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2 Web Security C. Evans
3 Internet-Draft C. Palmer
4 Intended status: Standards Track R. Sleevi
5 Expires: February 8, 2015 Google, Inc.
6 August 7, 2014
8 Public Key Pinning Extension for HTTP
9 draft-ietf-websec-key-pinning-20
11 Abstract
13 This document describes an extension to the HTTP protocol allowing
14 web host operators to instruct user agents to remember ("pin") the
15 hosts' cryptographic identities for a given period of time. During
16 that time, UAs will require that the host present a certificate chain
17 including at least one Subject Public Key Info structure whose
18 fingerprint matches one of the pinned fingerprints for that host. By
19 effectively reducing the number of authorities who can authenticate
20 the domain during the lifetime of the pin, pinning may reduce the
21 incidence of man-in-the-middle attacks due to compromised
22 Certification Authorities.
24 Status of This Memo
26 This Internet-Draft is submitted in full conformance with the
27 provisions of BCP 78 and BCP 79.
29 Internet-Drafts are working documents of the Internet Engineering
30 Task Force (IETF). Note that other groups may also distribute
31 working documents as Internet-Drafts. The list of current Internet-
32 Drafts is at http://datatracker.ietf.org/drafts/current/.
34 Internet-Drafts are draft documents valid for a maximum of six months
35 and may be updated, replaced, or obsoleted by other documents at any
36 time. It is inappropriate to use Internet-Drafts as reference
37 material or to cite them other than as "work in progress."
39 This Internet-Draft will expire on February 8, 2015.
41 Copyright Notice
43 Copyright (c) 2014 IETF Trust and the persons identified as the
44 document authors. All rights reserved.
46 This document is subject to BCP 78 and the IETF Trust's Legal
47 Provisions Relating to IETF Documents
48 (http://trustee.ietf.org/license-info) in effect on the date of
49 publication of this document. Please review these documents
50 carefully, as they describe your rights and restrictions with respect
51 to this document. Code Components extracted from this document must
52 include Simplified BSD License text as described in Section 4.e of
53 the Trust Legal Provisions and are provided without warranty as
54 described in the Simplified BSD License.
56 Table of Contents
58 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
59 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
60 2. Server and Client Behavior . . . . . . . . . . . . . . . . . 4
61 2.1. Response Header Field Syntax . . . . . . . . . . . . . . 4
62 2.1.1. The max-age Directive . . . . . . . . . . . . . . . . 5
63 2.1.2. The includeSubDomains Directive . . . . . . . . . . . 6
64 2.1.3. The report-uri Directive . . . . . . . . . . . . . . 6
65 2.1.4. Examples . . . . . . . . . . . . . . . . . . . . . . 7
66 2.2. Server Processing Model . . . . . . . . . . . . . . . . . 8
67 2.2.1. HTTP-over-Secure-Transport Request Type . . . . . . . 8
68 2.2.2. HTTP Request Type . . . . . . . . . . . . . . . . . . 8
69 2.3. User Agent Processing Model . . . . . . . . . . . . . . . 8
70 2.3.1. Public-Key-Pins Response Header Field Processing . . 8
71 2.3.2. Interaction of Public-Key-Pins and Public-Key-Pins-
72 Report-Only . . . . . . . . . . . . . . . . . . . . . 9
73 2.3.3. Noting a Pinned Host - Storage Model . . . . . . . . 10
74 2.3.4. HTTP-Equiv Element Attribute . . . . . . . . . 11
75 2.4. Semantics of Pins . . . . . . . . . . . . . . . . . . . . 11
76 2.5. Noting Pins . . . . . . . . . . . . . . . . . . . . . . . 12
77 2.6. Validating Pinned Connections . . . . . . . . . . . . . . 13
78 2.7. Interactions With Preloaded Pin Lists . . . . . . . . . . 14
79 2.8. Pinning Self-Signed End Entities . . . . . . . . . . . . 14
80 3. Reporting Pin Validation Failure . . . . . . . . . . . . . . 14
81 4. Security Considerations . . . . . . . . . . . . . . . . . . . 17
82 4.1. Maximum max-age . . . . . . . . . . . . . . . . . . . . . 18
83 4.2. Using includeSubDomains Safely . . . . . . . . . . . . . 18
84 4.3. Backup Pins . . . . . . . . . . . . . . . . . . . . . . . 19
85 4.4. Interactions With Cookie Scoping . . . . . . . . . . . . 20
86 5. Privacy Considerations . . . . . . . . . . . . . . . . . . . 20
87 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
88 7. Usability Considerations . . . . . . . . . . . . . . . . . . 22
89 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 22
90 9. What's Changed . . . . . . . . . . . . . . . . . . . . . . . 22
91 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
92 10.1. Normative References . . . . . . . . . . . . . . . . . . 23
93 10.2. Informative References . . . . . . . . . . . . . . . . . 24
94 Appendix A. Fingerprint Generation . . . . . . . . . . . . . . . 25
95 Appendix B. Deployment Guidance . . . . . . . . . . . . . . . . 25
96 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
98 1. Introduction
100 This document defines a new HTTP header that enables a web host to
101 express to user agents (UAs) which Subject Public Key Info (SPKI)
102 structure(s) UAs SHOULD expect to be present in the host's
103 certificate chain in future connections using TLS [RFC5246]. We call
104 this "public key pinning" (PKP); in particular, this document
105 describes HTTP-based public key pinning (HPKP). At least one UA
106 (Google Chrome) has experimented with the idea by shipping with a
107 user-extensible embedded set of Pins. Although effective, this does
108 not scale. This proposal addresses the scale problem.
110 Deploying PKP safely will require operational and organizational
111 maturity due to the risk that hosts may make themselves unavailable
112 by pinning to a (set of) SPKI(s) that becomes invalid (see
113 Section 4). With care, host operators can greatly reduce the risk of
114 main-in-the-middle (MITM) attacks and other false-authentication
115 problems for their users without incurring undue risk.
117 PKP is meant to be used together with HTTP Strict Transport Security
118 (HSTS) [RFC6797], but it is possible to pin keys without requiring
119 HSTS.
121 A Pin is a relationship between a hostname and a cryptographic
122 identity (in this document, 1 or more of the public keys in a chain
123 of X.509 certificates). Pin Validation is the process a UA performs
124 to ensure that a host is in fact authenticated with its previously-
125 established Pin.
127 Key pinning is a trust-on-first-use (TOFU) mechanism. The first time
128 a UA connects to a host, it lacks the information necessary to
129 perform Pin Validation; UAs can only apply their normal cryptographic
130 identity validation. (In this document, it is assumed that UAs apply
131 X.509 certificate chain validation in accord with [RFC5280].)
133 The UA will not be able to detect and thwart a MITM attacking the
134 UA's first connection to the host. (However, the requirement that
135 the MITM provide an X.509 certificate chain that can pass the UA's
136 validation requirements, without error, mitigates this risk
137 somewhat.) Worse, such a MITM can inject its own PKP header into the
138 HTTP stream, and pin the UA to its own keys. To avoid post facto
139 detection, the attacker would have to be in a position to intercept
140 all future requests to the host from that UA.
142 Thus, key pinning as described in this document is not a perfect
143 defense against MITM attackers capable of passing certificate chain
144 validation procedures -- nothing short of pre-shared keys can be.
145 However, it provides significant value by allowing host operators to
146 limit the number of certification authorities than can vouch for the
147 host's identity, and allows UAs to detect in-process MITM attacks
148 after the initial communication.
150 1.1. Requirements Language
152 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
153 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
154 document are to be interpreted as described in RFC 2119 [RFC2119].
156 2. Server and Client Behavior
158 2.1. Response Header Field Syntax
160 The "Public-Key-Pins" and "Public-Key-Pins-Report-Only" header
161 fields, also referred to within this specification as the PKP and
162 PKP-RO header fields, respectively, are new response headers defined
163 in this specification. They are used by a server to indicate that a
164 UA should perform Pin Validation (Section 2.6) for the host emitting
165 the response message, and to provide the necessary information for
166 the UA to do so.
168 Figure 1 describes the syntax (Augmented Backus-Naur Form) of the
169 header fields, using the grammar defined in [RFC5234] and the rules
170 defined in Section 3.2 of [RFC7230]. The field values of both header
171 fields conform to the same rules.
173 Public-Key-Directives = [ directive ] *( OWS ";" OWS [ directive ] )
175 directive = simple-directive
176 / pin-directive
178 simple-directive = directive-name [ "=" directive-value ]
179 directive-name = token
180 directive-value = token
181 / quoted-string
183 pin-directive = "pin-" token "=" quoted-string
185 Figure 1: HPKP Header Syntax
187 Optional white space (OWS) is used as defined in Section 3.2.3 of
188 [RFC7230]. token and quoted-string are used as defined in
189 Section 3.2.6 of [RFC7230].
191 The directives defined in this specification are described below.
192 The overall requirements for directives are:
194 1. The order of appearance of directives is not significant.
196 2. A given simple-directive MUST NOT appear more than once in a
197 given header field. Directives are either optional or required,
198 as stipulated in their definitions.
200 3. Directive names are case-insensitive.
202 4. UAs MUST ignore any header fields containing directives, or other
203 header field value data, that do not conform to the syntax
204 defined in this specification. In particular, UAs must not
205 attempt to fix malformed header fields.
207 5. If a header field contains any directive(s) the UA does not
208 recognize, the UA MUST ignore those directives.
210 6. If the PKP or PKP-RO header field otherwise satisfies the above
211 requirements (1 through 5), the UA MUST process the directives it
212 recognizes.
214 Additional directives extending the semantic functionality of the
215 header fields can be defined in other specifications. The first such
216 specification will need to define a reistry for such directives.
217 Such future directives will be ignored by UAs implementing only this
218 specification, as well as by generally non-conforming UAs.
220 In the pin-directive, the token is the name of a cryptographic hash
221 algorithm. The only algorithm allowed at this time is "sha256", i.e.
222 the hash algorithm SHA256 ([RFC4634]); additional algorithms may be
223 allowed for use in this context in the future. The quoted-string is
224 a sequence of base 64 digits: the base 64-encoded SPKI Fingerprint
225 [RFC4648] (see Section 2.4).
227 When a connection passes Pin Validation using the UA's noted Pins for
228 the host at the time, the host becomes a Known Pinned Host.
230 According to rule 5, above, the UA MUST ignore pin-directives with
231 tokens naming hash algorithms it does not recognize. If the set of
232 remaining effective pin-directives is empty, and if the host is a
233 Known Pinned Host, the UA MUST cease to consider the host as a Known
234 Pinned Host (the UA should fail open). The UA should indicate to
235 users that the host is no longer a Known Pinned Host.
237 2.1.1. The max-age Directive
239 The "max-age" directive specifies the number of seconds after the
240 reception of the PKP header field during which the UA SHOULD regard
241 the host (from whom the message was received) as a Known Pinned Host.
243 The "max-age" directive is REQUIRED to be present within a "Public-
244 Key-Pins" header field, and is OPTIONAL within a "Public-Key-Pins-
245 Report-Only" header field.
247 If present, the max-age directive is REQUIRED to have a directive
248 value, for which the the syntax (after quoted-string unescaping, if
249 necessary) is defined as:
251 max-age-value = delta-seconds
252 delta-seconds = 1*DIGIT
254 Figure 2: max-age Value Syntax
256 delta-seconds is used as defined in [RFC7234], Section 1.2.1.
258 2.1.2. The includeSubDomains Directive
260 The OPTIONAL includeSubDomains directive is a valueless directive
261 that, if present (i.e., it is "asserted"), signals to the UA that the
262 Pinning Policy applies to this Pinned Host as well as any subdomains
263 of the host's domain name.
265 2.1.3. The report-uri Directive
267 The OPTIONAL report-uri directive indicates the URI to which the UA
268 SHOULD report Pin Validation failures (Section 2.6). The UA POSTs
269 the reports to the given URI as described in Section 3.
271 When used in the PKP or PKP-RO headers, the presence of a report-uri
272 directive indicates to the UA that in the event of Pin Validation
273 failure it SHOULD POST a report to the report-uri. If the header is
274 Public-Key-Pins, the UA should do this in addition to terminating the
275 connection (as described in Section 2.6).
277 Hosts may set report-uris that use HTTP or HTTPS. If the scheme in
278 the report-uri is one that uses TLS (e.g. HTTPS), UAs MUST perform
279 Pinning Validation when the host in the report-uri is a Known Pinned
280 Host; similarly, UAs MUST apply HSTS if the host in the report-uri is
281 a Known HSTS Host.
283 Note that the report-uri need not necessarily be in the same Internet
284 domain or web origin as the host being reported about.
286 UAs SHOULD make their best effort to report Pin Validation failures
287 to the report-uri, but may fail to report in exceptional conditions.
288 For example, if connecting the report-uri itself incurs a Pinning
289 Validation failure or other certificate validation failure, the UA
290 MUST cancel the connection. Similarly, if Known Pinned Host A sets a
291 report-uri referring to Known Pinned Host B, and if B sets a report-
292 uri referring to A, and if both hosts fail Pin Validation, the UA
293 SHOULD detect and break the loop by failing to send reports to and
294 about those hosts.
296 In any case of report failure, the UA MAY attempt to re-send the
297 report later.
299 UAs SHOULD limit the rate at which they send reports. For example,
300 it is unnecessary to send the same report to the same report-uri more
301 than once per distinct set of declared Pins.
303 2.1.4. Examples
305 Figure 3 shows some example PKP and PKP-RO response header fields.
306 (Lines are folded to fit.)
308 Public-Key-Pins: max-age=3000;
309 pin-sha256="d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM=";
310 pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g=";
312 Public-Key-Pins: max-age=2592000;
313 pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g=";
314 pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ="
316 Public-Key-Pins: max-age=2592000;
317 pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g=";
318 pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ=";
319 report-uri="http://example.com/pkp-report"
321 Public-Key-Pins-Report-Only: max-age=2592000;
322 pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g=";
323 pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ=";
324 report-uri="https://other.example.net/pkp-report"
326 Public-Key-Pins:
327 pin-sha256="d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM=";
328 pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ=";
329 max-age=259200
331 Public-Key-Pins:
332 pin-sha256="d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM=";
333 pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g=";
334 pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ=";
335 max-age=10000; includeSubDomains
337 Figure 3: HPKP Header Examples
339 2.2. Server Processing Model
341 This section describes the processing model that Pinned Hosts
342 implement. The model has 2 parts: (1) the processing rules for HTTP
343 request messages received over a secure transport (e.g. TLS); and
344 (2) the processing rules for HTTP request messages received over non-
345 secure transports, such as TCP.
347 2.2.1. HTTP-over-Secure-Transport Request Type
349 When replying to an HTTP request that was conveyed over a secure
350 transport, a Pinned Host SHOULD include in its response exactly one
351 PKP header field, exactly one PKP-RO header field, or one of each.
352 Each instance of either header field MUST satisfy the grammar
353 specified in Section 2.1.
355 Establishing a given host as a Known Pinned Host, in the context of a
356 given UA, is accomplished as follows:
358 1. Over the HTTP protocol running over secure transport, by
359 correctly returning (per this specification) at least one valid
360 PKP header field to the UA.
362 2. Through other mechanisms, such as a client-side pre-loaded Known
363 Pinned Host List.
365 2.2.2. HTTP Request Type
367 Pinned Hosts SHOULD NOT include the PKP header field in HTTP
368 responses conveyed over non-secure transport. UAs MUST ignore any
369 PKP header received in an HTTP response conveyed over non-secure
370 transport.
372 2.3. User Agent Processing Model
374 The UA processing model relies on parsing domain names. Note that
375 internationalized domain names SHALL be canonicalized according to
376 the scheme in Section 10 of [RFC6797].
378 2.3.1. Public-Key-Pins Response Header Field Processing
380 If the UA receives, over a secure transport, an HTTP response that
381 includes a PKP header field conforming to the grammar specified in
382 Section 2.1, and there are no underlying secure transport errors or
383 warnings (see Section 2.5), the UA MUST either:
385 o Note the host as a Known Pinned Host if it is not already so noted
386 (see Section 2.3.3),
388 or,
390 o Update the UA's cached information for the Known Pinned Host if
391 any of of the max-age, includeSubDomains, or report-uri header
392 field value directives convey information different from that
393 already maintained by the UA.
395 The max-age value is essentially a "time to live" value relative to
396 the time of the most recent observation of the PKP header field. If
397 the max-age header field value token has a value of 0, the UA MUST
398 remove its cached Pinning Policy information (including the
399 includeSubDomains directive, if asserted) if the Pinned Host is
400 Known, or, MUST NOT note this Pinned Host if it is not yet Known.
402 If a UA receives more than one PKP header field or more than one PKP-
403 RO header fieled in an HTTP response message over secure transport,
404 then the UA MUST process only the first PKP header field (if present)
405 and only the first PKP-RO header field (if present).
407 If the UA receives the HTTP response over insecure transport, or if
408 the PKP header is not a Valid Pinning Header (see Section 2.5), the
409 UA MUST ignore any present PKP header field(s). Similarly, if the UA
410 receives the HTTP response over insecure transport, the UA MUST
411 ignore any present PKP-RO header field(s). The UA MUST ignore any
412 PKP or PKP-RO header fields not conforming to the grammar specified
413 in Section 2.1.
415 2.3.2. Interaction of Public-Key-Pins and Public-Key-Pins-Report-Only
417 A server MAY set both the "Public-Key-Pins" and "Public-Key-Pins-
418 Report-Only" headers simultaneously. The headers do not interact
419 with one another but the UA MUST process the PKP header and SHOULD
420 process both.
422 The headers are processed according to Section 2.3.1.
424 When the PKP-RO header is used with a report-uri, the UA SHOULD POST
425 reports for Pin Validation failures to the indicated report-uri,
426 although the UA MUST NOT enforce Pin Validation. That is, in the
427 event of Pin Validation failure when the host has set the PKP-RO
428 header, the UA performs Pin Validation only to check whether or not
429 it should POST a report, but not for causing connection failure.
431 Note: There is no purpose to using the PKP-RO header without the
432 report-uri directive. User Agents MAY discard such headers without
433 interpreting them further.
435 When the PKP header is used with a report-uri, the UA SHOULD POST
436 reports for Pin Validation failures to the indicated report-uri, as
437 well as enforcing Pin Validation.
439 If a host sets the PKP-RO header, the UA SHOULD note the Pins and
440 directives given in the PKP-RO header as specified by the max-age
441 directive. If the UA does note the Pins and directives in the PKP-RO
442 header it SHOULD evaluate the specified policy and SHOULD report any
443 would-be Pin Validation failures that would occur if the report-only
444 policy were enforced.
446 If a host sets both the PKP header and the PKP-RO header, the UA MUST
447 note and enforce Pin Validation as specified by the PKP header, and
448 SHOULD process the Pins and directives given in the PKP-RO header.
449 If the UA does process the Pins and directives in the PKP-RO header
450 it SHOULD evaluate the specified policy and SHOULD report any would-
451 be Pin Validation failures that would occur if the report-only policy
452 were enforced.
454 2.3.3. Noting a Pinned Host - Storage Model
456 The Effective Pin Date of a Known Pinned Host is the time that the UA
457 observed a Valid Pinning Header for the host. The Effective
458 Expiration Date of a Known Pinned Host is the Effective Pin Date plus
459 the max-age. A Known Pinned Host is "expired" if the Effective
460 Expiration Date refers to a date in the past. The UA MUST ignore any
461 expired Known Pinned Hosts in its cache.
463 For example, if a UA is beginning to perform Pin Validation for a
464 Known Pinned Host and finds that the cached pinning information for
465 the host indicates an Effective Expiration Date in the past, the UA
466 MUST NOT continue with Pin Validation for the host, and must consider
467 the host to no longer be a Known Pinned Host.
469 Known Pinned Hosts are identified only by domain names, and never IP
470 addresses. If the substring matching the host production from the
471 Request-URI (of the message to which the host responded)
472 syntactically matches the IP-literal or IPv4address productions from
473 Section 3.2.2 of [RFC3986], then the UA MUST NOT note this host as a
474 Known Pinned Host.
476 Otherwise, if the substring does not congruently match an existing
477 Known Pinned Host's domain name, per the matching procedure specified
478 in Section 8.2 of [RFC6797], then the UA MUST add this host to the
479 Known Pinned Host cache. The UA caches:
481 o the Pinned Host's domain name,
482 o the Effective Expiration Date, or enough information to calculate
483 it (the Effective Pin Date and the value of the max-age
484 directive),
486 o whether or not the includeSubDomains directive is asserted, and
488 o the value of the report-uri directive, if present.
490 If any other metadata from optional or future PKP header directives
491 are present in the Valid Pinning Header, and the UA understands them,
492 the UA MAY note them as well.
494 UAs MAY set an upper limit on the value of max-age, so that UAs that
495 have noted erroneous Pins (whether by accident or due to attack) have
496 some chance of recovering over time. If the server sets a max-age
497 greater than the UA's upper limit, the UA MAY behave as if the server
498 set the max-age to the UA's upper limit. For example, if the UA caps
499 max-age at 5184000 seconds (60 days), and a Pinned Host sets a max-
500 age directive of 90 days in its Valid Pinning Header, the UA MAY
501 behave as if the max-age were effectively 60 days. (One way to
502 achieve this behavior is for the UA to simply store a value of 60
503 days instead of the 90 day value provided by the Pinned Host.) For
504 UA implementation guidance on how to select a maximum max-age, see
505 Section 4.1.
507 The UA MUST NOT modify any pinning metadata of any superdomain
508 matched Known Pinned Host.
510 2.3.4. HTTP-Equiv Element Attribute
512 UAs MUST NOT heed http-equiv="Public-Key-Pins" or http-equiv="Public-
513 Key-Pins-Report-Only" attribute settings on elements
514 [W3C.REC-html401-19991224] in received content.
516 2.4. Semantics of Pins
518 An SPKI Fingerprint is defined as the output of a known cryptographic
519 hash algorithm whose input is the DER-encoded ASN.1 representation of
520 the subjectPublicKeyInfo (SPKI) field of an X.509 certificate. A Pin
521 is defined as the combination of the known algorithm identifier and
522 the SPKI Fingerprint computed using that algorithm.
524 The SPKI Fingerprint is encoded in base 64 for use in an HTTP header
525 [RFC4648].
527 In this version of the specification, the known cryptographic hash
528 algorithm is SHA-256, identified as "sha256" [RFC6234]. (Future
529 specifications may add new algorithms and deprecate old ones.) UAs
530 MUST ignore Pins for which they do not recognize the algorithm
531 identifier. UAs MUST continue to process the rest of a PKP response
532 header field and note Pins for algorithms they do recognize; UAs MUST
533 recognize "sha256".
535 Figure 4 reproduces the definition of the SubjectPublicKeyInfo
536 structure in [RFC5280].
538 SubjectPublicKeyInfo ::= SEQUENCE {
539 algorithm AlgorithmIdentifier,
540 subjectPublicKey BIT STRING }
542 AlgorithmIdentifier ::= SEQUENCE {
543 algorithm OBJECT IDENTIFIER,
544 parameters ANY DEFINED BY algorithm OPTIONAL }
546 Figure 4: SPKI Definition
548 If the certificate's subjectPublicKeyInfo is incomplete when taken in
549 isolation, such as when holding a DSA key without domain parameters,
550 a public key pin cannot be formed.
552 We pin public keys, rather than entire certificates, to enable
553 operators to generate new certificates containing old public keys
554 (see [why-pin-key]).
556 See Appendix A for an example non-normative program that generates
557 SPKI Fingerprints from certificates.
559 2.5. Noting Pins
561 Upon receipt of the PKP response header field, the UA notes the host
562 as a Known Pinned Host, storing the Pins and their associated
563 directives in non-volatile storage (for example, along with the HSTS
564 metadata). The Pins and their associated directives are collectively
565 known as Pinning Metadata.
567 The UA MUST note the Pins for a Host if and only if all three of the
568 following conditions hold:
570 o It received the PKP response header field over an error-free TLS
571 connection. If the host is a Pinned Host, this includes the
572 validation added in Section 2.6.
574 o The TLS connection was authenticated with a certificate chain
575 containing at least one of the SPKI structures indicated by at
576 least one of the given SPKI Fingerprints (see Section 2.6).
578 o The given set of Pins contains at least one Pin that does NOT
579 refer to an SPKI in the certificate chain. (That is, the host
580 must set a Backup Pin; see Section 4.3.)
582 If the PKP response header field does not meet all three of these
583 criteria, the UA MUST NOT note the host as a Pinned Host. A PKP
584 response header field that meets all these critera is known as a
585 Valid Pinning Header.
587 Whenever a UA receives a Valid Pinning Header, it MUST set its
588 Pinning Metadata to the exact Pins, Effective Expiration Date
589 (computed from max-age), and (if any) report-uri given in the most
590 recently received Valid Pinning Header.
592 For forward compatibility, the UA MUST ignore any unrecognized PKP
593 and PKP-RO header directives, while still processing those directives
594 it does recognize. Section 2.1 specifies the directives max-age,
595 Pins, includeSubDomains, and report-uri but future specifications and
596 implementations might use additional directives.
598 Upon receipt of a PKP-RO response header field, the UA SHOULD
599 evaluate the policy expressed in the field, and SHOULD generate and
600 send a report (see Section 3). However, failure to validate the Pins
601 in the field MUST have no effect on the validity or non-validity of
602 the policy expressed in the PKP field or in previously-noted Pins for
603 the Known Pinned Host.
605 The UA need not note any Pins or other policy expressed in the PKP-RO
606 response header field, except for the purpose of determining that it
607 has already sent a report for a given policy. UAs SHOULD make a best
608 effort not to inundate report-uris with redundant reports.
610 2.6. Validating Pinned Connections
612 When a UA connects to a Pinned Host, if the TLS connection has
613 errors, the UA MUST terminate the connection without allowing the
614 user to proceed anyway. (This behavior is the same as that required
615 by [RFC6797].)
617 If the connection has no errors, then the UA will determine whether
618 to apply a new, additional correctness check: Pin Validation. A UA
619 SHOULD perform Pin Validation whenever connecting to a Known Pinned
620 Host. It is acceptable to allow Pin Validation to be disabled for
621 some Hosts according to local policy. For example, a UA may disable
622 Pin Validation for Pinned Hosts whose validated certificate chain
623 terminates at a user-defined trust anchor, rather than a trust anchor
624 built-in to the UA.
626 To perform Pin Validation, the UA will compute the SPKI Fingerprints
627 for each certificate in the Pinned Host's validated certificate
628 chain, using each supported hash algorithm for each certificate. (As
629 described in Section 2.4, certificates whose SPKI cannot be taken in
630 isolation cannot be pinned.) The UA MUST ignore superfluous
631 certificates in the chain that do not form part of the validating
632 chain. The UA will then check that the set of these SPKI
633 Fingerprints intersects the set of SPKI Fingerprints in that Pinned
634 Host's Pinning Metadata. If there is set intersection, the UA
635 continues with the connection as normal. Otherwise, the UA MUST
636 treat this Pin Validation Failure as a non-recoverable error. Any
637 procedure that matches the results of this Pin Validation procedure
638 is considered equivalent.
640 A UA that has previous noted a host as a Known Pinned Host MUST
641 perform Pin Validation when setting up the TLS session, before
642 beginning an HTTP conversation over the TLS channel.
644 UAs send validation failure reports only when Pin Validation is
645 actually in effect. Pin Validation might not be in effect e.g.
646 because the user has elected to disable it, or because a presented
647 certificate chain chains up to a locally-installed anchor. In such
648 cases, UAs SHOULD NOT send reports.
650 2.7. Interactions With Preloaded Pin Lists
652 UAs MAY choose to implement additional sources of pinning
653 information, such as through built-in lists of pinning information.
654 Such UAs should allow users to override such additional sources,
655 including disabling them from consideration.
657 The effective policy for a Known Pinned Host that has both built-in
658 Pins and Pins from previously observed PKP header response fields is
659 implementation-defined.
661 2.8. Pinning Self-Signed End Entities
663 If UAs accept hosts that authenticate themselves with self-signed end
664 entity certificates, they MAY also allow hosts to pin the public keys
665 in such certificates. The usability and security implications of
666 this practice are outside the scope of this specification.
668 3. Reporting Pin Validation Failure
670 When a Known Pinned Host has set the report-uri directive, the UA
671 SHOULD report Pin Validation failures to the indicated URI. The UA
672 does this by POSTing a JSON [RFC4627] message to the URI; the JSON
673 message takes this form:
675 {
676 "date-time": date-time,
677 "hostname": hostname,
678 "port": port,
679 "effective-expiration-date": expiration-date,
680 "include-subdomains": include-subdomains,
681 "served-certificate-chain": [
682 pem1, ... pemN
683 ],
684 "validated-certificate-chain": [
685 pem1, ... pemN
686 ],
687 "known-pins": [
688 known-pin1, ... known-pinN
689 ]
690 }
692 Figure 5: JSON Report Format
694 Whitespace outside of quoted strings is not significant. The key/
695 value pairs may appear in any order, but each MUST appear only once.
697 The date-time indicates the time the UA observed the Pin Validation
698 failure. It is provided as a string formatted according to
699 Section 5.6, "Internet Date/Time Format", of [RFC3339].
701 The hostname is the hostname to which the UA made the original
702 request that failed Pin Validation. It is provided as a string.
704 The port is the port to which the UA made the original request that
705 failed Pin Validation. It is provided as an integer.
707 The effective-expiration-date is the Effective Expiration Date for
708 the noted Pins. It is provided as a string formatted according to
709 Section 5.6, "Internet Date/Time Format", of [RFC3339].
711 include-subdomains indicates whether or not the UA has noted the
712 includeSubDomains directive for the Known Pinned Host. It is
713 provided as one of the JSON identifiers "true" or "false".
715 The served-certificate-chain is the certificate chain, as served by
716 the Known Pinned Host during TLS session setup. It is provided as an
717 array of strings; each string pem1, ... pemN is the PEM
718 representation of each X.509 certificate as described in
719 [I-D.josefsson-pkix-textual].
721 The validated-certificate-chain is the certificate chain, as
722 constructed by the UA during certificate chain verification. (This
723 may differ from the served-certificate-chain.) It is provided as an
724 array of strings; each string pem1, ... pemN is the PEM
725 representation of each X.509 certificate as described in
726 [I-D.josefsson-pkix-textual]. For UAs that build certificate chains
727 in more than one way during the validation process, they SHOULD send
728 the last chain built. In this way they can avoid keeping too much
729 state during the validation process.
731 The known-pins are the Pins that the UA has noted for the Known
732 Pinned Host. They are provided as an array of strings with the
733 syntax:
735 known-pin = token "=" quoted-string
737 Figure 6: Known Pin Syntax
739 As in Section 2.4, the token refers to the algorithm name, and the
740 quoted-string refers to the base 64 encoding of the SPKI Fingerprint.
741 When formulating the JSON POST body, the UA MUST either use single-
742 quoted JSON strings, or use double-quoted JSON strings and \-escape
743 the embedded double quotes in the quoted-string part of the known-
744 pin.
746 Figure 7 shows an example of a Pin Validation failure report. (PEM
747 strings are shown on multiple lines for readability.)
749 {
750 "date-time": "2014-04-06T13:00:50Z",
751 "hostname": "www.example.com",
752 "port": 443,
753 "effective-expiration-date": "2014-05-01T12:40:50Z"
754 "include-subdomains": false,
755 "served-certificate-chain": [
756 "-----BEGIN CERTIFICATE-----\n
757 MIIEBDCCAuygAwIBAgIDAjppMA0GCSqGSIb3DQEBBQUAMEIxCzAJBgNVBAYTAlVT\n
758 ...
759 HFa9llF7b1cq26KqltyMdMKVvvBulRP/F/A8rLIQjcxz++iPAsbw+zOzlTvjwsto\n
760 WHPbqCRiOwY1nQ2pM714A5AuTHhdUDqB1O6gyHA43LL5Z/qHQF1hwFGPa4NrzQU6\n
761 yuGnBXj8ytqU0CwIPX4WecigUCAkVDNx\n
762 -----END CERTIFICATE-----",
763 ...
764 ],
765 "validated-certificate-chain": [
766 "-----BEGIN CERTIFICATE-----\n
767 MIIEBDCCAuygAwIBAgIDAjppMA0GCSqGSIb3DQEBBQUAMEIxCzAJBgNVBAYTAlVT\n
768 ...
769 HFa9llF7b1cq26KqltyMdMKVvvBulRP/F/A8rLIQjcxz++iPAsbw+zOzlTvjwsto\n
770 WHPbqCRiOwY1nQ2pM714A5AuTHhdUDqB1O6gyHA43LL5Z/qHQF1hwFGPa4NrzQU6\n
771 yuGnBXj8ytqU0CwIPX4WecigUCAkVDNx\n
772 -----END CERTIFICATE-----",
773 ...
774 ],
775 "known-pins": [
776 'pin-sha256="d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM="',
777 "pin-sha256=\"E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g=\""
778 ]
779 }
781 Figure 7: Pin Validation Failure Report Example
783 4. Security Considerations
785 Pinning public keys helps hosts strongly assert their cryptographic
786 identity even in the face of issuer error, malfeasance or compromise.
787 But there is some risk that a host operator could lose or lose
788 control of their host's private key (such as by operator error or
789 host compromise). If the operator had pinned only the key of the
790 host's end entity certificate, the operator would not be able to
791 serve their web site or application in a way that UAs would trust for
792 the duration of their pin's max-age. (Recall that UAs MUST close the
793 connection to a host upon Pin Failure.)
795 Therefore, there is a necessary trade-off between two competing
796 goods: pin specificity and maximal reduction of the scope of issuers
797 on the one hand; and flexibility and resilience of the host's
798 cryptographic identity on the other hand. One way to resolve this
799 trade-off is to compromise by pinning to the key(s) of the issuer(s)
800 of the host's end entity certificate(s). Often, a valid certificate
801 chain will have at least two certificates above the end entity
802 certificate: the intermediate issuer, and the trust anchor.
803 Operators can pin any one or more of the public keys in this chain,
804 and indeed could pin to issuers not in the chain (as, for example, a
805 Backup Pin). Pinning to an intermediate issuer, or even to a trust
806 anchor or root, still significantly reduces the number of issuers who
807 can issue end entity certificates for the Known Pinned Host, while
808 still giving that host flexibility to change keys without a
809 disruption of service.
811 4.1. Maximum max-age
813 As mentioned in Section 2.3.3, UAs MAY cap the max-age value at some
814 upper limit. There is a security trade-off in that low maximum
815 values provide a narrow window of protection for users who visit the
816 Known Pinned Host only infrequently, while high maximum values might
817 potentially result in a UA's inability to successfully perform Pin
818 Validation for a Known Pinned Host if the UA's noted Pins and the
819 host's true Pins diverge.
821 Such divergence could occur for several reasons, including: UA error;
822 host operator error; network attack; or a Known Pinned Host that
823 intentionally migrates all pinned keys, combined with a UA that has
824 noted true Pins with a high max-age value and has not had a chance to
825 observe the new true Pins for the host. (This last example
826 underscores the importance for host operators to phase in new keys
827 gradually, and to set the max-age value in accordance with their
828 planned key migration schedule.)
830 There is probably no ideal upper limit to the max-age directive that
831 would satisfy all use cases. However, a value on the order of 60
832 days (5,184,000 seconds) may be considered a balance between the two
833 competing security concerns.
835 4.2. Using includeSubDomains Safely
837 It may happen that Pinned Hosts whose hostnames share a parent domain
838 use different Valid Pinning Headers. If a host whose hostname is a
839 parent domain for another host sets the includeSubDomains directive,
840 the two hosts' Pins may conflict with each other. For example,
841 consider two Known Pinned Hosts, example.com and
842 subdomain.example.com. Assume example.com sets a Valid Pinning
843 Header such as this:
845 Public-Key-Pins: max-age=12000; pin-sha256="ABC...";
846 pin-sha256="DEF..."; includeSubDomains
848 Figure 8: example.com Valid Pinning Header
850 Assume subdomain.example.com sets a Valid Pinning Header such as
851 this:
853 Public-Key-Pins: pin-sha256="GHI..."; pin-sha256="JKL..."
855 Figure 9: subdomain.example.com Valid Pinning Header
857 Assume a UA that has not previously noted any Pins for either of
858 these hosts. If the UA first contacts subdomain.example.com, it will
859 note the Pins in the Valid Pinning Header, and perform Pin Validation
860 as normal on subsequent conections. If the UA then contacts
861 example.com, again it will note the Pins and perform Pin Validation
862 on future connections.
864 However, if the UA happened to visit example.com before
865 subdomain.example.com, the UA would, due to example.com's use of the
866 includeSubDomains directive, attempt to perform Pin Validation for
867 subdomain.example.com using the SPKI hashes ABC... and DEF..., which
868 are not valid for the certificate chains subdomain.example.com (which
869 uses certificates with SPKIs GHI... and JLK...). Thus, depending on
870 the order in which the UA observes the Valid Pinning Headers for
871 hosts example.com and subdomain.example.com, Pin Validation might or
872 might not fail for subdomain.example.com, even if the certificate
873 chain the UA receives for subdomain.example.com is perfectly valid.
875 Thus, Pinned Host operators must use the includeSubDomains directive
876 with care. For example, they may choose to use overlapping pin sets
877 for hosts under a parent domain that uses includeSubDomains, or to
878 not use the includeSubDomains directive in their effective-second-
879 level domains, or to simply use the same pin set for all hosts under
880 a given parent domain.
882 4.3. Backup Pins
884 The primary way to cope with the risk of inadvertent Pin Validation
885 Failure is to keep a Backup Pin. A Backup Pin is a fingerprint for
886 the public key of a secondary, not-yet-deployed key pair. The
887 operator keeps the backup key pair offline, and sets a pin for it in
888 the PKP header. Then, in case the operator loses control of their
889 primary private key, they can deploy the backup key pair. UAs, who
890 have had the backup key pair pinned (when it was set in previous
891 Valid Pinning Headers), can connect to the host without error.
893 Because having a backup key pair is so important to recovery, UAs
894 MUST require that hosts set a Backup Pin (see Section 2.5). The down
895 side of keeping a not-yet-deployed key pair is that if an attacker
896 gains control of the private key she will be able to perform a MITM
897 attack without being discovered. Operators must take care to avoid
898 leaking the key such as keeping it offline.
900 4.4. Interactions With Cookie Scoping
902 HTTP cookies [RFC6265] set by a Known Pinned Host can be stolen by a
903 network attacker who can forge web and DNS responses so as to cause a
904 client to send the cookies to a phony subdomain of the host. To
905 prevent this, hosts SHOULD set the "secure" attribute and omit the
906 "domain" attribute on all security-sensitive cookies, such as session
907 cookies. These settings tell the browser that the cookie should only
908 be sent back to the originating host (not its subdomains), and should
909 only be sent over HTTPS (not HTTP).
911 5. Privacy Considerations
913 Hosts can use HSTS or HPKP as a "super-cookie", by setting distinct
914 policies for a number of subdomains. For example, assume example.com
915 wishes to track distinct UAs without explicitly setting a cookie, or
916 if a previously-set cookie is deleted from the UA's cookie store.
917 Here are two attack scenarios.
919 o example.com can use report-uri and the ability to pin arbitrary
920 identifiers to distinguish UAs.
922 1. example.com sets a Valid Pinning Header in its response to
923 requests. The header asserts the includeSubDomains directive,
924 and specifies a report-uri directive as well. Pages served by
925 the host also include references to subresource
926 https://bad.example.com/foo.png.
928 2. The Valid Pinning Header includes a "pin" that is not really
929 the hash of an SPKI, but is instead an arbitrary
930 distinguishing string sent only in response to a particular
931 request. For each request, the host creates a new, distinct
932 distinguishing string and sets it as if it were a pin.
934 3. The certificate chain served by bad.example.com does not pass
935 Pin Validation given the pin set the host asserted in (1).
936 The HPKP-conforming UA attempts to report the Pin Validation
937 failure to the specified report-uri, including the certificate
938 chain it observed and the SPKI hashes it expected to see.
939 Among the SPKI hashes is the distinguishing string in step
940 (2).
942 4. Different site operators/origins can optionally collaborate by
943 setting the report-uri to be in an origin they share
944 administrative control of. UAs MAY, therefore, refuse to send
945 reports outside of the origin that set the PKP or PKP-RO
946 header.
948 o example.com can use server name indication (SNI; [RFC3546]) and
949 subdomains to distinguish UAs.
951 1. example.com sets a Valid Pinning Header in its response to
952 requests. The header asserts the includeSubDomains directive.
954 2. On a subsequent page view, the host responds with a page
955 including the subresource https://0.fingerprint.example.com/
956 foo.png, and the server responds using a certificate chain
957 that does not pass Pin Validation for the pin-set defined in
958 the Valid Pinning Header in step (1). The HPKP-conforming UA
959 will close the connection, never completing the request to
960 0.fingerprint.example.com. The host may thus note that this
961 particular UA had noted the (good) Pins for that subdomain.
963 3. example.com can distinguish 2^N UAs by serving Valid Pinning
964 Headers from an arbitrary number N distinct subdomains, giving
965 some UAs Valid Pinning Headers for some, but not all
966 subdomains (causing subsequent requests for
967 n.fingerprint.example.com to fail), and giving some UAs no
968 Valid Pinning Header for other subdomains (causing subsequent
969 requests for m.fingerprint.example.com to succeed).
971 Conforming implementations (as well as implementations conforming to
972 [RFC6797]) must store state about which domains have set policies,
973 hence which domains the UA has contacted. A forensic attacker might
974 find this information useful, even if the user has cleared other
975 parts of the UA's state.
977 6. IANA Considerations
979 IANA is requested to register the response headers described in this
980 document in the "Message Headers" registry ([permanent-headers] with
981 the following parameters:
983 o Header Field Names should be "Public-Key-Pins" and "Public-Key-
984 Pins-Report-Only".
986 o Protocol should be "http"
988 o Status should be "standard"
989 o Reference should be this document
991 7. Usability Considerations
993 When pinning works to detect impostor Pinned Hosts, users will
994 experience denial of service. It is advisable for UAs to explain the
995 reason why, i.e. that it was impossible to verify the confirmed
996 cryptographic identity of the host.
998 It is advisable that UAs have a way for users to clear current Pins
999 for Pinned Hosts, and to allow users to query the current state of
1000 Pinned Hosts.
1002 8. Acknowledgements
1004 Thanks to Tobias Gondrom, Jeff Hodges, Paul Hoffman, Ivan Krstic,
1005 Adam Langley, Barry Leiba, Nicolas Lidzborski, SM, James Manger, Yoav
1006 Nir, Trevor Perrin, Eric Rescorla, Tom Ritter, and Yan Zhu for
1007 suggestions and edits that clarified the text.
1009 9. What's Changed
1011 [RFC EDITOR: PLEASE REMOVE THIS SECTION]
1013 Clarified that max-age is REQUIRED for PKP, but OPTIONAL for PKP-RO
1014 (where it has no effect.
1016 Updated header field syntax and description to match that in
1017 [RFC7230].
1019 Updated normative references to current documents.
1021 Removed the strict directive.
1023 Removed the requirement that the server set the Valid Pinning Header
1024 on every response.
1026 Added normative references for SHA, JSON, and base-64.
1028 Added the Privacy Considerations section.
1030 Changed non-normative pin generation code from Go to POSIX shell
1031 script using openssl.
1033 Changed max-max-age from SHOULD to MAY, and used the example of 60
1034 days instead of 30.
1036 Removed the section "Pin Validity Times", which was intended to be in
1037 harmony with [I-D.perrin-tls-tack]. Now using max-age purely as
1038 specified in [RFC6797].
1040 Added new directives: includeSubDomains, report-uri and strict.
1042 Added a new variant of the PKP Header: Public-Key-Pins-Report-Only.
1044 Removed the section on pin break codes and verifiers, in favor the of
1045 most-recently-received policy (Section 2.5).
1047 Now using a new header field, Public-Key-Pins, separate from HSTS.
1048 This allows hosts to use pinning separately from Strict Transport
1049 Security.
1051 Explicitly requiring that UAs perform Pin Validation before the HTTP
1052 conversation begins.
1054 Backup Pins are now required.
1056 Separated normative from non-normative material. Removed tangential
1057 and out-of-scope non-normative discussion.
1059 10. References
1061 10.1. Normative References
1063 [I-D.josefsson-pkix-textual]
1064 Josefsson, S. and S. Leonard, "Text Encodings of PKIX and
1065 CMS Structures", draft-josefsson-pkix-textual-05 (work in
1066 progress), July 2014.
1068 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
1069 Requirement Levels", BCP 14, RFC 2119, March 1997.
1071 [RFC3339] Klyne, G., Ed. and C. Newman, "Date and Time on the
1072 Internet: Timestamps", RFC 3339, July 2002.
1074 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
1075 Resource Identifier (URI): Generic Syntax", STD 66, RFC
1076 3986, January 2005.
1078 [RFC4627] Crockford, D., "The application/json Media Type for
1079 JavaScript Object Notation (JSON)", RFC 4627, July 2006.
1081 [RFC4634] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms
1082 (SHA and HMAC-SHA)", RFC 4634, July 2006.
1084 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
1085 Encodings", RFC 4648, October 2006.
1087 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
1088 Specifications: ABNF", STD 68, RFC 5234, January 2008.
1090 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
1091 (TLS) Protocol Version 1.2", RFC 5246, August 2008.
1093 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
1094 Housley, R., and W. Polk, "Internet X.509 Public Key
1095 Infrastructure Certificate and Certificate Revocation List
1096 (CRL) Profile", RFC 5280, May 2008.
1098 [RFC6234] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms
1099 (SHA and SHA-based HMAC and HKDF)", RFC 6234, May 2011.
1101 [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
1102 April 2011.
1104 [RFC6797] Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
1105 Transport Security (HSTS)", RFC 6797, November 2012.
1107 [RFC7230] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
1108 (HTTP/1.1): Message Syntax and Routing", RFC 7230, June
1109 2014.
1111 [RFC7234] Fielding, R., Nottingham, M., and J. Reschke, "Hypertext
1112 Transfer Protocol (HTTP/1.1): Caching", RFC 7234, June
1113 2014.
1115 [W3C.REC-html401-19991224]
1116 Raggett, D., Hors, A., and I. Jacobs, "HTML 4.01
1117 Specification", World Wide Web Consortium Recommendation
1118 REC-html401-19991224, December 1999,
1119 .
1121 [permanent-headers]
1122 Klyne, G., "Permanent Message Header Field Names", July
1123 2014, .
1126 10.2. Informative References
1128 [I-D.perrin-tls-tack]
1129 Marlinspike, M., "Trust Assertions for Certificate Keys",
1130 draft-perrin-tls-tack-02 (work in progress), January 2013.
1132 [RFC3546] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
1133 and T. Wright, "Transport Layer Security (TLS)
1134 Extensions", RFC 3546, June 2003.
1136 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
1137 IANA Considerations Section in RFCs", BCP 26, RFC 5226,
1138 May 2008.
1140 [why-pin-key]
1141 Langley, A., "Public Key Pinning", May 2011,
1142 .
1144 Appendix A. Fingerprint Generation
1146 This POSIX shell program generates SPKI Fingerprints, suitable for
1147 use in pinning, from PEM-encoded certificates. It is non-normative.
1149 openssl x509 -noout -in certificate.pem -pubkey | \
1150 openssl asn1parse -noout -inform pem -out public.key
1151 openssl dgst -sha256 -binary public.key | base64
1153 Figure 10: Example SPKI Fingerprint Generation Code
1155 Appendix B. Deployment Guidance
1157 This section is non-normative guidance which may smooth the adoption
1158 of public key pinning.
1160 o Operators should get the backup public key signed by a different
1161 (root and/or intermediary) CA than their primary certificate, and
1162 store the backup key pair safely offline. The semantics of an
1163 SPKI Fingerprint do not require the issuance of a certificate to
1164 construct a valid Pin. However, in many deployment scenarios, in
1165 order to make a Backup Pin operational the server operator will
1166 need to have a certificate to deploy TLS on the host. Failure to
1167 obtain a certificate through prior arrangement will leave clients
1168 that recognize the site as a Known Pinned Host unable to
1169 successfully perform Pin Validation until such a time as the
1170 operator can obtain a new certificate from their desired
1171 certificate issuer.
1173 o It is most economical to have the backup certificate signed by a
1174 completely different signature chain than the live certificate, to
1175 maximize recoverability in the event of either root or
1176 intermediary signer compromise.
1178 o Operators should periodically exercise their Backup Pin plan -- an
1179 untested backup is no backup at all.
1181 o Operators should start small. Operators should first deploy
1182 public key pinning by using the report-only mode together with a
1183 report-uri directive that points to a reliable report collection
1184 endpoint. When moving out of report-only mode, operators should
1185 start by setting a max-age of minutes or a few hours, and
1186 gradually increase max-age as they gain confidence in their
1187 operational capability.
1189 Authors' Addresses
1191 Chris Evans
1192 Google, Inc.
1193 1600 Amphitheatre Pkwy
1194 Mountain View, CA 94043
1195 US
1197 Email: cevans@google.com
1199 Chris Palmer
1200 Google, Inc.
1201 1600 Amphitheatre Pkwy
1202 Mountain View, CA 94043
1203 US
1205 Email: palmer@google.com
1207 Ryan Sleevi
1208 Google, Inc.
1209 1600 Amphitheatre Pkwy
1210 Mountain View, CA 94043
1211 US
1213 Email: sleevi@google.com