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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Web Security C. Evans 3 Internet-Draft C. Palmer 4 Intended status: Standards Track R. Sleevi 5 Expires: August 10, 2014 Google, Inc. 6 February 6, 2014 8 Public Key Pinning Extension for HTTP 9 draft-ietf-websec-key-pinning-10 11 Abstract 13 This memo describes an extension to the HTTP protocol allowing web 14 host operators to instruct user agents (UAs) to remember ("pin") the 15 hosts' cryptographic identities for a given period of time. During 16 that time, UAs will require that the host present a certificate chain 17 including at least one Subject Public Key Info structure whose 18 fingerprint matches one of the pinned fingerprints for that host. By 19 effectively reducing the number of authorities who can authenticate 20 the domain during the lifetime of the pin, pinning may reduce the 21 incidence of man-in-the-middle attacks due to compromised 22 Certification Authorities. 24 Status of This Memo 26 This Internet-Draft is submitted in full conformance with the 27 provisions of BCP 78 and BCP 79. 29 Internet-Drafts are working documents of the Internet Engineering 30 Task Force (IETF). Note that other groups may also distribute 31 working documents as Internet-Drafts. The list of current Internet- 32 Drafts is at http://datatracker.ietf.org/drafts/current/. 34 Internet-Drafts are draft documents valid for a maximum of six months 35 and may be updated, replaced, or obsoleted by other documents at any 36 time. It is inappropriate to use Internet-Drafts as reference 37 material or to cite them other than as "work in progress." 39 This Internet-Draft will expire on August 10, 2014. 41 Copyright Notice 43 Copyright (c) 2014 IETF Trust and the persons identified as the 44 document authors. All rights reserved. 46 This document is subject to BCP 78 and the IETF Trust's Legal 47 Provisions Relating to IETF Documents 48 (http://trustee.ietf.org/license-info) in effect on the date of 49 publication of this document. Please review these documents 50 carefully, as they describe your rights and restrictions with respect 51 to this document. Code Components extracted from this document must 52 include Simplified BSD License text as described in Section 4.e of 53 the Trust Legal Provisions and are provided without warranty as 54 described in the Simplified BSD License. 56 Table of Contents 58 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 59 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 60 2. Server and Client Behavior . . . . . . . . . . . . . . . . . 3 61 2.1. Response Header Field Syntax . . . . . . . . . . . . . . 3 62 2.1.1. The max-age Directive . . . . . . . . . . . . . . . . 5 63 2.1.2. The includeSubDomains Directive . . . . . . . . . . . 5 64 2.1.3. The report-uri Directive . . . . . . . . . . . . . . 5 65 2.1.4. Examples . . . . . . . . . . . . . . . . . . . . . . 6 66 2.2. Server Processing Model . . . . . . . . . . . . . . . . . 7 67 2.2.1. HTTP-over-Secure-Transport Request Type . . . . . . . 7 68 2.2.2. HTTP Request Type . . . . . . . . . . . . . . . . . . 8 69 2.3. User Agent Processing Model . . . . . . . . . . . . . . . 8 70 2.3.1. Public-Key-Pins Response Header Field Processing . . 8 71 2.3.2. Noting a Pinned Host - Storage Model . . . . . . . . 9 72 2.3.3. HTTP-Equiv Element Attribute . . . . . . . . . 10 73 2.4. Semantics of Pins . . . . . . . . . . . . . . . . . . . . 10 74 2.5. Noting Pins . . . . . . . . . . . . . . . . . . . . . . . 11 75 2.6. Validating Pinned Connections . . . . . . . . . . . . . . 12 76 2.7. Interactions With Preloaded Pin Lists . . . . . . . . . . 12 77 2.8. Pinning Self-Signed End Entities . . . . . . . . . . . . 13 78 3. Reporting Pin Validation Failure . . . . . . . . . . . . . . 13 79 4. Security Considerations . . . . . . . . . . . . . . . . . . . 15 80 4.1. Maximum max-age . . . . . . . . . . . . . . . . . . . . . 15 81 4.2. Using includeSubDomains Safely . . . . . . . . . . . . . 16 82 4.3. Backup Pins . . . . . . . . . . . . . . . . . . . . . . . 17 83 5. Privacy Considerations . . . . . . . . . . . . . . . . . . . 17 84 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 85 7. Usability Considerations . . . . . . . . . . . . . . . . . . 19 86 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19 87 9. What's Changed . . . . . . . . . . . . . . . . . . . . . . . 19 88 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 20 89 10.1. Normative References . . . . . . . . . . . . . . . . . . 20 90 10.2. Informative References . . . . . . . . . . . . . . . . . 21 91 Appendix A. Fingerprint Generation . . . . . . . . . . . . . . . 21 92 Appendix B. Deployment Guidance . . . . . . . . . . . . . . . . 22 93 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22 95 1. Introduction 97 We propose a new HTTP header to enable a web host to express to user 98 agents (UAs) which Subject Public Key Info (SPKI) structure(s) UAs 99 SHOULD expect to be present in the host's certificate chain in future 100 connections using TLS (see [RFC5246]). We call this "public key 101 pinning". At least one UA (Google Chrome) has experimented with the 102 idea by shipping with a user-extensible embedded set of pins. 103 Although effective, this does not scale. This proposal addresses the 104 scale problem. 106 Deploying public key pinning safely will require operational and 107 organizational maturity due to the risk that hosts may make 108 themselves unavailable by pinning to a SPKI that becomes invalid. 109 (See Section 4.) We believe that, with care, host operators can 110 greatly reduce the risk of MITM attacks and other false- 111 authentication problems for their users without incurring undue risk. 113 We intend for hosts to use public key pinning together with HSTS 114 ([RFC6797]), but is possible to pin keys without requiring HSTS. 116 This draft is being discussed on the WebSec Working Group mailing 117 list, websec@ietf.org. 119 1.1. Requirements Language 121 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 122 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 123 document are to be interpreted as described in RFC 2119 [RFC2119]. 125 2. Server and Client Behavior 127 2.1. Response Header Field Syntax 129 The Public-Key-Pins HTTP response header field (PKP header field) 130 indicates to a UA that it should perform Pin Validation (Section 2.6) 131 in regards to the host emitting the response message containing this 132 header field, and provides the necessary information for the UA to do 133 so. 135 Figure 1 describes the ABNF (Augmented Backus-Naur Form) syntax of 136 the header field. It is based on the Generic Grammar defined in 137 Section 2 of [RFC2616] (which includes a notion of "implied linear 138 whitespace", also known as "implied *LWS"). 140 Public-Key-Pins = 141 "Public-Key-Pins" ":" [ directive ] *( ";" [ directive ] ) 142 Public-Key-Pins-Report-Only = 143 "Public-Key-Pins-Report-Only" ":" [ directive ] *( ";" [ directive ] ) 145 directive = simple-directive 146 / pin-directive 148 simple-directive = directive-name [ "=" directive-value ] 149 directive-name = token 150 directive-value = token 151 / quoted-string 153 pin-directive = "pin-" token "=" quoted-string 155 Figure 1: HPKP Header Syntax 157 token and quoted-string are used as defined in [RFC2616], 158 Section 2.2. 160 The directives defined in this specification are described below. 161 The overall requirements for directives are: 163 1. The order of appearance of directives is not significant. 165 2. All simple-directives MUST appear only once in a PKP header 166 field. Directives are either optional or required, as stipulated 167 in their definitions. 169 3. Directive names are case-insensitive. 171 4. UAs MUST ignore any PKP header fields containing directives, or 172 other header field value data, that do not conform to the syntax 173 defined in this specification. 175 5. If a PKP header field contains any directive(s) the UA does not 176 recognize, the UA MUST ignore the those directives. 178 6. If the PKP header field otherwise satisfies the above 179 requirements (1 through 5), the UA MUST process the directives it 180 recognizes. 182 Additional directives extending the semantic functionality of the PKP 183 header field can be defined in other specifications, with a registry 184 (having an IANA policy definition of IETF Review [RFC2616]) defined 185 for them at such time. Such future directives will be ignored by UAs 186 implementing only this specification, as well as by generally non- 187 conforming UAs. 189 In the pin-directive, the token is the name of a cryptographic hash 190 algorithm, and MUST be "sha256". (In the future, additional hash 191 algorithms MAY be registered and used.) The quoted-string is a 192 sequence of base 64 digits: the base 64-encoded SPKI Fingerprint 193 ([RFC4648]). See Section 2.4. 195 2.1.1. The max-age Directive 197 The REQUIRED "max-age" directive specifies the number of seconds, 198 after the reception of the PKP header field, during which the UA 199 SHOULD regard the host (from whom the message was received) as a 200 Known Pinned Host. The delta-seconds production is specified in 201 [RFC2616]. 203 The syntax of the max-age directive's REQUIRED value (after quoted- 204 string unescaping, if necessary) is defined as: 206 max-age-value = delta-seconds 207 delta-seconds = 1*DIGIT 209 Figure 2: max-age Value Syntax 211 delta-seconds is used as defined in [RFC2616], Section 3.3.2. 213 NOTE: A max-age value of zero (i.e., "max-age=0") signals the UA to 214 cease regarding the host as a Known Pinned Host, including the 215 includeSubDomains directive (if asserted for that Known Pinned Host). 216 See Section 2.3.1. 218 2.1.2. The includeSubDomains Directive 220 The OPTIONAL "includeSubDomains" directive is a valueless directive 221 which, if present (i.e., it is "asserted"), signals to the UA that 222 the Pinning Policy applies to this Pinned Host as well as any 223 subdomains of the host's domain name. 225 2.1.3. The report-uri Directive 227 The OPTIONAL "report-uri" directive indicates the URI to which the UA 228 SHOULD report Pin Validation failures (Section 2.6). The UA POSTs 229 the reports to the given URI as described in Section 3. 231 When used in the Public-Key-Pins-Report-Only header, the UA SHOULD 232 POST reports for Pin Validation failures to the indicated report-uri, 233 although the UA MUST NOT enforce Pin Validation. That is, in the 234 event of Pin Validation failure when the host has set the Public-Key- 235 Pins-Report-Only header, the UA performs Pin Validation only to check 236 whether or not it should POST a report, but not for causing 237 connection failure. 239 If a Host sets both the Public-Key-Pins header and the Public-Key- 240 Pins-Report-Only header, the UA MUST NOT enforce Pin Validation, and 241 MUST note only the pins and directives given in the Public-Key-Pins- 242 Report-Only header. 244 When used in the Public-Key-Pins header, the presence of a report-uri 245 directive indicates to the UA that the UA MUST enforce Pin 246 Validation, and the UA SHOULD also, in the event of Pin Validation 247 failure, POST a report to the report-uri. 249 Note that the report-uri need not necessarily be in the same Internet 250 domain or web origin as the Known Pinned Host. 252 Hosts may set report-uris that use HTTP, HTTPS, or other schemes. If 253 the scheme in the report-uri is HTTPS, UAs MUST perform Pinning 254 Validation when the host in the report-uri is a Known Pinned Host; 255 similarly, UAs MUST apply HSTS if the host in the report-uri is a 256 Known HSTS Host. 258 UAs SHOULD make their best effort to report Pin Validation failures 259 to the report-uri, but may fail to report in exceptional conditions. 260 For example, if connecting the report-uri itself incurs a Pinning 261 Validation failure or other certificate validation failure, the UA 262 MUST cancel the connection (and MAY attempt to re-send the report 263 later). Similarly, if Known Pinned Host A sets a report-uri 264 referring to Known Pinned Host B, and if B sets a report-uri 265 referring to A, and if both hosts fail Pin Validation, the UA SHOULD 266 detect and break the loop by failing to send reports to and about 267 those hosts. 269 UAs SHOULD limit the rate at which they send reports. For example, 270 it is unnecessary to send the same report to the same report-uri more 271 than once. 273 2.1.4. Examples 275 Figure 3 shows some example response header fields using the pins 276 extension (folded for clarity). 278 "d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM=" 279 "E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g=" 280 Public-Key-Pins: max-age=3000; 281 pin-sha256="d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM="; 282 pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g="; 284 Public-Key-Pins: max-age=2592000; 285 pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g="; 286 pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ=" 288 Public-Key-Pins: max-age=2592000; 289 pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g="; 290 pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ="; 291 report-uri="http://example.com/pkp-report" 293 Public-Key-Pins-Report-Only: max-age=2592000; 294 pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g="; 295 pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ="; 296 report-uri="http://example.com/pkp-report" 298 Public-Key-Pins: 299 pin-sha256="d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM="; 300 pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ="; 301 max-age=259200 303 Public-Key-Pins: 304 pin-sha256="d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM="; 305 pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g="; 306 pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ="; 307 max-age=10000; includeSubDomains 309 Figure 3: HPKP Header Examples 311 2.2. Server Processing Model 313 This section describes the processing model that Pinned Hosts 314 implement. The model comprises two facets: the processing rules for 315 HTTP request messages received over a secure transport (e.g. TLS 316 [RFC5246]); and the processing rules for HTTP request messages 317 received over non-secure transports, such as TCP. 319 2.2.1. HTTP-over-Secure-Transport Request Type 321 When replying to an HTTP request that was conveyed over a secure 322 transport, a Pinned Host SHOULD include in its response exactly one 323 PKP header field that MUST satisfy the grammar specified above in 324 Section 2.1. 326 Establishing a given host as a Known Pinned Host, in the context of a 327 given UA, MAY be accomplished over the HTTP protocol, which is in 328 turn running over secure transport, by correctly returning (per this 329 specification) at least one valid PKP header field to the UA. Other 330 mechanisms, such as a client-side pre-loaded Known Pinned Host list 331 MAY also be used. 333 2.2.2. HTTP Request Type 335 Pinned Hosts SHOULD NOT include the PKP header field in HTTP 336 responses conveyed over non-secure transport. UAs MUST ignore any 337 PKP header received in an HTTP response conveyed over non-secure 338 transport. 340 2.3. User Agent Processing Model 342 This section describes the HTTP Public Key Pinning processing model 343 for UAs. 345 The UA processing model relies on parsing domain names. Note that 346 internationalized domain names SHALL be canonicalized according to 347 the scheme in Section 10 of [RFC6797]. 349 2.3.1. Public-Key-Pins Response Header Field Processing 351 If the UA receives, over a secure transport, an HTTP response that 352 includes a PKP header field conforming to the grammar specified in 353 Section 2.1, and there are no underlying secure transport errors or 354 warnings (see Section 2.5), the UA MUST either: 356 o Note the host as a Known Pinned Host if it is not already so noted 357 (see Section 2.3.2), 359 or, 361 o Update the UA's cached information for the Known Pinned Host if 362 any of of the max-age, includeSubDomains, or report-uri header 363 field value directives convey information different than that 364 already maintained by the UA. 366 o The max-age value is essentially a "time to live" value relative 367 to the time of the most recent observation of the PKP header 368 field. 370 o If the max-age header field value token has a value of 0, the UA 371 MUST remove its cached Pinning Policy information (including the 372 includeSubDomains directive, if asserted) if the Pinned Host is 373 Known, or, MUST NOT note this Pinned Host if it is not yet Known. 375 o If a UA receives more than one PKP header field in an HTTP 376 response message over secure transport, then the UA MUST process 377 only the first such header field. 379 Otherwise: 381 o If the UA receives the HTTP response over insecure transport, or 382 if the PKP header is not a Valid Pinning Header (see Section 2.5), 383 the UA MUST ignore any present PKP header field(s). 385 o The UA MUST ignore any PKP header fields not conforming to the 386 grammar specified in Section 2.1. 388 2.3.2. Noting a Pinned Host - Storage Model 390 The Effective Pin Date of a Known Pinned Host is the time that the UA 391 observed a Valid Pinning Header for the host. The Effective 392 Expiration Date of a Known Pinned Host is the Effective Pin Date plus 393 the max-age. A Known Pinned Host is "expired" if the Effective 394 Expiration Date refers to a date in the past. The UA MUST ignore all 395 expired Known Pinned Hosts from its cache if, at any time, an expired 396 Known Pinned Host exists in the cache. 398 If the substring matching the host production from the Request-URI 399 (of the message to which the host responded) syntactically matches 400 the IP-literal or IPv4address productions from Section 3.2.2 of 401 [RFC3986], then the UA MUST NOT note this host as a Known Pinned 402 Host. 404 Otherwise, if the substring does not congruently match a Known Pinned 405 Host's domain name, per the matching procedure specified in 406 Section 8.2 of [RFC6797], then the UA MUST note this host as a Known 407 Pinned Host, caching the Pinned Host's domain name and noting along 408 with it the Effective Expiration Date (or enough information to 409 calculate it, i.e. the Effective Pin Date and the value of the max- 410 age directive), whether or not the includeSubDomains directive is 411 asserted, the value of the report-uri directive (if present). If any 412 other metadata from optional or future PKP header directives is 413 present in the Valid Pinning Header, the UA MAY note them if it 414 understands them, and need not note them if it does not understand 415 them. 417 UAs MAY set an upper limit on the value of max-age, so that UAs that 418 have noted erroneous pins (whether by accident or due to attack) have 419 some chance of recovering over time. If the server sets a max-age 420 greater than the UA's upper limit, the UA MAY behave as if the server 421 set the max-age to the UA's upper limit. For example, if the UA caps 422 max-age at 5184000 seconds (60 days), and a Pinned Host sets a max- 423 age directive of 90 days in its Valid Pinning Header, the UA MAY 424 behave as if the max-age were effectively 60 days. (One way to 425 achieve this behavior is for the UA to simply store a value of 60 426 days instead of the 90 day value provided by the Pinned Host.) For 427 UA implementation guidance on how to select a maximum max-age, see 428 Section 4.1. 430 The UA MUST NOT modify any pinning metadata of any superdomain 431 matched Known Pinned Host. 433 2.3.3. HTTP-Equiv Element Attribute 435 UAs MUST NOT heed http-equiv="Public-Key-Pins" attribute settings on 436 elements [W3C.REC-html401-19991224] in received content. 438 2.4. Semantics of Pins 440 An SPKI Fingerprint is defined as the output of a known cryptographic 441 hash algorithm whose input is the DER-encoded ASN.1 representation of 442 the SubjectPublicKeyInfo (SPKI) field of an X.509 certificate. A Pin 443 is defined as the combination of the known algorithm identifier and 444 the SPKI Fingerprint computed using that algorithm. 446 The SPKI Fingerprint is encoded in base 64 for use in an HTTP header. 447 (See [RFC4648].) 449 In this version of the specification, the known cryptographic hash 450 algorithm is SHA-256, identified as "sha256" ([RFC4634]). (Future 451 versions of this specification may add new algorithms and deprecate 452 old ones.) UAs MUST ignore Pins for which they do not recognize the 453 algorithm identifier. UAs MUST continue to process the rest of a PKP 454 response header field and note Pins for algorithms they do recognize; 455 UAs MUST recognize and "sha256". 457 Figure 4 reproduces the definition of the SubjectPublicKeyInfo 458 structure in [RFC5280]. 460 SubjectPublicKeyInfo ::= SEQUENCE { 461 algorithm AlgorithmIdentifier, 462 subjectPublicKey BIT STRING } 464 AlgorithmIdentifier ::= SEQUENCE { 465 algorithm OBJECT IDENTIFIER, 466 parameters ANY DEFINED BY algorithm OPTIONAL } 468 Figure 4: SPKI Definition 470 If the SubjectPublicKeyInfo of a certificate is incomplete when taken 471 in isolation, such as when holding a DSA key without domain 472 parameters, a public key pin cannot be formed. 474 We pin public keys, rather than entire certificates, to enable 475 operators to generate new certificates containing old public keys 476 (see [why-pin-key]). 478 See Appendix A for an example non-normative program that generates 479 SPKI Fingerprints from SubjectPublicKeyInfo fields in certificates. 481 2.5. Noting Pins 483 Upon receipt of the Public-Key-Pins response header field, the UA 484 notes the host as a Pinned Host, storing the Pins and their 485 associated directives in non-volatile storage (for example, along 486 with the HSTS metadata). The Pins and their associated directives 487 are collectively known as Pinning Metadata. 489 The UA MUST observe these conditions when noting a Host: 491 o The UA MUST note the Pins if and only if it received the Public- 492 Key-Pins response header field over an error-free TLS connection. 493 If the host is a Pinned Host, this includes the validation added 494 in Section 2.6. 496 o The UA MUST note the Pins if and only if the TLS connection was 497 authenticated with a certificate chain containing at least one of 498 the SPKI structures indicated by at least one of the given SPKI 499 Fingerprints. (See Section 2.6.) 501 o The UA MUST note the Pins if and only if the given set of Pins 502 contains at least one Pin that does NOT refer to an SPKI in the 503 certificate chain. (That is, the host must set a Backup Pin; see 504 Section 4.3.) 506 If the Public-Key-Pins response header field does not meet all three 507 of these criteria, the UA MUST NOT note the host as a Pinned Host. A 508 Public-Key-Pins response header field that meets all these critera is 509 known as a Valid Pinning Header. 511 Whenever a UA receives a Valid Pinning Header, it MUST set its 512 Pinning Metadata to the exact Pins, max-age, and (if any) report-uri 513 given in the most recently received Valid Pinning Header. 515 For forward compatibility, the UA MUST ignore any unrecognized 516 Public-Key-Pins header directives, while still processing those 517 directives it does recognize. Section 2.1 specifies the directives 518 max-age, pins, includeSubDomains, and report-uri but future 519 specifications and implementations might use additional directives. 521 2.6. Validating Pinned Connections 523 When a UA connects to a Pinned Host, if the TLS connection has 524 errors, the UA MUST terminate the connection without allowing the 525 user to proceed anyway. (This behavior is the same as that required 526 by [RFC6797].) 528 If the connection has no errors, then the UA will determine whether 529 to apply a new, additional correctness check: Pin Validation. A UA 530 SHOULD perform Pin Validation whenever connecting to a Known Pinned 531 Host, but MAY allow Pin Validation to be disabled for Hosts according 532 to local policy. For example, a UA may disable Pin Validation for 533 Pinned Hosts whose validated certificate chain terminates at a user- 534 defined trust anchor, rather than a trust anchor built-in to the UA. 536 To perform Pin Validation, the UA will compute the SPKI Fingerprints 537 for each certificate in the Pinned Host's validated certificate 538 chain, using each supported hash algorithm for each certificate. 539 (For the purposes of Pin Validation, the UA MUST ignore certificates 540 whose SPKI cannot be taken in isolation, and MUST ignore superfluous 541 certificates in the chain that do not form part of the validating 542 chain.) The UA will then check that the set of these SPKI 543 Fingerprints intersects the set of SPKI Fingerprints in that Pinned 544 Host's Pinning Metadata. If there is set intersection, the UA 545 continues with the connection as normal. Otherwise, the UA MUST 546 treat this Pin Failure as a non-recoverable error. Any procedure 547 that matches the results of this Pin Validation procedure is 548 considered equivalent. 550 Note that, although the UA has previously received Pins at the HTTP 551 layer, it can and MUST perform Pin Validation at the TLS layer, 552 before beginning an HTTP conversation over the TLS channel. The TLS 553 layer thus evaluates TLS connections with pinning information the UA 554 received previously, regardless of mechanism: statically preloaded, 555 via HTTP header, or some other means (possibly in the TLS layer 556 itself). 558 2.7. Interactions With Preloaded Pin Lists 560 UAs MAY choose to implement additional sources of pinning 561 information, such as through built-in lists of pinning information. 562 Such UAs SHOULD allow users to override such additional sources, 563 including disabling them from consideration. 565 UAs that support additional sources of pinning information MUST use 566 the most recently observed pinning information when performing Pin 567 Validation for a host. The most recently observed pinning 568 information is determined based upon the most recent Effective Pin 569 Date, as described in Section 2.3.2. 571 If the result of noting a Valid Pinning Header is to disable pinning 572 for the host, such as through supplying a max-age directive with a 573 value of 0, UAs MUST allow this new information to override any other 574 pinning data. That is, a host must be able to un-pin itself, even in 575 the presence of built-in pins. 577 Example: A UA may ship with a pre-configured list of pins that are 578 collected from past observations of Valid Pinning Headers supplied by 579 hosts. In such a solution, the pre-configured list should track when 580 the Valid Pinning Header was last observed, in order to permit site 581 operators to later update the value by supplying a new Valid Pinning 582 Header. Updates to such a pre-configured list should not update the 583 Effective Pin Dates for each host unless the list vendor has actually 584 observed a more recent header. This is to prevent situations where 585 updating the Effective Pin Date on a pre-configured list of pins may 586 effectively extend the max-age beyond the site operator's stated 587 policy. 589 Example: A UA may ship with a pre-configured list of pins that are 590 collected through out-of-band means, such as direct contact with the 591 site operator. In such a solution, the site operator accepts 592 responsibility for keeping the configured Valid Pinning Header in 593 sync with the vendor's list, allowing the UA vendor to have each 594 update to the list be treated as as an update of the Effective Pin 595 Date. 597 2.8. Pinning Self-Signed End Entities 599 If UAs accept hosts that authenticate themselves with self-signed end 600 entity certificates, they MAY also allow hosts to pin the public keys 601 in such certificates. The usability and security implications of 602 this practice are outside the scope of this specification. 604 3. Reporting Pin Validation Failure 606 When a Known Pinned Host has set the report-uri directive, the UA 607 SHOULD report Pin Validation failures to the indicated URI. The UA 608 does this by POSTing a JSON ([RFC4627]) message to the URI; the JSON 609 message takes this form: 611 { 612 "date-time": date-time, 613 "hostname": hostname, 614 "port": port, 615 "certificate-chain": [ 616 pem1, ... pemN 617 ], 618 "known-pins": [ 619 known-pin1, ... known-pinN 620 ] 621 } 623 Figure 5: JSON Report Format 625 Whitespace outside of quoted strings is not significant. The key/ 626 value pairs may appear in any order, but each SHOULD appear only 627 once. 629 The date-time indicates the time the UA observed the Pin Validation 630 failure. It is provided as a string formatted according to 631 Section 5.6, "Internet Date/Time Format", of [RFC3339]. 633 The hostname is the hostname to which the UA made the original 634 request that failed Pin Validation. It is provided as a string. 636 The port is the port to which the UA made the original request that 637 failed Pin Validation. It is provided either as a string or as an 638 integer. 640 The certificate-chain is the certificate chain, as constructed by the 641 UA during certificate chain verification. (This may differ from the 642 certificate chain as served by the Known Pinned Host, of course.) It 643 is provided as an array of strings; each string pem1, ... pemN is the 644 PEM representation of each X.509 certificate as described in 645 [I-D.josefsson-pkix-textual]. 647 The known-pins are the Pins that the UA has noted for the Known 648 Pinned Host. They are provided as an array of strings with the 649 syntax: 651 known-pin = token "=" quoted-string 653 Figure 6: Known Pin Syntax 655 As in Section 2.4, the token refers to the algorithm name, and the 656 quoted-string refers to the base 64 encoding of the SPKI Fingerprint. 658 4. Security Considerations 660 Pinning public keys helps hosts strongly assert their cryptographic 661 identity even in the face of issuer error, malfeasance or compromise. 662 But there is some risk that a host operator could lose or lose 663 control of their host's private key (such as by operator error or 664 host compromise). If the operator had pinned only the key of the 665 host's end entity certificate, the operator would not be able to 666 serve their web site or application in a way that UAs would trust for 667 the duration of their pin's max-age. (Recall that UAs MUST close the 668 connection to a host upon Pin Failure.) 670 Therefore, there is a necessary trade-off between two competing 671 goods: pin specificity and maximal reduction of the scope of issuers 672 on the one hand; and flexibility and resilience of the host's 673 cryptographic identity on the other hand. One way to resolve this 674 trade-off is to compromise by pinning to the key(s) of the issuer(s) 675 of the host's end entity certificate(s). Often, a valid certificate 676 chain will have at least two certificates above the end entity 677 certificate: the intermediate issuer, and the trust anchor. 678 Operators can pin any one or more of the public keys in this chain, 679 and indeed could pin to issuers not in the chain (as, for example, a 680 Backup Pin). Pinning to an intermediate issuer, or even to a trust 681 anchor or root, still significantly reduces the number of issuers who 682 can issue end entity certificates for the Known Pinned Host, while 683 still giving that host flexibility to change keys without a 684 disruption of service. 686 4.1. Maximum max-age 688 As mentioned in Section 2.3.2, UAs MAY cap the max-age value at some 689 upper limit. There is a security trade-off in that low maximum 690 values provide a narrow window of protection for users who visit the 691 Known Pinned Host only infrequently, while high maximum values might 692 potentially result in a UA's inability to successfully perform Pin 693 Validation for a Known Pinned Host if the UA's noted pins and the 694 Host's true pins diverge. 696 Such divergence could occur for several reasons, including: UA error; 697 Host operator error; network attack; or a Known Pinned Host that 698 intentionally migrates all pinned keys, combined with a UA that has 699 noted true pins with a high max-age value and has not had a chance to 700 observe the new true pins for the Host. (This last example 701 underscores the importance for Host operators to phase in new keys 702 gradually, and to set the max-age value in accordance with their 703 planned key migration schedule.) 704 There is probably no ideal upper limit to the max-age directive that 705 would satisfy all use cases. However, a value on the order of 60 706 days (5184000 seconds) may be considered a balance between the two 707 competing security concerns. 709 4.2. Using includeSubDomains Safely 711 It may happen that Pinned Hosts whose hostnames share a parent domain 712 use different Valid Pinning Headers. If a Host whose hostname is a 713 parent domain for another Host sets the includeSubDomains directive, 714 the two Hosts' pins may conflict with each other. For example, 715 consider two Known Pinned Hosts, example.com and 716 subdomain.example.com. Assume example.com sets a Valid Pinning 717 Header such as this: 719 Public-Key-Pins: pin-sha256="ABC..."; pin-sha256="DEF..."; includeSubDomains 721 Figure 7: example.com Valid Pinning Header 723 Assume subdomain.example.com sets a Valid Pinning Header such as 724 this: 726 Public-Key-Pins: pin-sha256="GHI..."; pin-sha256="JKL..." 728 Figure 8: subdomain.example.com Valid Pinning Header 730 Assume a UA that has not previously noted any pins for either of 731 these Hosts. If the UA first contacts subdomain.example.com, it will 732 note the pins in the Valid Pinning Header, and perform Pin Validation 733 as normal on subsequent conections. If the UA then contacts 734 example.com, again it will note the pins and perform Pin Validation 735 on future connections. However, if the UA happened to first 736 example.com before subdomain.example.com, the UA would, due to 737 example.com's use of the includeSubDomains directive, attempt to 738 perform Pin Validation for subdomain.example.com using the SPKI 739 hashes ABC... and DEF..., which are not valid for the certificate 740 chains subdomain.example.com (which uses certificates with SPKIs 741 GHI... and JLK...). Thus, depending on the order in which the UA 742 observes the Valid Pinning Headers for hosts example.com and 743 subdomain.example.com, Pin Validation might or might not fail for 744 subdomain.example.com, even if the certificate chain the UA receives 745 for subdomain.example.com is perfectly valid. 747 Thus, Pinned Host operators must use the includeSubDomains directive 748 with care. For example, they may choose to use overlapping pin sets 749 for hosts under a parent domain that uses includeSubDomains, or to 750 not use the includeSubDomains directive in their effective-second- 751 level domains, or to simply use the same pin set for all hosts under 752 a given parent domain. 754 4.3. Backup Pins 756 The primary way to cope with the risk of inadvertent Pin Failure is 757 to keep a Backup Pin. A Backup Pin is a fingerprint for the public 758 key of a secondary, not-yet-deployed key pair. The operator keeps 759 the backup key pair offline, and sets a pin for it in the Public-Key- 760 Pins header. Then, in case the operator loses control of their 761 primary private key, they can deploy the backup key pair. UAs, who 762 have had the backup key pair pinned (when it was set in previous 763 Valid Pinning Headers), can connect to the host without error. 765 Because having a backup key pair is so important to recovery, UAs 766 MUST require that hosts set a Backup Pin. (See Section 2.5.) 768 5. Privacy Considerations 770 Conforming implementations (as well as implementations conforming to 771 [RFC6797]) must store state about which domains have set policies, 772 hence which domains the UA has contacted. A forensic attacker might 773 find this information useful, even if the user has cleared other 774 parts of the UA's state. 776 More importantly, Hosts can use HSTS or HPKP as a "super-cookie", by 777 setting distinct policies for a number of subdomains. For example, 778 assume example.com wishes to track distinct UAs without explicitly 779 setting a cookie, or if a previously-set cookie is deleted from the 780 UA's cookie store. Here are two attack scenarios. 782 1. example.com can use report-uri and the ability to pin arbitrary 783 identifiers to distinguish UAs. 785 2. 787 1. example.com sets a Valid Pinning Header in its response to 788 requests. The header asserts the includeSubDomains 789 directive, and specifies a report-uri directive as well. 790 Pages served by the host also include references to 791 subresource https://bad.example.com/foo.png. 793 2. The Valid Pinning Header includes a "pin" that is not really 794 the hash of an SPKI, but is instead an arbitrary 795 distinguishing string sent only in response to a particular 796 request. For each request, the Host creates a new, distinct 797 distinguishing string and sets it as if it were a pin. 799 3. The certificate chain served by bad.example.com does not pass 800 Pin Validation given the pin set the Host asserted in (1). 801 The HPKP-conforming UA attempts to report the Pin Validation 802 failure to the specified report-uri, including the 803 certificate chain it observed and the SPKI hashes it expected 804 to see. Among the SPKI hashes is the distinguishing string 805 in step (2). 807 3. example.com can use SNI and subdomains to distinguish UAs. 809 4. 811 1. example.com sets a Valid Pinning Header in its response to 812 requests. The header asserts the includeSubDomains 813 directive. 815 2. On a subsequent page view, the Host responds with a page 816 including the subresource https://0.fingerprint.example.com/ 817 foo.png, and the server responds using a certificate chain 818 that does not pass Pin Validation for the pin-set defined in 819 the Valid Pinning Header in step (1). The HPKP-conforming UA 820 will close the connection, never completing the request to 821 0.fingerprint.example.com. The Host may thus note that this 822 particular UA had noted the (good) pins for that subdomain. 824 3. example.com can distinguish 2^N UAs by serving Valid Pinning 825 Headers from an arbitrary number N distinct subdomains, 826 giving some UAs Valid Pinning Headers for some, but not all 827 subdomains (causing subsequent requests for 828 n.fingerprint.example.com to fail), and giving some UAs no 829 Valid Pinning Header for other subdomains (causing subsequent 830 requests for m.fingerprint.example.com to succeed). 832 6. IANA Considerations 834 IANA is requested to register the header described in this document 835 in the "Message Headers" registry, with the following parameters: 837 o Header Field Name should be "Public-Key-Pins" 839 o Protocol should be "http" 841 o Status should be "standard" 843 o Reference should be this document 845 7. Usability Considerations 847 When pinning works to detect impostor Pinned Hosts, users will 848 experience denial of service. UAs MUST explain the reason why, i.e. 849 that it was impossible to verify the confirmed cryptographic identity 850 of the host. 852 UAs MUST have a way for users to clear current pins for Pinned Hosts. 853 UAs SHOULD have a way for users to query the current state of Pinned 854 Hosts. 856 8. Acknowledgements 858 Thanks to Tobias Gondrom, Jeff Hodges, Ivan Krstic, Adam Langley, 859 Nicolas Lidzborski, SM, James Manger, Eric Rescorla, Paul Hoffman, 860 and Yoav Nir for suggestions and edits that clarified the text. 861 Thanks to Trevor Perrin for suggesting a mechanism to affirmatively 862 break pins ([pin-break-codes]). 864 9. What's Changed 866 [RFC EDITOR: PLEASE REMOVE THIS SECTION] 868 Removed the strict directive. 870 Removed the requirement that the server set the Valid Pinning Header 871 on every response. 873 Added normative references for SHA, JSON, and base-64. 875 Added the Privacy Considerations section. 877 Changed non-normative pin generation code from Go to POSIX shell 878 script using openssl. 880 Changed max-max-age from SHOULD to MAY, and used the example of 60 881 days instead of 30. 883 Removed the section "Pin Validity Times", which was intended to be in 884 harmony with [I-D.perrin-tls-tack]. Now using max-age purely as 885 specified in [RFC6797]. 887 Added new directives: includeSubDomains, report-uri and strict. 889 Added a new variant of the PKP Header: Public-Key-Pins-Report-Only. 891 Removed the section on pin break codes and verifiers, in favor the of 892 most-recently-received policy (Section 2.5). 894 Now using a new header field, Public-Key-Pins, separate from HSTS. 895 This allows hosts to use pinning separately from Strict Transport 896 Security. 898 Explicitly requiring that UAs perform Pin Validation before the HTTP 899 conversation begins. 901 Backup Pins are now required. 903 Separated normative from non-normative material. Removed tangential 904 and out-of-scope non-normative discussion. 906 10. References 908 10.1. Normative References 910 [I-D.josefsson-pkix-textual] 911 Josefsson, S. and S. Leonard, "Text Encodings of PKIX and 912 CMS Structures", draft-josefsson-pkix-textual-02 (work in 913 progress), October 2013. 915 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 916 Requirement Levels", BCP 14, RFC 2119, March 1997. 918 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 919 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 920 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. 922 [RFC3339] Klyne, G., Ed. and C. Newman, "Date and Time on the 923 Internet: Timestamps", RFC 3339, July 2002. 925 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 926 Resource Identifier (URI): Generic Syntax", STD 66, RFC 927 3986, January 2005. 929 [RFC4627] Crockford, D., "The application/json Media Type for 930 JavaScript Object Notation (JSON)", RFC 4627, July 2006. 932 [RFC4634] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms 933 (SHA and HMAC-SHA)", RFC 4634, July 2006. 935 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data 936 Encodings", RFC 4648, October 2006. 938 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data 939 Encodings", RFC 4648, October 2006. 941 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 942 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 943 May 2008. 945 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 946 (TLS) Protocol Version 1.2", RFC 5246, August 2008. 948 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 949 Housley, R., and W. Polk, "Internet X.509 Public Key 950 Infrastructure Certificate and Certificate Revocation List 951 (CRL) Profile", RFC 5280, May 2008. 953 [RFC6797] Hodges, J., Jackson, C., and A. Barth, "HTTP Strict 954 Transport Security (HSTS)", RFC 6797, November 2012. 956 [W3C.REC-html401-19991224] 957 Raggett, D., Hors, A., and I. Jacobs, "HTML 4.01 958 Specification", World Wide Web Consortium Recommendation 959 REC-html401-19991224, December 1999, 960 . 962 10.2. Informative References 964 [I-D.perrin-tls-tack] 965 Marlinspike, M., "Trust Assertions for Certificate Keys", 966 draft-perrin-tls-tack-02 (work in progress), January 2013. 968 [pin-break-codes] 969 Perrin, T., "Self-Asserted Key Pinning", September 2011, 970 . 972 [why-pin-key] 973 Langley, A., "Public Key Pinning", May 2011, 974 . 976 Appendix A. Fingerprint Generation 978 This POSIX shell program generates SPKI Fingerprints, suitable for 979 use in pinning, from PEM-encoded certificates. It is non-normative. 981 openssl x509 -noout -in certificate.pem -pubkey | \ 982 openssl asn1parse -noout -inform pem -out public.key 983 openssl dgst -sha256 -binary public.key | base64 985 Figure 9: Example SPKI Fingerprint Generation Code 987 Appendix B. Deployment Guidance 989 This section is non-normative guidance which may smooth the adoption 990 of public key pinning. 992 o Operators SHOULD get the backup public key signed by a different 993 (root and/or intermediary) CA than their primary certificate, and 994 store the backup key pair safely offline. The semantics of an 995 SPKI Fingerprint do not require the issuance of a certificate to 996 construct a valid Pin. However, in many deployment scenarios, in 997 order to make a Backup Pin operational the server operator will 998 need to have a certificate to deploy TLS on the host. Failure to 999 obtain a certificate through prior arrangement will leave clients 1000 that recognize the site as a Known Pinned Host unable to 1001 successfully perform Pin Validation until such a time as the 1002 operator can obtain a new certificate from their desired 1003 certificate issuer. 1005 o It is most economical to have the backup certificate signed by a 1006 completely different signature chain than the live certificate, to 1007 maximize recoverability in the event of either root or 1008 intermediary signer compromise. 1010 o Operators SHOULD periodically exercise their Backup Pin plan -- an 1011 untested backup is no backup at all. 1013 o Operators SHOULD start small. Operators SHOULD first deploy 1014 public key pinning by using the report-only mode together with a 1015 report-uri directive that points to a reliable report collection 1016 endpoint. When moving out of report-only mode, operators should 1017 start by setting a max-age of minutes or a few hours, and 1018 gradually increase max-age as they gain confidence in their 1019 operational capability. 1021 Authors' Addresses 1023 Chris Evans 1024 Google, Inc. 1025 1600 Amphitheatre Pkwy 1026 Mountain View, CA 94043 1027 US 1029 Email: cevans@google.com 1030 Chris Palmer 1031 Google, Inc. 1032 1600 Amphitheatre Pkwy 1033 Mountain View, CA 94043 1034 US 1036 Email: palmer@google.com 1038 Ryan Sleevi 1039 Google, Inc. 1040 1600 Amphitheatre Pkwy 1041 Mountain View, CA 94043 1042 US 1044 Email: sleevi@google.com