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'ATIS-0300050' == Outdated reference: draft-ietf-stir-passport has been published as RFC 8225 == Outdated reference: draft-ietf-stir-rfc4474bis has been published as RFC 8224 ** Obsolete normative reference: RFC 3447 (Obsoleted by RFC 8017) ** Downref: Normative reference to an Informational RFC: RFC 3647 ** Downref: Normative reference to an Informational RFC: RFC 5912 ** Downref: Normative reference to an Informational RFC: RFC 6711 ** Obsolete normative reference: RFC 6961 (Obsoleted by RFC 8446) ** Downref: Normative reference to an Informational RFC: RFC 7093 ** Downref: Normative reference to an Informational RFC: RFC 7340 ** Downref: Normative reference to an Informational RFC: RFC 7375 Summary: 9 errors (**), 0 flaws (~~), 4 warnings (==), 4 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 S. Turner 5 Expires: March 13, 2017 sn3rd 6 September 9, 2016 8 Secure Telephone Identity Credentials: Certificates 9 draft-ietf-stir-certificates-08.txt 11 Abstract 13 In order to prevent the impersonation of telephone numbers on the 14 Internet, some kind of credential system needs to exist that 15 cryptographically asserts authority over telephone numbers. This 16 document describes the use of certificates in establishing authority 17 over telephone numbers, as a component of a broader architecture for 18 managing telephone numbers as identities in protocols like SIP. 20 Status of This Memo 22 This Internet-Draft is submitted in full conformance with the 23 provisions of BCP 78 and BCP 79. 25 Internet-Drafts are working documents of the Internet Engineering 26 Task Force (IETF). Note that other groups may also distribute 27 working documents as Internet-Drafts. The list of current Internet- 28 Drafts is at http://datatracker.ietf.org/drafts/current/. 30 Internet-Drafts are draft documents valid for a maximum of six months 31 and may be updated, replaced, or obsoleted by other documents at any 32 time. It is inappropriate to use Internet-Drafts as reference 33 material or to cite them other than as "work in progress." 35 This Internet-Draft will expire on March 13, 2017. 37 Copyright Notice 39 Copyright (c) 2016 IETF Trust and the persons identified as the 40 document authors. All rights reserved. 42 This document is subject to BCP 78 and the IETF Trust's Legal 43 Provisions Relating to IETF Documents 44 (http://trustee.ietf.org/license-info) in effect on the date of 45 publication of this document. Please review these documents 46 carefully, as they describe your rights and restrictions with respect 47 to this document. Code Components extracted from this document must 48 include Simplified BSD License text as described in Section 4.e of 49 the Trust Legal Provisions and are provided without warranty as 50 described in the Simplified BSD License. 52 Table of Contents 54 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 55 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 56 3. Authority for Telephone Numbers in Certificates . . . . . . . 3 57 4. Certificate Usage with STIR . . . . . . . . . . . . . . . . . 5 58 5. Enrollment and Authorization using the TN Authorization List 6 59 5.1. Levels Of Assurance . . . . . . . . . . . . . . . . . . . 7 60 5.2. Certificate Extension Scope and Structure . . . . . . . . 8 61 6. Provisioning Private Keying Material . . . . . . . . . . . . 8 62 7. Acquiring Credentials to Verify Signatures . . . . . . . . . 9 63 8. TN Authorization List Syntax . . . . . . . . . . . . . . . . 10 64 9. Certificate Freshness and Revocation . . . . . . . . . . . . 12 65 9.1. Choosing a Verification Method . . . . . . . . . . . . . 12 66 9.2. Using OCSP with TN Auth List . . . . . . . . . . . . . . 13 67 9.2.1. OCSP Extension Specification . . . . . . . . . . . . 14 68 9.3. Acquiring TN Lists By Reference . . . . . . . . . . . . . 16 69 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 70 11. Security Considerations . . . . . . . . . . . . . . . . . . . 18 71 12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18 72 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 18 73 Appendix A. ASN.1 Module . . . . . . . . . . . . . . . . . . . . 21 74 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23 76 1. Introduction 78 The STIR problem statement [RFC7340] identifies the primary enabler 79 of robocalling, vishing, swatting and related attacks as the 80 capability to impersonate a calling party number. The starkest 81 examples of these attacks are cases where automated callees on the 82 PSTN rely on the calling number as a security measure, for example to 83 access a voicemail system. Robocallers use impersonation as a means 84 of obscuring identity; while robocallers can, in the ordinary PSTN, 85 block (that is, withhold) their caller identity, callees are less 86 likely to pick up calls from blocked identities, and therefore 87 appearing to calling from some number, any number, is preferable. 88 Robocallers however prefer not to call from a number that can trace 89 back to the robocaller, and therefore they impersonate numbers that 90 are not assigned to them. 92 One of the most important components of a system to prevent 93 impersonation is the implementation of credentials which identify the 94 parties who control telephone numbers. With these credentials, 95 parties can assert that they are in fact authorized to use telephony 96 numbers, and thus distinguish themselves from impersonators unable to 97 present such credentials. For that reason the STIR threat model 98 [RFC7375] stipulates, "The design of the credential system envisioned 99 as a solution to these threats must, for example, limit the scope of 100 the credentials issued to carriers or national authorities to those 101 numbers that fall under their purview." This document describes 102 credential systems for telephone numbers based on X.509 version 3 103 certificates in accordance with [RFC5280]. While telephone numbers 104 have long been part of the X.509 standard (X.509 supports arbitrary 105 naming attributes to be included in a certificate; the 106 telephoneNumber attribute was defined in the 1988 [X.520] 107 specification) this document provides ways to determine authority 108 more aligned with telephone network requirements, including extending 109 X.509 with a Telephone Number Authorization List certificate 110 extension which binds certificates to asserted authority for 111 particular telephone numbers, or potentially telephone number blocks 112 or ranges. 114 In the STIR in-band architecture specified in 115 [I-D.ietf-stir-rfc4474bis], two basic types of entities need access 116 to these credentials: authentication services, and verification 117 services (or verifiers). An authentication service must be operated 118 by an entity enrolled with the certification authority (CA, see 119 Section 5), whereas a verifier need only trust the trust anchor of 120 the authority, and have a means to access and validate the public 121 keys associated with these certificates. Although the guidance in 122 this document is written with the STIR in-band architecture in mind, 123 the credential system described in this document could be useful for 124 other protocols that want to make use of certificates to assert 125 authority over telephone numbers on the Internet. 127 This document specifies only the credential syntax and semantics 128 necessary to support this architecture. It does not assume any 129 particular CA or deployment environment. We anticipate that some 130 deployment experience will be necessary to determine optimal 131 operational models. 133 2. Terminology 135 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 136 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 137 "OPTIONAL" in this document are to be interpreted as described in RFC 138 2119 [RFC2119]. 140 3. Authority for Telephone Numbers in Certificates 142 At a high level, this specification details two non-exclusive 143 approaches that can be employed to determine authority over telephone 144 numbers with certificates. 146 The first approach is to leverage the existing subject of the 147 certificate to ascertain that the holder of the certificate is 148 authorized to claim authority over a telephone number. The subject 149 might be represented as a domain name in the SubjectAltName, such as 150 an "example.net" where that domain is known to relying parties as a 151 carrier, or represented with other identifiers related to the 152 operation of the telephone network including Service Provider 153 Identifiers (SPIDs) could serve as a subject as well. A relying 154 party could then employ an external data set or service that 155 determines whether or not a specific telephone number is under the 156 authority of the carrier identified as the subject of the 157 certificate, and use that to ascertain whether or not the carrier 158 should have authority over a telephone number. Potentially, a 159 certificate extension to convey the URI of such an information 160 service trusted by the issuer of the certificate could be developed 161 (though this specification does not propose one). Alternatively, 162 some relying parties could form bilateral or multilateral trust 163 relationships with peer carriers, trusting one another's assertions 164 just as telephone carriers in the SS7 network today rely on 165 transitive trust when displaying the calling party telephone number 166 received through SS7 signaling. 168 The second approach is to extend the syntax of certificates to 169 include a new attribute, defined here as TN Authorization List, which 170 contains a list of telephone numbers defining the scope of authority 171 of the certificate. Relying parties, if they trust the issuer of the 172 certificate as a source of authoritative information on telephone 173 numbers, could therefore use the TN Authorization List instead of the 174 subject of the certificate to make a decision about whether or not 175 the signer has authority over a particular telephone number. The TN 176 Authorization List could be provided in one of two ways: as a literal 177 value in the certificate, or as a network service that allows relying 178 parties to query in real time to determine that a telephone number is 179 in the scope of a certificate. Using the TN Authorization list 180 rather than the certificate subject makes sense when, for example, 181 for privacy reasons, the certificate owner would prefer not to be 182 identified, or in cases where the holder of the certificate does not 183 participate in the sort of traditional carrier infrastructure that 184 the first approach assumes. 186 The first approach requires little change to existing Public Key 187 Infrastructure (PKI) certificates; for the second approach, we must 188 define an appropriate enrollment and authorization process. For the 189 purposes of STIR, the over-the-wire format specified in 190 [I-D.ietf-stir-rfc4474bis] accommodates either of these approaches: 191 the methods for canonicalizing, signing, for identifying and 192 accessing the certificate and so on remain the same; it is only the 193 verifier behavior and authorization decision that will change 194 depending on the approach to telephone number authority taken by the 195 certificate. For that reason, the two approaches are not mutually 196 exclusive, and in fact a certificate issued to a traditional 197 telephone network service provider could contain a TN Authorization 198 List or not, were it supported by the CA issuing the credential. 199 Regardless of which approach is used, certificates that assert 200 authority over telephone numbers are subject to the ordinary 201 operational procedures that govern certificate use per [RFC5280]. 202 This means that verification services must be mindful of the need to 203 ensure that they trust the trust anchor that issued the certificate, 204 and that they have some means to determine the freshness of the 205 certificate (see Section 9). 207 4. Certificate Usage with STIR 209 [I-D.ietf-stir-rfc4474bis] Section 7.4 requires that all credential 210 systems used by STIR explain how they address the requirements 211 enumerated below. Certificates as described in this document address 212 the STIR requirements as follows: 214 1. The URI schemes permitted in the SIP Identity header "info" 215 parameter, as well as any special procedures required to 216 dereference the URIs. While normative text is given below in 217 Section 7, this mechanism permits the HTTP, CID and SIP URI 218 schemes to appear in the "info" parameter. 220 2. Procedures required to extract keying material from the resources 221 designated by the URI. Implementations perform no special 222 procedures beyond dereferencing the "info" URI. See Section 7. 224 3. Procedures used by the verification service to determine the 225 scope of the credential. This specification effectively proposes 226 two methods, as outlined in Section 3: one where the subject (or 227 more properly subjectAltName) of the certificate indicates the 228 scope of authority through a domain name, and relying parties 229 either trust the subject entirely or have some direct means of 230 determining whether or not a number falls under a subject's 231 authority; and another where an extension to the certificate as 232 described in Section 8 identifies the scope of authority of the 233 certificate. 235 4. The cryptographic algorithms required to validate the 236 credentials. For this specification, that means the signature 237 algorithms used to sign certificates. This specification 238 REQUIRES that implementations support both ECDSA with the P-256 239 curve (see [RFC4754]) and RSA PKCS#1 v1.5 (see [RFC3447] 240 Section 8.2) for certificate signatures. Implementers are 241 advised that RS256 is mandated only as a transitional mechanism, 242 due to its widespread use in existing PKI, but we anticipate that 243 this mechanism will eventually be deprecated. 245 5. Finally, note that all certificates compliant with this 246 specification: 248 * MUST provide cryptographic keying material sufficient to 249 generate the ECDSA using P-256 and SHA-256 signatures 250 necessary to support the ES256 hashed signatures required by 251 PASSporT [I-D.ietf-stir-passport], which in turn follows JSON 252 Web Token (JWT) [RFC7519]. 254 * MUST support both ECDSA with P-256 and RSA PKCS#1 v1.5 for 255 certificate signature verification. 257 This document also includes additional certificate-related 258 requirements: 260 o See Section 5.1 for requirements related to the certificate 261 policies extension. 263 o See Section 7 for requirements related to the TN Query certificate 264 extension. 266 o See Section 9.2 and Section 9.3 for requirements related to the 267 Authority Information Access (AIA) certificate extension. 269 o See Section 9.2.1 for requirements related to the authority key 270 identifier and subject key identifier certificate extensions. 272 5. Enrollment and Authorization using the TN Authorization List 274 This document covers three models for enrollment when using the TN 275 Authorization List extension. 277 The first enrollment model is one where the CA acts in concert with 278 national numbering authorities to issue credentials to those parties 279 to whom numbers are assigned. In the United States, for example, 280 telephone number blocks are assigned to Local Exchange Carriers 281 (LECs) by the North American Numbering Plan Administrator (NANPA), 282 who is in turn directed by the national regulator. LECs may also 283 receive numbers in smaller allocations, through number pooling, or 284 via an individual assignment through number portability. LECs assign 285 numbers to customers, who may be private individuals or organizations 286 - and organizations take responsibility for assigning numbers within 287 their own enterprise. This model requires top-down adoption of the 288 model from regulators through to carriers. Assignees of E.164 289 numbering resources participating in this enrollment model should 290 take appropriate steps to establish trust anchors. 292 The second enrollment model is a bottom-up approach where a CA 293 requires that an entity prove control by means of some sort of test, 294 which, as with certification authorities for web PKI, might either be 295 automated or a manual administrative process. As an example of an 296 automated process, an authority might send a text message to a 297 telephone number containing a URL (which might be dereferenced by the 298 recipient) as a means of verifying that a user has control of 299 terminal corresponding to that number. Checks of this form are 300 frequently used in commercial systems today to validate telephone 301 numbers provided by users. This is comparable to existing enrollment 302 systems used by some certificate authorities for issuing S/MIME 303 credentials for email by verifying that the party applying for a 304 credential receives mail at the email address in question. 306 The third enrollment model is delegation: that is, the holder of a 307 certificate (assigned by either of the two methods above) might 308 delegate some or all of their authority to another party. In some 309 cases, multiple levels of delegation could occur: a LEC, for example, 310 might delegate authority to a customer organization for a block of 311 100 numbers used by an IP PBX, and the organization might in turn 312 delegate authority for a particular number to an individual employee. 313 This is analogous to delegation of organizational identities in 314 traditional hierarchical PKIs who use the name constraints extension 315 [RFC5280]; the root CA delegates names in sales to the sales 316 department CA, names in development to the development CA, etc. As 317 lengthy certificate delegation chains are brittle, however, and can 318 cause delays in the verification process, this document considers 319 optimizations to reduce the complexity of verification. 321 Future work might explore methods of partial delegation, where 322 certificate holders delegate only part of their authority. For 323 example, individual assignees may want to delegate to a service 324 authority for text messages associated with their telephone number, 325 but not for other functions. 327 5.1. Levels Of Assurance 329 This specification supports different level of assurance (LOA). The 330 LOA indications, which are Object Identifiers (OIDs) included in the 331 certificate's certificate policies extension [RFC5280], allow CAs to 332 differentiate those enrolled from proof-of-possession versus 333 delegation. A Certification Policy and a Certification Practice 334 Statement [RFC3647] are produced as part of the normal PKI 335 bootstrapping process (i.e., the CP is written first and then the CAs 336 say how they conform to the CP in the CPS). OIDs are used to 337 reference the CP and if the CA wishes it can also include a reference 338 to the CPS with the certificate policies extension. CAs that wish to 339 express different LOAs MUST include the certificate policies 340 extension in issued certificates. See Section 11 for additional 341 information about the LOA registry. 343 5.2. Certificate Extension Scope and Structure 345 This specification places no limits on the number of telephone 346 numbers that can be associated with any given certificate. Some 347 service providers may be assigned millions of numbers, and may wish 348 to have a single certificate that can be applied to signing for any 349 one of those numbers. Others may wish to compartmentalize authority 350 over subsets of the numbers they control. 352 Moreover, service providers may wish to have multiple certificates 353 with the same scope of authority. For example, a service provider 354 with several regional gateway systems may want each system to be 355 capable of signing for each of their numbers, but not want to have 356 each system share the same private key. 358 The set of telephone numbers for which a particular certificate is 359 valid is expressed in the certificate through a certificate 360 extension; the certificate's extensibility mechanism is defined in 361 [RFC5280] but the TN Authorization List extension is specified in 362 this document. 364 The subjects of certificates containing the TN Authorization List 365 extension are typically the administrative entities to whom numbers 366 are assigned or delegated. For example, a LEC might hold a 367 certificate for a range of telephone numbers. In some cases, the 368 organization or individual issued such a certificate may not want to 369 associate themselves with a certificate; for example, a private 370 individual with a certificate for a single telephone number might not 371 want to distribute that certificate publicly if every verifier 372 immediately knew their name. The certification authorities issuing 373 certificates with the TN Authorization List extensions may, in 374 accordance with their policies, obscure the identity of the subject, 375 though mechanisms for doing so are outside the scope of this 376 document. 378 6. Provisioning Private Keying Material 380 In order for authentication services to sign calls via the procedures 381 described in [I-D.ietf-stir-rfc4474bis], they must hold a private key 382 corresponding to a certificate with authority over the calling 383 number. [I-D.ietf-stir-rfc4474bis] does not require that any 384 particular entity in a SIP deployment architecture sign requests, 385 only that it be an entity with an appropriate private key; the 386 authentication service role may be instantiated by any entity in a 387 SIP network. For a certificate granting authority only over a 388 particular number which has been issued to an end user, for example, 389 an end user device might hold the private key and generate the 390 signature. In the case of a service provider with authority over 391 large blocks of numbers, an intermediary might hold the private key 392 and sign calls. 394 The specification recommends distribution of private keys through 395 PKCS#8 objects signed by a trusted entity, for example through the 396 CMS package specified in [RFC5958]. 398 7. Acquiring Credentials to Verify Signatures 400 This specification documents multiple ways that a verifier can gain 401 access to the credentials needed to verify a request. As the 402 validity of certificates does not depend on the method of their 403 acquisition, there is no need to standardize any single mechanism for 404 this purpose. All entities that comply with 405 [I-D.ietf-stir-rfc4474bis] necessarily support SIP, and consequently 406 SIP itself can serve as a way to deliver certificates. 407 [I-D.ietf-stir-rfc4474bis] provides an "info" parameter of the 408 Identity header which contains a URI for the credential used to 409 generate the Identity header; [I-D.ietf-stir-rfc4474bis] also 410 requires documents which define credential systems list the URI 411 schemes that may be present in the "info" parameter. For 412 implementations compliant with this specification, three URI schemes 413 are REQUIRED: the CID URI, the SIP URI, and the HTTP URI. 415 The simplest way for a verifier to acquire the certificate needed to 416 verify a signature is for the certificate be conveyed in a SIP 417 request along with the signature itself. In SIP, for example, a 418 certificate could be carried in a multipart MIME body [RFC2046], and 419 the URI in the Identity header "info" parameter could specify that 420 body with a CID URI [RFC2392]. However, in many environments this is 421 not feasible due to message size restrictions or lack of necessary 422 support for multipart MIME. 424 The Identity header "info" parameter in a SIP request may contain a 425 URI that the verifier dereferences. Implementations of this 426 specification are required to support the use of SIP for this 427 function (via the SUBSCRIBE/NOTIFY mechanism), as well as HTTP, via 428 the Enrollment over Secure Transport mechanisms described in RFC 7030 429 [RFC7030]. 431 Note well that as an optimization, a verifier may have access to a 432 service, a cache or other local store that grants access to 433 certificates for a particular telephone number. However, there may 434 be multiple valid certificates that can sign a call setup request for 435 a telephone number, and as a consequence, there needs to be some 436 discriminator that the signer uses to identify their credentials. 437 The Identity header "info" parameter itself can serve as such a 438 discriminator, provided implementations use that parameter as a key 439 when accessing certificates from caches or other sources. 441 8. TN Authorization List Syntax 443 The subjects of certificates containing the TN Authorization List 444 extension are the administrative entities to whom numbers are 445 assigned or delegated. When a verifier is validating a caller's 446 identity, local policy always determines the circumstances under 447 which any particular subject may be trusted, but the purpose of the 448 TN Authorization List extension in particular is to allow a verifier 449 to ascertain when the CA has designated that the subject has 450 authority over a particular telephone number or number range. The 451 Telephony Number (TN) Authorization List certificate extension is 452 included in the Certificate's extension field [RFC5280]. The 453 extension is defined with ASN.1, defined in [X.680] through [X.683]. 454 What follows is the syntax and semantics of the extension. 456 The Telephony Number (TN) Authorization List certificate extension is 457 identified by the following object identifier (OID), which is defined 458 under the id-ce OID arc defined in [RFC5280] and managed by IANA (see 459 Section 10): 461 id-ce-TNAuthList OBJECT IDENTIFIER ::= { id-ce TBD } 463 The TN Authorization List certificate extension has the following 464 syntax: 466 TNAuthorizationList ::= SEQUENCE SIZE (1..MAX) OF TNAuthorization 468 TNAuthorization ::= SEQUENCE SIZE (1..MAX) OF TNEntry 470 TNEntry ::= CHOICE { 471 spid [0] ServiceProviderIdentifierList, 472 range [1] TelephoneNumberRange, 473 one E164Number } 475 ServiceProviderIdentifierList ::= SEQUENCE SIZE (1..3) OF 476 OCTET STRING 478 -- When all three are present: SPID, Alt SPID, and Last Alt SPID 480 TelephoneNumberRange ::= SEQUENCE { 481 start E164Number, 482 count INTEGER } 484 E164Number ::= IA5String (SIZE (1..15)) (FROM ("0123456789")) 486 The TN Authorization List certificate extension indicates the 487 authorized phone numbers for the call setup signer. It indicates one 488 or more blocks of telephone number entries that have been authorized 489 for use by the call setup signer. There are three ways to identify 490 the block: 492 1. A Service Provider Identifier (SPID, also known as an Operating 493 Company Number (OCN) or Carrier Identification Code (CIC), as 494 specified in [ATIS-0300050]) can be used to indirectly name all 495 of the telephone numbers associated with that service provider, 497 2. Telephone numbers can be listed in a range (in the 498 TelephoneNumberRange format), or 500 3. A single telephone number can be listed (as an E164Number). 502 Note that because large-scale service providers may want to associate 503 many numbers, possibly millions of numbers, with a particular 504 certificate, optimizations are required for those cases to prevent 505 certificate size from becoming unmanageable. In these cases, the TN 506 Authorization List may be given by reference rather than by value, 507 through the presence of a separate certificate extension that permits 508 verifiers to either securely download the list of numbers associated 509 with a certificate, or to verify that a single number is under the 510 authority of this certificate. This optimization is left for future 511 work. 513 9. Certificate Freshness and Revocation 515 Regardless of which of the approaches in Section 3 is followed for 516 using certificates, a certificate verification mechanism is required. 517 However, the traditional problem of certificate freshness gains a new 518 wrinkle when using the TN Authorization List extension, because 519 verifiers must establish not only that a certificate remains valid, 520 but also that the certificate's scope contains the telephone number 521 that the verifier is validating. Dynamic changes to number 522 assignments can occur due to number portability, for example. So 523 even if a verifier has a valid cached certificate for a telephone 524 number (or a range containing the number), the verifier must 525 determine that the entity that signed is still a proper authority for 526 that number. 528 To verify the status of the certificate, the verifier needs to 529 acquire the certificate if necessary (via the methods described in 530 Section 7), and then would need to either: 532 (a) Rely on short-lived certificates and not check the certificate's 533 status, or 535 (b) Rely on status information from the authority (e.g. OCSP, see 536 Section 9.2) 538 The tradeoff between short lived certificates and using status 539 information is that the former's burden is on the front end (i.e., 540 enrollment) and the latter's burden is on the back end (i.e., 541 verification). Both impact call setup time, but it is assumed that 542 generating a short-lived certificate for each all, and consequently 543 performing enrollment for each call, is more of an impact than 544 acquiring status information. This document therefore recommends 545 relying on status information. 547 9.1. Choosing a Verification Method 549 There are three common certificate verification mechanisms employed 550 by CAs: 552 1. Certificate Revocation Lists (CRLs) [RFC5280] 554 2. Online Certificate Status Protocol (OCSP) [RFC6960], and 556 3. Server-based Certificate Validation Protocol (SCVP) [RFC5055]. 558 When relying on status information, the verifier needs to obtain the 559 status information - but before that can happen, the verifier needs 560 to know where to locate it. Placing the location of the status 561 information in the certificate makes the certificate larger, but it 562 eases the client workload. The CRL Distribution Point certificate 563 extension includes the location of the CRL and the Authority 564 Information Access certificate extension includes the location of 565 OCSP and/or SCVP servers; both of these extensions are defined in 566 [RFC5280]. In all cases, the status information location is provided 567 in the form of an URI. 569 CRLs are an obviously attractive solution because they are supported 570 by every CA. CRLs have a reputation of being quite large (10s of 571 MBytes), because CAs maintain and issue one monolithic CRL with all 572 of their revoked certificates, but CRLs do support a variety of 573 mechanisms to scope the size of the CRLs based on revocation reasons 574 (e.g., key compromise vs CA compromise), user certificates only, and 575 CA certificates only as well as just operationally deciding to keep 576 the CRLs small. However, scoping the CRL introduces other issues 577 (i.e., does the RP have all of the CRL partitions). 579 CAs in the STIR architecture will likely all create CRLs for audit 580 purposes, but it NOT RECOMMENDED that they be relied upon for status 581 information. Instead, one of the two "online" options is preferred. 582 Between the two, OCSP is much more widely deployed and this document 583 therefore recommends the use of OCSP in high-volume environments 584 (HVE) for validating the freshness of certificates, based on 585 [RFC6960], incorporating some (but not all) of the optimizations of 586 [RFC5019]. CRLs MUST be signed with the same algorithm as the 587 certificate. 589 9.2. Using OCSP with TN Auth List 591 Certificates compliant with this specification therefore SHOULD 592 include a URL pointing to an OCSP service in the Authority 593 Information Access (AIA) certificate extension, via the "id-ad-ocsp" 594 accessMethod specified in [RFC5280]. It is RECOMMENDED that entities 595 that issue certificates with the Telephone Number Authorization List 596 certificate extension run an OCSP server for this purpose. Baseline 597 OCSP however supports only three possible response values: good, 598 revoked, or unknown. Without some extension, OCSP would not indicate 599 whether the certificate is authorized for a particular telephone 600 number that the verifier is validating. 602 At a high level, there are two ways that a client might pose this 603 authorization question: 605 For this certificate, is the following number currently in its 606 scope of validity? 607 What are all the telephone numbers associated with this 608 certificate, or this certificate subject? 610 Only the former lends itself to piggybacking on the OCSP status 611 mechanism; since the verifier is already asking an authority about 612 the certificate's status, why not reuse that mechanism, instead of 613 creating a new service that requires additional round trips? Like 614 most PKIX-developed protocols, OCSP is extensible; OCSP supports 615 request extensions (including sending multiple requests at once) and 616 per-request extensions. It seems unlikely that the verifier will be 617 requesting authorization checks on multiple telephone numbers in one 618 request, so a per-request extension is what is needed. 620 The requirement to consult OCSP in real time results in a network 621 round-trip time of day, which is something to consider because it 622 will add to the call setup time. OCSP server implementations 623 commonly pre-generate responses, and to speed up HTTPS connections, 624 servers often provide OCSP responses for each certificate in their 625 hierarchy. If possible, both of these OCSP concepts should be 626 adopted for use with STIR. 628 9.2.1. OCSP Extension Specification 630 The extension mechanism for OCSP follows X.509 v3 certificate 631 extensions, and thus requires an OID, a criticality flag, and ASN.1 632 syntax as defined by the OID. The criticality specified here is 633 optional: per [RFC6960] Section 4.4, support for all OCSP extensions 634 is optional. If the OCSP server does not understand the requested 635 extension, it will still provide the baseline validation of the 636 certificate itself. Moreover, in practical STIR deployments, the 637 issuer of the certificate will set the accessLocation for the OCSP 638 AIA extension to point to an OCSP service that supports this 639 extension, so the risk of interoperability failure due to lack of 640 support for this extension is minimal. 642 The OCSP TNQuery extension is included as one of the request's 643 singleRequestExtensions. It may also appear in the response's 644 singleExtensions. When an OCSP server includes a number in the 645 response's singleExtensions, this informs the client that the 646 certificate is still valid for the number that appears in the TNQuery 647 extension field. If the TNQuery is absent from a response to a query 648 containing a TNQuery in its singleRequestExtension, then the server 649 is not able to validate that the number is still in the scope of 650 authority of the certificate. 652 id-pkix-ocsp-stir-tn OBJECT IDENTIFIER ::= { id-pkix-ocsp TBD } 654 TNQuery ::= E164Number 656 The HVE OCSP profile [RFC5019] prohibits the use of per-request 657 extensions. As it is anticipated that STIR will use OCSP in a high- 658 volume environment, many of the optimizations recommended by HVE are 659 desirable for the STIR environment. This document therefore uses the 660 HVE optimizations augmented as follows: 662 o Implementations MUST use SHA-256 as the hashing algorithm for the 663 CertID.issuerNameHash and the CertID.issuerKeyHash values. That 664 is CertID.hashAlgorithm is id-sha256 [RFC4055] and the values are 665 truncated to 160-bits as specified Option 1 in Section 2 of 666 [RFC7093]. 668 o Clients MUST include the OCSP TNQuery extension in requests' 669 singleRequestExtensions. 671 o Servers MUST include the OCSP TNQuery extension in responses' 672 singleExtensions. 674 o Servers SHOULD return responses that would otherwise have been 675 "unknown" as "not good" (i.e., return only "good" and "not good" 676 responses). 678 o Clients MUST treat returned "unknown" responses as "not good". 680 o If the server uses ResponderID, it MUST generate the KeyHash using 681 SHA-256 and truncate the value to 160-bits as specified in Option 682 1 in Section 2 of [RFC7093]. 684 o Implementations MUST support ECDSA using P-256 and SHA-256. Note 685 that [RFC6960] requires RSA with SHA-256 be supported. 687 o There is no requirement to support SHA-1, RSA with SHA-1, or DSA 688 with SHA-1. 690 OCSP responses MUST be signed using the same algorithm as the 691 certificate being checked. 693 To facilitate matching the authority key identifier values found in 694 CA certificates with the KeyHash used in the OCSP response, 695 certificates compliant with this specification MUST generate 696 authority key identifiers and subject key identifiers using the 697 SHA-256 and truncate the value to 160-bits as specified in Option 1 698 in Section 2 of [RFC7093]. 700 Ideally, once a certificate has been acquired by a verifier, some 701 sort of asynchronous mechanism could notify and update the verifier 702 if the scope of the certificate changes so that verifiers could 703 implement a cache. While not all possible categories of verifiers 704 could implement such behavior, some sort of event-driven notification 705 of certificate status is another potential subject of future work. 706 One potential direction is that a future SIP SUBSCRIBE/NOTIFY-based 707 accessMethod for AIA might be defined (which would also be applicable 708 to the method described in the following section) by some future 709 specification. 711 Strategies for stapling OCSP [RFC6961] have become common in some web 712 PKI environments as an optimization which allows web servers to send 713 up-to-date certificate status information acquired from OCSP to 714 clients as TLS is negotiated. A similar mechanism could be 715 implemented for SIP requests, in which the authentication service 716 adds status information for its certificate to the SIP request, which 717 would save the verifier the trouble of performing the OCSP dip 718 itself. Especially for high-volume authentication and verification 719 services, this could result in significant performance improvements. 720 This is left as an optimization for future work. 722 9.3. Acquiring TN Lists By Reference 724 Acquiring a list of the telephone numbers associated with a 725 certificate or its subject lends itself to an application-layer 726 query/response interaction outside of OCSP, one which could be 727 initiated through a separate URI included in the certificate. The 728 AIA extension (see [RFC5280]) supports such a mechanism: it 729 designates an OID to identify the accessMethod and an accessLocation, 730 which would most likely be a URI. A verifier would then follow the 731 URI to ascertain whether the list of TNs are authorized for use by 732 the caller. 734 HTTPS is the most obvious candidate for a protocol to be used for 735 fetching the list of telephone numbers associated with a particular 736 certificate. This document defines a new AIA accessMethod, called 737 "id-ad-stir-tn", which uses the following AIA OID: 739 id-ad-stir-tn OBJECT IDENTIFIER ::= { id-ad TBD } 741 When the "id-ad-stir-tn" accessMethod is used, the accessLocation 742 MUST be an HTTPS URI. The document returned by dereferencing that 743 URI will contain the complete TN Authorization List (see Section 8) 744 for the certificate. 746 Delivering the entire list of telephone numbers associated with a 747 particular certificate will divulge to STIR verifiers information 748 about telephone numbers other than the one associated with the 749 particular call that the verifier is checking. In some environments, 750 where STIR verifiers handle a high volume of calls, maintaining an 751 up-to-date and complete cache for the numbers associated with crucial 752 certificate holders could give an important boost to performance. 754 10. IANA Considerations 756 This document makes use of object identifiers for the TN Certificate 757 Extension defined in Section 8, TN-HVE OCSP extension in 758 Section 9.2.1, the TN by reference AIA access descriptor defined in 759 Section 9.3, and the ASN.1 module identifier defined in Appendix A. 760 It therefore requests that the IANA make the following assignments: 762 o TN Certificate Extension in the SMI Security for PKIX Certificate 763 Extension registry: http://www.iana.org/assignments/smi-numbers/ 764 smi-numbers.xhtml#smi-numbers-1.3.6.1.5.5.7.1 766 o TN-HVE OCSP extension in the SMI Security for PKIX Online 767 Certificate Status Protocol (OCSP) registry: 768 http://www.iana.org/assignments/smi-numbers/smi-numbers.xhtml#smi- 769 numbers-1.3.6.1.5.5.7.48.1 771 o TNS by reference access descriptor in the SMI Security for PKIX 772 Access Descriptor registry: http://www.iana.org/assignments/smi- 773 numbers/smi-numbers.xhtml#smi-numbers-1.3.6.1.5.5.7.48 775 o The TN ASN.1 module in SMI Security for PKIX Module Identifier 776 registry: http://www.iana.org/assignments/smi-numbers/smi- 777 numbers.xhtml#smi-numbers-1.3.6.1.5.5.7.0 779 This document also makes use of the Level of Assurance (LoA) Profiles 780 registry defined in [RFC6711] because as is stated in RFC 6711: "Use 781 of the registry by protocols other than SAML is encouraged." IANA is 782 requested to creae the STIR Levels of Assurance (LOA) sub-registry in 783 the Level of Assurance (LoA) Profile registry. Instead of 784 registering a SAML Context Class, the Certificate Policy's Object 785 Identifier representing the LOA is included in the registry. An 786 example registration is as follows: 788 To: loa-profiles-experts@icann.org 790 From: jrandom@example.com 792 1. Name of requester: J. Random User 794 2. Email address of requester: jrandom@example.com 796 3. Organization of requester: Example Trust Frameworks LLP 798 4. Requested registration: 800 URI http://foo.example.com/assurance/loa-1 802 Name foo-loa-1 804 Informational URL https://foo.example.com/assurance/ 806 Certificate Policy Object Identifier: 0.0.0.0 808 NOTE: Do not register this example. The OID is purposely invalid. 810 Experts are expected to ensure the reference CP includes the OID 811 being registered. 813 11. Security Considerations 815 This document is entirely about security. For further information on 816 certificate security and practices, see [RFC5280], in particular its 817 Security Considerations. For OCSP-related security considerations 818 see [RFC6960] and [RFC5019] 820 12. Acknowledgments 822 Russ Housley, Brian Rosen, Cullen Jennings, Dave Crocker, Tony 823 Rutkowski, John Braunberger, and Eric Rescorla provided key input to 824 the discussions leading to this document. 826 13. References 828 [ATIS-0300050] 829 ATIS Recommendation 0300050, "Carrier Identification Code 830 (CIC) Assignment Guidelines", 2012. 832 [I-D.ietf-stir-passport] 833 Wendt, C. and J. Peterson, "Persona Assertion Token", 834 draft-ietf-stir-passport-07 (work in progress), September 835 2016. 837 [I-D.ietf-stir-rfc4474bis] 838 Peterson, J., Jennings, C., Rescorla, E., and C. Wendt, 839 "Authenticated Identity Management in the Session 840 Initiation Protocol (SIP)", draft-ietf-stir-rfc4474bis-11 841 (work in progress), August 2016. 843 [RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 844 Extensions (MIME) Part Two: Media Types", RFC 2046, 845 DOI 10.17487/RFC2046, November 1996, 846 . 848 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 849 Requirement Levels", BCP 14, RFC 2119, 850 DOI 10.17487/RFC2119, March 1997, 851 . 853 [RFC2392] Levinson, E., "Content-ID and Message-ID Uniform Resource 854 Locators", RFC 2392, DOI 10.17487/RFC2392, August 1998, 855 . 857 [RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography 858 Standards (PKCS) #1: RSA Cryptography Specifications 859 Version 2.1", RFC 3447, DOI 10.17487/RFC3447, February 860 2003, . 862 [RFC3647] Chokhani, S., Ford, W., Sabett, R., Merrill, C., and S. 863 Wu, "Internet X.509 Public Key Infrastructure Certificate 864 Policy and Certification Practices Framework", RFC 3647, 865 DOI 10.17487/RFC3647, November 2003, 866 . 868 [RFC4055] Schaad, J., Kaliski, B., and R. Housley, "Additional 869 Algorithms and Identifiers for RSA Cryptography for use in 870 the Internet X.509 Public Key Infrastructure Certificate 871 and Certificate Revocation List (CRL) Profile", RFC 4055, 872 DOI 10.17487/RFC4055, June 2005, 873 . 875 [RFC4754] Fu, D. and J. Solinas, "IKE and IKEv2 Authentication Using 876 the Elliptic Curve Digital Signature Algorithm (ECDSA)", 877 RFC 4754, DOI 10.17487/RFC4754, January 2007, 878 . 880 [RFC5019] Deacon, A. and R. Hurst, "The Lightweight Online 881 Certificate Status Protocol (OCSP) Profile for High-Volume 882 Environments", RFC 5019, DOI 10.17487/RFC5019, September 883 2007, . 885 [RFC5055] Freeman, T., Housley, R., Malpani, A., Cooper, D., and W. 886 Polk, "Server-Based Certificate Validation Protocol 887 (SCVP)", RFC 5055, DOI 10.17487/RFC5055, December 2007, 888 . 890 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 891 Housley, R., and W. Polk, "Internet X.509 Public Key 892 Infrastructure Certificate and Certificate Revocation List 893 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, 894 . 896 [RFC5912] Hoffman, P. and J. Schaad, "New ASN.1 Modules for the 897 Public Key Infrastructure Using X.509 (PKIX)", RFC 5912, 898 DOI 10.17487/RFC5912, June 2010, 899 . 901 [RFC5958] Turner, S., "Asymmetric Key Packages", RFC 5958, 902 DOI 10.17487/RFC5958, August 2010, 903 . 905 [RFC6711] Johansson, L., "An IANA Registry for Level of Assurance 906 (LoA) Profiles", RFC 6711, DOI 10.17487/RFC6711, August 907 2012, . 909 [RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A., 910 Galperin, S., and C. Adams, "X.509 Internet Public Key 911 Infrastructure Online Certificate Status Protocol - OCSP", 912 RFC 6960, DOI 10.17487/RFC6960, June 2013, 913 . 915 [RFC6961] Pettersen, Y., "The Transport Layer Security (TLS) 916 Multiple Certificate Status Request Extension", RFC 6961, 917 DOI 10.17487/RFC6961, June 2013, 918 . 920 [RFC7030] Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed., 921 "Enrollment over Secure Transport", RFC 7030, 922 DOI 10.17487/RFC7030, October 2013, 923 . 925 [RFC7093] Turner, S., Kent, S., and J. Manger, "Additional Methods 926 for Generating Key Identifiers Values", RFC 7093, 927 DOI 10.17487/RFC7093, December 2013, 928 . 930 [RFC7340] Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure 931 Telephone Identity Problem Statement and Requirements", 932 RFC 7340, DOI 10.17487/RFC7340, September 2014, 933 . 935 [RFC7375] Peterson, J., "Secure Telephone Identity Threat Model", 936 RFC 7375, DOI 10.17487/RFC7375, October 2014, 937 . 939 [RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token 940 (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015, 941 . 943 [X.509] ITU-T Recommendation X.520 (10/2012) | ISO/IEC 9594-8, 944 "Information technology - Open Systems Interconnection - 945 The Directory: Public-key and attribute certificate 946 frameworks", 2012. 948 [X.520] ITU-T Recommendation X.520 (10/2012) | ISO/IEC 9594-6, 949 "Information technology - Open Systems Interconnection - 950 The Directory: Selected Attribute Types", 2012. 952 [X.680] ITU-T Recommendation X.680 (08/2015) | ISO/IEC 8824-1, 953 "Information Technology - Abstract Syntax Notation One: 954 Specification of basic notation". 956 [X.681] ITU-T Recommendation X.681 (08/2015) | ISO/IEC 8824-2, 957 "Information Technology - Abstract Syntax Notation One: 958 Information Object Specification". 960 [X.682] ITU-T Recommendation X.682 (08/2015) | ISO/IEC 8824-2, 961 "Information Technology - Abstract Syntax Notation One: 962 Constraint Specification". 964 [X.683] ITU-T Recommendation X.683 (08/2015) | ISO/IEC 8824-3, 965 "Information Technology - Abstract Syntax Notation One: 966 Parameterization of ASN.1 Specifications". 968 Appendix A. ASN.1 Module 970 This appendix provides the normative ASN.1 [X.680] definitions for 971 the structures described in this specification using ASN.1, as 972 defined in [X.680] through [X.683]. 974 The modules defined in this document are compatible with the most 975 current ASN.1 specification published in 2015 (see [X.680], [X.681], 976 [X.682], [X.683]). None of the newly defined tokens in the 2008 977 ASN.1 (DATE, DATE-TIME, DURATION, NOT-A-NUMBER, OID-IRI, RELATIVE- 978 OID-IRI, TIME, TIME-OF-DAY)) are currently used in any of the ASN.1 979 specifications referred to here. 981 This ASN.1 module imports ASN.1 from [RFC5912]. 983 TN-Module { 984 iso(1) identified-organization(3) dod(6) internet(1) 985 security(5) mechanisms(5) pkix(7) id-mod(0) 986 id-mod-tn-module(TBD) } 988 DEFINITIONS EXPLICIT TAGS ::= BEGIN 989 IMPORTS 990 id-ad, id-ad-ocsp -- From [RFC5912] 991 FROM PKIX1Explicit-2009 { 992 iso(1) identified-organization(3) dod(6) internet(1) security(5) 993 mechanisms(5) pkix(7) id-mod(0) id-mod-pkix1-explicit-02(51) } 995 id-ce -- From [RFC5912] 996 FROM PKIX1Implicit-2009 { 997 iso(1) identified-organization(3) dod(6) internet(1) security(5) 998 mechanisms(5) pkix(7) id-mod(0) id-mod-pkix1-implicit-02(59) } 1000 EXTENSION -- From [RFC5912] 1001 FROM PKIX-CommonTypes-2009 { 1002 iso(1) identified-organization(3) dod(6) internet(1) 1003 security(5) mechanisms(5) pkix(7) id-mod(0) 1004 id-mod-pkixCommon-02(57) } 1006 ; 1008 -- TN Entry Certificate Extension 1010 ext-tnAuthList EXTENSION ::= { 1011 SYNTAX TNAuthorizationList IDENTIFIED BY id-ce-TNAuthList } 1013 TNAuthorizationList ::= SEQUENCE SIZE (1..MAX) OF TNAuthorization 1015 TNAuthorization ::= SEQUENCE SIZE (1..MAX) OF TNEntry 1017 TNEntry ::= CHOICE { 1018 spid [0] ServiceProviderIdentifierList, 1019 range [1] TelephoneNumberRange, 1020 one E164Number } 1022 ServiceProviderIdentifierList ::= SEQUENCE SIZE (1..3) OF 1023 OCTET STRING 1025 -- When all three are present: SPID, Alt SPID, and Last Alt SPID 1027 TelephoneNumberRange ::= SEQUENCE { 1028 start E164Number, 1029 count INTEGER } 1031 E164Number ::= IA5String (SIZE (1..15)) (FROM ("0123456789")) 1033 -- TN OCSP Extension 1035 re-ocsp-tn-query EXTENSION ::= { 1036 SYNTAX TNQuery IDENTIFIED BY id-pkix-ocsp-stir-tn } 1038 TNQuery ::= E164Number 1040 -- TN Access Descriptor 1042 id-ad-stir-tn OBJECT IDENTIFIER ::= { id-ad TBD } 1044 -- 1045 -- Object Identifiers 1046 -- 1048 id-pkix-ocsp OBJECT IDENTIFIER ::= id-ad-ocsp 1049 id-ce-TNAuthList OBJECT IDENTIFIER ::= { id-ce TBD } 1050 id-pkix-ocsp-stir-tn OBJECT IDENTIFIER ::= { id-pkix-ocsp TBD } 1052 END 1054 Authors' Addresses 1056 Jon Peterson 1057 Neustar, Inc. 1059 Email: jon.peterson@neustar.biz 1061 Sean Turner 1062 sn3rd 1064 Email: sean@sn3rd.com