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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: February 8, 2015 Google, Inc. 6 August 7, 2014 8 Public Key Pinning Extension for HTTP 9 draft-ietf-websec-key-pinning-20 11 Abstract 13 This document describes an extension to the HTTP protocol allowing 14 web host operators to instruct user agents 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 February 8, 2015. 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 . . . . . . . . . . . . . . . . . . 4 60 2. Server and Client Behavior . . . . . . . . . . . . . . . . . 4 61 2.1. Response Header Field Syntax . . . . . . . . . . . . . . 4 62 2.1.1. The max-age Directive . . . . . . . . . . . . . . . . 5 63 2.1.2. The includeSubDomains Directive . . . . . . . . . . . 6 64 2.1.3. The report-uri Directive . . . . . . . . . . . . . . 6 65 2.1.4. Examples . . . . . . . . . . . . . . . . . . . . . . 7 66 2.2. Server Processing Model . . . . . . . . . . . . . . . . . 8 67 2.2.1. HTTP-over-Secure-Transport Request Type . . . . . . . 8 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. Interaction of Public-Key-Pins and Public-Key-Pins- 72 Report-Only . . . . . . . . . . . . . . . . . . . . . 9 73 2.3.3. Noting a Pinned Host - Storage Model . . . . . . . . 10 74 2.3.4. HTTP-Equiv Element Attribute . . . . . . . . . 11 75 2.4. Semantics of Pins . . . . . . . . . . . . . . . . . . . . 11 76 2.5. Noting Pins . . . . . . . . . . . . . . . . . . . . . . . 12 77 2.6. Validating Pinned Connections . . . . . . . . . . . . . . 13 78 2.7. Interactions With Preloaded Pin Lists . . . . . . . . . . 14 79 2.8. Pinning Self-Signed End Entities . . . . . . . . . . . . 14 80 3. Reporting Pin Validation Failure . . . . . . . . . . . . . . 14 81 4. Security Considerations . . . . . . . . . . . . . . . . . . . 17 82 4.1. Maximum max-age . . . . . . . . . . . . . . . . . . . . . 18 83 4.2. Using includeSubDomains Safely . . . . . . . . . . . . . 18 84 4.3. Backup Pins . . . . . . . . . . . . . . . . . . . . . . . 19 85 4.4. Interactions With Cookie Scoping . . . . . . . . . . . . 20 86 5. Privacy Considerations . . . . . . . . . . . . . . . . . . . 20 87 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21 88 7. Usability Considerations . . . . . . . . . . . . . . . . . . 22 89 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 22 90 9. What's Changed . . . . . . . . . . . . . . . . . . . . . . . 22 91 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 23 92 10.1. Normative References . . . . . . . . . . . . . . . . . . 23 93 10.2. Informative References . . . . . . . . . . . . . . . . . 24 94 Appendix A. Fingerprint Generation . . . . . . . . . . . . . . . 25 95 Appendix B. Deployment Guidance . . . . . . . . . . . . . . . . 25 96 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26 98 1. Introduction 100 This document defines a new HTTP header that enables a web host to 101 express to user agents (UAs) which Subject Public Key Info (SPKI) 102 structure(s) UAs SHOULD expect to be present in the host's 103 certificate chain in future connections using TLS [RFC5246]. We call 104 this "public key pinning" (PKP); in particular, this document 105 describes HTTP-based public key pinning (HPKP). At least one UA 106 (Google Chrome) has experimented with the idea by shipping with a 107 user-extensible embedded set of Pins. Although effective, this does 108 not scale. This proposal addresses the scale problem. 110 Deploying PKP safely will require operational and organizational 111 maturity due to the risk that hosts may make themselves unavailable 112 by pinning to a (set of) SPKI(s) that becomes invalid (see 113 Section 4). With care, host operators can greatly reduce the risk of 114 main-in-the-middle (MITM) attacks and other false-authentication 115 problems for their users without incurring undue risk. 117 PKP is meant to be used together with HTTP Strict Transport Security 118 (HSTS) [RFC6797], but it is possible to pin keys without requiring 119 HSTS. 121 A Pin is a relationship between a hostname and a cryptographic 122 identity (in this document, 1 or more of the public keys in a chain 123 of X.509 certificates). Pin Validation is the process a UA performs 124 to ensure that a host is in fact authenticated with its previously- 125 established Pin. 127 Key pinning is a trust-on-first-use (TOFU) mechanism. The first time 128 a UA connects to a host, it lacks the information necessary to 129 perform Pin Validation; UAs can only apply their normal cryptographic 130 identity validation. (In this document, it is assumed that UAs apply 131 X.509 certificate chain validation in accord with [RFC5280].) 133 The UA will not be able to detect and thwart a MITM attacking the 134 UA's first connection to the host. (However, the requirement that 135 the MITM provide an X.509 certificate chain that can pass the UA's 136 validation requirements, without error, mitigates this risk 137 somewhat.) Worse, such a MITM can inject its own PKP header into the 138 HTTP stream, and pin the UA to its own keys. To avoid post facto 139 detection, the attacker would have to be in a position to intercept 140 all future requests to the host from that UA. 142 Thus, key pinning as described in this document is not a perfect 143 defense against MITM attackers capable of passing certificate chain 144 validation procedures -- nothing short of pre-shared keys can be. 145 However, it provides significant value by allowing host operators to 146 limit the number of certification authorities than can vouch for the 147 host's identity, and allows UAs to detect in-process MITM attacks 148 after the initial communication. 150 1.1. Requirements Language 152 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 153 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 154 document are to be interpreted as described in RFC 2119 [RFC2119]. 156 2. Server and Client Behavior 158 2.1. Response Header Field Syntax 160 The "Public-Key-Pins" and "Public-Key-Pins-Report-Only" header 161 fields, also referred to within this specification as the PKP and 162 PKP-RO header fields, respectively, are new response headers defined 163 in this specification. They are used by a server to indicate that a 164 UA should perform Pin Validation (Section 2.6) for the host emitting 165 the response message, and to provide the necessary information for 166 the UA to do so. 168 Figure 1 describes the syntax (Augmented Backus-Naur Form) of the 169 header fields, using the grammar defined in [RFC5234] and the rules 170 defined in Section 3.2 of [RFC7230]. The field values of both header 171 fields conform to the same rules. 173 Public-Key-Directives = [ directive ] *( OWS ";" OWS [ directive ] ) 175 directive = simple-directive 176 / pin-directive 178 simple-directive = directive-name [ "=" directive-value ] 179 directive-name = token 180 directive-value = token 181 / quoted-string 183 pin-directive = "pin-" token "=" quoted-string 185 Figure 1: HPKP Header Syntax 187 Optional white space (OWS) is used as defined in Section 3.2.3 of 188 [RFC7230]. token and quoted-string are used as defined in 189 Section 3.2.6 of [RFC7230]. 191 The directives defined in this specification are described below. 192 The overall requirements for directives are: 194 1. The order of appearance of directives is not significant. 196 2. A given simple-directive MUST NOT appear more than once in a 197 given header field. Directives are either optional or required, 198 as stipulated in their definitions. 200 3. Directive names are case-insensitive. 202 4. UAs MUST ignore any header fields containing directives, or other 203 header field value data, that do not conform to the syntax 204 defined in this specification. In particular, UAs must not 205 attempt to fix malformed header fields. 207 5. If a header field contains any directive(s) the UA does not 208 recognize, the UA MUST ignore those directives. 210 6. If the PKP or PKP-RO header field otherwise satisfies the above 211 requirements (1 through 5), the UA MUST process the directives it 212 recognizes. 214 Additional directives extending the semantic functionality of the 215 header fields can be defined in other specifications. The first such 216 specification will need to define a reistry for such directives. 217 Such future directives will be ignored by UAs implementing only this 218 specification, as well as by generally non-conforming UAs. 220 In the pin-directive, the token is the name of a cryptographic hash 221 algorithm. The only algorithm allowed at this time is "sha256", i.e. 222 the hash algorithm SHA256 ([RFC4634]); additional algorithms may be 223 allowed for use in this context in the future. The quoted-string is 224 a sequence of base 64 digits: the base 64-encoded SPKI Fingerprint 225 [RFC4648] (see Section 2.4). 227 When a connection passes Pin Validation using the UA's noted Pins for 228 the host at the time, the host becomes a Known Pinned Host. 230 According to rule 5, above, the UA MUST ignore pin-directives with 231 tokens naming hash algorithms it does not recognize. If the set of 232 remaining effective pin-directives is empty, and if the host is a 233 Known Pinned Host, the UA MUST cease to consider the host as a Known 234 Pinned Host (the UA should fail open). The UA should indicate to 235 users that the host is no longer a Known Pinned Host. 237 2.1.1. The max-age Directive 239 The "max-age" directive specifies the number of seconds after the 240 reception of the PKP header field during which the UA SHOULD regard 241 the host (from whom the message was received) as a Known Pinned Host. 243 The "max-age" directive is REQUIRED to be present within a "Public- 244 Key-Pins" header field, and is OPTIONAL within a "Public-Key-Pins- 245 Report-Only" header field. 247 If present, the max-age directive is REQUIRED to have a directive 248 value, for which the the syntax (after quoted-string unescaping, if 249 necessary) is defined as: 251 max-age-value = delta-seconds 252 delta-seconds = 1*DIGIT 254 Figure 2: max-age Value Syntax 256 delta-seconds is used as defined in [RFC7234], Section 1.2.1. 258 2.1.2. The includeSubDomains Directive 260 The OPTIONAL includeSubDomains directive is a valueless directive 261 that, if present (i.e., it is "asserted"), signals to the UA that the 262 Pinning Policy applies to this Pinned Host as well as any subdomains 263 of the host's domain name. 265 2.1.3. The report-uri Directive 267 The OPTIONAL report-uri directive indicates the URI to which the UA 268 SHOULD report Pin Validation failures (Section 2.6). The UA POSTs 269 the reports to the given URI as described in Section 3. 271 When used in the PKP or PKP-RO headers, the presence of a report-uri 272 directive indicates to the UA that in the event of Pin Validation 273 failure it SHOULD POST a report to the report-uri. If the header is 274 Public-Key-Pins, the UA should do this in addition to terminating the 275 connection (as described in Section 2.6). 277 Hosts may set report-uris that use HTTP or HTTPS. If the scheme in 278 the report-uri is one that uses TLS (e.g. HTTPS), UAs MUST perform 279 Pinning Validation when the host in the report-uri is a Known Pinned 280 Host; similarly, UAs MUST apply HSTS if the host in the report-uri is 281 a Known HSTS Host. 283 Note that the report-uri need not necessarily be in the same Internet 284 domain or web origin as the host being reported about. 286 UAs SHOULD make their best effort to report Pin Validation failures 287 to the report-uri, but may fail to report in exceptional conditions. 288 For example, if connecting the report-uri itself incurs a Pinning 289 Validation failure or other certificate validation failure, the UA 290 MUST cancel the connection. Similarly, if Known Pinned Host A sets a 291 report-uri referring to Known Pinned Host B, and if B sets a report- 292 uri referring to A, and if both hosts fail Pin Validation, the UA 293 SHOULD detect and break the loop by failing to send reports to and 294 about those hosts. 296 In any case of report failure, the UA MAY attempt to re-send the 297 report later. 299 UAs SHOULD limit the rate at which they send reports. For example, 300 it is unnecessary to send the same report to the same report-uri more 301 than once per distinct set of declared Pins. 303 2.1.4. Examples 305 Figure 3 shows some example PKP and PKP-RO response header fields. 306 (Lines are folded to fit.) 308 Public-Key-Pins: max-age=3000; 309 pin-sha256="d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM="; 310 pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g="; 312 Public-Key-Pins: max-age=2592000; 313 pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g="; 314 pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ=" 316 Public-Key-Pins: max-age=2592000; 317 pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g="; 318 pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ="; 319 report-uri="http://example.com/pkp-report" 321 Public-Key-Pins-Report-Only: max-age=2592000; 322 pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g="; 323 pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ="; 324 report-uri="https://other.example.net/pkp-report" 326 Public-Key-Pins: 327 pin-sha256="d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM="; 328 pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ="; 329 max-age=259200 331 Public-Key-Pins: 332 pin-sha256="d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM="; 333 pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g="; 334 pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ="; 335 max-age=10000; includeSubDomains 337 Figure 3: HPKP Header Examples 339 2.2. Server Processing Model 341 This section describes the processing model that Pinned Hosts 342 implement. The model has 2 parts: (1) the processing rules for HTTP 343 request messages received over a secure transport (e.g. TLS); and 344 (2) the processing rules for HTTP request messages received over non- 345 secure transports, such as TCP. 347 2.2.1. HTTP-over-Secure-Transport Request Type 349 When replying to an HTTP request that was conveyed over a secure 350 transport, a Pinned Host SHOULD include in its response exactly one 351 PKP header field, exactly one PKP-RO header field, or one of each. 352 Each instance of either header field MUST satisfy the grammar 353 specified in Section 2.1. 355 Establishing a given host as a Known Pinned Host, in the context of a 356 given UA, is accomplished as follows: 358 1. Over the HTTP protocol running over secure transport, by 359 correctly returning (per this specification) at least one valid 360 PKP header field to the UA. 362 2. Through other mechanisms, such as a client-side pre-loaded Known 363 Pinned Host List. 365 2.2.2. HTTP Request Type 367 Pinned Hosts SHOULD NOT include the PKP header field in HTTP 368 responses conveyed over non-secure transport. UAs MUST ignore any 369 PKP header received in an HTTP response conveyed over non-secure 370 transport. 372 2.3. User Agent Processing Model 374 The UA processing model relies on parsing domain names. Note that 375 internationalized domain names SHALL be canonicalized according to 376 the scheme in Section 10 of [RFC6797]. 378 2.3.1. Public-Key-Pins Response Header Field Processing 380 If the UA receives, over a secure transport, an HTTP response that 381 includes a PKP header field conforming to the grammar specified in 382 Section 2.1, and there are no underlying secure transport errors or 383 warnings (see Section 2.5), the UA MUST either: 385 o Note the host as a Known Pinned Host if it is not already so noted 386 (see Section 2.3.3), 388 or, 390 o Update the UA's cached information for the Known Pinned Host if 391 any of of the max-age, includeSubDomains, or report-uri header 392 field value directives convey information different from that 393 already maintained by the UA. 395 The max-age value is essentially a "time to live" value relative to 396 the time of the most recent observation of the PKP header field. If 397 the max-age header field value token has a value of 0, the UA MUST 398 remove its cached Pinning Policy information (including the 399 includeSubDomains directive, if asserted) if the Pinned Host is 400 Known, or, MUST NOT note this Pinned Host if it is not yet Known. 402 If a UA receives more than one PKP header field or more than one PKP- 403 RO header fieled in an HTTP response message over secure transport, 404 then the UA MUST process only the first PKP header field (if present) 405 and only the first PKP-RO header field (if present). 407 If the UA receives the HTTP response over insecure transport, or if 408 the PKP header is not a Valid Pinning Header (see Section 2.5), the 409 UA MUST ignore any present PKP header field(s). Similarly, if the UA 410 receives the HTTP response over insecure transport, the UA MUST 411 ignore any present PKP-RO header field(s). The UA MUST ignore any 412 PKP or PKP-RO header fields not conforming to the grammar specified 413 in Section 2.1. 415 2.3.2. Interaction of Public-Key-Pins and Public-Key-Pins-Report-Only 417 A server MAY set both the "Public-Key-Pins" and "Public-Key-Pins- 418 Report-Only" headers simultaneously. The headers do not interact 419 with one another but the UA MUST process the PKP header and SHOULD 420 process both. 422 The headers are processed according to Section 2.3.1. 424 When the PKP-RO header is used with a report-uri, the UA SHOULD POST 425 reports for Pin Validation failures to the indicated report-uri, 426 although the UA MUST NOT enforce Pin Validation. That is, in the 427 event of Pin Validation failure when the host has set the PKP-RO 428 header, the UA performs Pin Validation only to check whether or not 429 it should POST a report, but not for causing connection failure. 431 Note: There is no purpose to using the PKP-RO header without the 432 report-uri directive. User Agents MAY discard such headers without 433 interpreting them further. 435 When the PKP header is used with a report-uri, the UA SHOULD POST 436 reports for Pin Validation failures to the indicated report-uri, as 437 well as enforcing Pin Validation. 439 If a host sets the PKP-RO header, the UA SHOULD note the Pins and 440 directives given in the PKP-RO header as specified by the max-age 441 directive. If the UA does note the Pins and directives in the PKP-RO 442 header it SHOULD evaluate the specified policy and SHOULD report any 443 would-be Pin Validation failures that would occur if the report-only 444 policy were enforced. 446 If a host sets both the PKP header and the PKP-RO header, the UA MUST 447 note and enforce Pin Validation as specified by the PKP header, and 448 SHOULD process the Pins and directives given in the PKP-RO header. 449 If the UA does process the Pins and directives in the PKP-RO header 450 it SHOULD evaluate the specified policy and SHOULD report any would- 451 be Pin Validation failures that would occur if the report-only policy 452 were enforced. 454 2.3.3. Noting a Pinned Host - Storage Model 456 The Effective Pin Date of a Known Pinned Host is the time that the UA 457 observed a Valid Pinning Header for the host. The Effective 458 Expiration Date of a Known Pinned Host is the Effective Pin Date plus 459 the max-age. A Known Pinned Host is "expired" if the Effective 460 Expiration Date refers to a date in the past. The UA MUST ignore any 461 expired Known Pinned Hosts in its cache. 463 For example, if a UA is beginning to perform Pin Validation for a 464 Known Pinned Host and finds that the cached pinning information for 465 the host indicates an Effective Expiration Date in the past, the UA 466 MUST NOT continue with Pin Validation for the host, and must consider 467 the host to no longer be a Known Pinned Host. 469 Known Pinned Hosts are identified only by domain names, and never IP 470 addresses. If the substring matching the host production from the 471 Request-URI (of the message to which the host responded) 472 syntactically matches the IP-literal or IPv4address productions from 473 Section 3.2.2 of [RFC3986], then the UA MUST NOT note this host as a 474 Known Pinned Host. 476 Otherwise, if the substring does not congruently match an existing 477 Known Pinned Host's domain name, per the matching procedure specified 478 in Section 8.2 of [RFC6797], then the UA MUST add this host to the 479 Known Pinned Host cache. The UA caches: 481 o the Pinned Host's domain name, 482 o the Effective Expiration Date, or enough information to calculate 483 it (the Effective Pin Date and the value of the max-age 484 directive), 486 o whether or not the includeSubDomains directive is asserted, and 488 o the value of the report-uri directive, if present. 490 If any other metadata from optional or future PKP header directives 491 are present in the Valid Pinning Header, and the UA understands them, 492 the UA MAY note them as well. 494 UAs MAY set an upper limit on the value of max-age, so that UAs that 495 have noted erroneous Pins (whether by accident or due to attack) have 496 some chance of recovering over time. If the server sets a max-age 497 greater than the UA's upper limit, the UA MAY behave as if the server 498 set the max-age to the UA's upper limit. For example, if the UA caps 499 max-age at 5184000 seconds (60 days), and a Pinned Host sets a max- 500 age directive of 90 days in its Valid Pinning Header, the UA MAY 501 behave as if the max-age were effectively 60 days. (One way to 502 achieve this behavior is for the UA to simply store a value of 60 503 days instead of the 90 day value provided by the Pinned Host.) For 504 UA implementation guidance on how to select a maximum max-age, see 505 Section 4.1. 507 The UA MUST NOT modify any pinning metadata of any superdomain 508 matched Known Pinned Host. 510 2.3.4. HTTP-Equiv Element Attribute 512 UAs MUST NOT heed http-equiv="Public-Key-Pins" or http-equiv="Public- 513 Key-Pins-Report-Only" attribute settings on elements 514 [W3C.REC-html401-19991224] in received content. 516 2.4. Semantics of Pins 518 An SPKI Fingerprint is defined as the output of a known cryptographic 519 hash algorithm whose input is the DER-encoded ASN.1 representation of 520 the subjectPublicKeyInfo (SPKI) field of an X.509 certificate. A Pin 521 is defined as the combination of the known algorithm identifier and 522 the SPKI Fingerprint computed using that algorithm. 524 The SPKI Fingerprint is encoded in base 64 for use in an HTTP header 525 [RFC4648]. 527 In this version of the specification, the known cryptographic hash 528 algorithm is SHA-256, identified as "sha256" [RFC6234]. (Future 529 specifications may add new algorithms and deprecate old ones.) UAs 530 MUST ignore Pins for which they do not recognize the algorithm 531 identifier. UAs MUST continue to process the rest of a PKP response 532 header field and note Pins for algorithms they do recognize; UAs MUST 533 recognize "sha256". 535 Figure 4 reproduces the definition of the SubjectPublicKeyInfo 536 structure in [RFC5280]. 538 SubjectPublicKeyInfo ::= SEQUENCE { 539 algorithm AlgorithmIdentifier, 540 subjectPublicKey BIT STRING } 542 AlgorithmIdentifier ::= SEQUENCE { 543 algorithm OBJECT IDENTIFIER, 544 parameters ANY DEFINED BY algorithm OPTIONAL } 546 Figure 4: SPKI Definition 548 If the certificate's subjectPublicKeyInfo is incomplete when taken in 549 isolation, such as when holding a DSA key without domain parameters, 550 a public key pin cannot be formed. 552 We pin public keys, rather than entire certificates, to enable 553 operators to generate new certificates containing old public keys 554 (see [why-pin-key]). 556 See Appendix A for an example non-normative program that generates 557 SPKI Fingerprints from certificates. 559 2.5. Noting Pins 561 Upon receipt of the PKP response header field, the UA notes the host 562 as a Known Pinned Host, storing the Pins and their associated 563 directives in non-volatile storage (for example, along with the HSTS 564 metadata). The Pins and their associated directives are collectively 565 known as Pinning Metadata. 567 The UA MUST note the Pins for a Host if and only if all three of the 568 following conditions hold: 570 o It received the PKP response header field over an error-free TLS 571 connection. If the host is a Pinned Host, this includes the 572 validation added in Section 2.6. 574 o The TLS connection was authenticated with a certificate chain 575 containing at least one of the SPKI structures indicated by at 576 least one of the given SPKI Fingerprints (see Section 2.6). 578 o The given set of Pins contains at least one Pin that does NOT 579 refer to an SPKI in the certificate chain. (That is, the host 580 must set a Backup Pin; see Section 4.3.) 582 If the PKP response header field does not meet all three of these 583 criteria, the UA MUST NOT note the host as a Pinned Host. A PKP 584 response header field that meets all these critera is known as a 585 Valid Pinning Header. 587 Whenever a UA receives a Valid Pinning Header, it MUST set its 588 Pinning Metadata to the exact Pins, Effective Expiration Date 589 (computed from max-age), and (if any) report-uri given in the most 590 recently received Valid Pinning Header. 592 For forward compatibility, the UA MUST ignore any unrecognized PKP 593 and PKP-RO header directives, while still processing those directives 594 it does recognize. Section 2.1 specifies the directives max-age, 595 Pins, includeSubDomains, and report-uri but future specifications and 596 implementations might use additional directives. 598 Upon receipt of a PKP-RO response header field, the UA SHOULD 599 evaluate the policy expressed in the field, and SHOULD generate and 600 send a report (see Section 3). However, failure to validate the Pins 601 in the field MUST have no effect on the validity or non-validity of 602 the policy expressed in the PKP field or in previously-noted Pins for 603 the Known Pinned Host. 605 The UA need not note any Pins or other policy expressed in the PKP-RO 606 response header field, except for the purpose of determining that it 607 has already sent a report for a given policy. UAs SHOULD make a best 608 effort not to inundate report-uris with redundant reports. 610 2.6. Validating Pinned Connections 612 When a UA connects to a Pinned Host, if the TLS connection has 613 errors, the UA MUST terminate the connection without allowing the 614 user to proceed anyway. (This behavior is the same as that required 615 by [RFC6797].) 617 If the connection has no errors, then the UA will determine whether 618 to apply a new, additional correctness check: Pin Validation. A UA 619 SHOULD perform Pin Validation whenever connecting to a Known Pinned 620 Host. It is acceptable to allow Pin Validation to be disabled for 621 some Hosts according to local policy. For example, a UA may disable 622 Pin Validation for Pinned Hosts whose validated certificate chain 623 terminates at a user-defined trust anchor, rather than a trust anchor 624 built-in to the UA. 626 To perform Pin Validation, the UA will compute the SPKI Fingerprints 627 for each certificate in the Pinned Host's validated certificate 628 chain, using each supported hash algorithm for each certificate. (As 629 described in Section 2.4, certificates whose SPKI cannot be taken in 630 isolation cannot be pinned.) The UA MUST ignore superfluous 631 certificates in the chain that do not form part of the validating 632 chain. The UA will then check that the set of these SPKI 633 Fingerprints intersects the set of SPKI Fingerprints in that Pinned 634 Host's Pinning Metadata. If there is set intersection, the UA 635 continues with the connection as normal. Otherwise, the UA MUST 636 treat this Pin Validation Failure as a non-recoverable error. Any 637 procedure that matches the results of this Pin Validation procedure 638 is considered equivalent. 640 A UA that has previous noted a host as a Known Pinned Host MUST 641 perform Pin Validation when setting up the TLS session, before 642 beginning an HTTP conversation over the TLS channel. 644 UAs send validation failure reports only when Pin Validation is 645 actually in effect. Pin Validation might not be in effect e.g. 646 because the user has elected to disable it, or because a presented 647 certificate chain chains up to a locally-installed anchor. In such 648 cases, UAs SHOULD NOT send reports. 650 2.7. Interactions With Preloaded Pin Lists 652 UAs MAY choose to implement additional sources of pinning 653 information, such as through built-in lists of pinning information. 654 Such UAs should allow users to override such additional sources, 655 including disabling them from consideration. 657 The effective policy for a Known Pinned Host that has both built-in 658 Pins and Pins from previously observed PKP header response fields is 659 implementation-defined. 661 2.8. Pinning Self-Signed End Entities 663 If UAs accept hosts that authenticate themselves with self-signed end 664 entity certificates, they MAY also allow hosts to pin the public keys 665 in such certificates. The usability and security implications of 666 this practice are outside the scope of this specification. 668 3. Reporting Pin Validation Failure 670 When a Known Pinned Host has set the report-uri directive, the UA 671 SHOULD report Pin Validation failures to the indicated URI. The UA 672 does this by POSTing a JSON [RFC4627] message to the URI; the JSON 673 message takes this form: 675 { 676 "date-time": date-time, 677 "hostname": hostname, 678 "port": port, 679 "effective-expiration-date": expiration-date, 680 "include-subdomains": include-subdomains, 681 "served-certificate-chain": [ 682 pem1, ... pemN 683 ], 684 "validated-certificate-chain": [ 685 pem1, ... pemN 686 ], 687 "known-pins": [ 688 known-pin1, ... known-pinN 689 ] 690 } 692 Figure 5: JSON Report Format 694 Whitespace outside of quoted strings is not significant. The key/ 695 value pairs may appear in any order, but each MUST appear only once. 697 The date-time indicates the time the UA observed the Pin Validation 698 failure. It is provided as a string formatted according to 699 Section 5.6, "Internet Date/Time Format", of [RFC3339]. 701 The hostname is the hostname to which the UA made the original 702 request that failed Pin Validation. It is provided as a string. 704 The port is the port to which the UA made the original request that 705 failed Pin Validation. It is provided as an integer. 707 The effective-expiration-date is the Effective Expiration Date for 708 the noted Pins. It is provided as a string formatted according to 709 Section 5.6, "Internet Date/Time Format", of [RFC3339]. 711 include-subdomains indicates whether or not the UA has noted the 712 includeSubDomains directive for the Known Pinned Host. It is 713 provided as one of the JSON identifiers "true" or "false". 715 The served-certificate-chain is the certificate chain, as served by 716 the Known Pinned Host during TLS session setup. It is provided as an 717 array of strings; each string pem1, ... pemN is the PEM 718 representation of each X.509 certificate as described in 719 [I-D.josefsson-pkix-textual]. 721 The validated-certificate-chain is the certificate chain, as 722 constructed by the UA during certificate chain verification. (This 723 may differ from the served-certificate-chain.) It is provided as an 724 array of strings; each string pem1, ... pemN is the PEM 725 representation of each X.509 certificate as described in 726 [I-D.josefsson-pkix-textual]. For UAs that build certificate chains 727 in more than one way during the validation process, they SHOULD send 728 the last chain built. In this way they can avoid keeping too much 729 state during the validation process. 731 The known-pins are the Pins that the UA has noted for the Known 732 Pinned Host. They are provided as an array of strings with the 733 syntax: 735 known-pin = token "=" quoted-string 737 Figure 6: Known Pin Syntax 739 As in Section 2.4, the token refers to the algorithm name, and the 740 quoted-string refers to the base 64 encoding of the SPKI Fingerprint. 741 When formulating the JSON POST body, the UA MUST either use single- 742 quoted JSON strings, or use double-quoted JSON strings and \-escape 743 the embedded double quotes in the quoted-string part of the known- 744 pin. 746 Figure 7 shows an example of a Pin Validation failure report. (PEM 747 strings are shown on multiple lines for readability.) 749 { 750 "date-time": "2014-04-06T13:00:50Z", 751 "hostname": "www.example.com", 752 "port": 443, 753 "effective-expiration-date": "2014-05-01T12:40:50Z" 754 "include-subdomains": false, 755 "served-certificate-chain": [ 756 "-----BEGIN CERTIFICATE-----\n 757 MIIEBDCCAuygAwIBAgIDAjppMA0GCSqGSIb3DQEBBQUAMEIxCzAJBgNVBAYTAlVT\n 758 ... 759 HFa9llF7b1cq26KqltyMdMKVvvBulRP/F/A8rLIQjcxz++iPAsbw+zOzlTvjwsto\n 760 WHPbqCRiOwY1nQ2pM714A5AuTHhdUDqB1O6gyHA43LL5Z/qHQF1hwFGPa4NrzQU6\n 761 yuGnBXj8ytqU0CwIPX4WecigUCAkVDNx\n 762 -----END CERTIFICATE-----", 763 ... 764 ], 765 "validated-certificate-chain": [ 766 "-----BEGIN CERTIFICATE-----\n 767 MIIEBDCCAuygAwIBAgIDAjppMA0GCSqGSIb3DQEBBQUAMEIxCzAJBgNVBAYTAlVT\n 768 ... 769 HFa9llF7b1cq26KqltyMdMKVvvBulRP/F/A8rLIQjcxz++iPAsbw+zOzlTvjwsto\n 770 WHPbqCRiOwY1nQ2pM714A5AuTHhdUDqB1O6gyHA43LL5Z/qHQF1hwFGPa4NrzQU6\n 771 yuGnBXj8ytqU0CwIPX4WecigUCAkVDNx\n 772 -----END CERTIFICATE-----", 773 ... 774 ], 775 "known-pins": [ 776 'pin-sha256="d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM="', 777 "pin-sha256=\"E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g=\"" 778 ] 779 } 781 Figure 7: Pin Validation Failure Report Example 783 4. Security Considerations 785 Pinning public keys helps hosts strongly assert their cryptographic 786 identity even in the face of issuer error, malfeasance or compromise. 787 But there is some risk that a host operator could lose or lose 788 control of their host's private key (such as by operator error or 789 host compromise). If the operator had pinned only the key of the 790 host's end entity certificate, the operator would not be able to 791 serve their web site or application in a way that UAs would trust for 792 the duration of their pin's max-age. (Recall that UAs MUST close the 793 connection to a host upon Pin Failure.) 795 Therefore, there is a necessary trade-off between two competing 796 goods: pin specificity and maximal reduction of the scope of issuers 797 on the one hand; and flexibility and resilience of the host's 798 cryptographic identity on the other hand. One way to resolve this 799 trade-off is to compromise by pinning to the key(s) of the issuer(s) 800 of the host's end entity certificate(s). Often, a valid certificate 801 chain will have at least two certificates above the end entity 802 certificate: the intermediate issuer, and the trust anchor. 803 Operators can pin any one or more of the public keys in this chain, 804 and indeed could pin to issuers not in the chain (as, for example, a 805 Backup Pin). Pinning to an intermediate issuer, or even to a trust 806 anchor or root, still significantly reduces the number of issuers who 807 can issue end entity certificates for the Known Pinned Host, while 808 still giving that host flexibility to change keys without a 809 disruption of service. 811 4.1. Maximum max-age 813 As mentioned in Section 2.3.3, UAs MAY cap the max-age value at some 814 upper limit. There is a security trade-off in that low maximum 815 values provide a narrow window of protection for users who visit the 816 Known Pinned Host only infrequently, while high maximum values might 817 potentially result in a UA's inability to successfully perform Pin 818 Validation for a Known Pinned Host if the UA's noted Pins and the 819 host's true Pins diverge. 821 Such divergence could occur for several reasons, including: UA error; 822 host operator error; network attack; or a Known Pinned Host that 823 intentionally migrates all pinned keys, combined with a UA that has 824 noted true Pins with a high max-age value and has not had a chance to 825 observe the new true Pins for the host. (This last example 826 underscores the importance for host operators to phase in new keys 827 gradually, and to set the max-age value in accordance with their 828 planned key migration schedule.) 830 There is probably no ideal upper limit to the max-age directive that 831 would satisfy all use cases. However, a value on the order of 60 832 days (5,184,000 seconds) may be considered a balance between the two 833 competing security concerns. 835 4.2. Using includeSubDomains Safely 837 It may happen that Pinned Hosts whose hostnames share a parent domain 838 use different Valid Pinning Headers. If a host whose hostname is a 839 parent domain for another host sets the includeSubDomains directive, 840 the two hosts' Pins may conflict with each other. For example, 841 consider two Known Pinned Hosts, example.com and 842 subdomain.example.com. Assume example.com sets a Valid Pinning 843 Header such as this: 845 Public-Key-Pins: max-age=12000; pin-sha256="ABC..."; 846 pin-sha256="DEF..."; includeSubDomains 848 Figure 8: example.com Valid Pinning Header 850 Assume subdomain.example.com sets a Valid Pinning Header such as 851 this: 853 Public-Key-Pins: pin-sha256="GHI..."; pin-sha256="JKL..." 855 Figure 9: subdomain.example.com Valid Pinning Header 857 Assume a UA that has not previously noted any Pins for either of 858 these hosts. If the UA first contacts subdomain.example.com, it will 859 note the Pins in the Valid Pinning Header, and perform Pin Validation 860 as normal on subsequent conections. If the UA then contacts 861 example.com, again it will note the Pins and perform Pin Validation 862 on future connections. 864 However, if the UA happened to visit example.com before 865 subdomain.example.com, the UA would, due to example.com's use of the 866 includeSubDomains directive, attempt to perform Pin Validation for 867 subdomain.example.com using the SPKI hashes ABC... and DEF..., which 868 are not valid for the certificate chains subdomain.example.com (which 869 uses certificates with SPKIs GHI... and JLK...). Thus, depending on 870 the order in which the UA observes the Valid Pinning Headers for 871 hosts example.com and subdomain.example.com, Pin Validation might or 872 might not fail for subdomain.example.com, even if the certificate 873 chain the UA receives for subdomain.example.com is perfectly valid. 875 Thus, Pinned Host operators must use the includeSubDomains directive 876 with care. For example, they may choose to use overlapping pin sets 877 for hosts under a parent domain that uses includeSubDomains, or to 878 not use the includeSubDomains directive in their effective-second- 879 level domains, or to simply use the same pin set for all hosts under 880 a given parent domain. 882 4.3. Backup Pins 884 The primary way to cope with the risk of inadvertent Pin Validation 885 Failure is to keep a Backup Pin. A Backup Pin is a fingerprint for 886 the public key of a secondary, not-yet-deployed key pair. The 887 operator keeps the backup key pair offline, and sets a pin for it in 888 the PKP header. Then, in case the operator loses control of their 889 primary private key, they can deploy the backup key pair. UAs, who 890 have had the backup key pair pinned (when it was set in previous 891 Valid Pinning Headers), can connect to the host without error. 893 Because having a backup key pair is so important to recovery, UAs 894 MUST require that hosts set a Backup Pin (see Section 2.5). The down 895 side of keeping a not-yet-deployed key pair is that if an attacker 896 gains control of the private key she will be able to perform a MITM 897 attack without being discovered. Operators must take care to avoid 898 leaking the key such as keeping it offline. 900 4.4. Interactions With Cookie Scoping 902 HTTP cookies [RFC6265] set by a Known Pinned Host can be stolen by a 903 network attacker who can forge web and DNS responses so as to cause a 904 client to send the cookies to a phony subdomain of the host. To 905 prevent this, hosts SHOULD set the "secure" attribute and omit the 906 "domain" attribute on all security-sensitive cookies, such as session 907 cookies. These settings tell the browser that the cookie should only 908 be sent back to the originating host (not its subdomains), and should 909 only be sent over HTTPS (not HTTP). 911 5. Privacy Considerations 913 Hosts can use HSTS or HPKP as a "super-cookie", by setting distinct 914 policies for a number of subdomains. For example, assume example.com 915 wishes to track distinct UAs without explicitly setting a cookie, or 916 if a previously-set cookie is deleted from the UA's cookie store. 917 Here are two attack scenarios. 919 o example.com can use report-uri and the ability to pin arbitrary 920 identifiers to distinguish UAs. 922 1. example.com sets a Valid Pinning Header in its response to 923 requests. The header asserts the includeSubDomains directive, 924 and specifies a report-uri directive as well. Pages served by 925 the host also include references to subresource 926 https://bad.example.com/foo.png. 928 2. The Valid Pinning Header includes a "pin" that is not really 929 the hash of an SPKI, but is instead an arbitrary 930 distinguishing string sent only in response to a particular 931 request. For each request, the host creates a new, distinct 932 distinguishing string and sets it as if it were a pin. 934 3. The certificate chain served by bad.example.com does not pass 935 Pin Validation given the pin set the host asserted in (1). 936 The HPKP-conforming UA attempts to report the Pin Validation 937 failure to the specified report-uri, including the certificate 938 chain it observed and the SPKI hashes it expected to see. 939 Among the SPKI hashes is the distinguishing string in step 940 (2). 942 4. Different site operators/origins can optionally collaborate by 943 setting the report-uri to be in an origin they share 944 administrative control of. UAs MAY, therefore, refuse to send 945 reports outside of the origin that set the PKP or PKP-RO 946 header. 948 o example.com can use server name indication (SNI; [RFC3546]) and 949 subdomains to distinguish UAs. 951 1. example.com sets a Valid Pinning Header in its response to 952 requests. The header asserts the includeSubDomains directive. 954 2. On a subsequent page view, the host responds with a page 955 including the subresource https://0.fingerprint.example.com/ 956 foo.png, and the server responds using a certificate chain 957 that does not pass Pin Validation for the pin-set defined in 958 the Valid Pinning Header in step (1). The HPKP-conforming UA 959 will close the connection, never completing the request to 960 0.fingerprint.example.com. The host may thus note that this 961 particular UA had noted the (good) Pins for that subdomain. 963 3. example.com can distinguish 2^N UAs by serving Valid Pinning 964 Headers from an arbitrary number N distinct subdomains, giving 965 some UAs Valid Pinning Headers for some, but not all 966 subdomains (causing subsequent requests for 967 n.fingerprint.example.com to fail), and giving some UAs no 968 Valid Pinning Header for other subdomains (causing subsequent 969 requests for m.fingerprint.example.com to succeed). 971 Conforming implementations (as well as implementations conforming to 972 [RFC6797]) must store state about which domains have set policies, 973 hence which domains the UA has contacted. A forensic attacker might 974 find this information useful, even if the user has cleared other 975 parts of the UA's state. 977 6. IANA Considerations 979 IANA is requested to register the response headers described in this 980 document in the "Message Headers" registry ([permanent-headers] with 981 the following parameters: 983 o Header Field Names should be "Public-Key-Pins" and "Public-Key- 984 Pins-Report-Only". 986 o Protocol should be "http" 988 o Status should be "standard" 989 o Reference should be this document 991 7. Usability Considerations 993 When pinning works to detect impostor Pinned Hosts, users will 994 experience denial of service. It is advisable for UAs to explain the 995 reason why, i.e. that it was impossible to verify the confirmed 996 cryptographic identity of the host. 998 It is advisable that UAs have a way for users to clear current Pins 999 for Pinned Hosts, and to allow users to query the current state of 1000 Pinned Hosts. 1002 8. Acknowledgements 1004 Thanks to Tobias Gondrom, Jeff Hodges, Paul Hoffman, Ivan Krstic, 1005 Adam Langley, Barry Leiba, Nicolas Lidzborski, SM, James Manger, Yoav 1006 Nir, Trevor Perrin, Eric Rescorla, Tom Ritter, and Yan Zhu for 1007 suggestions and edits that clarified the text. 1009 9. What's Changed 1011 [RFC EDITOR: PLEASE REMOVE THIS SECTION] 1013 Clarified that max-age is REQUIRED for PKP, but OPTIONAL for PKP-RO 1014 (where it has no effect. 1016 Updated header field syntax and description to match that in 1017 [RFC7230]. 1019 Updated normative references to current documents. 1021 Removed the strict directive. 1023 Removed the requirement that the server set the Valid Pinning Header 1024 on every response. 1026 Added normative references for SHA, JSON, and base-64. 1028 Added the Privacy Considerations section. 1030 Changed non-normative pin generation code from Go to POSIX shell 1031 script using openssl. 1033 Changed max-max-age from SHOULD to MAY, and used the example of 60 1034 days instead of 30. 1036 Removed the section "Pin Validity Times", which was intended to be in 1037 harmony with [I-D.perrin-tls-tack]. Now using max-age purely as 1038 specified in [RFC6797]. 1040 Added new directives: includeSubDomains, report-uri and strict. 1042 Added a new variant of the PKP Header: Public-Key-Pins-Report-Only. 1044 Removed the section on pin break codes and verifiers, in favor the of 1045 most-recently-received policy (Section 2.5). 1047 Now using a new header field, Public-Key-Pins, separate from HSTS. 1048 This allows hosts to use pinning separately from Strict Transport 1049 Security. 1051 Explicitly requiring that UAs perform Pin Validation before the HTTP 1052 conversation begins. 1054 Backup Pins are now required. 1056 Separated normative from non-normative material. Removed tangential 1057 and out-of-scope non-normative discussion. 1059 10. References 1061 10.1. Normative References 1063 [I-D.josefsson-pkix-textual] 1064 Josefsson, S. and S. Leonard, "Text Encodings of PKIX and 1065 CMS Structures", draft-josefsson-pkix-textual-05 (work in 1066 progress), July 2014. 1068 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1069 Requirement Levels", BCP 14, RFC 2119, March 1997. 1071 [RFC3339] Klyne, G., Ed. and C. Newman, "Date and Time on the 1072 Internet: Timestamps", RFC 3339, July 2002. 1074 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 1075 Resource Identifier (URI): Generic Syntax", STD 66, RFC 1076 3986, January 2005. 1078 [RFC4627] Crockford, D., "The application/json Media Type for 1079 JavaScript Object Notation (JSON)", RFC 4627, July 2006. 1081 [RFC4634] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms 1082 (SHA and HMAC-SHA)", RFC 4634, July 2006. 1084 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data 1085 Encodings", RFC 4648, October 2006. 1087 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 1088 Specifications: ABNF", STD 68, RFC 5234, January 2008. 1090 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 1091 (TLS) Protocol Version 1.2", RFC 5246, August 2008. 1093 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 1094 Housley, R., and W. Polk, "Internet X.509 Public Key 1095 Infrastructure Certificate and Certificate Revocation List 1096 (CRL) Profile", RFC 5280, May 2008. 1098 [RFC6234] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms 1099 (SHA and SHA-based HMAC and HKDF)", RFC 6234, May 2011. 1101 [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265, 1102 April 2011. 1104 [RFC6797] Hodges, J., Jackson, C., and A. Barth, "HTTP Strict 1105 Transport Security (HSTS)", RFC 6797, November 2012. 1107 [RFC7230] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol 1108 (HTTP/1.1): Message Syntax and Routing", RFC 7230, June 1109 2014. 1111 [RFC7234] Fielding, R., Nottingham, M., and J. Reschke, "Hypertext 1112 Transfer Protocol (HTTP/1.1): Caching", RFC 7234, June 1113 2014. 1115 [W3C.REC-html401-19991224] 1116 Raggett, D., Hors, A., and I. Jacobs, "HTML 4.01 1117 Specification", World Wide Web Consortium Recommendation 1118 REC-html401-19991224, December 1999, 1119 . 1121 [permanent-headers] 1122 Klyne, G., "Permanent Message Header Field Names", July 1123 2014, . 1126 10.2. Informative References 1128 [I-D.perrin-tls-tack] 1129 Marlinspike, M., "Trust Assertions for Certificate Keys", 1130 draft-perrin-tls-tack-02 (work in progress), January 2013. 1132 [RFC3546] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., 1133 and T. Wright, "Transport Layer Security (TLS) 1134 Extensions", RFC 3546, June 2003. 1136 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 1137 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 1138 May 2008. 1140 [why-pin-key] 1141 Langley, A., "Public Key Pinning", May 2011, 1142 . 1144 Appendix A. Fingerprint Generation 1146 This POSIX shell program generates SPKI Fingerprints, suitable for 1147 use in pinning, from PEM-encoded certificates. It is non-normative. 1149 openssl x509 -noout -in certificate.pem -pubkey | \ 1150 openssl asn1parse -noout -inform pem -out public.key 1151 openssl dgst -sha256 -binary public.key | base64 1153 Figure 10: Example SPKI Fingerprint Generation Code 1155 Appendix B. Deployment Guidance 1157 This section is non-normative guidance which may smooth the adoption 1158 of public key pinning. 1160 o Operators should get the backup public key signed by a different 1161 (root and/or intermediary) CA than their primary certificate, and 1162 store the backup key pair safely offline. The semantics of an 1163 SPKI Fingerprint do not require the issuance of a certificate to 1164 construct a valid Pin. However, in many deployment scenarios, in 1165 order to make a Backup Pin operational the server operator will 1166 need to have a certificate to deploy TLS on the host. Failure to 1167 obtain a certificate through prior arrangement will leave clients 1168 that recognize the site as a Known Pinned Host unable to 1169 successfully perform Pin Validation until such a time as the 1170 operator can obtain a new certificate from their desired 1171 certificate issuer. 1173 o It is most economical to have the backup certificate signed by a 1174 completely different signature chain than the live certificate, to 1175 maximize recoverability in the event of either root or 1176 intermediary signer compromise. 1178 o Operators should periodically exercise their Backup Pin plan -- an 1179 untested backup is no backup at all. 1181 o Operators should start small. Operators should first deploy 1182 public key pinning by using the report-only mode together with a 1183 report-uri directive that points to a reliable report collection 1184 endpoint. When moving out of report-only mode, operators should 1185 start by setting a max-age of minutes or a few hours, and 1186 gradually increase max-age as they gain confidence in their 1187 operational capability. 1189 Authors' Addresses 1191 Chris Evans 1192 Google, Inc. 1193 1600 Amphitheatre Pkwy 1194 Mountain View, CA 94043 1195 US 1197 Email: cevans@google.com 1199 Chris Palmer 1200 Google, Inc. 1201 1600 Amphitheatre Pkwy 1202 Mountain View, CA 94043 1203 US 1205 Email: palmer@google.com 1207 Ryan Sleevi 1208 Google, Inc. 1209 1600 Amphitheatre Pkwy 1210 Mountain View, CA 94043 1211 US 1213 Email: sleevi@google.com