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2 Web Security C. Evans
3 Internet-Draft C. Palmer
4 Intended status: Standards Track R. Sleevi
5 Expires: December 09, 2013 Google, Inc.
6 June 07, 2013
8 Public Key Pinning Extension for HTTP
9 draft-ietf-websec-key-pinning-05
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 December 09, 2013.
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 . . . . . . . . . . . . . . . . . . 7
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 . . . . . . . . . 9
74 2.3.4. UA Processing Examples . . . . . . . . . . . . . . . 9
75 2.4. Semantics of Pins . . . . . . . . . . . . . . . . . . . . 10
76 2.5. Noting Pins . . . . . . . . . . . . . . . . . . . . . . . 11
77 2.6. Validating Pinned Connections . . . . . . . . . . . . . . 11
78 2.7. Interactions With Preloaded Pin Lists . . . . . . . . . . 12
79 2.8. Pinning Self-Signed End Entities . . . . . . . . . . . . 13
80 3. Reporting Pin Validation Failure . . . . . . . . . . . . . . 13
81 4. Security Considerations . . . . . . . . . . . . . . . . . . . 14
82 4.1. Maximum max-age . . . . . . . . . . . . . . . . . . . . . 15
83 4.2. Using includeSubdomains Safely . . . . . . . . . . . . . 16
84 4.3. Backup Pins . . . . . . . . . . . . . . . . . . . . . . . 16
85 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
86 6. Usability Considerations . . . . . . . . . . . . . . . . . . 17
87 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17
88 8. What's Changed . . . . . . . . . . . . . . . . . . . . . . . 17
89 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
90 9.1. Normative References . . . . . . . . . . . . . . . . . . 18
91 9.2. Informative References . . . . . . . . . . . . . . . . . 19
92 Appendix A. Fingerprint Generation . . . . . . . . . . . . . . . 19
93 Appendix B. Deployment Guidance . . . . . . . . . . . . . . . . 20
94 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
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
103 shipping with a user-extensible embedded set of pins. Although
104 effective, this does not scale. This proposal addresses the scale
105 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
193 Fingerprint. 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 the Pinning
277 Policy contained should be applied to the Pinned Host exactly as
278 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=500;
286 pin-sha1="4n972HfV354KP560yw4uqe/baXc=";
287 pin-sha1="IvGeLsbqzPxdI0b0wuj2xVTdXgc="
289 Public-Key-Pins: max-age=31536000;
290 pin-sha1="4n972HfV354KP560yw4uqe/baXc=";
291 pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ="
293 Public-Key-Pins: pin-sha1="4n972HfV354KP560yw4uqe/baXc=";
294 pin-sha1="qvTGHdzF6KLavt4PO0gs2a6pQ00=";
295 pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ=";
296 max-age=2592000
298 Public-Key-Pins: pin-sha1="4n972HfV354KP560yw4uqe/baXc=";
299 pin-sha1="qvTGHdzF6KLavt4PO0gs2a6pQ00=";
300 pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ=";
301 max-age=2592000; includeSubDomains
303 Figure 3: HPKP Header Examples
305 2.2. Server Processing Model
307 This section describes the processing model that Pinned Hosts
308 implement. The model comprises two facets: the processing rules for
309 HTTP request messages received over a secure transport (e.g. TLS
310 [RFC5246]); and the processing rules for HTTP request messages
311 received over non-secure transports, such as TCP.
313 2.2.1. HTTP-over-Secure-Transport Request Type
315 When replying to an HTTP request that was conveyed over a secure
316 transport, a Pinned Host SHOULD include in its response exactly one
317 PKP header field that MUST satisfy the grammar specified above in
318 Section 2.1. If the Pinned Host does not include the PKP header
319 field, and if the connection passed Pin Validation, UAs MUST treat
320 the host as if it had set its max-age to 0 (see Section 2.3.1).
322 Establishing a given host as a Known Pinned Host, in the context of a
323 given UA, MAY be accomplished over the HTTP protocol, which is in
324 turn running over secure transport, by correctly returning (per this
325 specification) at least one valid PKP header field to the UA. Other
326 mechanisms, such as a client-side pre-loaded Known Pinned Host list
327 MAY also be used.
329 2.2.2. HTTP Request Type
331 Pinned Hosts SHOULD NOT include the PKP header field in HTTP
332 responses conveyed over non-secure transport. UAs MUST ignore any
333 PKP header received in an HTTP response conveyed over non-secure
334 transport.
336 2.3. User Agent Processing Model
338 This section describes the HTTP Public Key Pinning processing model
339 for UAs.
341 The UA processing model relies on parsing domain names. Note that
342 internationalized domain names SHALL be canonicalized according to
343 the scheme in Section 10 of [RFC6797].
345 2.3.1. Public-Key-Pins Response Header Field Processing
347 If the UA receives, over a secure transport, an HTTP response that
348 includes a PKP header field conforming to the grammar specified in
349 Section 2.1, and there are no underlying secure transport errors or
350 warnings (see Section 2.5), the UA MUST either:
352 o Note the host as a Known HSTS Host if it is not already so noted
353 (see Section 2.3.2),
355 or,
357 o Update the UA's cached information for the Known Pinned Host if
358 any of of the max-age, includeSubDomains, strict, or report-uri
359 header field value directives convey information different than
360 that already maintained by the UA.
362 o The max-age value is essentially a "time to live" value relative
363 to the time of the most recent observation of the PKP header
364 field.
366 o If the max-age header field value token has a value of 0, the UA
367 MUST remove its cached Pinning Policy information (including the
368 includeSubDomains and strict directives, if asserted) if the
369 Pinned Host is Known, or, MUST NOT note this Pinned Host if it is
370 not yet Known.
372 o If a UA receives more than one PKP header field in an HTTP
373 response message over secure transport, then the UA MUST process
374 only the first such header field.
376 Otherwise:
378 o If the UA receives the HTTP response over insecure transport, or
379 if the PKP header is not a Valid Pinning Header (see Section 2.5),
380 the UA MUST ignore any present PKP header field(s).
382 o The UA MUST ignore any PKP header fields not conforming to the
383 grammar specified in Section 2.1.
385 2.3.2. Noting a Pinned Host - Storage Model
387 If the substring matching the host production from the Request-URI
388 (of the message to which the host responded) syntactically matches
389 the IP-literal or IPv4address productions from Section 3.2.2 of
390 [RFC3986], then the UA MUST NOT note this host as a Known Pinned
391 Host.
393 Otherwise, if the substring does not congruently match a Known Pinned
394 Host's domain name, per the matching procedure specified in
395 Section 8.2 of [RFC6797], then the UA MUST note this host as a Known
396 Pinned Host, caching the Pinned Host's domain name and noting along
397 with it the time of the observation (also known as the Effective Pin
398 Date), the value of the max-age directive, whether or not the
399 includeSubDomains or strict directives are asserted, the value of the
400 report-uri directive (if present), and any other metadata from
401 optional or future PKP header directives.
403 UAs SHOULD set an upper limit on the value of max-age, so that UAs
404 that have noted erroneous pins (whether by accident or due to attack)
405 have some chance of recovering over time. If the server sets a max-
406 age greater than the UA's upper limit, the UA SHOULD behave as if the
407 server set the max-age to the UA's upper limit. For example, if the
408 UA caps max-age at 2592000 seconds (30 days), and a Pinned Host sets
409 a max-age directive of 60 days in its Valid Pinning Header, the UA
410 SHOULD behave as if the max-age were effectively 30 days. (One way
411 to achieve this behavior is for the UA to simply store a value of 30
412 days instead of the 60 day value provided by the Pinned Host.) For
413 UA implementation guidance on how to select a maximum max-age, see
414 Section 4.1.
416 The UA MUST NOT modify any pinning metadata of any superdomain
417 matched Known Pinned Host.
419 A Known Pinned Host is "expired" if the Effective Pin Date plus the
420 max-age refers to a date in the past. The UA MUST ignore all expired
421 Known Pinned Hosts from its cache, if at any time, an expired Known
422 Pinned Host exists in the cache.
424 2.3.3. HTTP-Equiv Element Attribute
426 UAs MUST NOT heed http-equiv="Public-Key-Pins" attribute settings on
427 elements [W3C.REC-html401-19991224] in received content.
429 2.3.4. UA Processing Examples
431 TODO.
433 2.4. Semantics of Pins
435 An SPKI Fingerprint is defined as the output of a known cryptographic
436 hash algorithm whose input is the DER-encoded ASN.1 representation of
437 the SubjectPublicKeyInfo (SPKI) field of an X.509 certificate. A Pin
438 is defined as the combination of the known algorithm identifier and
439 the SPKI Fingerprint computed using that algorithm.
441 The SPKI Fingerprint is encoded in base 64 for use in an HTTP header.
442 (See [RFC4648].)
444 In this version of the specification, the known cryptographic hash
445 algorithms are SHA-1, identified as "sha1", and SHA-256, identified
446 as "sha256". (Future versions of this specification may add new
447 algorithms and deprecate old ones.) UAs MUST ignore Pins for which
448 they do not recognize the algorithm identifier. UAs MUST continue to
449 process the rest of a PKP response header field and note Pins for
450 algorithms they do recognize; UAs MUST recognize "sha1" and "sha256".
452 Figure 4 reproduces the definition of the SubjectPublicKeyInfo
453 structure in [RFC5280].
455 SubjectPublicKeyInfo ::= SEQUENCE {
456 algorithm AlgorithmIdentifier,
457 subjectPublicKey BIT STRING }
459 AlgorithmIdentifier ::= SEQUENCE {
460 algorithm OBJECT IDENTIFIER,
461 parameters ANY DEFINED BY algorithm OPTIONAL }
463 Figure 4: SPKI Definition
465 If the SubjectPublicKeyInfo of a certificate is incomplete when taken
466 in isolation, such as when holding a DSA key without domain
467 parameters, a public key pin cannot be formed.
469 We pin public keys, rather than entire certificates, to enable
470 operators to generate new certificates containing old public keys
471 (see [why-pin-key]).
473 See Appendix A for an example non-normative program that generates
474 SPKI Fingerprints from SubjectPublicKeyInfo fields in certificates.
476 2.5. Noting Pins
478 Upon receipt of the Public-Key-Pins response header field, the UA
479 notes the host as a Pinned Host, storing the Pins and their
480 associated directives in non-volatile storage (for example, along
481 with the HSTS metadata). The Pins and their associated directives
482 collectively known as Pinning Metadata.
484 The UA MUST observe these conditions when noting a Host:
486 o The UA MUST note the Pins if and only if it received the Public-
487 Key-Pins response header field over an error-free TLS connection.
488 If the host is a Pinned Host, this includes the validation added
489 in Section 2.6.
491 o The UA MUST note the Pins if and only if the TLS connection was
492 authenticated with a certificate chain containing at least one of
493 the SPKI structures indicated by at least one of the given SPKI
494 Fingerprints. (See Section 2.6.)
496 o The UA MUST note the Pins if and only if the given set of Pins
497 contains at least one Pin that does NOT refer to an SPKI in the
498 certificate chain. (That is, the host must set a Backup Pin; see
499 Section 4.3.)
501 If the Public-Key-Pins response header field does not meet all three
502 of these criteria, the UA MUST NOT note the host as a Pinned Host. A
503 Public-Key-Pins response header field that meets all these critera is
504 known as a Valid Pinning Header.
506 The UA MUST ignore Public-Key-Pins response header fields received on
507 connections that do not meet the first criterion.
509 Whenever a UA receives a Valid Pinning Header, it MUST set its
510 Pinning Metadata to the exact Pins, max-age, and (if any) report-uri
511 and strict mode given in the most recently received Valid Pinning
512 Header.
514 For forward compatibility, the UA MUST ignore any unrecognized
515 Public-Key-Pins header directives, while still processing those
516 directives it does recognize. Section 2.1 specifies the directives
517 max-age, pins, includeSubDomains, report-uri, and strict, but future
518 specifications and implementations might use additional directives.
520 2.6. Validating Pinned Connections
522 When a UA connects to a Pinned Host, if the TLS connection has
523 errors, the UA MUST terminate the connection without allowing the
524 user to proceed anyway. (This behavior is the same as that required
525 by [RFC6797].)
527 If the connection has no errors, then the UA will determine whether
528 to apply a new, additional correctness check: Pin Validation. A UA
529 SHOULD perform Pin Validation whenever connecting to a Known Pinned
530 Host, but MAY allow Pin Validation to be disabled for Hosts according
531 to local policy. For example, a UA may disable Pin Validation for
532 Pinned Hosts whose validated certificate chain terminates at a user-
533 defined trust anchor, rather than a trust anchor built-in to the UA.
534 However, if the Pinned Host Metadata indicates that the Pinned Host
535 is operating in "strict mode" (see Section 2.1.4), then the UA MUST
536 perform Pin Validation.
538 To perform Pin Validation, the UA will compute the SPKI Fingerprints
539 for each certificate in the Pinned Host's validated certificate
540 chain, using each supported hash algorithm for each certificate.
541 (For the purposes of Pin Validation, the UA MUST ignore certificates
542 whose SPKI cannot be taken in isolation and superfluous certificates
543 in the chain that do not form part of the validating chain.) The UA
544 will then check that the set of these SPKI Fingerprints intersects
545 the set of SPKI Fingerprints in that Pinned Host's Pinning Metadata.
546 If there is set intersection, the UA continues with the connection as
547 normal. Otherwise, the UA MUST treat this Pin Failure as a non-
548 recoverable error. Any procedure that matches the results of this
549 Pin Validation procedure is considered equivalent.
551 Note that, although the UA has previously received Pins at the HTTP
552 layer, it can and MUST perform Pin Validation at the TLS layer,
553 before beginning an HTTP conversation over the TLS channel. The TLS
554 layer thus evaluates TLS connections with pinning information the UA
555 received previously, regardless of mechanism: statically preloaded,
556 via HTTP header, or some other means (possibly in the TLS layer
557 itself).
559 2.7. Interactions With Preloaded Pin Lists
561 UAs MAY choose to implement additional sources of pinning
562 information, such as through built-in lists of pinning information.
563 Such UAs SHOULD allow users to override such additional sources,
564 including disabling them from consideration.
566 UAs that support additional sources of pinning information MUST use
567 the most recently observed pinning information when performing Pin
568 Validation for a host. The most recently observed pinning
569 information is determined based upon the most recent Effective Pin
570 Date, as described in Section 2.3.2.
572 If the result of noting a Valid Pinning Header is to disable pinning
573 for the host, such as through supplying a max-age directive with a
574 value of 0, UAs MUST allow this new information to override any other
575 pinning data. That is, a host must be able to un-pin itself, even in
576 the presence of built-in pins.
578 Example: A UA may ship with a pre-configured list of pins that are
579 collected from past observations of Valid Pinning Headers supplied by
580 hosts. In such a solution, the pre-configured list should track when
581 the Valid Pinning Header was last observed, in order to permit site
582 operators to later update the value by supplying a new Valid Pinning
583 Header. Updates to such a pre-configured list should not update the
584 Effective Pin Dates for each host unless the list vendor has actually
585 observed a more recent header. This is to prevent situations where
586 updating the Effective Pin Date on a pre-configured list of pins may
587 effectively extend the max-age beyond the site operator's stated
588 policy.
590 Example: An alternative example would be a UA that ships with a pre-
591 configured list of pins that are collected through out-of-band means,
592 such as direct contact with the site operator. In such a solution,
593 the site operator accepts responsibility for keeping the configured
594 Valid Pinning Header in sync with the vendor's list, allowing the UA
595 vendor to have each update to the list be treated as as an update of
596 the Effective Pin Date.
598 2.8. Pinning Self-Signed End Entities
600 If UAs accept hosts that authenticate themselves with self-signed end
601 entity certificates, they MAY also allow hosts to pin the public keys
602 in such certificates. The usability and security implications of
603 this practice are outside the scope of this specification.
605 3. Reporting Pin Validation Failure
607 When a Known Pinned Host has set the report-uri directive, the UA
608 SHOULD report Pin Validation failures to the indicated URI. The UA
609 does this by POSTing a JSON message to the URI; the JSON message
610 takes this form:
612 {
613 "date-time": date-time,
614 "hostname": hostname,
615 "port": port,
616 "certificate-chain": [
617 pem1, ... pemN
618 ],
619 "known-pins": [
620 known-pin1, ... known-pinN
621 ]
622 }
624 Figure 5: JSON Report Format
626 Whitespace outside of quoted strings is not significant. The key/
627 value pairs may appear in any order, but each SHOULD appear only
628 once.
630 The date-time indicates the time the UA observed the Pin Validation
631 failure. It is provided as a string formatted according to
632 Section 5.6, "Internet Date/Time Format", of [RFC3339].
634 The hostname is the hostname to which the UA made the original
635 request that failed Pin Validation. It is provided as a string.
637 The port is the port to which the UA made the original request that
638 failed Pin Validation. It is provided either as a string or as an
639 integer.
641 The certificate-chain is the certificate chain, as constructed by the
642 UA during certificate chain verification. (This may differ from the
643 certificate chain as served by the Known Pinned Host, of course.) It
644 is provided as an array of strings; each string pem1, ... pemN is the
645 PEM representation of each X.509 certificate as described in
646 [I-D.josefsson-pkix-textual].
648 The known-pins are the Pins that the UA has noted for the Known
649 Pinned Host. They are provided as an array of strings with the
650 syntax:
652 known-pin = token "=" quoted-string
654 Figure 6: Known Pin Syntax
656 As in Section 2.4, the token refers to the algorithm name, and the
657 quoted-string refers to the base 64 encoding of the SPKI Fingerprint.
659 4. Security Considerations
661 Pinning public keys helps hosts strongly assert their cryptographic
662 identity even in the face of issuer error, malfeasance or compromise.
663 But there is some risk that a host operator could lose or lose
664 control of their host's private key (such as by operator error or
665 host compromise). If the operator had pinned only the key of the
666 host's end entity certificate, the operator would not be able to
667 serve their web site or application in a way that UAs would trust for
668 the duration of their pin's max-age. (Recall that UAs MUST close the
669 connection to a host upon Pin Failure.)
671 Therefore, there is a necessary trade-off between two competing
672 goods: pin specificity and maximal reduction of the scope of issuers
673 on the one hand; and flexibility and resilience of the host's
674 cryptographic identity on the other hand. One way to resolve this
675 trade-off is to compromise by pinning to the key(s) of the issuer(s)
676 of the host's end entity certificate(s). Often, a valid certificate
677 chain will have at least two certificates above the end entity
678 certificate: the intermediate issuer, and the trust anchor.
679 Operators can pin any one or more of the public keys in this chain,
680 and indeed could pin to issuers not in the chain (as, for example, a
681 Backup Pin). Pinning to an intermediate issuer, or even to a trust
682 anchor or root, still significantly reduces the number of issuers who
683 can issue end entity certificates for the Known Pinned Host, while
684 still giving that host flexibility to change keys without a
685 disruption of service.
687 4.1. Maximum max-age
689 As mentioned in Section 2.3.2, UAs SHOULD cap the max-age value at
690 some upper limit. There is a security trade-off in that low maximum
691 values provide a narrow window of protection for users who visit the
692 Known Pinned Host only infrequently, while high maximum values might
693 potentially result in a UA's inability to successfully perform Pin
694 Validation for a Known Pinned Host if the UA's noted pins and the
695 Host's true pins diverge.
697 Such divergence could occur for several reasons, including: UA error;
698 Host operator error; network attack; or a Known Pinned Host that
699 intentionally migrates all pinned keys, combined with a UA that has
700 noted true pins with a high max-age value and has not had a chance to
701 observe the new true pins for the Host. (This last example
702 underscores the importance for Host operators to phase in new keys
703 gradually, and to set the max-age value in accordance with their
704 planned key migration schedule.)
706 There is probably no ideal upper limit to the max-age directive that
707 would satisfy all use cases. However, a value on the order of 30
708 days (2592000 seconds) may be considered a balance between the two
709 competing security concerns.
711 4.2. Using includeSubdomains Safely
713 It may happen that Pinned Hosts whose hostnames share a parent domain
714 use different Valid Pinning Headers. If a Host whose hostname is a
715 parent domain for another Host sets the includeSubdomains directive,
716 the Hosts' pins may conflict with each other. For example, consider
717 two Known Pinned Hosts, example.com and subdomain.example.com.
718 Assume example.com sets a Valid Pinning Header such as this:
720 Public-Key-Pins: pin-sha1="ABC..."; pin-sha1="DEF..."; includeSubdomains
722 Figure 7: example.com Valid Pinning Header
724 Assume subdomain.example.com sets a Valid Pinning Header such as
725 this:
727 Public-Key-Pins: pin-sha1="GHI..."; pin-sha1="JKL..."
729 Figure 8: subdomain.example.com Valid Pinning Header
731 Assume a UA that has not previously noted any pins for either of
732 these Hosts. If the UA first contacts subdomain.example.com, it will
733 note the pins in the Valid Pinning Header, and perform Pin Validation
734 as normal on subsequent conections. If the UA then contacts
735 example.com, again it will note the pins and perform Pin Validation
736 on future connections. However, if the UA happened to first
737 example.com before subdomain.example.com, the UA would, due to
738 example.com's use of the includeSubdomains directive, attempt to
739 perform Pin Validation for subdomain.example.com using the SPKI
740 hashes ABC... and DEF..., which are not valid for the certificate
741 chains subdomain.example.com (which uses certificates with SPKIs
742 GHI... and JLK...). Thus, depending on the order in which the UA
743 observes the Valid Pinning Headers for hosts example.com and
744 subdomain.example.com, Pin Validation might or might not fail for
745 subdomain.example.com, even if the certificate chain the UA receives
746 for subdomain.example.com is perfectly valid.
748 Thus, Pinned Host operators must take care that they use the
749 includeSubdomains directive with care. For example, they may choose
750 to use overlapping pin sets for hosts under a parent domain that uses
751 includeSubdomains, or to not use the includeSubdomains directive in
752 their effective-second-level domains, or to simply use the same pin
753 set for all hosts under a given parent domain.
755 4.3. Backup Pins
757 The primary way to cope with the risk of inadvertant Pin Failure is
758 to keep a Backup Pin. A Backup Pin is a fingerprint for the public
759 key of a secondary, not-yet-deployed key pair. The operator keeps
760 the backup key pair offline, and sets a pin for it in the Public-Key-
761 Pins header. Then, in case the operator loses control of their
762 primary private key, they can deploy the backup key pair. UAs, who
763 have had the backup key pair pinned (when it was set in previous
764 Valid Pinning Headers), can connect to the host without error.
766 Because having a backup key pair is so important to recovery, UAs
767 MUST require that hosts set a Backup Pin. (See Section 2.5.)
769 5. IANA Considerations
771 This document has no actions for IANA.
773 6. Usability Considerations
775 When pinning works to detect impostor Pinned Hosts, users will
776 experience denial of service. UAs MUST explain the reason why, i.e.
777 that it was impossible to verify the confirmed cryptographic identity
778 of the host.
780 UAs MUST have a way for users to clear current pins for Pinned Hosts.
781 UAs SHOULD have a way for users to query the current state of Pinned
782 Hosts.
784 7. Acknowledgements
786 Thanks to Tobias Gondrom, Jeff Hodges, Adam Langley, Nicolas
787 Lidzborski, SM, James Manger, Eric Rescorla, Paul Hoffman, and Yoav
788 Nir for suggestions and edits that clarified the text. Thanks to
789 Trevor Perrin for suggesting a mechanism to affirmatively break pins
790 ([pin-break-codes]). Adam Langley provided the SPKI fingerprint
791 generation code.
793 8. What's Changed
795 Removed the section "Pin Validity Times", which was intended to be in
796 harmony with [I-D.perrin-tls-tack]. Now using max-age purely as
797 specified in [RFC6797].
799 Added new directives: includeSubDomains, report-uri and strict.
801 Added a new variant of the PKP Header: Public-Key-Pins-Report-Only.
803 Removed the section on pin break codes and verifiers, in favor the of
804 most-recently-received policy (Section 2.5).
806 Now using a new header field, Public-Key-Pins, separate from HSTS.
807 This allows hosts to use pinning separately from Strict Transport
808 Security.
810 Explicitly requiring that UAs perform Pin Validation before the HTTP
811 conversation begins.
813 Backup Pins are now required.
815 Separated normative from non-normative material. Removed tangential
816 and out-of-scope non-normative discussion.
818 9. References
820 9.1. Normative References
822 [I-D.josefsson-pkix-textual]
823 Josefsson, S. and S. Leonard, "Text Encodings of PKIX and
824 CMS Structures", draft-josefsson-pkix-textual-01 (work in
825 progress), July 2012.
827 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
828 Requirement Levels", BCP 14, RFC 2119, March 1997.
830 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
831 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
832 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
834 [RFC3339] Klyne, G., Ed. and C. Newman, "Date and Time on the
835 Internet: Timestamps", RFC 3339, July 2002.
837 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
838 Resource Identifier (URI): Generic Syntax", STD 66, RFC
839 3986, January 2005.
841 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
842 Encodings", RFC 4648, October 2006.
844 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
845 IANA Considerations Section in RFCs", BCP 26, RFC 5226,
846 May 2008.
848 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
849 (TLS) Protocol Version 1.2", RFC 5246, August 2008.
851 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
852 Housley, R., and W. Polk, "Internet X.509 Public Key
853 Infrastructure Certificate and Certificate Revocation List
854 (CRL) Profile", RFC 5280, May 2008.
856 [RFC6797] Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
857 Transport Security (HSTS)", RFC 6797, November 2012.
859 [W3C.REC-html401-19991224]
860 Hors, A., Raggett, D., and I. Jacobs, "HTML 4.01
861 Specification", World Wide Web Consortium Recommendation
862 REC-html401-19991224, December 1999,
863 .
865 9.2. Informative References
867 [I-D.perrin-tls-tack]
868 Marlinspike, M., "Trust Assertions for Certificate Keys",
869 draft-perrin-tls-tack-02 (work in progress), January 2013.
871 [pin-break-codes]
872 Perrin, T., "Self-Asserted Key Pinning", September 2011,
873 .
875 [why-pin-key]
876 Langley, A., "Public Key Pinning", May 2011,
877 .
879 Appendix A. Fingerprint Generation
881 This Go program generates SPKI Fingerprints, suitable for use in
882 pinning, from PEM-encoded certificates. It is non-normative.
884 package main
886 import (
887 "io/ioutil"
888 "os"
889 "crypto/sha1"
890 "crypto/x509"
891 "encoding/base64"
892 "encoding/pem"
893 "fmt"
894 )
896 func main() {
897 if len(os.Args) < 2 {
898 fmt.Printf("Usage: %s PEM-filename\n", os.Args[0])
899 os.Exit(1)
900 }
901 pemBytes, err := ioutil.ReadFile(os.Args[1])
902 if err != nil {
903 panic(err.String())
904 }
905 block, _ := pem.Decode(pemBytes)
906 if block == nil {
907 panic("No PEM structure found")
908 }
909 derBytes := block.Bytes
910 certs, err := x509.ParseCertificates(derBytes)
911 if err != nil {
912 panic(err.String())
913 }
914 cert := certs[0]
915 h := sha1.New()
916 h.Write(cert.RawSubjectPublicKeyInfo)
917 digest := h.Sum()
919 fmt.Printf("Hex: %x\nBase64: %s\n", digest,
920 base64.StdEncoding.EncodeToString(digest))
921 }
923 Figure 9: Example SPKI Fingerprint Generation Code
925 Appendix B. Deployment Guidance
927 This section is non-normative guidance which may smooth the adoption
928 of public key pinning.
930 o Operators SHOULD get the backup public key signed by a different
931 (root and/or intermediary) CA than their primary certificate, and
932 store the backup key pair safely offline. The semantics of an
933 SPKI Fingerprint do not require the issuance of a certificate to
934 construct a valid Pin. However, in many deployment scenarios, in
935 order to make a Backup Pin operational the server operator will
936 need to have a certificate to deploy TLS on the host. Failure to
937 obtain a certificate through prior arrangement will leave clients
938 that recognize the site as a Known Pinned Host unable to
939 successfully perform Pin Validation until such a time as the
940 operator can obtain a new certificate from their desired
941 certificate issuer.
943 o It is most economical to have the backup certificate signed by a
944 completely different signature chain than the live certificate, to
945 maximize recoverability in the event of either root or
946 intermediary signer compromise.
948 o Operators SHOULD periodically exercise their Backup Pin plan
949 -\u002D an untested backup is no backup at all.
951 o Operators SHOULD start small. Operators SHOULD first deploy
952 public key pinning by using the report-only mode together with a
953 report-uri directive that points to a reliable report collection
954 endpoint. When moving out of report-only mode, operators should
955 start by setting a max-age of minutes or a few hours, and
956 gradually increase max-age as they gain confidence in their
957 operational capability.
959 Authors' Addresses
961 Chris Evans
962 Google, Inc.
963 1600 Amphitheatre Pkwy
964 Mountain View, CA 94043
965 US
967 Email: cevans@google.com
969 Chris Palmer
970 Google, Inc.
971 1600 Amphitheatre Pkwy
972 Mountain View, CA 94043
973 US
975 Email: palmer@google.com
977 Ryan Sleevi
978 Google, Inc.
979 1600 Amphitheatre Pkwy
980 Mountain View, CA 94043
981 US
983 Email: sleevi@google.com