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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