1676 Cullen Jennings 1677 Cisco 1678 400 3rd Avenue SW, Suite 350 1679 Calgary, AB T2P 4H2 1680 Canada 1682 Email: fluffy@iii.ca 1684 Eric Rescorla 1685 RTFM, Inc. 1686 2064 Edgewood Drive 1687 Palo Alto, CA 94303 1688 USA 1690 Email: ekr@rtfm.com 1692 Chris Wendt 1693 Comcast 1694 One Comcast Center 1695 Philadelphia, PA 19103 1696 USA 1698 Email: chris-ietf@chriswendt.net