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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: January 4, 2015 Google, Inc. 6 July 3, 2014 8 Public Key Pinning Extension for HTTP 9 draft-ietf-websec-key-pinning-18 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 January 4, 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 . . . . . . . . . . . . . . . . . . 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. 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). At least one UA (Google Chrome) has 105 experimented with the idea by shipping with a user-extensible 106 embedded set of Pins. Although effective, this does not scale. This 107 proposal addresses the scale problem. 109 Deploying PKP safely will require operational and organizational 110 maturity due to the risk that hosts may make themselves unavailable 111 by pinning to a (set of) SPKI(s) that becomes invalid (see 112 Section 4). With care, host operators can greatly reduce the risk of 113 main-in-the-middle (MITM) attacks and other false-authentication 114 problems for their users without incurring undue risk. 116 PKP is meant to be used toegether with HTTP Strict Transport Security 117 (HSTS) [RFC6797], but is possible to pin keys without requiring HSTS. 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" and "Public-Key-Pins-Report-Only" header 130 fields, also referred to within this specification as the PKP and 131 PKP-RO header fields, respectively, are new response headers defined 132 in this specification. They are used by a server to indicate that a 133 UA should perform Pin Validation (Section 2.6) for the host emitting 134 the response message, and to provide the necessary information for 135 the UA to do so. 137 Figure 1 describes the syntax (Augmented Backus-Naur Form) of the 138 header fields, using the grammar defined in [RFC5234] and the rules 139 defined in Section 3.2 of [RFC7230]. The field values of both header 140 fields conform to the same rules. 142 Public-Key-Directives = [ directive ] *( OWS ";" OWS [ directive ] ) 144 directive = simple-directive 145 / pin-directive 147 simple-directive = directive-name [ "=" directive-value ] 148 directive-name = token 149 directive-value = token 150 / quoted-string 152 pin-directive = "pin-" token "=" quoted-string 154 Figure 1: HPKP Header Syntax 156 OWS is used as defined in Section 3.2.3 of [RFC7230]. token and 157 quoted-string are used as defined in Section 3.2.6 of [RFC7230]. 159 The directives defined in this specification are described below. 160 The overall requirements for directives are: 162 1. The order of appearance of directives is not significant. 164 2. A given simple-directive MUST NOT appear more than once in a 165 given header field. Directives are either optional or required, 166 as stipulated in their definitions. 168 3. Directive names are case-insensitive. 170 4. UAs MUST ignore any header fields containing directives, or other 171 header field value data, that do not conform to the syntax 172 defined in this specification. In particular, UAs must not 173 attempt to fix malformed header fields. 175 5. If a header field contains any directive(s) the UA does not 176 recognize, the UA MUST ignore those directives. 178 6. If the PKP or PKP-RO 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 183 header fields can be defined in other specifications. The first such 184 specification will need to define a reistry for such directives. 185 Such future directives will be ignored by UAs implementing only this 186 specification, as well as by generally non-conforming UAs. 188 In the pin-directive, the token is the name of a cryptographic hash 189 algorithm. The only algorithm allowed at this time is "sha256"; 190 additional algorithms may be defined in the future. The quoted- 191 string is a sequence of base 64 digits: the base 64-encoded SPKI 192 Fingerprint [RFC4648] (see Section 2.4). 194 When a connection passes Pin Validation using the UA's noted Pins for 195 the host at the time, the host becomes a Known Pinned Host. 197 According to rule 5, above, the UA MUST ignore pin-directives with 198 tokens naming hahs algorithms it does not recognize. If the set of 199 remaining effective pin-directives is empty, and if the host is a 200 Known Pinned Host, the UA MUST cease to consider the host as a Known 201 Pinned Host (the UA should fail open). The UA SHOULD indicate to 202 users that the host is no longer a Known Pinned Host. 204 2.1.1. The max-age Directive 206 The "max-age" directive specifies the number of seconds after the 207 reception of the PKP header field during which the UA SHOULD regard 208 the host (from whom the message was received) as a Known Pinned Host. 210 The "max-age" directive is REQUIRED to be present within a "Public- 211 Key-Pins" header field, and is OPTIONAL within a "Public-Key-Pins- 212 Report-Only" header field. 214 If present, the max-age directive is REQUIRED to have a directive 215 value, for which the the syntax (after quoted-string unescaping, if 216 necessary) is defined as: 218 max-age-value = delta-seconds 219 delta-seconds = 1*DIGIT 221 Figure 2: max-age Value Syntax 223 delta-seconds is used as defined in [RFC7234], Section 1.2.1. 225 2.1.2. The includeSubDomains Directive 227 The OPTIONAL includeSubDomains directive is a valueless directive 228 that, if present (i.e., it is "asserted"), signals to the UA that the 229 Pinning Policy applies to this Pinned Host as well as any subdomains 230 of the host's domain name. 232 2.1.3. The report-uri Directive 234 The OPTIONAL report-uri directive indicates the URI to which the UA 235 SHOULD report Pin Validation failures (Section 2.6). The UA POSTs 236 the reports to the given URI as described in Section 3. 238 When used in the PKP or PKP-RO headers, the presence of a report-uri 239 directive indicates to the UA that in the event of Pin Validation 240 failure it SHOULD POST a report to the report-uri. If the header is 241 Public-Key-Pins, the UA should do this in addition to terminating the 242 connection (as described in Section 2.6). 244 Hosts may set report-uris that use HTTP, HTTPS, or other schemes. If 245 the scheme in the report-uri is one that uses TLS (e.g. HTTPS or 246 WSS), UAs MUST perform Pinning Validation when the host in the 247 report-uri is a Known Pinned Host; similarly, UAs MUST apply HSTS if 248 the host in the report-uri is a Known HSTS Host. 250 Note that the report-uri need not necessarily be in the same Internet 251 domain or web origin as the host being reported about. 253 UAs SHOULD make their best effort to report Pin Validation failures 254 to the report-uri, but may fail to report in exceptional conditions. 255 For example, if connecting the report-uri itself incurs a Pinning 256 Validation failure or other certificate validation failure, the UA 257 MUST cancel the connection. Similarly, if Known Pinned Host A sets a 258 report-uri referring to Known Pinned Host B, and if B sets a report- 259 uri referring to A, and if both hosts fail Pin Validation, the UA 260 SHOULD detect and break the loop by failing to send reports to and 261 about those hosts. 263 In any case of report failure, the UA MAY attempt to re-send the 264 report later. 266 UAs SHOULD limit the rate at which they send reports. For example, 267 it is unnecessary to send the same report to the same report-uri more 268 than once per distinct set of declared Pins. 270 2.1.4. Examples 272 Figure 3 shows some example PKP and PKP-RO response header fields. 273 (Lines are folded to fit.) 274 Public-Key-Pins: max-age=3000; 275 pin-sha256="d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM="; 276 pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g="; 278 Public-Key-Pins: max-age=2592000; 279 pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g="; 280 pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ=" 282 Public-Key-Pins: max-age=2592000; 283 pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g="; 284 pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ="; 285 report-uri="http://example.com/pkp-report" 287 Public-Key-Pins-Report-Only: max-age=2592000; 288 pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g="; 289 pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ="; 290 report-uri="https://other.example.net/pkp-report" 292 Public-Key-Pins: 293 pin-sha256="d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM="; 294 pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ="; 295 max-age=259200 297 Public-Key-Pins: 298 pin-sha256="d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM="; 299 pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g="; 300 pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ="; 301 max-age=10000; includeSubDomains 303 Figure 3: HPKP Header Examples 305 2.2. Server Processing Model 307 This section describes the processing model that Pinned Hosts 308 implement. The model has 2 parts: (1) the processing rules for HTTP 309 request messages received over a secure transport (e.g. TLS); and 310 (2) the processing rules for HTTP request messages received over non- 311 secure transports, such as TCP. 313 2.2.1. HTTP-over-Secure-Transport Request Type 315 When replying to an HTTP request that was conveyed over a secure 316 transport, a Pinned Host SHOULD include in its response exactly one 317 PKP header field, exactly one PKP-RO header field, or one of each. 318 Each instance of either header field MUST satisfy the grammar 319 specified in Section 2.1. 321 Establishing a given host as a Known Pinned Host, in the context of a 322 given UA, is accomplished as follows: 324 1. Over the HTTP protocol running over secure transport, by 325 correctly returning (per this specification) at least one valid 326 PKP header field to the UA. 328 2. Through other mechanisms, such as a client-side pre-loaded Known 329 Pinned Host List. 331 2.2.2. HTTP Request Type 333 Pinned Hosts SHOULD NOT include the PKP header field in HTTP 334 responses conveyed over non-secure transport. UAs MUST ignore any 335 PKP header received in an HTTP response conveyed over non-secure 336 transport. 338 2.3. User Agent Processing Model 340 The UA processing model relies on parsing domain names. Note that 341 internationalized domain names SHALL be canonicalized according to 342 the scheme in Section 10 of [RFC6797]. 344 2.3.1. Public-Key-Pins Response Header Field Processing 346 If the UA receives, over a secure transport, an HTTP response that 347 includes a PKP header field conforming to the grammar specified in 348 Section 2.1, and there are no underlying secure transport errors or 349 warnings (see Section 2.5), the UA MUST either: 351 o Note the host as a Known Pinned Host if it is not already so noted 352 (see Section 2.3.3), 354 or, 356 o Update the UA's cached information for the Known Pinned Host if 357 any of of the max-age, includeSubDomains, or report-uri header 358 field value directives convey information different from that 359 already maintained by the UA. 361 The max-age value is essentially a "time to live" value relative to 362 the time of the most recent observation of the PKP header field. If 363 the max-age header field value token has a value of 0, the UA MUST 364 remove its cached Pinning Policy information (including the 365 includeSubDomains directive, if asserted) if the Pinned Host is 366 Known, or, MUST NOT note this Pinned Host if it is not yet Known. 368 If a UA receives more than one PKP header field or more than one PKP- 369 RO header fieled in an HTTP response message over secure transport, 370 then the UA MUST process only the first PKP header field (if present) 371 and only the first PKP-RO header field (if present). 373 If the UA receives the HTTP response over insecure transport, or if 374 the PKP header is not a Valid Pinning Header (see Section 2.5), the 375 UA MUST ignore any present PKP header field(s). Similarly, if the UA 376 receives the HTTP response over insecure transport, the UA MUST 377 ignore any present PKP-RO header field(s). The UA MUST ignore any 378 PKP or PKP-RO header fields not conforming to the grammar specified 379 in Section 2.1. 381 2.3.2. Interaction of Public-Key-Pins and Public-Key-Pins-Report-Only 383 A server MAY set both the "Public-Key-Pins" and "Public-Key-Pins- 384 Report-Only" headers simultaneously. The headers do not interact 385 with one another but the UA MUST process the PKP header and SHOULD 386 process both. 388 The headers are processed according to Section 2.3.1. 390 When the PKP-RO header is used with a report-uri, the UA SHOULD POST 391 reports for Pin Validation failures to the indicated report-uri, 392 although the UA MUST NOT enforce Pin Validation. That is, in the 393 event of Pin Validation failure when the host has set the PKP-RO 394 header, the UA performs Pin Validation only to check whether or not 395 it should POST a report, but not for causing connection failure. 397 Note: There is no purpose to using the PKP-RO header without the 398 report-uri directive. User Agents MAY discard such headers without 399 interpreting them further. 401 When the PKP header is used with a report-uri, the UA SHOULD POST 402 reports for Pin Validation failures to the indicated report-uri, as 403 well as enforcing Pin Validation. 405 If a host sets the PKP-RO header, the UA SHOULD note the Pins and 406 directives given in the PKP-RO header as specified by the max-age 407 directive. If the UA does note the Pins and directives in the PKP-RO 408 header it SHOULD evaluate the specified policy and SHOULD report any 409 would-be Pin Validation failures that would occur if the report-only 410 policy were enforced. 412 If a host sets both the PKP header and the PKP-RO header, the UA MUST 413 note and enforce Pin Validation as specified by the PKP header, and 414 SHOULD process the Pins and directives given in the PKP-RO header. 415 If the UA does process the Pins and directives in the PKP-RO header 416 it SHOULD evaluate the specified policy and SHOULD report any would- 417 be Pin Validation failures that would occur if the report-only policy 418 were enforced. 420 2.3.3. Noting a Pinned Host - Storage Model 422 The Effective Pin Date of a Known Pinned Host is the time that the UA 423 observed a Valid Pinning Header for the host. The Effective 424 Expiration Date of a Known Pinned Host is the Effective Pin Date plus 425 the max-age. A Known Pinned Host is "expired" if the Effective 426 Expiration Date refers to a date in the past. The UA MUST ignore any 427 expired Known Pinned Hosts in its cache. 429 For example, if a UA is beginning to perform Pin Validation for a 430 Known Pinned Host and finds that the cached pinning information for 431 the host indicates an Effective Expiration Date in the past, the UA 432 MUST NOT continue with Pin Validation for the host, and must consider 433 the host to no longer be a Known Pinned Host. 435 Known Pinned Hosts are identified only by domain names, and never IP 436 addresses. If the substring matching the host production from the 437 Request-URI (of the message to which the host responded) 438 syntactically matches the IP-literal or IPv4address productions from 439 Section 3.2.2 of [RFC3986], then the UA MUST NOT note this host as a 440 Known Pinned Host. 442 Otherwise, if the substring does not congruently match an existing 443 Known Pinned Host's domain name, per the matching procedure specified 444 in Section 8.2 of [RFC6797], then the UA MUST add this host to the 445 Known Pinned Host cache. The UA caches: 447 o the Pinned Host's domain name, 449 o the Effective Expiration Date, or enough information to calculate 450 it (the Effective Pin Date and the value of the max-age 451 directive), 453 o whether or not the includeSubDomains directive is asserted, and 455 o the value of the report-uri directive, if present. 457 If any other metadata from optional or future PKP header directives 458 are present in the Valid Pinning Header, and the UA understands them, 459 the UA MAY note them as well. 461 UAs MAY set an upper limit on the value of max-age, so that UAs that 462 have noted erroneous Pins (whether by accident or due to attack) have 463 some chance of recovering over time. If the server sets a max-age 464 greater than the UA's upper limit, the UA MAY behave as if the server 465 set the max-age to the UA's upper limit. For example, if the UA caps 466 max-age at 5184000 seconds (60 days), and a Pinned Host sets a max- 467 age directive of 90 days in its Valid Pinning Header, the UA MAY 468 behave as if the max-age were effectively 60 days. (One way to 469 achieve this behavior is for the UA to simply store a value of 60 470 days instead of the 90 day value provided by the Pinned Host.) For 471 UA implementation guidance on how to select a maximum max-age, see 472 Section 4.1. 474 The UA MUST NOT modify any pinning metadata of any superdomain 475 matched Known Pinned Host. 477 2.3.4. HTTP-Equiv Element Attribute 479 UAs MUST NOT heed http-equiv="Public-Key-Pins" or http-equiv="Public- 480 Key-Pins-Report-Only" attribute settings on elements 481 [W3C.REC-html401-19991224] in received content. 483 2.4. Semantics of Pins 485 An SPKI Fingerprint is defined as the output of a known cryptographic 486 hash algorithm whose input is the DER-encoded ASN.1 representation of 487 the subjectPublicKeyInfo (SPKI) field of an X.509 certificate. A Pin 488 is defined as the combination of the known algorithm identifier and 489 the SPKI Fingerprint computed using that algorithm. 491 The SPKI Fingerprint is encoded in base 64 for use in an HTTP header 492 [RFC4648]. 494 In this version of the specification, the known cryptographic hash 495 algorithm is SHA-256, identified as "sha256" [RFC6234]. (Future 496 specifications may add new algorithms and deprecate old ones.) UAs 497 MUST ignore Pins for which they do not recognize the algorithm 498 identifier. UAs MUST continue to process the rest of a PKP response 499 header field and note Pins for algorithms they do recognize; UAs MUST 500 recognize "sha256". 502 Figure 4 reproduces the definition of the SubjectPublicKeyInfo 503 structure in [RFC5280]. 505 SubjectPublicKeyInfo ::= SEQUENCE { 506 algorithm AlgorithmIdentifier, 507 subjectPublicKey BIT STRING } 509 AlgorithmIdentifier ::= SEQUENCE { 510 algorithm OBJECT IDENTIFIER, 511 parameters ANY DEFINED BY algorithm OPTIONAL } 513 Figure 4: SPKI Definition 515 If the certificate's subjectPublicKeyInfo is incomplete when taken in 516 isolation, such as when holding a DSA key without domain parameters, 517 a public key pin cannot be formed. 519 We pin public keys, rather than entire certificates, to enable 520 operators to generate new certificates containing old public keys 521 (see [why-pin-key]). 523 See Appendix A for an example non-normative program that generates 524 SPKI Fingerprints from certificates. 526 2.5. Noting Pins 528 Upon receipt of the PKP response header field, the UA notes the host 529 as a Known Pinned Host, storing the Pins and their associated 530 directives in non-volatile storage (for example, along with the HSTS 531 metadata). The Pins and their associated directives are collectively 532 known as Pinning Metadata. 534 The UA MUST note the Pins for a Host if and only if all three of the 535 following conditions hold: 537 o It received the PKP response header field over an error-free TLS 538 connection. If the host is a Pinned Host, this includes the 539 validation added in Section 2.6. 541 o The TLS connection was authenticated with a certificate chain 542 containing at least one of the SPKI structures indicated by at 543 least one of the given SPKI Fingerprints (see Section 2.6). 545 o The given set of Pins contains at least one Pin that does NOT 546 refer to an SPKI in the certificate chain. (That is, the host 547 must set a Backup Pin; see Section 4.3.) 549 If the PKP response header field does not meet all three of these 550 criteria, the UA MUST NOT note the host as a Pinned Host. A PKP 551 response header field that meets all these critera is known as a 552 Valid Pinning Header. 554 Whenever a UA receives a Valid Pinning Header, it MUST set its 555 Pinning Metadata to the exact Pins, Effective Expiration Date 556 (computed from max-age), and (if any) report-uri given in the most 557 recently received Valid Pinning Header. 559 For forward compatibility, the UA MUST ignore any unrecognized PKP 560 and PKP-RO header directives, while still processing those directives 561 it does recognize. Section 2.1 specifies the directives max-age, 562 Pins, includeSubDomains, and report-uri but future specifications and 563 implementations might use additional directives. 565 Upon receipt of a PKP-RO response header field, the UA SHOULD 566 evaluate the policy expressed in the field, and SHOULD generate and 567 send a report (see Section 3). However, failure to validate the Pins 568 in the field MUST have no effect on the validity or non-validity of 569 the policy expressed in the PKP field or in previously-noted Pins for 570 the Known Pinned Host. 572 The UA need not note any Pins or other policy expressed in the PKP-RO 573 response header field, except for the purpose of determining that it 574 has already sent a report for a given policy. UAs SHOULD make a best 575 effort not to inundate report-uris with redundant reports. 577 2.6. Validating Pinned Connections 579 When a UA connects to a Pinned Host, if the TLS connection has 580 errors, the UA MUST terminate the connection without allowing the 581 user to proceed anyway. (This behavior is the same as that required 582 by [RFC6797].) 584 If the connection has no errors, then the UA will determine whether 585 to apply a new, additional correctness check: Pin Validation. A UA 586 SHOULD perform Pin Validation whenever connecting to a Known Pinned 587 Host. It is acceptable to allow Pin Validation to be disabled for 588 some Hosts according to local policy. For example, a UA may disable 589 Pin Validation for Pinned Hosts whose validated certificate chain 590 terminates at a user-defined trust anchor, rather than a trust anchor 591 built-in to the UA. 593 To perform Pin Validation, the UA will compute the SPKI Fingerprints 594 for each certificate in the Pinned Host's validated certificate 595 chain, using each supported hash algorithm for each certificate. (As 596 described in Section 2.4, certificates whose SPKI cannot be taken in 597 isolation cannot be pinned.) The UA MUST ignore superfluous 598 certificates in the chain that do not form part of the validating 599 chain. The UA will then check that the set of these SPKI 600 Fingerprints intersects the set of SPKI Fingerprints in that Pinned 601 Host's Pinning Metadata. If there is set intersection, the UA 602 continues with the connection as normal. Otherwise, the UA MUST 603 treat this Pin Validation Failure as a non-recoverable error. Any 604 procedure that matches the results of this Pin Validation procedure 605 is considered equivalent. 607 Note that the UA MUST perform Pin Validation when setting up the TLS 608 session, before beginning an HTTP conversation over the TLS channel. 610 UAs send validation failure reports only when Pin Validation is 611 actually in effect. Pin Validation might not be in effect e.g. 612 because the user has elected to disable it, or because a presented 613 certificate chain chains up to a locally-installed anchor. In such 614 cases, UAs SHOULD NOT send reports. 616 2.7. Interactions With Preloaded Pin Lists 618 UAs MAY choose to implement additional sources of pinning 619 information, such as through built-in lists of pinning information. 620 Such UAs SHOULD allow users to override such additional sources, 621 including disabling them from consideration. 623 The effective policy for a Known Pinned Host that has both built-in 624 Pins and Pins from previously observed PKP header response fields is 625 implementation-defined. 627 2.8. Pinning Self-Signed End Entities 629 If UAs accept hosts that authenticate themselves with self-signed end 630 entity certificates, they MAY also allow hosts to pin the public keys 631 in such certificates. The usability and security implications of 632 this practice are outside the scope of this specification. 634 3. Reporting Pin Validation Failure 636 When a Known Pinned Host has set the report-uri directive, the UA 637 SHOULD report Pin Validation failures to the indicated URI. The UA 638 does this by POSTing a JSON [RFC4627] message to the URI; the JSON 639 message takes this form: 641 { 642 "date-time": date-time, 643 "hostname": hostname, 644 "port": port, 645 "effective-expiration-date": expiration-date, 646 "include-subdomains": include-subdomains, 647 "served-certificate-chain": [ 648 pem1, ... pemN 649 ], 650 "validated-certificate-chain": [ 651 pem1, ... pemN 652 ], 653 "known-pins": [ 654 known-pin1, ... known-pinN 655 ] 656 } 658 Figure 5: JSON Report Format 660 Whitespace outside of quoted strings is not significant. The key/ 661 value pairs may appear in any order, but each MUST appear only once. 663 The date-time indicates the time the UA observed the Pin Validation 664 failure. It is provided as a string formatted according to 665 Section 5.6, "Internet Date/Time Format", of [RFC3339]. 667 The hostname is the hostname to which the UA made the original 668 request that failed Pin Validation. It is provided as a string. 670 The port is the port to which the UA made the original request that 671 failed Pin Validation. It is provided as an integer. 673 The effective-expiration-date is the Effective Expiration Date for 674 the noted Pins. It is provided as a string formatted according to 675 Section 5.6, "Internet Date/Time Format", of [RFC3339]. 677 include-subdomains indicates whether or not the UA has noted the 678 includeSubDomains directive for the Known Pinned Host. It is 679 provided as one of the JSON identifiers "true" or "false". 681 The served-certificate-chain is the certificate chain, as served by 682 the Known Pinned Host during TLS session setup. It is provided as an 683 array of strings; each string pem1, ... pemN is the PEM 684 representation of each X.509 certificate as described in 685 [I-D.josefsson-pkix-textual]. 687 The validated-certificate-chain is the certificate chain, as 688 constructed by the UA during certificate chain verification. (This 689 may differ from the served-certificate-chain.) It is provided as an 690 array of strings; each string pem1, ... pemN is the PEM 691 representation of each X.509 certificate as described in 692 [I-D.josefsson-pkix-textual]. For UAs that build certificate chains 693 in more than one way during the validation process, they SHOULD send 694 the last chain built. In this way they can avoid keeping too much 695 state during the validation process. 697 The known-pins are the Pins that the UA has noted for the Known 698 Pinned Host. They are provided as an array of strings with the 699 syntax: 701 known-pin = token "=" quoted-string 703 Figure 6: Known Pin Syntax 705 As in Section 2.4, the token refers to the algorithm name, and the 706 quoted-string refers to the base 64 encoding of the SPKI Fingerprint. 707 When formulating the JSON POST body, the UA MUST either use single- 708 quoted JSON strings, or use double-quoted JSON strings and \-escape 709 the embedded double quotes in the quoted-string part of the known- 710 pin. 712 Figure 7 shows an example of a Pin Validation failure report. (PEM 713 strings are shown on multiple lines for readability.) 715 { 716 "date-time": "2014-04-06T13:00:50Z", 717 "hostname": "www.example.com", 718 "port": 443, 719 "effective-expiration-date": "2014-05-01T12:40:50Z" 720 "include-subdomains": false, 721 "served-certificate-chain": [ 722 "-----BEGIN CERTIFICATE-----\n 723 MIIEBDCCAuygAwIBAgIDAjppMA0GCSqGSIb3DQEBBQUAMEIxCzAJBgNVBAYTAlVT\n 724 ... 725 HFa9llF7b1cq26KqltyMdMKVvvBulRP/F/A8rLIQjcxz++iPAsbw+zOzlTvjwsto\n 726 WHPbqCRiOwY1nQ2pM714A5AuTHhdUDqB1O6gyHA43LL5Z/qHQF1hwFGPa4NrzQU6\n 727 yuGnBXj8ytqU0CwIPX4WecigUCAkVDNx\n 728 -----END CERTIFICATE-----", 729 ... 730 ], 731 "validated-certificate-chain": [ 732 "-----BEGIN CERTIFICATE-----\n 733 MIIEBDCCAuygAwIBAgIDAjppMA0GCSqGSIb3DQEBBQUAMEIxCzAJBgNVBAYTAlVT\n 734 ... 735 HFa9llF7b1cq26KqltyMdMKVvvBulRP/F/A8rLIQjcxz++iPAsbw+zOzlTvjwsto\n 736 WHPbqCRiOwY1nQ2pM714A5AuTHhdUDqB1O6gyHA43LL5Z/qHQF1hwFGPa4NrzQU6\n 737 yuGnBXj8ytqU0CwIPX4WecigUCAkVDNx\n 738 -----END CERTIFICATE-----", 739 ... 740 ], 741 "known-pins": [ 742 'pin-sha256="d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM="', 743 "pin-sha256=\"E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g=\"" 744 ] 745 } 747 Figure 7: Pin Validation Failure Report Example 749 4. Security Considerations 751 Pinning public keys helps hosts strongly assert their cryptographic 752 identity even in the face of issuer error, malfeasance or compromise. 753 But there is some risk that a host operator could lose or lose 754 control of their host's private key (such as by operator error or 755 host compromise). If the operator had pinned only the key of the 756 host's end entity certificate, the operator would not be able to 757 serve their web site or application in a way that UAs would trust for 758 the duration of their pin's max-age. (Recall that UAs MUST close the 759 connection to a host upon Pin Failure.) 761 Therefore, there is a necessary trade-off between two competing 762 goods: pin specificity and maximal reduction of the scope of issuers 763 on the one hand; and flexibility and resilience of the host's 764 cryptographic identity on the other hand. One way to resolve this 765 trade-off is to compromise by pinning to the key(s) of the issuer(s) 766 of the host's end entity certificate(s). Often, a valid certificate 767 chain will have at least two certificates above the end entity 768 certificate: the intermediate issuer, and the trust anchor. 769 Operators can pin any one or more of the public keys in this chain, 770 and indeed could pin to issuers not in the chain (as, for example, a 771 Backup Pin). Pinning to an intermediate issuer, or even to a trust 772 anchor or root, still significantly reduces the number of issuers who 773 can issue end entity certificates for the Known Pinned Host, while 774 still giving that host flexibility to change keys without a 775 disruption of service. 777 4.1. Maximum max-age 779 As mentioned in Section 2.3.3, UAs MAY cap the max-age value at some 780 upper limit. There is a security trade-off in that low maximum 781 values provide a narrow window of protection for users who visit the 782 Known Pinned Host only infrequently, while high maximum values might 783 potentially result in a UA's inability to successfully perform Pin 784 Validation for a Known Pinned Host if the UA's noted Pins and the 785 host's true Pins diverge. 787 Such divergence could occur for several reasons, including: UA error; 788 host operator error; network attack; or a Known Pinned Host that 789 intentionally migrates all pinned keys, combined with a UA that has 790 noted true Pins with a high max-age value and has not had a chance to 791 observe the new true Pins for the host. (This last example 792 underscores the importance for host operators to phase in new keys 793 gradually, and to set the max-age value in accordance with their 794 planned key migration schedule.) 796 There is probably no ideal upper limit to the max-age directive that 797 would satisfy all use cases. However, a value on the order of 60 798 days (5,184,000 seconds) may be considered a balance between the two 799 competing security concerns. 801 4.2. Using includeSubDomains Safely 803 It may happen that Pinned Hosts whose hostnames share a parent domain 804 use different Valid Pinning Headers. If a host whose hostname is a 805 parent domain for another host sets the includeSubDomains directive, 806 the two hosts' Pins may conflict with each other. For example, 807 consider two Known Pinned Hosts, example.com and 808 subdomain.example.com. Assume example.com sets a Valid Pinning 809 Header such as this: 811 Public-Key-Pins: max-age=12000; pin-sha256="ABC..."; pin-sha256="DEF..."; 812 includeSubDomains 814 Figure 8: example.com Valid Pinning Header 816 Assume subdomain.example.com sets a Valid Pinning Header such as 817 this: 819 Public-Key-Pins: pin-sha256="GHI..."; pin-sha256="JKL..." 821 Figure 9: subdomain.example.com Valid Pinning Header 823 Assume a UA that has not previously noted any Pins for either of 824 these hosts. If the UA first contacts subdomain.example.com, it will 825 note the Pins in the Valid Pinning Header, and perform Pin Validation 826 as normal on subsequent conections. If the UA then contacts 827 example.com, again it will note the Pins and perform Pin Validation 828 on future connections. 830 However, if the UA happened to visit example.com before 831 subdomain.example.com, the UA would, due to example.com's use of the 832 includeSubDomains directive, attempt to perform Pin Validation for 833 subdomain.example.com using the SPKI hashes ABC... and DEF..., which 834 are not valid for the certificate chains subdomain.example.com (which 835 uses certificates with SPKIs GHI... and JLK...). Thus, depending on 836 the order in which the UA observes the Valid Pinning Headers for 837 hosts example.com and subdomain.example.com, Pin Validation might or 838 might not fail for subdomain.example.com, even if the certificate 839 chain the UA receives for subdomain.example.com is perfectly valid. 841 Thus, Pinned Host operators must use the includeSubDomains directive 842 with care. For example, they may choose to use overlapping pin sets 843 for hosts under a parent domain that uses includeSubDomains, or to 844 not use the includeSubDomains directive in their effective-second- 845 level domains, or to simply use the same pin set for all hosts under 846 a given parent domain. 848 4.3. Backup Pins 850 The primary way to cope with the risk of inadvertent Pin Validation 851 Failure is to keep a Backup Pin. A Backup Pin is a fingerprint for 852 the public key of a secondary, not-yet-deployed key pair. The 853 operator keeps the backup key pair offline, and sets a pin for it in 854 the PKP header. Then, in case the operator loses control of their 855 primary private key, they can deploy the backup key pair. UAs, who 856 have had the backup key pair pinned (when it was set in previous 857 Valid Pinning Headers), can connect to the host without error. 859 Because having a backup key pair is so important to recovery, UAs 860 MUST require that hosts set a Backup Pin (see Section 2.5). The down 861 side of keeping a not-yet-deployed key pair is that if an attacker 862 gains control of the private key she will be able to perform a MITM 863 attack without being discovered. Operators must take care to avoid 864 leaking the key such as keeping it offline. 866 4.4. Interactions With Cookie Scoping 868 HTTP cookies [RFC6265] set by a Known Pinned Host can be stolen by a 869 network attacker who can forge web and DNS responses so as to cause a 870 client to send the cookies to a phony subdomain of the host. To 871 prevent this, hosts SHOULD set the "secure" attribute and omit the 872 "domain" attribute on all security-sensitive cookies, such as session 873 cookies. These settings tell the browser that the cookie should only 874 be sent back to the originating host (not its subdomains), and should 875 only be sent over HTTPS (not HTTP). 877 5. Privacy Considerations 879 Hosts can use HSTS or HPKP as a "super-cookie", by setting distinct 880 policies for a number of subdomains. For example, assume example.com 881 wishes to track distinct UAs without explicitly setting a cookie, or 882 if a previously-set cookie is deleted from the UA's cookie store. 883 Here are two attack scenarios. 885 o example.com can use report-uri and the ability to pin arbitrary 886 identifiers to distinguish UAs. 888 1. example.com sets a Valid Pinning Header in its response to 889 requests. The header asserts the includeSubDomains directive, 890 and specifies a report-uri directive as well. Pages served by 891 the host also include references to subresource 892 https://bad.example.com/foo.png. 894 2. The Valid Pinning Header includes a "pin" that is not really 895 the hash of an SPKI, but is instead an arbitrary 896 distinguishing string sent only in response to a particular 897 request. For each request, the host creates a new, distinct 898 distinguishing string and sets it as if it were a pin. 900 3. The certificate chain served by bad.example.com does not pass 901 Pin Validation given the pin set the host asserted in (1). 902 The HPKP-conforming UA attempts to report the Pin Validation 903 failure to the specified report-uri, including the certificate 904 chain it observed and the SPKI hashes it expected to see. 905 Among the SPKI hashes is the distinguishing string in step 906 (2). 908 4. Different site operators/origins can optionally collaborate by 909 setting the report-uri to be in an origin they share 910 administrative control of. UAs MAY, therefore, refuse to send 911 reports outside of the origin that set the PKP or PKP-RO 912 header. 914 o example.com can use SNI and subdomains to distinguish UAs. 916 1. example.com sets a Valid Pinning Header in its response to 917 requests. The header asserts the includeSubDomains directive. 919 2. On a subsequent page view, the host responds with a page 920 including the subresource https://0.fingerprint.example.com/ 921 foo.png, and the server responds using a certificate chain 922 that does not pass Pin Validation for the pin-set defined in 923 the Valid Pinning Header in step (1). The HPKP-conforming UA 924 will close the connection, never completing the request to 925 0.fingerprint.example.com. The host may thus note that this 926 particular UA had noted the (good) Pins for that subdomain. 928 3. example.com can distinguish 2^N UAs by serving Valid Pinning 929 Headers from an arbitrary number N distinct subdomains, giving 930 some UAs Valid Pinning Headers for some, but not all 931 subdomains (causing subsequent requests for 932 n.fingerprint.example.com to fail), and giving some UAs no 933 Valid Pinning Header for other subdomains (causing subsequent 934 requests for m.fingerprint.example.com to succeed). 936 Conforming implementations (as well as implementations conforming to 937 [RFC6797]) must store state about which domains have set policies, 938 hence which domains the UA has contacted. A forensic attacker might 939 find this information useful, even if the user has cleared other 940 parts of the UA's state. 942 6. IANA Considerations 944 IANA is requested to register the header described in this document 945 in the "Message Headers" registry, with the following parameters: 947 o Header Field Names should be "Public-Key-Pins" and "Public-Key- 948 Pins-Report-Only". 950 o Protocol should be "http" 952 o Status should be "standard" 954 o Reference should be this document 956 7. Usability Considerations 958 When pinning works to detect impostor Pinned Hosts, users will 959 experience denial of service. It is advisable for UAs to explain the 960 reason why, i.e. that it was impossible to verify the confirmed 961 cryptographic identity of the host. 963 It is advisable that UAs have a way for users to clear current Pins 964 for Pinned Hosts, and to allow users to query the current state of 965 Pinned Hosts. 967 8. Acknowledgements 969 Thanks to Tobias Gondrom, Jeff Hodges, Paul Hoffman, Ivan Krstic, 970 Adam Langley, Barry Leiba, Nicolas Lidzborski, SM, James Manger, Yoav 971 Nir, Trevor Perrin, Eric Rescorla, Tom Ritter, and Yan Zhu for 972 suggestions and edits that clarified the text. 974 9. What's Changed 976 [RFC EDITOR: PLEASE REMOVE THIS SECTION] 978 Clarified that max-age is REQUIRED for PKP, but OPTIONAL for PKP-RO 979 (where it has no effect. 981 Updated header field syntax and description to match that in 982 [RFC7230]. 984 Updated normative references to current documents. 986 Removed the strict directive. 988 Removed the requirement that the server set the Valid Pinning Header 989 on every response. 991 Added normative references for SHA, JSON, and base-64. 993 Added the Privacy Considerations section. 995 Changed non-normative pin generation code from Go to POSIX shell 996 script using openssl. 998 Changed max-max-age from SHOULD to MAY, and used the example of 60 999 days instead of 30. 1001 Removed the section "Pin Validity Times", which was intended to be in 1002 harmony with [I-D.perrin-tls-tack]. Now using max-age purely as 1003 specified in [RFC6797]. 1005 Added new directives: includeSubDomains, report-uri and strict. 1007 Added a new variant of the PKP Header: Public-Key-Pins-Report-Only. 1009 Removed the section on pin break codes and verifiers, in favor the of 1010 most-recently-received policy (Section 2.5). 1012 Now using a new header field, Public-Key-Pins, separate from HSTS. 1013 This allows hosts to use pinning separately from Strict Transport 1014 Security. 1016 Explicitly requiring that UAs perform Pin Validation before the HTTP 1017 conversation begins. 1019 Backup Pins are now required. 1021 Separated normative from non-normative material. Removed tangential 1022 and out-of-scope non-normative discussion. 1024 10. References 1026 10.1. Normative References 1028 [I-D.josefsson-pkix-textual] 1029 Josefsson, S. and S. Leonard, "Text Encodings of PKIX and 1030 CMS Structures", draft-josefsson-pkix-textual-04 (work in 1031 progress), July 2014. 1033 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1034 Requirement Levels", BCP 14, RFC 2119, March 1997. 1036 [RFC3339] Klyne, G., Ed. and C. Newman, "Date and Time on the 1037 Internet: Timestamps", RFC 3339, July 2002. 1039 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 1040 Resource Identifier (URI): Generic Syntax", STD 66, RFC 1041 3986, January 2005. 1043 [RFC4627] Crockford, D., "The application/json Media Type for 1044 JavaScript Object Notation (JSON)", RFC 4627, July 2006. 1046 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data 1047 Encodings", RFC 4648, October 2006. 1049 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 1050 Specifications: ABNF", STD 68, RFC 5234, January 2008. 1052 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 1053 (TLS) Protocol Version 1.2", RFC 5246, August 2008. 1055 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 1056 Housley, R., and W. Polk, "Internet X.509 Public Key 1057 Infrastructure Certificate and Certificate Revocation List 1058 (CRL) Profile", RFC 5280, May 2008. 1060 [RFC6234] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms 1061 (SHA and SHA-based HMAC and HKDF)", RFC 6234, May 2011. 1063 [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265, 1064 April 2011. 1066 [RFC6797] Hodges, J., Jackson, C., and A. Barth, "HTTP Strict 1067 Transport Security (HSTS)", RFC 6797, November 2012. 1069 [RFC7230] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol 1070 (HTTP/1.1): Message Syntax and Routing", RFC 7230, June 1071 2014. 1073 [RFC7234] Fielding, R., Nottingham, M., and J. Reschke, "Hypertext 1074 Transfer Protocol (HTTP/1.1): Caching", RFC 7234, June 1075 2014. 1077 [W3C.REC-html401-19991224] 1078 Raggett, D., Hors, A., and I. Jacobs, "HTML 4.01 1079 Specification", World Wide Web Consortium Recommendation 1080 REC-html401-19991224, December 1999, 1081 . 1083 10.2. Informative References 1085 [I-D.perrin-tls-tack] 1086 Marlinspike, M., "Trust Assertions for Certificate Keys", 1087 draft-perrin-tls-tack-02 (work in progress), January 2013. 1089 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 1090 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 1091 May 2008. 1093 [why-pin-key] 1094 Langley, A., "Public Key Pinning", May 2011, 1095 . 1097 Appendix A. Fingerprint Generation 1099 This POSIX shell program generates SPKI Fingerprints, suitable for 1100 use in pinning, from PEM-encoded certificates. It is non-normative. 1102 openssl x509 -noout -in certificate.pem -pubkey | \ 1103 openssl asn1parse -noout -inform pem -out public.key 1104 openssl dgst -sha256 -binary public.key | base64 1106 Figure 10: Example SPKI Fingerprint Generation Code 1108 Appendix B. Deployment Guidance 1110 This section is non-normative guidance which may smooth the adoption 1111 of public key pinning. 1113 o Operators should get the backup public key signed by a different 1114 (root and/or intermediary) CA than their primary certificate, and 1115 store the backup key pair safely offline. The semantics of an 1116 SPKI Fingerprint do not require the issuance of a certificate to 1117 construct a valid Pin. However, in many deployment scenarios, in 1118 order to make a Backup Pin operational the server operator will 1119 need to have a certificate to deploy TLS on the host. Failure to 1120 obtain a certificate through prior arrangement will leave clients 1121 that recognize the site as a Known Pinned Host unable to 1122 successfully perform Pin Validation until such a time as the 1123 operator can obtain a new certificate from their desired 1124 certificate issuer. 1126 o It is most economical to have the backup certificate signed by a 1127 completely different signature chain than the live certificate, to 1128 maximize recoverability in the event of either root or 1129 intermediary signer compromise. 1131 o Operators should periodically exercise their Backup Pin plan -- an 1132 untested backup is no backup at all. 1134 o Operators should start small. Operators should first deploy 1135 public key pinning by using the report-only mode together with a 1136 report-uri directive that points to a reliable report collection 1137 endpoint. When moving out of report-only mode, operators should 1138 start by setting a max-age of minutes or a few hours, and 1139 gradually increase max-age as they gain confidence in their 1140 operational capability. 1142 Authors' Addresses 1144 Chris Evans 1145 Google, Inc. 1146 1600 Amphitheatre Pkwy 1147 Mountain View, CA 94043 1148 US 1150 Email: cevans@google.com 1152 Chris Palmer 1153 Google, Inc. 1154 1600 Amphitheatre Pkwy 1155 Mountain View, CA 94043 1156 US 1158 Email: palmer@google.com 1160 Ryan Sleevi 1161 Google, Inc. 1162 1600 Amphitheatre Pkwy 1163 Mountain View, CA 94043 1164 US 1166 Email: sleevi@google.com