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2 Web Security C. Evans
3 Internet-Draft C. Palmer
4 Intended status: Standards Track R. Sleevi
5 Expires: January 10, 2014 Google, Inc.
6 July 09, 2013
8 Public Key Pinning Extension for HTTP
9 draft-ietf-websec-key-pinning-07
11 Abstract
13 This memo describes an extension to the HTTP protocol allowing web
14 host operators to instruct user agents (UAs) to remember ("pin") the
15 hosts' cryptographic identities for a given period of time. During
16 that time, UAs will require that the host present a certificate chain
17 including at least one Subject Public Key Info structure whose
18 fingerprint matches one of the pinned fingerprints for that host. By
19 effectively reducing the number of authorities who can authenticate
20 the domain during the lifetime of the pin, pinning may reduce the
21 incidence of man-in-the-middle attacks due to compromised
22 Certification Authorities.
24 Status of This Memo
26 This Internet-Draft is submitted in full conformance with the
27 provisions of BCP 78 and BCP 79.
29 Internet-Drafts are working documents of the Internet Engineering
30 Task Force (IETF). Note that other groups may also distribute
31 working documents as Internet-Drafts. The list of current Internet-
32 Drafts is at http://datatracker.ietf.org/drafts/current/.
34 Internet-Drafts are draft documents valid for a maximum of six months
35 and may be updated, replaced, or obsoleted by other documents at any
36 time. It is inappropriate to use Internet-Drafts as reference
37 material or to cite them other than as "work in progress."
39 This Internet-Draft will expire on January 10, 2014.
41 Copyright Notice
43 Copyright (c) 2013 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. The strict Directive . . . . . . . . . . . . . . . . 6
66 2.1.5. Examples . . . . . . . . . . . . . . . . . . . . . . 6
67 2.2. Server Processing Model . . . . . . . . . . . . . . . . . 7
68 2.2.1. HTTP-over-Secure-Transport Request Type . . . . . . . 7
69 2.2.2. HTTP Request Type . . . . . . . . . . . . . . . . . . 8
70 2.3. User Agent Processing Model . . . . . . . . . . . . . . . 8
71 2.3.1. Public-Key-Pins Response Header Field Processing . . 8
72 2.3.2. Noting a Pinned Host - Storage Model . . . . . . . . 9
73 2.3.3. HTTP-Equiv Element Attribute . . . . . . . . . 10
74 2.4. Semantics of Pins . . . . . . . . . . . . . . . . . . . . 10
75 2.5. Noting Pins . . . . . . . . . . . . . . . . . . . . . . . 11
76 2.6. Validating Pinned Connections . . . . . . . . . . . . . . 12
77 2.7. Interactions With Preloaded Pin Lists . . . . . . . . . . 13
78 2.8. Pinning Self-Signed End Entities . . . . . . . . . . . . 13
79 3. Reporting Pin Validation Failure . . . . . . . . . . . . . . 13
80 4. Security Considerations . . . . . . . . . . . . . . . . . . . 15
81 4.1. Maximum max-age . . . . . . . . . . . . . . . . . . . . . 15
82 4.2. Using includeSubDomains Safely . . . . . . . . . . . . . 16
83 4.3. Backup Pins . . . . . . . . . . . . . . . . . . . . . . . 17
84 5. Privacy Considerations . . . . . . . . . . . . . . . . . . . 17
85 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
86 7. Usability Considerations . . . . . . . . . . . . . . . . . . 19
87 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19
88 9. What's Changed . . . . . . . . . . . . . . . . . . . . . . . 19
89 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
90 10.1. Normative References . . . . . . . . . . . . . . . . . . 20
91 10.2. Informative References . . . . . . . . . . . . . . . . . 21
92 Appendix A. Fingerprint Generation . . . . . . . . . . . . . . . 21
93 Appendix B. Deployment Guidance . . . . . . . . . . . . . . . . 21
94 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
96 1. Introduction
98 We propose a new HTTP header to enable a web host to express to user
99 agents (UAs) which Subject Public Key Info (SPKI) structure(s) UAs
100 SHOULD expect to be present in the host's certificate chain in future
101 connections using TLS (see [RFC5246]). We call this "public key
102 pinning". At least one UA (Google Chrome) has experimented with the
103 idea by shipping with a user-extensible embedded set of pins.
104 Although effective, this does not scale. This proposal addresses the
105 scale problem.
107 Deploying public key pinning safely will require operational and
108 organizational maturity due to the risk that hosts may make
109 themselves unavailable by pinning to a SPKI that becomes invalid.
110 (See Section 4.) We believe that, with care, host operators can
111 greatly reduce the risk of MITM attacks and other false-
112 authentication problems for their users without incurring undue risk.
114 We intend for hosts to use public key pinning together with HSTS
115 ([RFC6797]), but is possible to pin keys without requiring HSTS.
117 This draft is being discussed on the WebSec Working Group mailing
118 list, websec@ietf.org.
120 1.1. Requirements Language
122 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
123 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
124 document are to be interpreted as described in RFC 2119 [RFC2119].
126 2. Server and Client Behavior
128 2.1. Response Header Field Syntax
130 The Public-Key-Pins HTTP response header field (PKP header field)
131 indicates to a UA that it should perform Pin Validation (Section 2.6)
132 in regards to the host emitting the response message containing this
133 header field, and provides the necessary information for the UA to do
134 so.
136 Figure 1 describes the ABNF (Augmented Backus-Naur Form) syntax of
137 the header field. It is based on the Generic Grammar defined in
138 Section 2 of [RFC2616] (which includes a notion of "implied linear
139 whitespace", also known as "implied *LWS").
141 Public-Key-Pins =
142 "Public-Key-Pins" ":" [ directive ] *( ";" [ directive ] )
143 Public-Key-Pins-Report-Only =
144 "Public-Key-Pins-Report-Only" ":" [ directive ] *( ";" [ directive ] )
146 directive = simple-directive
147 / pin-directive
149 simple-directive = directive-name [ "=" directive-value ]
150 directive-name = token
151 directive-value = token
152 / quoted-string
154 pin-directive = "pin-" token "=" quoted-string
156 Figure 1: HPKP Header Syntax
158 token and quoted-string are used as defined in [RFC2616],
159 Section 2.2.
161 The directives defined in this specification are described below.
162 The overall requirements for directives are:
164 1. The order of appearance of directives is not significant.
166 2. All simple-directives MUST appear only once in a PKP header
167 field. Directives are either optional or required, as stipulated
168 in their definitions.
170 3. Directive names are case-insensitive.
172 4. UAs MUST ignore any PKP header fields containing directives, or
173 other header field value data, that do not conform to the syntax
174 defined in this specification.
176 5. If a PKP header field contains any directive(s) the UA does not
177 recognize, the UA MUST ignore the those directives.
179 6. If the PKP header field otherwise satisfies the above
180 requirements (1 through 5), the UA MUST process the directives it
181 recognizes.
183 Additional directives extending the semantic functionality of the PKP
184 header field can be defined in other specifications, with a registry
185 (having an IANA policy definition of IETF Review [RFC2616]) defined
186 for them at such time. Such future directives will be ignored by UAs
187 implementing only this specification, as well as by generally non-
188 conforming UAs.
190 In the pin-directive, the token is the name of a cryptographic hash
191 algorithm, and MUST be either "sha1" or "sha256". The quoted-string
192 is a sequence of base 64 digits: the base 64-encoded SPKI Fingerprint
193 ([RFC4648]). See Section 2.4.
195 2.1.1. The max-age Directive
197 The REQUIRED "max-age" directive specifies the number of seconds,
198 after the reception of the PKP header field, during which the UA
199 SHOULD regard the host (from whom the message was received) as a
200 Known Pinned Host. The delta-seconds production is specified in
201 [RFC2616].
203 The syntax of the max-age directive's REQUIRED value (after quoted-
204 string unescaping, if necessary) is defined as:
206 max-age-value = delta-seconds
207 delta-seconds = 1*DIGIT
209 Figure 2: max-age Value Syntax
211 delta-seconds is used as defined in [RFC2616], Section 3.3.2.
213 NOTE: A max-age value of zero (i.e., "max-age=0") signals the UA to
214 cease regarding the host as a Known Pinned Host, including the
215 includeSubDomains directive (if asserted for that Known Pinned Host).
216 See Section 2.3.1.
218 2.1.2. The includeSubDomains Directive
220 The OPTIONAL "includeSubDomains" directive is a valueless directive
221 which, if present (i.e., it is "asserted"), signals to the UA that
222 the Pinning Policy applies to this Pinned Host as well as any
223 subdomains of the host's domain name.
225 2.1.3. The report-uri Directive
227 The OPTIONAL "report-uri" directive indicates the URI to which the UA
228 SHOULD report Pin Validation failures (Section 2.6). The UA POSTs
229 the reports to the given URI as described in Section 3.
231 When used in the Public-Key-Pins-Report-Only header, the UA SHOULD
232 POST reports for Pin Validation failures to the indicated report-uri,
233 although the UA MUST NOT enforce Pin Validation. That is, in the
234 event of Pin Validation failure when the host has set the Public-Key-
235 Pins-Report-Only header, the UA performs Pin Validation only to check
236 whether or not it should POST a report, but not for causing
237 connection failure.
239 If a Host sets both the Public-Key-Pins header and the Public-Key-
240 Pins-Report-Only header, the UA MUST NOT enforce Pin Validation, and
241 MUST note only the pins and directives given in the Public-Key-Pins-
242 Report-Only header.
244 When used in the Public-Key-Pins header, the presence of a report-uri
245 directive indicates to the UA that the UA MUST enforce Pin
246 Validation, and the UA SHOULD also, in the event of Pin Validation
247 failure, POST a report to the report-uri.
249 Note that the report-uri need not necessarily be in the same Internet
250 domain or web origin as the Known Pinned Host.
252 Hosts may set report-uris that use HTTP, HTTPS, or other schemes. If
253 the scheme in the report-uri is HTTPS, UAs MUST perform Pinning
254 Validation when the host in the report-uri is a Known Pinned Host;
255 similarly, UAs MUST apply HSTS if the host in the report-uri is a
256 Known HSTS Host.
258 UAs SHOULD make their best effort to report Pin Validation failures
259 to the report-uri, but may fail to report in exceptional conditions.
260 For example, if connecting the report-uri itself incurs a Pinning
261 Validation failure or other certificate validation failure, the UA
262 MUST cancel the connection (and MAY attempt to re-send the report
263 later). Similarly, if Known Pinned Host A sets a report-uri
264 referring to Known Pinned Host B, and if B sets a report-uri
265 referring to A, and if both hosts fail Pin Validation, the UA SHOULD
266 detect and break the loop by failing to send reports to and about
267 those hosts.
269 UAs SHOULD limit the rate at which they send reports. For example,
270 it is unnecessary to send the same report to the same report-uri more
271 than once.
273 2.1.4. The strict Directive
275 The OPTIONAL "strict" directive is a valueless directive which, if
276 present (i.e., it is "asserted"), signals to the UA that it should
277 apply to the Pinned Host the Pinning Policy expressed in the PKP
278 header exactly as specified, ignoring local client policy.
280 2.1.5. Examples
282 Figure 3 shows some example response header fields using the pins
283 extension (folded for clarity).
285 Public-Key-Pins: max-age=3000;
286 pin-sha1="4n972HfV354KP560yw4uqe/baXc=";
287 pin-sha1="IvGeLsbqzPxdI0b0wuj2xVTdXgc="
289 Public-Key-Pins: max-age=2592000;
290 pin-sha1="4n972HfV354KP560yw4uqe/baXc=";
291 pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ="
293 Public-Key-Pins: max-age=2592000;
294 pin-sha1="4n972HfV354KP560yw4uqe/baXc=";
295 pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ=";
296 report-uri="http://example.com/pkp-report"
298 Public-Key-Pins-Report-Only: max-age=2592000;
299 pin-sha1="4n972HfV354KP560yw4uqe/baXc=";
300 pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ=";
301 report-uri="http://example.com/pkp-report"
303 Public-Key-Pins: pin-sha1="4n972HfV354KP560yw4uqe/baXc=";
304 pin-sha1="qvTGHdzF6KLavt4PO0gs2a6pQ00=";
305 pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ=";
306 max-age=259200
308 Public-Key-Pins: pin-sha1="4n972HfV354KP560yw4uqe/baXc=";
309 pin-sha1="qvTGHdzF6KLavt4PO0gs2a6pQ00=";
310 pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ=";
311 max-age=10000; includeSubDomains
313 Figure 3: HPKP Header Examples
315 2.2. Server Processing Model
317 This section describes the processing model that Pinned Hosts
318 implement. The model comprises two facets: the processing rules for
319 HTTP request messages received over a secure transport (e.g. TLS
320 [RFC5246]); and the processing rules for HTTP request messages
321 received over non-secure transports, such as TCP.
323 2.2.1. HTTP-over-Secure-Transport Request Type
325 When replying to an HTTP request that was conveyed over a secure
326 transport, a Pinned Host SHOULD include in its response exactly one
327 PKP header field that MUST satisfy the grammar specified above in
328 Section 2.1. If the Pinned Host does not include the PKP header
329 field, and if the connection passed Pin Validation, UAs MUST treat
330 the host as if it had set its max-age to 0 (see Section 2.3.1).
332 Establishing a given host as a Known Pinned Host, in the context of a
333 given UA, MAY be accomplished over the HTTP protocol, which is in
334 turn running over secure transport, by correctly returning (per this
335 specification) at least one valid PKP header field to the UA. Other
336 mechanisms, such as a client-side pre-loaded Known Pinned Host list
337 MAY also be used.
339 2.2.2. HTTP Request Type
341 Pinned Hosts SHOULD NOT include the PKP header field in HTTP
342 responses conveyed over non-secure transport. UAs MUST ignore any
343 PKP header received in an HTTP response conveyed over non-secure
344 transport.
346 2.3. User Agent Processing Model
348 This section describes the HTTP Public Key Pinning processing model
349 for UAs.
351 The UA processing model relies on parsing domain names. Note that
352 internationalized domain names SHALL be canonicalized according to
353 the scheme in Section 10 of [RFC6797].
355 2.3.1. Public-Key-Pins Response Header Field Processing
357 If the UA receives, over a secure transport, an HTTP response that
358 includes a PKP header field conforming to the grammar specified in
359 Section 2.1, and there are no underlying secure transport errors or
360 warnings (see Section 2.5), the UA MUST either:
362 o Note the host as a Known HSTS Host if it is not already so noted
363 (see Section 2.3.2),
365 or,
367 o Update the UA's cached information for the Known Pinned Host if
368 any of of the max-age, includeSubDomains, strict, or report-uri
369 header field value directives convey information different than
370 that already maintained by the UA.
372 o The max-age value is essentially a "time to live" value relative
373 to the time of the most recent observation of the PKP header
374 field.
376 o If the max-age header field value token has a value of 0, the UA
377 MUST remove its cached Pinning Policy information (including the
378 includeSubDomains and strict directives, if asserted) if the
379 Pinned Host is Known, or, MUST NOT note this Pinned Host if it is
380 not yet Known.
382 o If a UA receives more than one PKP header field in an HTTP
383 response message over secure transport, then the UA MUST process
384 only the first such header field.
386 Otherwise:
388 o If the UA receives the HTTP response over insecure transport, or
389 if the PKP header is not a Valid Pinning Header (see Section 2.5),
390 the UA MUST ignore any present PKP header field(s).
392 o The UA MUST ignore any PKP header fields not conforming to the
393 grammar specified in Section 2.1.
395 2.3.2. Noting a Pinned Host - Storage Model
397 If the substring matching the host production from the Request-URI
398 (of the message to which the host responded) syntactically matches
399 the IP-literal or IPv4address productions from Section 3.2.2 of
400 [RFC3986], then the UA MUST NOT note this host as a Known Pinned
401 Host.
403 Otherwise, if the substring does not congruently match a Known Pinned
404 Host's domain name, per the matching procedure specified in
405 Section 8.2 of [RFC6797], then the UA MUST note this host as a Known
406 Pinned Host, caching the Pinned Host's domain name and noting along
407 with it the time of the observation (also known as the Effective Pin
408 Date), the value of the max-age directive, whether or not the
409 includeSubDomains or strict directives are asserted, the value of the
410 report-uri directive (if present), and any other metadata from
411 optional or future PKP header directives.
413 UAs MAY set an upper limit on the value of max-age, so that UAs that
414 have noted erroneous pins (whether by accident or due to attack) have
415 some chance of recovering over time. If the server sets a max-age
416 greater than the UA's upper limit, the UA MAY behave as if the server
417 set the max-age to the UA's upper limit. For example, if the UA caps
418 max-age at 5184000 seconds (60 days), and a Pinned Host sets a max-
419 age directive of 90 days in its Valid Pinning Header, the UA MAY
420 behave as if the max-age were effectively 60 days. (One way to
421 achieve this behavior is for the UA to simply store a value of 60
422 days instead of the 90 day value provided by the Pinned Host.) For
423 UA implementation guidance on how to select a maximum max-age, see
424 Section 4.1.
426 The UA MUST NOT modify any pinning metadata of any superdomain
427 matched Known Pinned Host.
429 A Known Pinned Host is "expired" if the Effective Pin Date plus the
430 max-age refers to a date in the past. The UA MUST ignore all expired
431 Known Pinned Hosts from its cache if, at any time, an expired Known
432 Pinned Host exists in the cache.
434 2.3.3. HTTP-Equiv Element Attribute
436 UAs MUST NOT heed http-equiv="Public-Key-Pins" attribute settings on
437 elements [W3C.REC-html401-19991224] in received content.
439 2.4. Semantics of Pins
441 An SPKI Fingerprint is defined as the output of a known cryptographic
442 hash algorithm whose input is the DER-encoded ASN.1 representation of
443 the SubjectPublicKeyInfo (SPKI) field of an X.509 certificate. A Pin
444 is defined as the combination of the known algorithm identifier and
445 the SPKI Fingerprint computed using that algorithm.
447 The SPKI Fingerprint is encoded in base 64 for use in an HTTP header.
448 (See [RFC4648].)
450 In this version of the specification, the known cryptographic hash
451 algorithms are SHA-1, identified as "sha1", and SHA-256, identified
452 as "sha256" ([RFC4634]). (Future versions of this specification may
453 add new algorithms and deprecate old ones.) UAs MUST ignore Pins for
454 which they do not recognize the algorithm identifier. UAs MUST
455 continue to process the rest of a PKP response header field and note
456 Pins for algorithms they do recognize; UAs MUST recognize "sha1" and
457 "sha256".
459 Figure 4 reproduces the definition of the SubjectPublicKeyInfo
460 structure in [RFC5280].
462 SubjectPublicKeyInfo ::= SEQUENCE {
463 algorithm AlgorithmIdentifier,
464 subjectPublicKey BIT STRING }
466 AlgorithmIdentifier ::= SEQUENCE {
467 algorithm OBJECT IDENTIFIER,
468 parameters ANY DEFINED BY algorithm OPTIONAL }
470 Figure 4: SPKI Definition
472 If the SubjectPublicKeyInfo of a certificate is incomplete when taken
473 in isolation, such as when holding a DSA key without domain
474 parameters, a public key pin cannot be formed.
476 We pin public keys, rather than entire certificates, to enable
477 operators to generate new certificates containing old public keys
478 (see [why-pin-key]).
480 See Appendix A for an example non-normative program that generates
481 SPKI Fingerprints from SubjectPublicKeyInfo fields in certificates.
483 2.5. Noting Pins
485 Upon receipt of the Public-Key-Pins response header field, the UA
486 notes the host as a Pinned Host, storing the Pins and their
487 associated directives in non-volatile storage (for example, along
488 with the HSTS metadata). The Pins and their associated directives
489 are collectively known as Pinning Metadata.
491 The UA MUST observe these conditions when noting a Host:
493 o The UA MUST note the Pins if and only if it received the Public-
494 Key-Pins response header field over an error-free TLS connection.
495 If the host is a Pinned Host, this includes the validation added
496 in Section 2.6.
498 o The UA MUST note the Pins if and only if the TLS connection was
499 authenticated with a certificate chain containing at least one of
500 the SPKI structures indicated by at least one of the given SPKI
501 Fingerprints. (See Section 2.6.)
503 o The UA MUST note the Pins if and only if the given set of Pins
504 contains at least one Pin that does NOT refer to an SPKI in the
505 certificate chain. (That is, the host must set a Backup Pin; see
506 Section 4.3.)
508 If the Public-Key-Pins response header field does not meet all three
509 of these criteria, the UA MUST NOT note the host as a Pinned Host. A
510 Public-Key-Pins response header field that meets all these critera is
511 known as a Valid Pinning Header.
513 The UA MUST ignore Public-Key-Pins response header fields received on
514 connections that do not meet the first criterion.
516 Whenever a UA receives a Valid Pinning Header, it MUST set its
517 Pinning Metadata to the exact Pins, max-age, and (if any) report-uri
518 and strict mode given in the most recently received Valid Pinning
519 Header.
521 For forward compatibility, the UA MUST ignore any unrecognized
522 Public-Key-Pins header directives, while still processing those
523 directives it does recognize. Section 2.1 specifies the directives
524 max-age, pins, includeSubDomains, report-uri, and strict, but future
525 specifications and implementations might use additional directives.
527 2.6. Validating Pinned Connections
529 When a UA connects to a Pinned Host, if the TLS connection has
530 errors, the UA MUST terminate the connection without allowing the
531 user to proceed anyway. (This behavior is the same as that required
532 by [RFC6797].)
534 If the connection has no errors, then the UA will determine whether
535 to apply a new, additional correctness check: Pin Validation. A UA
536 SHOULD perform Pin Validation whenever connecting to a Known Pinned
537 Host, but MAY allow Pin Validation to be disabled for Hosts according
538 to local policy. For example, a UA may disable Pin Validation for
539 Pinned Hosts whose validated certificate chain terminates at a user-
540 defined trust anchor, rather than a trust anchor built-in to the UA.
541 However, if the Pinned Host Metadata indicates that the Pinned Host
542 is operating in "strict mode" (see Section 2.1.4), then the UA MUST
543 perform Pin Validation.
545 To perform Pin Validation, the UA will compute the SPKI Fingerprints
546 for each certificate in the Pinned Host's validated certificate
547 chain, using each supported hash algorithm for each certificate.
548 (For the purposes of Pin Validation, the UA MUST ignore certificates
549 whose SPKI cannot be taken in isolation, and MUST ignore superfluous
550 certificates in the chain that do not form part of the validating
551 chain.) The UA will then check that the set of these SPKI
552 Fingerprints intersects the set of SPKI Fingerprints in that Pinned
553 Host's Pinning Metadata. If there is set intersection, the UA
554 continues with the connection as normal. Otherwise, the UA MUST
555 treat this Pin Failure as a non-recoverable error. Any procedure
556 that matches the results of this Pin Validation procedure is
557 considered equivalent.
559 Note that, although the UA has previously received Pins at the HTTP
560 layer, it can and MUST perform Pin Validation at the TLS layer,
561 before beginning an HTTP conversation over the TLS channel. The TLS
562 layer thus evaluates TLS connections with pinning information the UA
563 received previously, regardless of mechanism: statically preloaded,
564 via HTTP header, or some other means (possibly in the TLS layer
565 itself).
567 2.7. Interactions With Preloaded Pin Lists
569 UAs MAY choose to implement additional sources of pinning
570 information, such as through built-in lists of pinning information.
571 Such UAs SHOULD allow users to override such additional sources,
572 including disabling them from consideration.
574 UAs that support additional sources of pinning information MUST use
575 the most recently observed pinning information when performing Pin
576 Validation for a host. The most recently observed pinning
577 information is determined based upon the most recent Effective Pin
578 Date, as described in Section 2.3.2.
580 If the result of noting a Valid Pinning Header is to disable pinning
581 for the host, such as through supplying a max-age directive with a
582 value of 0, UAs MUST allow this new information to override any other
583 pinning data. That is, a host must be able to un-pin itself, even in
584 the presence of built-in pins.
586 Example: A UA may ship with a pre-configured list of pins that are
587 collected from past observations of Valid Pinning Headers supplied by
588 hosts. In such a solution, the pre-configured list should track when
589 the Valid Pinning Header was last observed, in order to permit site
590 operators to later update the value by supplying a new Valid Pinning
591 Header. Updates to such a pre-configured list should not update the
592 Effective Pin Dates for each host unless the list vendor has actually
593 observed a more recent header. This is to prevent situations where
594 updating the Effective Pin Date on a pre-configured list of pins may
595 effectively extend the max-age beyond the site operator's stated
596 policy.
598 Example: A UA may ship with a pre-configured list of pins that are
599 collected through out-of-band means, such as direct contact with the
600 site operator. In such a solution, the site operator accepts
601 responsibility for keeping the configured Valid Pinning Header in
602 sync with the vendor's list, allowing the UA vendor to have each
603 update to the list be treated as as an update of the Effective Pin
604 Date.
606 2.8. Pinning Self-Signed End Entities
608 If UAs accept hosts that authenticate themselves with self-signed end
609 entity certificates, they MAY also allow hosts to pin the public keys
610 in such certificates. The usability and security implications of
611 this practice are outside the scope of this specification.
613 3. Reporting Pin Validation Failure
614 When a Known Pinned Host has set the report-uri directive, the UA
615 SHOULD report Pin Validation failures to the indicated URI. The UA
616 does this by POSTing a JSON ([RFC4627]) message to the URI; the JSON
617 message takes this form:
619 {
620 "date-time": date-time,
621 "hostname": hostname,
622 "port": port,
623 "certificate-chain": [
624 pem1, ... pemN
625 ],
626 "known-pins": [
627 known-pin1, ... known-pinN
628 ]
629 }
631 Figure 5: JSON Report Format
633 Whitespace outside of quoted strings is not significant. The key/
634 value pairs may appear in any order, but each SHOULD appear only
635 once.
637 The date-time indicates the time the UA observed the Pin Validation
638 failure. It is provided as a string formatted according to
639 Section 5.6, "Internet Date/Time Format", of [RFC3339].
641 The hostname is the hostname to which the UA made the original
642 request that failed Pin Validation. It is provided as a string.
644 The port is the port to which the UA made the original request that
645 failed Pin Validation. It is provided either as a string or as an
646 integer.
648 The certificate-chain is the certificate chain, as constructed by the
649 UA during certificate chain verification. (This may differ from the
650 certificate chain as served by the Known Pinned Host, of course.) It
651 is provided as an array of strings; each string pem1, ... pemN is the
652 PEM representation of each X.509 certificate as described in
653 [I-D.josefsson-pkix-textual].
655 The known-pins are the Pins that the UA has noted for the Known
656 Pinned Host. They are provided as an array of strings with the
657 syntax:
659 known-pin = token "=" quoted-string
661 Figure 6: Known Pin Syntax
663 As in Section 2.4, the token refers to the algorithm name, and the
664 quoted-string refers to the base 64 encoding of the SPKI Fingerprint.
666 4. Security Considerations
668 Pinning public keys helps hosts strongly assert their cryptographic
669 identity even in the face of issuer error, malfeasance or compromise.
670 But there is some risk that a host operator could lose or lose
671 control of their host's private key (such as by operator error or
672 host compromise). If the operator had pinned only the key of the
673 host's end entity certificate, the operator would not be able to
674 serve their web site or application in a way that UAs would trust for
675 the duration of their pin's max-age. (Recall that UAs MUST close the
676 connection to a host upon Pin Failure.)
678 Therefore, there is a necessary trade-off between two competing
679 goods: pin specificity and maximal reduction of the scope of issuers
680 on the one hand; and flexibility and resilience of the host's
681 cryptographic identity on the other hand. One way to resolve this
682 trade-off is to compromise by pinning to the key(s) of the issuer(s)
683 of the host's end entity certificate(s). Often, a valid certificate
684 chain will have at least two certificates above the end entity
685 certificate: the intermediate issuer, and the trust anchor.
686 Operators can pin any one or more of the public keys in this chain,
687 and indeed could pin to issuers not in the chain (as, for example, a
688 Backup Pin). Pinning to an intermediate issuer, or even to a trust
689 anchor or root, still significantly reduces the number of issuers who
690 can issue end entity certificates for the Known Pinned Host, while
691 still giving that host flexibility to change keys without a
692 disruption of service.
694 4.1. Maximum max-age
696 As mentioned in Section 2.3.2, UAs MAY cap the max-age value at some
697 upper limit. There is a security trade-off in that low maximum
698 values provide a narrow window of protection for users who visit the
699 Known Pinned Host only infrequently, while high maximum values might
700 potentially result in a UA's inability to successfully perform Pin
701 Validation for a Known Pinned Host if the UA's noted pins and the
702 Host's true pins diverge.
704 Such divergence could occur for several reasons, including: UA error;
705 Host operator error; network attack; or a Known Pinned Host that
706 intentionally migrates all pinned keys, combined with a UA that has
707 noted true pins with a high max-age value and has not had a chance to
708 observe the new true pins for the Host. (This last example
709 underscores the importance for Host operators to phase in new keys
710 gradually, and to set the max-age value in accordance with their
711 planned key migration schedule.)
713 There is probably no ideal upper limit to the max-age directive that
714 would satisfy all use cases. However, a value on the order of 60
715 days (5184000 seconds) may be considered a balance between the two
716 competing security concerns.
718 4.2. Using includeSubDomains Safely
720 It may happen that Pinned Hosts whose hostnames share a parent domain
721 use different Valid Pinning Headers. If a Host whose hostname is a
722 parent domain for another Host sets the includeSubDomains directive,
723 the two Hosts' pins may conflict with each other. For example,
724 consider two Known Pinned Hosts, example.com and
725 subdomain.example.com. Assume example.com sets a Valid Pinning
726 Header such as this:
728 Public-Key-Pins: pin-sha1="ABC..."; pin-sha1="DEF..."; includeSubDomains
730 Figure 7: example.com Valid Pinning Header
732 Assume subdomain.example.com sets a Valid Pinning Header such as
733 this:
735 Public-Key-Pins: pin-sha1="GHI..."; pin-sha1="JKL..."
737 Figure 8: subdomain.example.com Valid Pinning Header
739 Assume a UA that has not previously noted any pins for either of
740 these Hosts. If the UA first contacts subdomain.example.com, it will
741 note the pins in the Valid Pinning Header, and perform Pin Validation
742 as normal on subsequent conections. If the UA then contacts
743 example.com, again it will note the pins and perform Pin Validation
744 on future connections. However, if the UA happened to first
745 example.com before subdomain.example.com, the UA would, due to
746 example.com's use of the includeSubDomains directive, attempt to
747 perform Pin Validation for subdomain.example.com using the SPKI
748 hashes ABC... and DEF..., which are not valid for the certificate
749 chains subdomain.example.com (which uses certificates with SPKIs
750 GHI... and JLK...). Thus, depending on the order in which the UA
751 observes the Valid Pinning Headers for hosts example.com and
752 subdomain.example.com, Pin Validation might or might not fail for
753 subdomain.example.com, even if the certificate chain the UA receives
754 for subdomain.example.com is perfectly valid.
756 Thus, Pinned Host operators must use the includeSubDomains directive
757 with care. For example, they may choose to use overlapping pin sets
758 for hosts under a parent domain that uses includeSubDomains, or to
759 not use the includeSubDomains directive in their effective-second-
760 level domains, or to simply use the same pin set for all hosts under
761 a given parent domain.
763 4.3. Backup Pins
765 The primary way to cope with the risk of inadvertent Pin Failure is
766 to keep a Backup Pin. A Backup Pin is a fingerprint for the public
767 key of a secondary, not-yet-deployed key pair. The operator keeps
768 the backup key pair offline, and sets a pin for it in the Public-Key-
769 Pins header. Then, in case the operator loses control of their
770 primary private key, they can deploy the backup key pair. UAs, who
771 have had the backup key pair pinned (when it was set in previous
772 Valid Pinning Headers), can connect to the host without error.
774 Because having a backup key pair is so important to recovery, UAs
775 MUST require that hosts set a Backup Pin. (See Section 2.5.)
777 5. Privacy Considerations
779 Conforming implementations (as well as implementations conforming to
780 [RFC6797]) must store state about which domains have set policies,
781 hence which domains the UA has contacted. A forensic attacker might
782 find this information useful, even if the user has cleared other
783 parts of the UA's state.
785 More importantly, Hosts can use HSTS or HPKP as a "super-cookie", by
786 setting distinct policies for a number of subdomains. For example,
787 assume example.com wishes to track distinct UAs without explicitly
788 setting a cookie, or if a previously-set cookie is deleted from the
789 UA's cookie store. Here are two attack scenarios.
791 1. example.com can use report-uri and the ability to pin arbitrary
792 identifiers to distinguish UAs.
794 2.
796 1. example.com sets a Valid Pinning Header in its response to
797 requests. The header asserts the includeSubDomains
798 directive, and specifies a report-uri directive as well.
799 Pages served by the host also include references to
800 subresource https://bad.example.com/foo.png.
802 2. The Valid Pinning Header includes a "pin" that is not really
803 the hash of an SPKI, but is instead an arbitrary
804 distinguishing string sent only in response to a particular
805 request. For each request, the Host creates a new, distinct
806 distinguishing string and sets it as if it were a pin.
808 3. The certificate chain served by bad.example.com does not pass
809 Pin Validation given the pin set the Host asserted in (1).
810 The HPKP-conforming UA attempts to report the Pin Validation
811 failure to the specified report-uri, including the
812 certificate chain it observed and the SPKI hashes it expected
813 to see. Among the SPKI hashes is the distinguishing string
814 in step (2).
816 3. example.com can use SNI and subdomains to distinguish UAs.
818 4.
820 1. example.com sets a Valid Pinning Header in its response to
821 requests. The header asserts the includeSubDomains
822 directive.
824 2. On a subsequent page view, the Host responds with a page
825 including the subresource https://0.fingerprint.example.com/
826 foo.png, and the server responds using a certificate chain
827 that does not pass Pin Validation for the pin-set defined in
828 the Valid Pinning Header in step (1). The HPKP-conforming UA
829 will close the connection, never completing the request to
830 0.fingerprint.example.com. The Host may thus note that this
831 particular UA had noted the (good) pins for that subdomain.
833 3. example.com can distinguish 2^N UAs by serving Valid Pinning
834 Headers from an arbitrary number N distinct subdomains,
835 giving some UAs Valid Pinning Headers for some, but not all
836 subdomains (causing subsequent requests for
837 n.fingerprint.example.com to fail), and giving some UAs no
838 Valid Pinning Header for other subdomains (causing subsequent
839 requests for m.fingerprint.example.com to succeed).
841 6. IANA Considerations
842 This document has no actions for IANA. However, in the future there
843 may arise a need for a registry of extension directives (see
844 Section 2.1).
846 7. Usability Considerations
848 When pinning works to detect impostor Pinned Hosts, users will
849 experience denial of service. UAs MUST explain the reason why, i.e.
850 that it was impossible to verify the confirmed cryptographic identity
851 of the host.
853 UAs MUST have a way for users to clear current pins for Pinned Hosts.
854 UAs SHOULD have a way for users to query the current state of Pinned
855 Hosts.
857 8. Acknowledgements
859 Thanks to Tobias Gondrom, Jeff Hodges, Ivan Krstic, Adam Langley,
860 Nicolas Lidzborski, SM, James Manger, Eric Rescorla, Paul Hoffman,
861 and Yoav Nir for suggestions and edits that clarified the text.
862 Thanks to Trevor Perrin for suggesting a mechanism to affirmatively
863 break pins ([pin-break-codes]).
865 9. What's Changed
867 Added normative references for SHA, JSON, and base-64.
869 Added the Privacy Considerations section.
871 Changed non-normative pin generation code from Go to POSIX shell
872 script using openssl.
874 Changed max-max-age from SHOULD to MAY, and used the example of 60
875 days instead of 30.
877 Removed the section "Pin Validity Times", which was intended to be in
878 harmony with [I-D.perrin-tls-tack]. Now using max-age purely as
879 specified in [RFC6797].
881 Added new directives: includeSubDomains, report-uri and strict.
883 Added a new variant of the PKP Header: Public-Key-Pins-Report-Only.
885 Removed the section on pin break codes and verifiers, in favor the of
886 most-recently-received policy (Section 2.5).
888 Now using a new header field, Public-Key-Pins, separate from HSTS.
889 This allows hosts to use pinning separately from Strict Transport
890 Security.
892 Explicitly requiring that UAs perform Pin Validation before the HTTP
893 conversation begins.
895 Backup Pins are now required.
897 Separated normative from non-normative material. Removed tangential
898 and out-of-scope non-normative discussion.
900 10. References
902 10.1. Normative References
904 [I-D.josefsson-pkix-textual]
905 Josefsson, S. and S. Leonard, "Text Encodings of PKIX and
906 CMS Structures", draft-josefsson-pkix-textual-01 (work in
907 progress), July 2012.
909 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
910 Requirement Levels", BCP 14, RFC 2119, March 1997.
912 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
913 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
914 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
916 [RFC3339] Klyne, G., Ed. and C. Newman, "Date and Time on the
917 Internet: Timestamps", RFC 3339, July 2002.
919 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
920 Resource Identifier (URI): Generic Syntax", STD 66, RFC
921 3986, January 2005.
923 [RFC4627] Crockford, D., "The application/json Media Type for
924 JavaScript Object Notation (JSON)", RFC 4627, July 2006.
926 [RFC4634] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms
927 (SHA and HMAC-SHA)", RFC 4634, July 2006.
929 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
930 Encodings", RFC 4648, October 2006.
932 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
933 Encodings", RFC 4648, October 2006.
935 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
936 IANA Considerations Section in RFCs", BCP 26, RFC 5226,
937 May 2008.
939 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
940 (TLS) Protocol Version 1.2", RFC 5246, August 2008.
942 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
943 Housley, R., and W. Polk, "Internet X.509 Public Key
944 Infrastructure Certificate and Certificate Revocation List
945 (CRL) Profile", RFC 5280, May 2008.
947 [RFC6797] Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
948 Transport Security (HSTS)", RFC 6797, November 2012.
950 [W3C.REC-html401-19991224]
951 Hors, A., Raggett, D., and I. Jacobs, "HTML 4.01
952 Specification", World Wide Web Consortium Recommendation
953 REC-html401-19991224, December 1999,
954 .
956 10.2. Informative References
958 [I-D.perrin-tls-tack]
959 Marlinspike, M., "Trust Assertions for Certificate Keys",
960 draft-perrin-tls-tack-02 (work in progress), January 2013.
962 [pin-break-codes]
963 Perrin, T., "Self-Asserted Key Pinning", September 2011,
964 .
966 [why-pin-key]
967 Langley, A., "Public Key Pinning", May 2011,
968 .
970 Appendix A. Fingerprint Generation
972 This POSIX shell program generates SPKI Fingerprints, suitable for
973 use in pinning, from PEM-encoded certificates. It is non-normative.
975 openssl x509 -noout -in certificate.pem -pubkey | \
976 openssl asn1parse -inform pem -out public.key
977 openssl dgst -sha1 -binary public.key | base64
979 Figure 9: Example SPKI Fingerprint Generation Code
981 Appendix B. Deployment Guidance
982 This section is non-normative guidance which may smooth the adoption
983 of public key pinning.
985 o Operators SHOULD get the backup public key signed by a different
986 (root and/or intermediary) CA than their primary certificate, and
987 store the backup key pair safely offline. The semantics of an
988 SPKI Fingerprint do not require the issuance of a certificate to
989 construct a valid Pin. However, in many deployment scenarios, in
990 order to make a Backup Pin operational the server operator will
991 need to have a certificate to deploy TLS on the host. Failure to
992 obtain a certificate through prior arrangement will leave clients
993 that recognize the site as a Known Pinned Host unable to
994 successfully perform Pin Validation until such a time as the
995 operator can obtain a new certificate from their desired
996 certificate issuer.
998 o It is most economical to have the backup certificate signed by a
999 completely different signature chain than the live certificate, to
1000 maximize recoverability in the event of either root or
1001 intermediary signer compromise.
1003 o Operators SHOULD periodically exercise their Backup Pin plan
1004 -\u002D an untested backup is no backup at all.
1006 o Operators SHOULD start small. Operators SHOULD first deploy
1007 public key pinning by using the report-only mode together with a
1008 report-uri directive that points to a reliable report collection
1009 endpoint. When moving out of report-only mode, operators should
1010 start by setting a max-age of minutes or a few hours, and
1011 gradually increase max-age as they gain confidence in their
1012 operational capability.
1014 Authors' Addresses
1016 Chris Evans
1017 Google, Inc.
1018 1600 Amphitheatre Pkwy
1019 Mountain View, CA 94043
1020 US
1022 Email: cevans@google.com
1023 Chris Palmer
1024 Google, Inc.
1025 1600 Amphitheatre Pkwy
1026 Mountain View, CA 94043
1027 US
1029 Email: palmer@google.com
1031 Ryan Sleevi
1032 Google, Inc.
1033 1600 Amphitheatre Pkwy
1034 Mountain View, CA 94043
1035 US
1037 Email: sleevi@google.com