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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: August 18, 2014 IECA 6 February 14, 2014 8 Secure Telephone Identity Credentials: Certificates 9 draft-peterson-stir-certificates-00.txt 11 Abstract 13 In order to provide a means of proving ownership of telephone numbers 14 on the Internet, some kind of public structure needs to exist that 15 binds cryptographic keys to authority over telephone numbers. This 16 document describes a certificate-based credential system for 17 telephone numbers, which could be used as a part of a broader 18 architecture for managing telephone numbers as identities in 19 protocols like SIP. 21 Status of This Memo 23 This Internet-Draft is submitted in full conformance with the 24 provisions of BCP 78 and BCP 79. 26 Internet-Drafts are working documents of the Internet Engineering 27 Task Force (IETF). Note that other groups may also distribute 28 working documents as Internet-Drafts. The list of current Internet- 29 Drafts is at http://datatracker.ietf.org/drafts/current/. 31 Internet-Drafts are draft documents valid for a maximum of six months 32 and may be updated, replaced, or obsoleted by other documents at any 33 time. It is inappropriate to use Internet-Drafts as reference 34 material or to cite them other than as "work in progress." 36 This Internet-Draft will expire on August 18, 2014. 38 Copyright Notice 40 Copyright (c) 2014 IETF Trust and the persons identified as the 41 document authors. All rights reserved. 43 This document is subject to BCP 78 and the IETF Trust's Legal 44 Provisions Relating to IETF Documents 45 (http://trustee.ietf.org/license-info) in effect on the date of 46 publication of this document. Please review these documents 47 carefully, as they describe your rights and restrictions with respect 48 to this document. Code Components extracted from this document must 49 include Simplified BSD License text as described in Section 4.e of 50 the Trust Legal Provisions and are provided without warranty as 51 described in the Simplified BSD License. 53 Table of Contents 55 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 56 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 57 3. Enrollment and Authorization . . . . . . . . . . . . . . . . 3 58 3.1. Certificate Scope and Structure . . . . . . . . . . . . . 4 59 3.2. Provisioning Private Keying Material . . . . . . . . . . 5 60 4. Acquiring Credentials to Verify Signatures . . . . . . . . . 5 61 4.1. Verifying Certificate Scope . . . . . . . . . . . . . . . 6 62 4.2. Certificate Freshness and Revocation . . . . . . . . . . 8 63 5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8 64 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 65 7. Security Considerations . . . . . . . . . . . . . . . . . . . 8 66 8. Informative References . . . . . . . . . . . . . . . . . . . 8 67 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 69 1. Introduction 71 As is discussed in the STIR problem statement [13], the primary 72 enabler of robocalling, vishing, swatting and related attacks is the 73 capability to impersonate a calling party number. The starkest 74 examples of these attacks are cases where automated callees on the 75 PSTN rely on the calling number as a security measure, for example to 76 access a voicemail system. Robocallers use impersonation as a means 77 of obscuring identity; while robocallers can, in the ordinary PSTN, 78 block (that is, withhold) their caller identity, callees are less 79 likely to pick up calls from blocked identities, and therefore 80 appearing to calling from some number, any number, is preferable. 81 Robocallers however prefer not to call from a number that can trace 82 back to the robocaller, and therefore they impersonate numbers that 83 are not assigned to them. 85 One of the most important components of a system to prevent 86 impersonation is an authority responsible for issuing credentials to 87 parties who control telephone numbers. With these credentials, 88 parties can prove that they are in fact authorized to use telephony 89 numbers, and thus distinguish themselves from impersonators unable to 90 present credentials. This document describes a credential system for 91 telephone numbers based on X.509 version 3 certificates in accordance 92 with [7]. While telephone numbers have long been a part of the X.509 93 standard, the certificates described in this document may contain 94 telephone number blocks or ranges, and accordingly it uses an 95 alternate syntax. 97 In the STIR in-band architecture, two basic types of entities need 98 access to these credentials: authentication services, and 99 verification services (or verifiers); see [15]. An authentication 100 service must be operated by an entity enrolled with the certificate 101 authority (see Section 3), whereas a verifier need only trust the 102 root certificate of the authority, and have a means to acquire and 103 validate certificates. 105 The STIR out-of-band architecture is not considered in this document. 106 [TBD] 108 2. Terminology 110 In this document, the key words "MUST", "MUST NOT", "REQUIRED", 111 "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT 112 RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as 113 described in RFC 2119 [1] and RFC 6919 [2]. 115 3. Enrollment and Authorization 117 This document assumes a threefold model for certificate enrollment. 119 The first enrollment model is one where the certificate authority 120 acts in concert with national numbering authorities to issue 121 credentials to those parties to whom numbers are assigned. In the 122 United States, for example, telephone number blocks are assigned to 123 Local Exchange Carriers (LECs) by the North American Numbering Plan 124 Administrator (NANPA), who is in turn directed by the national 125 regulator. LECs may also receive numbers in smaller allocations, 126 through number pooling, or via an individual assignment through 127 number portability. LECs assign numbers to customers, who may be 128 private individuals or organizations - and organizations take 129 responsibility for assigning numbers within their own enterprise. 131 The second enrollment model is one where a certificate authority 132 requires that an entity prove control by means of some sort of test. 133 For example, an authority might send a text message to a telephone 134 number containing a URL (which might be deferenced by the recipient) 135 as a means of verifying that a user has control of terminal 136 corresponding to that number. Checks of this form are frequently 137 used in commercial systems today to validate telephone numbers 138 provided by users. This is comparable to existing enrollment systems 139 used by some certificate authorities for issuing S/MIME credentials 140 for email by verifying that the party applying for a credential 141 receives mail at the email address in question. 143 The third enrollment model is delegation: that is, the holder of a 144 certificate (assigned by either of the two methods above) may 145 delegate some or all of their authority to another party. In some 146 cases, multiple levels of delegation could occur: a LEC, for example, 147 might delegate authority to customer organization for a block of 100 148 numbers, and the organization might in turn delegate authority for a 149 particular number to an individual employee. This is analogous to 150 delegation of organizational identities in traditional hierarchical 151 PKIs who use the name constraints extension [3]; the root CA 152 delegates names in sales to the sales department CA, names in 153 development to the development CA, etc. As lengthy certificate 154 delegation chains are brittle, however, and can cause delays in the 155 verification process, this document considers optimizations to reduce 156 the complexity of verification. 158 [TBD] Future versions of this specification will also discuss methods 159 of partial delegation, where certificate holders delegate only part 160 of their authority. For example, an individual assignee may want to 161 delegate authority to an entity for text messages associated with 162 their telephone number, but not for other functions. 164 3.1. Certificate Scope and Structure 166 The subjects of telephone number certificates are the administrative 167 entities to whom numbers are assigned or delegated. For example, a 168 LEC might hold a certificate for a range of telephone numbers. 170 This specification places no limits on the number of telephone 171 numbers that can be associated with any given certificate. Some 172 service providers may be assigned millions of numbers, and may wish 173 to have a single certificate that is capable of signing for any one 174 of those numbers. Others may wish to compartmentalize authority over 175 subsets of the numbers they control. 177 Moreover, service providers may wish to have multiple certificates 178 with the same scope of authority. For example, a service provider 179 with several regional gateway systems may want each system to be 180 capable of signing for each of their numbers, but not want to have 181 each system share the same private key. 183 The set of telephone numbers for which a particular certificate is 184 valid is expressed in the certificate through a certificate 185 extension; the certificate's extensibility mechanism is defined in 186 RFC 5280 but the telephone number authorization extension is defined 187 in this document. 189 3.2. Provisioning Private Keying Material 191 In order for authentication services to sign calls via the procedures 192 described in [15], they must possess a private key corresponding to a 193 certificate with authority over the calling number. This 194 specification does not require that any particular entity sign 195 requests, only that it be an entity with an appropriate private key; 196 the authentication service role may be instantiated by any entity in 197 a SIP network. For a certificate granting authority only over a 198 particular number which has been issued to an end user, for example, 199 an end user device might hold the private key and generate the 200 signature. In the case of a service provider with authority over 201 large blocks of numbers, an intermediary might old the private key 202 and sign calls. 204 The specification recommends distribution of private keys through 205 PKCS#8 objects signed by a trusted entity, for example through the 206 CMS package specified in [8]. 208 4. Acquiring Credentials to Verify Signatures 210 This specification documents multiple ways that a verifier can gain 211 access to the credentials needed to verify a request. As the 212 validity of certificates does not depend on the circumstances of 213 their acquistion, there is no need to standardize any single 214 mechanism for this purpose. All entities that comply with [15] 215 necessarily support SIP, and consequently SIP itself can serve as a 216 way to acquire certificates. This specific does allow delivery 217 through alternate means as well. 219 The simplest way for a verifier to acquire the certificate needed to 220 verify a signature is for the certificate be conveyed along with the 221 signature itself. In SIP, for example, a certificate could be 222 carried in a multipart MIME body [9], and the URI in the Identity- 223 Info header could specify that body with a CID URI [10]. However, in 224 many environments this is not feasible due to message size 225 restrictions or lack of necessary support for multipart MIME. 227 Alternatively, the Identity-Info header of a SIP request may contain 228 a URI that the verifier dereferences with a network call. 229 Implementations of this specification are required to support the use 230 of SIP for this function (via the SUBSCRIBE/NOTIFY mechanism), as 231 well as HTTP, via the Enrollment over Secure Transport mechanisms 232 described in RFC 7030 [11]. 234 A verifier can however have access to a service that grants access to 235 certificates for a particular telephone number. Note however that 236 there may be multiple valid certificates that can sign a call setup 237 request for a telephone number, and that as a consequence, there 238 needs to be some discriminator that the signer uses to identify their 239 credentials. The Identity-Info header itself can serve just such a 240 discriminator. 242 4.1. Verifying Certificate Scope 244 The subjects of these certificates are the administrative entities to 245 whom numbers are assigned or delegated. When a verifier is 246 validating a caller's identity, local policy always determines the 247 circumstances under which any particular subject may be trusted, but 248 for the purpose of validating a caller's identity, this certificate 249 extension establishes whether or not a signer is authorized to sign 250 for a particular number. 252 The TN Authorization List certificate extension is identified by the 253 following object identifier: 255 id-ce-TNAuthList OBJECT IDENTIFIER ::= { TBD } 257 The TN Authorization List certificate extension has the following 258 syntax: 260 TNAuthorizationList ::= SEQUENCE SIZE (1..MAX) OF TNAuthorization 262 TNAuthorization ::= SEQUENCE SIZE (1..MAX) OF TNEntry 264 TNEntry ::= CHOICE { 266 spid ServiceProviderIdentifierList, 268 range TelephoneNumberRange, 270 one E164Number } 272 ServiceProviderIdentifierList ::= SEQUENCE SIZE (1..3) OF 274 OCTET STRING 276 -- When all three are present: SPID, Alt SPID, and Last Alt SPID 278 TelephoneNumberRange ::= SEQUENCE { 280 start E164Number, 282 count INTEGER } 284 E164Number ::= IA5String (SIZE (1..15)) (FROM ("0123456789")) 286 [TBD- do we really need to do IA5String? The alternative would be 287 UTF8String, e.g.: UTF8String (SIZE (1..15)) (FROM ("0123456789")) ] 289 The TN Authorization List certificate extension indicates the 290 authorized phone numbers for the call setup signer. It indicates one 291 or more blocks of telephone number entries that have been authorized 292 for use by the call setup signer. There are three ways to identify 293 the block: 1) a Service Provider Identifier (SPID) can be used to 294 indirectly name all of the telephone numbers associated with that 295 service provider, 2) telephone numbers can be listed in a range, and 296 3) a single telephone number can be listed. 298 Note that because large-scale service providers may want to associate 299 many numbers, possibly millions of numbers, with a particular 300 certificate, optimizations are required for those cases to prevent 301 certificate size from becoming unmanageable. In these cases, the TN 302 Authorization List may be given by reference rather than by value, 303 through the presence of a separate certificate extension that permits 304 verifiers to either securely download the list of numbers associated 305 with a certificate, or to verify that a single number is under the 306 authority of this certificate. This optimization will be detailed in 307 future version of this specification. 309 4.2. Certificate Freshness and Revocation 311 The problem of certificate freshness gains a new wrinkle in the 312 telephone number context, because verifiers must establish not only 313 that a certificate remains valid, but also that the certificate's 314 scope contains the telephone number that the verifier is validating. 315 Dynamic changes to number assignments can occur due to number 316 portability, for example. So even if a verifier has a valid cached 317 certificate for a telephone number (or a range containing the 318 number), the verifier must determine that the entity that the signer 319 is still a proper authority for that number. 321 This document therefore recommends the use of OCSP in high-volume 322 environments for validating the freshness of certificates, per [12]. 323 [TBD - depending on our algorithm choices this profile may need to be 324 further profiled.] 326 5. Acknowledgments 328 Russ Housley, Brian Rosen, Cullen Jennings and Eric Rescorla provided 329 key input to the discussions leading to this document. 331 6. IANA Considerations 333 This memo includes no request to IANA. 335 7. Security Considerations 337 This document is entirely about security. For further information on 338 certificate security and practices, see RFC 3280 [5], in particular 339 its Security Considerations. 341 8. Informative References 343 [1] Bradner, S., "Key words for use in RFCs to Indicate 344 Requirement Levels", BCP 14, RFC 2119, March 1997. 346 [2] Barnes, R., Kent, S., and E. Rescorla, "Further Key Words 347 for Use in RFCs to Indicate Requirement Levels", RFC 6919, 348 April 1 2013. 350 [3] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 351 A., Peterson, J., Sparks, R., Handley, M., and E. 352 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 353 June 2002. 355 [4] Rosenberg, J. and H. Schulzrinne, "Session Initiation 356 Protocol (SIP): Locating SIP Servers", RFC 3263, June 357 2002. 359 [5] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet 360 X.509 Public Key Infrastructure Certificate and 361 Certificate Revocation List (CRL) Profile", RFC 3280, 362 April 2002. 364 [6] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000. 366 [7] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 367 Housley, R., and W. Polk, "Internet X.509 Public Key 368 Infrastructure Certificate and Certificate Revocation List 369 (CRL) Profile", RFC 5280, May 2008. 371 [8] Turner, S., "Asymmetric Key Packages", RFC 5958, August 372 2010. 374 [9] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 375 Extensions (MIME) Part Two: Media Types", RFC 2046, 376 November 1996. 378 [10] Levinson, E., "Content-ID and Message-ID Uniform Resource 379 Locators", RFC 2392, August 1998. 381 [11] Pritikin, M., Yee, P., and D. Harkins, "Enrollment over 382 Secure Transport", RFC 7030, October 2013. 384 [12] Deacon, A. and R. Hurst, "The Lightweight Online 385 Certificate Status Protocol (OCSP) Profile for High-Volume 386 Environments", RFC 5019, September 2007. 388 [13] Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure 389 Telephone Identity Problem Statement", draft-ietf-stir- 390 problem-statement-03 (work in progress), January 2014. 392 [14] Peterson, J., "Retargeting and Security in SIP: A 393 Framework and Requirements", draft-peterson-sipping- 394 retarget-00 (work in progress), February 2005. 396 [15] Peterson, J., Jennings, C., and E. Rescorla, 397 "Authenticated Identity Management in the Session 398 Initiation Protocol (SIP)", draft-jennings-stir- 399 rfc4474bis-00 (work in progress), October 2013. 401 Authors' Addresses 403 Jon Peterson 404 Neustar, Inc. 405 1800 Sutter St Suite 570 406 Concord, CA 94520 407 US 409 Email: jon.peterson@neustar.biz 411 Sean Turner 412 IECA, Inc. 413 3057 Nutley Street, Suite 106 414 Farifax, VA 22031 415 US 417 Email: turners@ieca.com