draft-ietf-websec-key-pinning-21.txt   rfc7469.txt 
Web Security C. Evans Internet Engineering Task Force (IETF) C. Evans
Internet-Draft C. Palmer Request for Comments: 7469 C. Palmer
Intended status: Standards Track R. Sleevi Category: Standards Track R. Sleevi
Expires: April 8, 2015 Google, Inc. ISSN: 2070-1721 Google, Inc.
October 5, 2014 April 2015
Public Key Pinning Extension for HTTP Public Key Pinning Extension for HTTP
draft-ietf-websec-key-pinning-21
Abstract Abstract
This document defines a new HTTP header that allows web host This document defines a new HTTP header that allows web host
operators to instruct user agents to remember ("pin") the hosts' operators to instruct user agents to remember ("pin") the hosts'
cryptographic identities over a period of time. During that time, cryptographic identities over a period of time. During that time,
UAs will require that the host presents a certificate chain including user agents (UAs) will require that the host presents a certificate
at least one Subject Public Key Info structure whose fingerprint chain including at least one Subject Public Key Info structure whose
matches one of the pinned fingerprints for that host. By effectively fingerprint matches one of the pinned fingerprints for that host. By
reducing the number of trusted authorities who can authenticate the effectively reducing the number of trusted authorities who can
domain during the lifetime of the pin, pinning may reduce the authenticate the domain during the lifetime of the pin, pinning may
incidence of man-in-the-middle attacks due to compromised reduce the incidence of man-in-the-middle attacks due to compromised
Certification Authorities. Certification Authorities.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This is an Internet Standards Track document.
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months This document is a product of the Internet Engineering Task Force
and may be updated, replaced, or obsoleted by other documents at any (IETF). It represents the consensus of the IETF community. It has
time. It is inappropriate to use Internet-Drafts as reference received public review and has been approved for publication by the
material or to cite them other than as "work in progress." Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
This Internet-Draft will expire on April 8, 2015. Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7469.
Copyright Notice Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5
2. Server and Client Behavior . . . . . . . . . . . . . . . . . 4 2. Server and Client Behavior . . . . . . . . . . . . . . . . . 5
2.1. Response Header Field Syntax . . . . . . . . . . . . . . 4 2.1. Response Header Field Syntax . . . . . . . . . . . . . . 5
2.1.1. The pin- Directive . . . . . . . . . . . . . . . . . 5 2.1.1. The Pin Directive . . . . . . . . . . . . . . . . . . 6
2.1.2. The max-age Directive . . . . . . . . . . . . . . . . 6 2.1.2. The max-age Directive . . . . . . . . . . . . . . . . 7
2.1.3. The includeSubDomains Directive . . . . . . . . . . . 6 2.1.3. The includeSubDomains Directive . . . . . . . . . . . 7
2.1.4. The report-uri Directive . . . . . . . . . . . . . . 6 2.1.4. The report-uri Directive . . . . . . . . . . . . . . 7
2.1.5. Examples . . . . . . . . . . . . . . . . . . . . . . 7 2.1.5. Examples . . . . . . . . . . . . . . . . . . . . . . 8
2.2. Server Processing Model . . . . . . . . . . . . . . . . . 8 2.2. Server Processing Model . . . . . . . . . . . . . . . . . 9
2.2.1. HTTP-over-Secure-Transport Request Type . . . . . . . 8 2.2.1. HTTP-over-Secure-Transport Request Type . . . . . . . 9
2.2.2. HTTP Request Type . . . . . . . . . . . . . . . . . . 9 2.2.2. HTTP Request Type . . . . . . . . . . . . . . . . . . 9
2.3. User Agent Processing Model . . . . . . . . . . . . . . . 9 2.3. User Agent Processing Model . . . . . . . . . . . . . . . 10
2.3.1. Public-Key-Pins Response Header Field Processing . . 9 2.3.1. Public-Key-Pins Response Header Field Processing . . 10
2.3.2. Interaction of Public-Key-Pins and Public-Key-Pins- 2.3.2. Interaction of Public-Key-Pins and Public-Key-Pins-
Report-Only . . . . . . . . . . . . . . . . . . . . . 10 Report-Only . . . . . . . . . . . . . . . . . . . . . 11
2.3.3. Noting a Pinned Host - Storage Model . . . . . . . . 11 2.3.3. Noting a Pinned Host - Storage Model . . . . . . . . 11
2.3.4. HTTP-Equiv <Meta> Element Attribute . . . . . . . . . 12 2.3.4. HTTP-Equiv <Meta> Element Attribute . . . . . . . . . 13
2.4. Semantics of Pins . . . . . . . . . . . . . . . . . . . . 12 2.4. Semantics of Pins . . . . . . . . . . . . . . . . . . . . 13
2.5. Noting Pins . . . . . . . . . . . . . . . . . . . . . . . 13 2.5. Noting Pins . . . . . . . . . . . . . . . . . . . . . . . 14
2.6. Validating Pinned Connections . . . . . . . . . . . . . . 14 2.6. Validating Pinned Connections . . . . . . . . . . . . . . 15
2.7. Interactions With Preloaded Pin Lists . . . . . . . . . . 15 2.7. Interactions with Preloaded Pin Lists . . . . . . . . . . 16
2.8. Pinning Self-Signed End Entities . . . . . . . . . . . . 15 2.8. Pinning Self-Signed End Entities . . . . . . . . . . . . 16
3. Reporting Pin Validation Failure . . . . . . . . . . . . . . 15 3. Reporting Pin Validation Failure . . . . . . . . . . . . . . 16
4. Security Considerations . . . . . . . . . . . . . . . . . . . 18 4. Security Considerations . . . . . . . . . . . . . . . . . . . 19
4.1. Maximum max-age . . . . . . . . . . . . . . . . . . . . . 19 4.1. Maximum max-age . . . . . . . . . . . . . . . . . . . . . 19
4.2. Using includeSubDomains Safely . . . . . . . . . . . . . 19 4.2. Using includeSubDomains Safely . . . . . . . . . . . . . 20
4.3. Backup Pins . . . . . . . . . . . . . . . . . . . . . . . 20 4.3. Backup Pins . . . . . . . . . . . . . . . . . . . . . . . 21
4.4. Interactions With Cookie Scoping . . . . . . . . . . . . 21 4.4. Interactions With Cookie Scoping . . . . . . . . . . . . 21
4.5. Hostile Pinning . . . . . . . . . . . . . . . . . . . . . 21 4.5. Hostile Pinning . . . . . . . . . . . . . . . . . . . . . 21
5. Privacy Considerations . . . . . . . . . . . . . . . . . . . 21 5. Privacy Considerations . . . . . . . . . . . . . . . . . . . 22
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
7. Usability Considerations . . . . . . . . . . . . . . . . . . 23 7. Usability Considerations . . . . . . . . . . . . . . . . . . 24
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 24 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
9. What's Changed . . . . . . . . . . . . . . . . . . . . . . . 24 8.1. Normative References . . . . . . . . . . . . . . . . . . 24
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 25 8.2. Informative References . . . . . . . . . . . . . . . . . 26
10.1. Normative References . . . . . . . . . . . . . . . . . . 25
10.2. Informative References . . . . . . . . . . . . . . . . . 26
Appendix A. Fingerprint Generation . . . . . . . . . . . . . . . 27 Appendix A. Fingerprint Generation . . . . . . . . . . . . . . . 27
Appendix B. Deployment Guidance . . . . . . . . . . . . . . . . 27 Appendix B. Deployment Guidance . . . . . . . . . . . . . . . . 27
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
1. Introduction 1. Introduction
This document defines a new HTTP header that enables user agents This document defines a new HTTP header that enables UAs to determine
(UAs) to determine which Subject Public Key Info (SPKI) structures which Subject Public Key Info (SPKI) structures will be present in a
will be present in a web host's certificate chain in future TLS web host's certificate chain in future Transport Layer Security (TLS)
[RFC5246] connections. [RFC5246] connections.
Deploying PKP safely will require operational and organizational Deploying Public Key Pinning (PKP) safely will require operational
maturity due to the risk that hosts may make themselves unavailable and organizational maturity due to the risk that hosts may make
by pinning to a (set of) SPKI(s) that becomes invalid (see themselves unavailable by pinning to a set of SPKIs that becomes
Section 4). With care, host operators can greatly reduce the risk of invalid (see Section 4). With care, host operators can greatly
main-in-the-middle (MITM) attacks and other false-authentication reduce the risk of man-in-the-middle (MITM) attacks and other false-
problems for their users without incurring undue risk. authentication problems for their users without incurring undue risk.
PKP is meant to be used together with HTTP Strict Transport Security PKP is meant to be used together with HTTP Strict Transport Security
(HSTS) [RFC6797], but it is possible to pin keys without requiring (HSTS) [RFC6797], but it is possible to pin keys without requiring
HSTS. HSTS.
A Pin is a relationship between a hostname and a cryptographic A Pin is a relationship between a hostname and a cryptographic
identity (in this document, 1 or more of the public keys in a chain identity (in this document, one or more of the public keys in a chain
of X.509 certificates). Pin Validation is the process a UA performs of X.509 certificates). Pin Validation is the process a UA performs
to ensure that a host is in fact authenticated with its previously- to ensure that a host is in fact authenticated with its previously
established Pin. established Pin.
Key pinning is a trust-on-first-use (TOFU) mechanism. The first time Key pinning is a trust-on-first-use (TOFU) mechanism. The first time
a UA connects to a host, it lacks the information necessary to a UA connects to a host, it lacks the information necessary to
perform Pin Validation; UAs can only apply their normal cryptographic perform Pin Validation; UAs can only apply their normal cryptographic
identity validation. (In this document, it is assumed that UAs apply identity validation. (In this document, it is assumed that UAs apply
X.509 certificate chain validation in accord with [RFC5280].) X.509 certificate chain validation in accord with [RFC5280].)
The UA will not be able to detect and thwart a MITM attacking the The UA will not be able to detect and thwart a MITM attacking the
UA's first connection to the host. (However, the requirement that UA's first connection to the host. (However, the requirement that
skipping to change at page 3, line 50 skipping to change at page 4, line 48
validation requirements, without error, mitigates this risk validation requirements, without error, mitigates this risk
somewhat.) Worse, such a MITM can inject its own PKP header into the somewhat.) Worse, such a MITM can inject its own PKP header into the
HTTP stream, and pin the UA to its own keys. To avoid post facto HTTP stream, and pin the UA to its own keys. To avoid post facto
detection, the attacker would have to be in a position to intercept detection, the attacker would have to be in a position to intercept
all future requests to the host from that UA. all future requests to the host from that UA.
Thus, key pinning as described in this document is not a perfect Thus, key pinning as described in this document is not a perfect
defense against MITM attackers capable of passing certificate chain defense against MITM attackers capable of passing certificate chain
validation procedures -- nothing short of pre-shared keys can be. validation procedures -- nothing short of pre-shared keys can be.
However, it provides significant value by allowing host operators to However, it provides significant value by allowing host operators to
limit the number of certification authorities than can vouch for the limit the number of certification authorities that can vouch for the
host's identity, and allows UAs to detect in-process MITM attacks host's identity, and allows UAs to detect in-process MITM attacks
after the initial communication. after the initial communication.
1.1. Requirements Language 1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
2. Server and Client Behavior 2. Server and Client Behavior
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Optional white space (OWS) is used as defined in Section 3.2.3 of Optional white space (OWS) is used as defined in Section 3.2.3 of
[RFC7230]. token and quoted-string are used as defined in [RFC7230]. token and quoted-string are used as defined in
Section 3.2.6 of [RFC7230]. Section 3.2.6 of [RFC7230].
The directives defined in this specification are described below. The directives defined in this specification are described below.
The overall requirements for directives are: The overall requirements for directives are:
1. The order of appearance of directives is not significant. 1. The order of appearance of directives is not significant.
2. A given directive MUST NOT appear more than once in a given 2. With the exception of pin-directives with the same pin-directive-
header field. Directives are either optional or required, as name (see below), a given directive MUST NOT appear more than
stipulated in their definitions. once in a given header field. Directives are either optional or
required, as stipulated in their definitions.
3. Directive names are case-insensitive. 3. Directive names are case insensitive.
4. UAs MUST ignore any header fields containing directives, or other 4. UAs MUST ignore any header fields containing directives, or other
header field value data, that do not conform to the syntax header field value data, that do not conform to the syntax
defined in this specification. In particular, UAs must not defined in this specification. In particular, UAs must not
attempt to fix malformed header fields. attempt to fix malformed header fields.
5. If a header field contains any directive(s) the UA does not 5. If a header field contains any directive(s) the UA does not
recognize, the UA MUST ignore those directives. recognize, the UA MUST ignore those directives.
6. If the PKP or PKP-RO header field otherwise satisfies the above 6. If the PKP or PKP-RO header field otherwise satisfies the above
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Additional directives extending the semantic functionality of the Additional directives extending the semantic functionality of the
header fields can be defined in other specifications. The first such header fields can be defined in other specifications. The first such
specification will need to define a registry for such directives. specification will need to define a registry for such directives.
Such future directives will be ignored by UAs implementing only this Such future directives will be ignored by UAs implementing only this
specification, as well as by generally non-conforming UAs. specification, as well as by generally non-conforming UAs.
When a connection passes Pin Validation using the UA's noted Pins for When a connection passes Pin Validation using the UA's noted Pins for
the host at the time, the host becomes a Known Pinned Host. the host at the time, the host becomes a Known Pinned Host.
2.1.1. The pin- Directive 2.1.1. The Pin Directive
The pin directive specifies a way for web host operators to indicate The pin directive specifies a way for web host operators to indicate
a cryptographic identity that should be bound to a given web host. a cryptographic identity that should be bound to a given web host.
The syntax of a pin directive is as follows: The syntax of a pin directive is as follows:
pin-directive = pin-directive-name "=" pin-directive-value pin-directive = pin-directive-name "=" pin-directive-value
pin-directive-name = "pin-" token pin-directive-name = "pin-" token
pin-directive-value = quoted-string pin-directive-value = quoted-string
Figure 2: Pin Directive Syntax Figure 2: Pin Directive Syntax
In the pin-directive, the token is the name of a cryptographic hash In the pin-directive, the token is the name of a cryptographic hash
algorithm. The only algorithm allowed at this time is "sha256", i.e. algorithm. The only algorithm allowed at this time is "sha256",
the hash algorithm SHA256 ([RFC4634]); additional algorithms may be i.e., the hash algorithm SHA256 [RFC6234]; additional algorithms may
allowed for use in this context in the future. The quoted-string is be allowed for use in this context in the future. The quoted-string
a sequence of base 64 digits: the base 64-encoded SPKI Fingerprint is a sequence of base 64 digits: the base64-encoded SPKI Fingerprint
[RFC4648] (see Section 2.4). [RFC4648] (see Section 2.4).
According to the processing rules of Section 2.1, the UA MUST ignore According to the processing rules of Section 2.1, the UA MUST ignore
pin-directives with tokens naming hash algorithms it does not pin-directives with tokens naming hash algorithms it does not
recognize. If the set of remaining effective pin-directives is recognize. If the set of remaining effective pin-directives is
empty, and if the host is a Known Pinned Host, the UA MUST cease to empty, and if the host is a Known Pinned Host, the UA MUST cease to
consider the host as a Known Pinned Host (the UA should fail open). consider the host as a Known Pinned Host (the UA should fail open).
The UA should indicate to users that the host is no longer a Known The UA should indicate to users that the host is no longer a Known
Pinned Host. Pinned Host.
Note, per the processing rules of Section 2.1, the pin-directive-name Note, per the processing rules of Section 2.1, the pin-directive-name
is case insensitive. is case insensitive.
2.1.2. The max-age Directive 2.1.2. The max-age Directive
The "max-age" directive specifies the number of seconds after the The "max-age" directive specifies the number of seconds after the
reception of the PKP header field during which the UA SHOULD regard reception of the PKP header field during which the UA SHOULD regard
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SHOULD report Pin Validation failures (Section 2.6). The UA POSTs SHOULD report Pin Validation failures (Section 2.6). The UA POSTs
the reports to the given URI as described in Section 3. the reports to the given URI as described in Section 3.
When used in the PKP or PKP-RO headers, the presence of a report-uri When used in the PKP or PKP-RO headers, the presence of a report-uri
directive indicates to the UA that in the event of Pin Validation directive indicates to the UA that in the event of Pin Validation
failure it SHOULD POST a report to the report-uri. If the header is failure it SHOULD POST a report to the report-uri. If the header is
Public-Key-Pins, the UA should do this in addition to terminating the Public-Key-Pins, the UA should do this in addition to terminating the
connection (as described in Section 2.6). connection (as described in Section 2.6).
Hosts may set report-uris that use HTTP or HTTPS. If the scheme in Hosts may set report-uris that use HTTP or HTTPS. If the scheme in
the report-uri is one that uses TLS (e.g. HTTPS), UAs MUST perform the report-uri is one that uses TLS (e.g., HTTPS), UAs MUST perform
Pinning Validation when the host in the report-uri is a Known Pinned Pinning Validation when the host in the report-uri is a Known Pinned
Host; similarly, UAs MUST apply HSTS if the host in the report-uri is Host; similarly, UAs MUST apply HSTS if the host in the report-uri is
a Known HSTS Host. a Known HSTS Host.
Note that the report-uri need not necessarily be in the same Internet Note that the report-uri need not necessarily be in the same Internet
domain or web origin as the host being reported about. domain or web origin as the host being reported about.
UAs SHOULD make their best effort to report Pin Validation failures UAs SHOULD make their best effort to report Pin Validation failures
to the report-uri, but may fail to report in exceptional conditions. to the report-uri, but they may fail to report in exceptional
For example, if connecting the report-uri itself incurs a Pinning conditions. For example, if connecting the report-uri itself incurs
Validation failure or other certificate validation failure, the UA a Pinning Validation failure or other certificate validation failure,
MUST cancel the connection. Similarly, if Known Pinned Host A sets a the UA MUST cancel the connection. Similarly, if Known Pinned Host A
report-uri referring to Known Pinned Host B, and if B sets a report- sets a report-uri referring to Known Pinned Host B, and if B sets a
uri referring to A, and if both hosts fail Pin Validation, the UA report-uri referring to A, and if both hosts fail Pin Validation, the
SHOULD detect and break the loop by failing to send reports to and UA SHOULD detect and break the loop by failing to send reports to and
about those hosts. about those hosts.
In any case of report failure, the UA MAY attempt to re-send the In any case of report failure, the UA MAY attempt to re-send the
report later. report later.
UAs SHOULD limit the rate at which they send reports. For example, UAs SHOULD limit the rate at which they send reports. For example,
it is unnecessary to send the same report to the same report-uri more it is unnecessary to send the same report to the same report-uri more
than once per distinct set of declared Pins. than once per distinct set of declared Pins.
2.1.5. Examples 2.1.5. Examples
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report later. report later.
UAs SHOULD limit the rate at which they send reports. For example, UAs SHOULD limit the rate at which they send reports. For example,
it is unnecessary to send the same report to the same report-uri more it is unnecessary to send the same report to the same report-uri more
than once per distinct set of declared Pins. than once per distinct set of declared Pins.
2.1.5. Examples 2.1.5. Examples
Figure 4 shows some example PKP and PKP-RO response header fields. Figure 4 shows some example PKP and PKP-RO response header fields.
(Lines are folded to fit.) (Lines are folded to fit.)
Public-Key-Pins: max-age=3000; Public-Key-Pins: max-age=3000;
pin-sha256="d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM="; pin-sha256="d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM=";
pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g="; pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g="
Public-Key-Pins: max-age=2592000; Public-Key-Pins: max-age=2592000;
pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g="; pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g=";
pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ=" pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ="
Public-Key-Pins: max-age=2592000; Public-Key-Pins: max-age=2592000;
pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g="; pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g=";
pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ="; pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ=";
report-uri="http://example.com/pkp-report" report-uri="http://example.com/pkp-report"
skipping to change at page 8, line 33 skipping to change at page 9, line 16
pin-sha256="d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM="; pin-sha256="d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM=";
pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ="; pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ=";
max-age=259200 max-age=259200
Public-Key-Pins: Public-Key-Pins:
pin-sha256="d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM="; pin-sha256="d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM=";
pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g="; pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g=";
pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ="; pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ=";
max-age=10000; includeSubDomains max-age=10000; includeSubDomains
Figure 4: HPKP Header Examples Figure 4: HTTP Public Key Pinning (HPKP) Header Examples
2.2. Server Processing Model 2.2. Server Processing Model
This section describes the processing model that Pinned Hosts This section describes the processing model that Pinned Hosts
implement. The model has 2 parts: (1) the processing rules for HTTP implement. The model has 2 parts: (1) the processing rules for HTTP
request messages received over a secure transport (e.g. request messages received over a secure transport (e.g.,
authenticated, non-anonymous TLS); and (2) the processing rules for authenticated, non-anonymous TLS); and (2) the processing rules for
HTTP request messages received over non-secure transports, such as HTTP request messages received over non-secure transports, such as
TCP. TCP.
2.2.1. HTTP-over-Secure-Transport Request Type 2.2.1. HTTP-over-Secure-Transport Request Type
When replying to an HTTP request that was conveyed over a secure When replying to an HTTP request that was conveyed over a secure
transport, a Pinned Host SHOULD include in its response exactly one transport, a Pinned Host SHOULD include in its response exactly one
PKP header field, exactly one PKP-RO header field, or one of each. PKP header field, exactly one PKP-RO header field, or one of each.
Each instance of either header field MUST satisfy the grammar Each instance of either header field MUST satisfy the grammar
specified in Section 2.1. specified in Section 2.1.
Establishing a given host as a Known Pinned Host, in the context of a Establishing a given host as a Known Pinned Host, in the context of a
given UA, is accomplished as follows: given UA, is accomplished as follows:
1. Over the HTTP protocol running over secure transport, by 1. Over the HTTP protocol running over secure transport, by
correctly returning (per this specification) at least one valid correctly returning (per this specification) at least one valid
PKP header field to the UA. PKP header field to the UA.
2. Through other mechanisms, such as a client-side pre-loaded Known 2. Through other mechanisms, such as a client-side preloaded Known
Pinned Host List. Pinned Host List.
2.2.2. HTTP Request Type 2.2.2. HTTP Request Type
Pinned Hosts SHOULD NOT include the PKP header field in HTTP Pinned Hosts SHOULD NOT include the PKP header field in HTTP
responses conveyed over non-secure transport. UAs MUST ignore any responses conveyed over non-secure transport. UAs MUST ignore any
PKP header received in an HTTP response conveyed over non-secure PKP header received in an HTTP response conveyed over non-secure
transport. transport.
2.3. User Agent Processing Model 2.3. User Agent Processing Model
skipping to change at page 9, line 41 skipping to change at page 10, line 24
includes a PKP header field conforming to the grammar specified in includes a PKP header field conforming to the grammar specified in
Section 2.1, and there are no underlying secure transport errors or Section 2.1, and there are no underlying secure transport errors or
warnings (see Section 2.5), the UA MUST either: warnings (see Section 2.5), the UA MUST either:
o Note the host as a Known Pinned Host if it is not already so noted o Note the host as a Known Pinned Host if it is not already so noted
(see Section 2.3.3), (see Section 2.3.3),
or, or,
o Update the UA's cached information for the Known Pinned Host if o Update the UA's cached information for the Known Pinned Host if
any of of the max-age, includeSubDomains, or report-uri header any of the max-age, includeSubDomains, or report-uri header field
field value directives convey information different from that value directives convey information different from that already
already maintained by the UA. maintained by the UA.
The max-age value is essentially a "time to live" value relative to The max-age value is essentially a "time to live" value relative to
the time of the most recent observation of the PKP header field. If the time of the most recent observation of the PKP header field. If
the max-age header field value token has a value of 0, the UA MUST the max-age header field value token has a value of 0, the UA MUST
remove its cached Pinning Policy information (including the remove its cached Pinning Policy information (including the
includeSubDomains directive, if asserted) if the Pinned Host is includeSubDomains directive, if asserted) if the Pinned Host is
Known, or, MUST NOT note this Pinned Host if it is not yet Known. Known, or, MUST NOT note this Pinned Host if it is not yet Known.
If a UA receives more than one PKP header field or more than one PKP- If a UA receives more than one PKP header field or more than one PKP-
RO header field in an HTTP response message over secure transport, RO header field in an HTTP response message over secure transport,
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UA MUST ignore any present PKP header field(s). Similarly, if the UA UA MUST ignore any present PKP header field(s). Similarly, if the UA
receives the HTTP response over insecure transport, the UA MUST receives the HTTP response over insecure transport, the UA MUST
ignore any present PKP-RO header field(s). The UA MUST ignore any ignore any present PKP-RO header field(s). The UA MUST ignore any
PKP or PKP-RO header fields not conforming to the grammar specified PKP or PKP-RO header fields not conforming to the grammar specified
in Section 2.1. in Section 2.1.
2.3.2. Interaction of Public-Key-Pins and Public-Key-Pins-Report-Only 2.3.2. Interaction of Public-Key-Pins and Public-Key-Pins-Report-Only
A server MAY set both the "Public-Key-Pins" and "Public-Key-Pins- A server MAY set both the "Public-Key-Pins" and "Public-Key-Pins-
Report-Only" headers simultaneously. The headers do not interact Report-Only" headers simultaneously. The headers do not interact
with one another but the UA MUST process the PKP header and SHOULD with one another, but the UA MUST process the PKP header and SHOULD
process both. process both.
The headers are processed according to Section 2.3.1. The headers are processed according to Section 2.3.1.
When the PKP-RO header is used with a report-uri, the UA SHOULD POST When the PKP-RO header is used with a report-uri, the UA SHOULD POST
reports for Pin Validation failures to the indicated report-uri, reports for Pin Validation failures to the indicated report-uri,
although the UA MUST NOT enforce Pin Validation. That is, in the although the UA MUST NOT enforce Pin Validation. That is, in the
event of Pin Validation failure when the host has set the PKP-RO event of Pin Validation failure when the host has set the PKP-RO
header, the UA performs Pin Validation only to check whether or not header, the UA performs Pin Validation to check whether or not it
it should POST a report, but not for causing connection failure. should POST a report, but not whether it should cause a connection
failure.
Note: There is no purpose to using the PKP-RO header without the Note: There is no purpose to using the PKP-RO header without the
report-uri directive. User Agents MAY discard such headers without report-uri directive. User Agents MAY discard such headers without
interpreting them further. interpreting them further.
When the PKP header is used with a report-uri, the UA SHOULD POST When the PKP header is used with a report-uri, the UA SHOULD POST
reports for Pin Validation failures to the indicated report-uri, as reports for Pin Validation failures to the indicated report-uri, as
well as enforcing Pin Validation. well as enforce Pin Validation.
If a host sets the PKP-RO header, the UA SHOULD note the Pins and If a host sets the PKP-RO header, the UA SHOULD note the Pins and
directives given in the PKP-RO header, ignoring any max-age directives given in the PKP-RO header, ignoring any max-age
directive. If the UA does note the Pins and directives in the PKP-RO directive. If the UA does note the Pins and directives in the PKP-RO
header it SHOULD evaluate the specified policy and SHOULD report any header, it SHOULD evaluate the specified policy and SHOULD report any
would-be Pin Validation failures that would occur if the report-only would-be Pin Validation failures that would occur if the report-only
policy were enforced. policy were enforced.
If a host sets both the PKP header and the PKP-RO header, the UA MUST If a host sets both the PKP header and the PKP-RO header, the UA MUST
note and enforce Pin Validation as specified by the PKP header, and note and enforce Pin Validation as specified by the PKP header, and
SHOULD process the Pins and directives given in the PKP-RO header. SHOULD process the Pins and directives given in the PKP-RO header.
If the UA does process the Pins and directives in the PKP-RO header If the UA does process the Pins and directives in the PKP-RO header,
it SHOULD evaluate the specified policy and SHOULD report any would- it SHOULD evaluate the specified policy and SHOULD report any would-
be Pin Validation failures that would occur if the report-only policy be Pin Validation failures that would occur if the report-only policy
were enforced. were enforced.
2.3.3. Noting a Pinned Host - Storage Model 2.3.3. Noting a Pinned Host - Storage Model
The Effective Pin Date of a Known Pinned Host is the time that the UA The Effective Pin Date of a Known Pinned Host is the time that the UA
observed a Valid Pinning Header for the host. The Effective observed a Valid Pinning Header for the host. The Effective
Expiration Date of a Known Pinned Host is the Effective Pin Date plus Expiration Date of a Known Pinned Host is the Effective Pin Date plus
the max-age. A Known Pinned Host is "expired" if the Effective the max-age. A Known Pinned Host is "expired" if the Effective
Expiration Date refers to a date in the past. The UA MUST ignore any Expiration Date refers to a date in the past. The UA MUST ignore any
expired Known Pinned Hosts in its cache. expired Known Pinned Hosts in its cache.
For example, if a UA is beginning to perform Pin Validation for a For example, if a UA is beginning to perform Pin Validation for a
Known Pinned Host and finds that the cached pinning information for Known Pinned Host and finds that the cached pinning information for
the host indicates an Effective Expiration Date in the past, the UA the host indicates an Effective Expiration Date in the past, the UA
MUST NOT continue with Pin Validation for the host, and must consider MUST NOT continue with Pin Validation for the host, and MUST consider
the host to no longer be a Known Pinned Host. the host to no longer be a Known Pinned Host.
Known Pinned Hosts are identified only by domain names, and never IP Known Pinned Hosts are identified only by domain names, and never IP
addresses. If the substring matching the host production from the addresses. If the substring matching the host production from the
Request-URI (of the message to which the host responded) Request-URI (of the message to which the host responded)
syntactically matches the IP-literal or IPv4address productions from syntactically matches the IP-literal or IPv4address productions from
Section 3.2.2 of [RFC3986], then the UA MUST NOT note this host as a Section 3.2.2 of [RFC3986], then the UA MUST NOT note this host as a
Known Pinned Host. Known Pinned Host.
Otherwise, if the substring does not congruently match an existing Otherwise, if the substring does not congruently match an existing
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If any other metadata from optional or future PKP header directives If any other metadata from optional or future PKP header directives
are present in the Valid Pinning Header, and the UA understands them, are present in the Valid Pinning Header, and the UA understands them,
the UA MAY note them as well. the UA MAY note them as well.
UAs MAY set an upper limit on the value of max-age, so that UAs that UAs MAY set an upper limit on the value of max-age, so that UAs that
have noted erroneous Pins (whether by accident or due to attack) have have noted erroneous Pins (whether by accident or due to attack) have
some chance of recovering over time. If the server sets a max-age some chance of recovering over time. If the server sets a max-age
greater than the UA's upper limit, the UA MAY behave as if the server greater than the UA's upper limit, the UA MAY behave as if the server
set the max-age to the UA's upper limit. For example, if the UA caps set the max-age to the UA's upper limit. For example, if the UA caps
max-age at 5184000 seconds (60 days), and a Pinned Host sets a max- max-age at 5,184,000 seconds (60 days), and a Pinned Host sets a max-
age directive of 90 days in its Valid Pinning Header, the UA MAY age directive of 90 days in its Valid Pinning Header, the UA MAY
behave as if the max-age were effectively 60 days. (One way to behave as if the max-age were effectively 60 days. (One way to
achieve this behavior is for the UA to simply store a value of 60 achieve this behavior is for the UA to simply store a value of 60
days instead of the 90 day value provided by the Pinned Host.) For days instead of the 90-day value provided by the Pinned Host.) For
UA implementation guidance on how to select a maximum max-age, see UA implementation guidance on how to select a maximum max-age, see
Section 4.1. Section 4.1.
The UA MUST NOT modify any pinning metadata of any superdomain The UA MUST NOT modify any pinning metadata of any superdomain
matched Known Pinned Host. matched Known Pinned Host.
The UA MUST NOT cache information derived from a PKP-RO header. The UA MUST NOT cache information derived from a PKP-RO header.
(PKP-RO headers are useful only at the time of receipt and (PKP-RO headers are useful only at the time of receipt and
processing.) processing.)
2.3.4. HTTP-Equiv <Meta> Element Attribute 2.3.4. HTTP-Equiv <Meta> Element Attribute
UAs MUST NOT heed http-equiv="Public-Key-Pins" or http-equiv="Public- UAs MUST NOT heed http-equiv="Public-Key-Pins" or
Key-Pins-Report-Only" attribute settings on <meta> elements http-equiv="Public-Key-Pins-Report-Only" attribute settings on <meta>
[W3C.REC-html401-19991224] in received content. elements [W3C.REC-html401-19991224] in received content.
2.4. Semantics of Pins 2.4. Semantics of Pins
An SPKI Fingerprint is defined as the output of a known cryptographic An SPKI Fingerprint is defined as the output of a known cryptographic
hash algorithm whose input is the DER-encoded ASN.1 representation of hash algorithm whose input is the DER-encoded ASN.1 representation of
the subjectPublicKeyInfo (SPKI) field of an X.509 certificate. A Pin the Subject Public Key Info (SPKI) of an X.509 certificate. A Pin is
is defined as the combination of the known algorithm identifier and defined as the combination of the known algorithm identifier and the
the SPKI Fingerprint computed using that algorithm. SPKI Fingerprint computed using that algorithm.
The SPKI Fingerprint is encoded in base 64 for use in an HTTP header The SPKI Fingerprint is encoded in base 64 for use in an HTTP header
[RFC4648]. [RFC4648].
In this version of the specification, the known cryptographic hash In this version of the specification, the known cryptographic hash
algorithm is SHA-256, identified as "sha256" [RFC6234]. (Future algorithm is SHA-256, identified as "sha256" [RFC6234]. (Future
specifications may add new algorithms and deprecate old ones.) UAs specifications may add new algorithms and deprecate old ones.) UAs
MUST ignore Pins for which they do not recognize the algorithm MUST ignore Pins for which they do not recognize the algorithm
identifier. UAs MUST continue to process the rest of a PKP response identifier. UAs MUST continue to process the rest of a PKP response
header field and note Pins for algorithms they do recognize. header field and note Pins for algorithms they do recognize.
skipping to change at page 13, line 15 skipping to change at page 13, line 46
SubjectPublicKeyInfo ::= SEQUENCE { SubjectPublicKeyInfo ::= SEQUENCE {
algorithm AlgorithmIdentifier, algorithm AlgorithmIdentifier,
subjectPublicKey BIT STRING } subjectPublicKey BIT STRING }
AlgorithmIdentifier ::= SEQUENCE { AlgorithmIdentifier ::= SEQUENCE {
algorithm OBJECT IDENTIFIER, algorithm OBJECT IDENTIFIER,
parameters ANY DEFINED BY algorithm OPTIONAL } parameters ANY DEFINED BY algorithm OPTIONAL }
Figure 5: SPKI Definition Figure 5: SPKI Definition
If the certificate's subjectPublicKeyInfo is incomplete when taken in If the certificate's Subject Public Key Info is incomplete when taken
isolation, such as when holding a DSA key without domain parameters, in isolation, such as when holding a DSA key without domain
a public key pin cannot be formed. parameters, a public key pin cannot be formed.
We pin public keys, rather than entire certificates, to enable We pin public keys, rather than entire certificates, to enable
operators to generate new certificates containing old public keys operators to generate new certificates containing old public keys
(see [why-pin-key]). (see [why-pin-key]).
See Appendix A for an example non-normative program that generates See Appendix A for an example non-normative program that generates
SPKI Fingerprints from certificates. SPKI Fingerprints from certificates.
2.5. Noting Pins 2.5. Noting Pins
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o The TLS connection was authenticated with a certificate chain o The TLS connection was authenticated with a certificate chain
containing at least one of the SPKI structures indicated by at containing at least one of the SPKI structures indicated by at
least one of the given SPKI Fingerprints (see Section 2.6). least one of the given SPKI Fingerprints (see Section 2.6).
o The given set of Pins contains at least one Pin that does NOT o The given set of Pins contains at least one Pin that does NOT
refer to an SPKI in the certificate chain. (That is, the host refer to an SPKI in the certificate chain. (That is, the host
must set a Backup Pin; see Section 4.3.) must set a Backup Pin; see Section 4.3.)
If the PKP response header field does not meet all three of these If the PKP response header field does not meet all three of these
criteria, the UA MUST NOT note the host as a Pinned Host. A PKP criteria, the UA MUST NOT note the host as a Pinned Host. A PKP
response header field that meets all these critera is known as a response header field that meets all these criteria is known as a
Valid Pinning Header. Valid Pinning Header.
Whenever a UA receives a Valid Pinning Header, it MUST set its Whenever a UA receives a Valid Pinning Header, it MUST set its
Pinning Metadata to the exact Pins, Effective Expiration Date Pinning Metadata to the exact Pins, Effective Expiration Date
(computed from max-age), and (if any) report-uri given in the most (computed from max-age), and (if any) report-uri given in the most
recently received Valid Pinning Header. recently received Valid Pinning Header.
For forward compatibility, the UA MUST ignore any unrecognized PKP For forward compatibility, the UA MUST ignore any unrecognized PKP
and PKP-RO header directives, while still processing those directives and PKP-RO header directives, while still processing those directives
it does recognize. Section 2.1 specifies the directives max-age, it does recognize. Section 2.1 specifies the directives max-age,
Pins, includeSubDomains, and report-uri but future specifications and Pins, includeSubDomains, and report-uri, but future specifications
implementations might use additional directives. and implementations might use additional directives.
Upon receipt of a PKP-RO response header field, the UA SHOULD Upon receipt of a PKP-RO response header field, the UA SHOULD
evaluate the policy expressed in the field, and SHOULD generate and evaluate the policy expressed in the field, and SHOULD generate and
send a report (see Section 3). However, failure to validate the Pins send a report (see Section 3). However, failure to validate the Pins
in the field MUST have no effect on the validity or non-validity of in the field MUST have no effect on the validity or non-validity of
the policy expressed in the PKP field or in previously-noted Pins for the policy expressed in the PKP field or in previously noted Pins for
the Known Pinned Host. the Known Pinned Host.
The UA need not note any Pins or other policy expressed in the PKP-RO The UA need not note any Pins or other policy expressed in the PKP-RO
response header field, except for the purpose of determining that it response header field, except for the purpose of determining that it
has already sent a report for a given policy. UAs SHOULD make a best has already sent a report for a given policy. UAs SHOULD make a best
effort not to inundate report-uris with redundant reports. effort not to inundate report-uris with redundant reports.
2.6. Validating Pinned Connections 2.6. Validating Pinned Connections
When a UA connects to a Pinned Host using a TLS connection, if the When a UA connects to a Pinned Host using a TLS connection, if the
TLS connection has errors, the UA MUST terminate the connection TLS connection has errors, the UA MUST terminate the connection
without allowing the user to proceed anyway. (This behavior is the without allowing the user to proceed anyway. (This behavior is the
same as that required by [RFC6797].) same as that required by [RFC6797].)
If the connection has no errors, then the UA will determine whether If the connection has no errors, then the UA will determine whether
to apply a new, additional correctness check: Pin Validation. A UA to apply a new, additional correctness check: Pin Validation. A UA
SHOULD perform Pin Validation whenever connecting to a Known Pinned SHOULD perform Pin Validation whenever connecting to a Known Pinned
Host, as soon as possible (e.g. immediately after receiving the Host, as soon as possible (e.g., immediately after receiving the
Server Certificate message). It is acceptable to allow Pin Server Certificate message). It is acceptable to allow Pin
Validation to be disabled for some Hosts according to local policy. Validation to be disabled for some Hosts according to local policy.
For example, a UA may disable Pin Validation for Pinned Hosts whose For example, a UA may disable Pin Validation for Pinned Hosts whose
validated certificate chain terminates at a user-defined trust validated certificate chain terminates at a user-defined trust
anchor, rather than a trust anchor built-in to the UA (or underlying anchor, rather than a trust anchor built-in to the UA (or underlying
platform). platform).
To perform Pin Validation, the UA will compute the SPKI Fingerprints To perform Pin Validation, the UA will compute the SPKI Fingerprints
for each certificate in the Pinned Host's validated certificate for each certificate in the Pinned Host's validated certificate
chain, using each supported hash algorithm for each certificate. (As chain, using each supported hash algorithm for each certificate. (As
described in Section 2.4, certificates whose SPKI cannot be taken in described in Section 2.4, certificates whose SPKI cannot be taken in
isolation cannot be pinned.) The UA MUST ignore superfluous isolation cannot be pinned.) The UA MUST ignore superfluous
certificates in the chain that do not form part of the validating certificates in the chain that do not form part of the validating
chain. The UA will then check that the set of these SPKI chain. The UA will then check that the set of these SPKI
Fingerprints intersects the set of SPKI Fingerprints in that Pinned Fingerprints intersects the set of SPKI Fingerprints in that Pinned
Host's Pinning Metadata. If there is set intersection, the UA Host's Pinning Metadata. If there is set intersection, the UA
continues with the connection as normal. Otherwise, the UA MUST continues with the connection as normal. Otherwise, the UA MUST
treat this Pin Validation Failure as a non-recoverable error. Any treat this Pin Validation failure as a non-recoverable error. Any
procedure that matches the results of this Pin Validation procedure procedure that matches the results of this Pin Validation procedure
is considered equivalent. is considered equivalent.
A UA that has previously noted a host as a Known Pinned Host MUST A UA that has previously noted a host as a Known Pinned Host MUST
perform Pin Validation when setting up the TLS session, before perform Pin Validation when setting up the TLS session, before
beginning an HTTP conversation over the TLS channel. beginning an HTTP conversation over the TLS channel.
UAs send validation failure reports only when Pin Validation is UAs send validation failure reports only when Pin Validation is
actually in effect. Pin Validation might not be in effect e.g. actually in effect. Pin Validation might not be in effect, e.g.,
because the user has elected to disable it, or because a presented because the user has elected to disable it, or because a presented
certificate chain chains up to a user-defined trust anchor. In such certificate chain chains up to a user-defined trust anchor. In such
cases, UAs SHOULD NOT send reports. cases, UAs SHOULD NOT send reports.
2.7. Interactions With Preloaded Pin Lists 2.7. Interactions with Preloaded Pin Lists
UAs MAY choose to implement additional sources of pinning UAs MAY choose to implement additional sources of pinning
information, such as through built-in lists of pinning information. information, such as through built-in lists of pinning information.
Such UAs should allow users to override such additional sources, Such UAs should allow users to override such additional sources,
including disabling them from consideration. including disabling them from consideration.
The effective policy for a Known Pinned Host that has both built-in The effective policy for a Known Pinned Host that has both built-in
Pins and Pins from previously observed PKP header response fields is Pins and Pins from previously observed PKP header response fields is
implementation-defined. implementation-defined.
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The effective-expiration-date is the Effective Expiration Date for The effective-expiration-date is the Effective Expiration Date for
the noted Pins. It is provided as a string formatted according to the noted Pins. It is provided as a string formatted according to
Section 5.6, "Internet Date/Time Format", of [RFC3339]. Section 5.6, "Internet Date/Time Format", of [RFC3339].
include-subdomains indicates whether or not the UA has noted the include-subdomains indicates whether or not the UA has noted the
includeSubDomains directive for the Known Pinned Host. It is includeSubDomains directive for the Known Pinned Host. It is
provided as one of the JSON identifiers "true" or "false". provided as one of the JSON identifiers "true" or "false".
noted-hostname indicates the hostname that the UA noted when it noted noted-hostname indicates the hostname that the UA noted when it noted
the Known Pinned Host. This field allows operators to understand why the Known Pinned Host. This field allows operators to understand why
Pin Validation was performed for e.g. foo.example.com when the noted Pin Validation was performed for, e.g., foo.example.com when the
Known Pinned Host was example.com with includeSubDomains set. noted Known Pinned Host was example.com with includeSubDomains set.
The served-certificate-chain is the certificate chain, as served by The served-certificate-chain is the certificate chain, as served by
the Known Pinned Host during TLS session setup. It is provided as an the Known Pinned Host during TLS session setup. It is provided as an
array of strings; each string pem1, ... pemN is the PEM array of strings; each string pem1, ... pemN is the Privacy-Enhanced
representation of each X.509 certificate as described in Mail (PEM) representation of each X.509 certificate as described in
[I-D.josefsson-pkix-textual]. [RFC7468].
The validated-certificate-chain is the certificate chain, as The validated-certificate-chain is the certificate chain, as
constructed by the UA during certificate chain verification. (This constructed by the UA during certificate chain verification. (This
may differ from the served-certificate-chain.) It is provided as an may differ from the served-certificate-chain.) It is provided as an
array of strings; each string pem1, ... pemN is the PEM array of strings; each string pem1, ... pemN is the PEM
representation of each X.509 certificate as described in representation of each X.509 certificate as described in [RFC7468].
[I-D.josefsson-pkix-textual]. For UAs that build certificate chains UAs that build certificate chains in more than one way during the
in more than one way during the validation process, they SHOULD send validation process SHOULD send the last chain built. In this way,
the last chain built. In this way they can avoid keeping too much they can avoid keeping too much state during the validation process.
state during the validation process.
The known-pins are the Pins that the UA has noted for the Known The known-pins are the Pins that the UA has noted for the Known
Pinned Host. They are provided as an array of strings with the Pinned Host. They are provided as an array of strings with the
syntax: syntax:
known-pin = token "=" quoted-string known-pin = token "=" quoted-string
Figure 7: Known Pin Syntax Figure 7: Known Pin Syntax
As in Section 2.4, the token refers to the algorithm name, and the As in Section 2.4, the token refers to the algorithm name, and the
quoted-string refers to the base 64 encoding of the SPKI Fingerprint. quoted-string refers to the base64 encoding of the SPKI Fingerprint.
When formulating the JSON POST body, the UA MUST either use single- When formulating the JSON POST body, the UA MUST either use single-
quoted JSON strings, or use double-quoted JSON strings and \-escape quoted JSON strings or use double-quoted JSON strings and backslash-
the embedded double quotes in the quoted-string part of the known- escape the embedded double quotes in the quoted-string part of the
pin. known-pin.
Figure 8 shows an example of a Pin Validation failure report. (PEM Figure 8 shows an example of a Pin Validation failure report. (PEM
strings are shown on multiple lines for readability.) strings are shown on multiple lines for readability.)
{ {
"date-time": "2014-04-06T13:00:50Z", "date-time": "2014-04-06T13:00:50Z",
"hostname": "www.example.com", "hostname": "www.example.com",
"port": 443, "port": 443,
"effective-expiration-date": "2014-05-01T12:40:50Z" "effective-expiration-date": "2014-05-01T12:40:50Z"
"include-subdomains": false, "include-subdomains": false,
skipping to change at page 18, line 42 skipping to change at page 19, line 8
'pin-sha256="d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM="', 'pin-sha256="d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM="',
"pin-sha256=\"E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g=\"" "pin-sha256=\"E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g=\""
] ]
} }
Figure 8: Pin Validation Failure Report Example Figure 8: Pin Validation Failure Report Example
4. Security Considerations 4. Security Considerations
Pinning public keys helps hosts strongly assert their cryptographic Pinning public keys helps hosts strongly assert their cryptographic
identity even in the face of issuer error, malfeasance or compromise. identity even in the face of issuer error, malfeasance, or
But there is some risk that a host operator could lose or lose compromise. But, there is some risk that a host operator could lose
control of their host's private key (such as by operator error or (or lose control of) their host's private key (such as by operator
host compromise). If the operator had pinned only the key of the error or host compromise). If the operator had pinned only the key
host's end entity certificate, the operator would not be able to of the host's end-entity certificate, the operator would not be able
serve their web site or application in a way that UAs would trust for to serve their web site or application in a way that UAs would trust
the duration of their pin's max-age. (Recall that UAs MUST close the for the duration of their pin's max-age. (Recall that UAs MUST close
connection to a host upon Pin Failure.) the connection to a host upon Pin Failure.)
Therefore, there is a necessary trade-off between two competing Therefore, there is a necessary trade-off between two competing
goods: pin specificity and maximal reduction of the scope of issuers goods: pin specificity and maximal reduction of the scope of issuers
on the one hand; and flexibility and resilience of the host's on the one hand; and flexibility and resilience of the host's
cryptographic identity on the other hand. One way to resolve this cryptographic identity on the other hand. One way to resolve this
trade-off is to compromise by pinning to the key(s) of the issuer(s) trade-off is to compromise by pinning to the key(s) of the issuer(s)
of the host's end entity certificate(s). Often, a valid certificate of the host's end-entity certificate(s). Often, a valid certificate
chain will have at least two certificates above the end entity chain will have at least two certificates above the end-entity
certificate: the intermediate issuer, and the trust anchor. certificate: the intermediate issuer and the trust anchor. Operators
Operators can pin any one or more of the public keys in this chain, can pin any one or more of the public keys in this chain, and indeed
and indeed MUST pin to issuers not in the chain (as, for example, a MUST pin to issuers not in the chain (as, for example, a Backup Pin).
Backup Pin). Pinning to an intermediate issuer, or even to a trust Pinning to an intermediate issuer, or even to a trust anchor or root,
anchor or root, still significantly reduces the number of issuers who still significantly reduces the number of issuers who can issue end-
can issue end entity certificates for the Known Pinned Host, while entity certificates for the Known Pinned Host, while still giving
still giving that host flexibility to change keys without a that host flexibility to change keys without a disruption of service.
disruption of service.
4.1. Maximum max-age 4.1. Maximum max-age
As mentioned in Section 2.3.3, UAs MAY cap the max-age value at some As mentioned in Section 2.3.3, UAs MAY cap the max-age value at some
upper limit. There is a security trade-off in that low maximum upper limit. There is a security trade-off in that low maximum
values provide a narrow window of protection for users who visit the values provide a narrow window of protection for users who visit the
Known Pinned Host only infrequently, while high maximum values might Known Pinned Host only infrequently, while high maximum values might
potentially result in a UA's inability to successfully perform Pin result in a UA's inability to successfully perform Pin Validation for
Validation for a Known Pinned Host if the UA's noted Pins and the a Known Pinned Host if the UA's noted Pins and the host's true Pins
host's true Pins diverge. diverge.
Such divergence could occur for several reasons, including: UA error; Such divergence could occur for several reasons, including: UA error;
host operator error; network attack; or a Known Pinned Host that host operator error; network attack; or a Known Pinned Host that
intentionally migrates all pinned keys, combined with a UA that has intentionally migrates all pinned keys, combined with a UA that has
noted true Pins with a high max-age value and has not had a chance to noted true Pins with a high max-age value and has not had a chance to
observe the new true Pins for the host. (This last example observe the new true Pins for the host. (This last example
underscores the importance for host operators to phase in new keys underscores the importance for host operators to phase in new keys
gradually, and to set the max-age value in accordance with their gradually and to set the max-age value in accordance with their
planned key migration schedule.) planned key migration schedule.)
There is probably no ideal upper limit to the max-age directive that There is probably no ideal upper limit to the max-age directive that
would satisfy all use cases. However, a value on the order of 60 would satisfy all use cases. However, a value on the order of 60
days (5,184,000 seconds) may be considered a balance between the two days (5,184,000 seconds) may be considered a balance between the two
competing security concerns. competing security concerns.
4.2. Using includeSubDomains Safely 4.2. Using includeSubDomains Safely
It may happen that Pinned Hosts whose hostnames share a parent domain It may happen that Pinned Hosts whose hostnames share a parent domain
use different Valid Pinning Headers. If a host whose hostname is a use different Valid Pinning Headers. If a host whose hostname is a
parent domain for another host sets the includeSubDomains directive, parent domain for another host sets the includeSubDomains directive,
skipping to change at page 20, line 20 skipping to change at page 20, line 34
Assume subdomain.example.com sets a Valid Pinning Header such as Assume subdomain.example.com sets a Valid Pinning Header such as
this: this:
Public-Key-Pins: pin-sha256="GHI..."; pin-sha256="JKL..." Public-Key-Pins: pin-sha256="GHI..."; pin-sha256="JKL..."
Figure 10: subdomain.example.com Valid Pinning Header Figure 10: subdomain.example.com Valid Pinning Header
Assume a UA that has not previously noted any Pins for either of Assume a UA that has not previously noted any Pins for either of
these hosts. If the UA first contacts subdomain.example.com, it will these hosts. If the UA first contacts subdomain.example.com, it will
note the Pins in the Valid Pinning Header, and perform Pin Validation note the Pins in the Valid Pinning Header, and perform Pin Validation
as normal on subsequent conections. If the UA then contacts as normal on subsequent connections. If the UA then contacts
example.com, again it will note the Pins and perform Pin Validation example.com, again it will note the Pins and perform Pin Validation
on future connections. on future connections.
However, if the UA happened to visit example.com before However, if the UA happened to visit example.com before
subdomain.example.com, the UA would, due to example.com's use of the subdomain.example.com, the UA would, due to example.com's use of the
includeSubDomains directive, attempt to perform Pin Validation for includeSubDomains directive, attempt to perform Pin Validation for
subdomain.example.com using the SPKI hashes ABC... and DEF..., which subdomain.example.com using the SPKI hashes ABC... and DEF..., which
are not valid for the certificate chains subdomain.example.com (which are not valid for the certificate chains subdomain.example.com (which
uses certificates with SPKIs GHI... and JLK...). Thus, depending on uses certificates with SPKIs GHI... and JLK...). Thus, depending on
the order in which the UA observes the Valid Pinning Headers for the order in which the UA observes the Valid Pinning Headers for
skipping to change at page 20, line 45 skipping to change at page 21, line 11
Thus, Pinned Host operators must use the includeSubDomains directive Thus, Pinned Host operators must use the includeSubDomains directive
with care. For example, they may choose to use overlapping pin sets with care. For example, they may choose to use overlapping pin sets
for hosts under a parent domain that uses includeSubDomains, or to for hosts under a parent domain that uses includeSubDomains, or to
not use the includeSubDomains directive in their effective-second- not use the includeSubDomains directive in their effective-second-
level domains, or to simply use the same pin set for all hosts under level domains, or to simply use the same pin set for all hosts under
a given parent domain. a given parent domain.
4.3. Backup Pins 4.3. Backup Pins
The primary way to cope with the risk of inadvertent Pin Validation The primary way to cope with the risk of inadvertent Pin Validation
Failure is to keep a Backup Pin. A Backup Pin is a fingerprint for failure is to keep a Backup Pin. A Backup Pin is a fingerprint for
the public key of a secondary, not-yet-deployed key pair. The the public key of a secondary, not-yet-deployed key pair. The
operator keeps the backup key pair offline, and sets a pin for it in operator keeps the backup key pair offline, and sets a pin for it in
the PKP header. Then, in case the operator loses control of their the PKP header. Then, in case the operator loses control of their
primary private key, they can deploy the backup key pair. UAs, who primary private key, they can deploy the backup key pair. UAs, who
have had the backup key pair pinned (when it was set in previous have had the backup key pair pinned (when it was set in previous
Valid Pinning Headers), can connect to the host without error. Valid Pinning Headers), can connect to the host without error.
Because having a backup key pair is so important to recovery, UAs Because having a backup key pair is so important to recovery, UAs
MUST require that hosts set a Backup Pin (see Section 2.5). The down MUST require that hosts set a Backup Pin (see Section 2.5). The down
side of keeping a not-yet-deployed key pair is that if an attacker side of keeping a not-yet-deployed key pair is that, if an attacker
gains control of the private key she will be able to perform a MITM gains control of the private key, she will be able to perform a MITM
attack without being discovered. Operators must take care to avoid attack without being discovered. Operators must take care to avoid
leaking the key such as keeping it offline. leaking the key such as keeping it offline.
4.4. Interactions With Cookie Scoping 4.4. Interactions With Cookie Scoping
HTTP cookies [RFC6265] set by a Known Pinned Host can be stolen by a HTTP cookies [RFC6265] set by a Known Pinned Host can be stolen by a
network attacker who can forge web and DNS responses so as to cause a network attacker who can forge web and DNS responses so as to cause a
client to send the cookies to a phony subdomain of the host. To client to send the cookies to a phony subdomain of the host. To
prevent this, hosts SHOULD set the "secure" attribute and precisely prevent this, hosts SHOULD set the "secure" attribute and precisely
scope the "domain" attribute on all security-sensitive cookies, such scope the "domain" attribute on all security-sensitive cookies, such
as session cookies. These settings tell the browser that the cookie as session cookies. These settings tell the browser that the cookie
should only be sent back to the specific host(s) (and not e.g. all should only be sent back to the specific host(s) (and not, e.g., all
subdomains of a given domain), and should only be sent over HTTPS subdomains of a given domain), and should only be sent over HTTPS
(not HTTP). (not HTTP).
4.5. Hostile Pinning 4.5. Hostile Pinning
An attacker who is able to obtain a valid certificate for a domain, An attacker who is able to obtain a valid certificate for a domain,
either through misissuance by a Certification Authority or through either through misissuance by a Certification Authority or through
other means, such as being the prior owner of a given domain, may other means, such as being the prior owner of a given domain, may
attempt to perform 'hostile' pinning. In this scenario, the attacker attempt to perform 'hostile' pinning. In this scenario, the attacker
provides a Valid Pinning Header that pins to a set of SPKIs of the provides a Valid Pinning Header that pins to a set of SPKIs of the
attacker's choice. If a UA has not previously noted pins for that attacker's choice. If a UA has not previously noted pins for that
host, it may note the attacker's pins, preventing access to the host, it may note the attacker's pins, preventing access to the
legitimate site. legitimate site.
This attack is mitigated through several means. Most prominantly, This attack is mitigated through several means. Most prominently,
the attack can only persist for the maximum max-age (see the attack can only persist for the maximum max-age (see
Section 4.1). Web host operators can reduce the opportunity for Section 4.1). Web host operators can reduce the opportunity for
attack by working to preload the host's pins within the UA. attack by working to preload the host's pins within the UA.
Operators may further detect such misissuance through other means, Operators may further detect such misissuance through other means,
such as Certificate Transparency ([RFC6962]). such as certificate transparency ([RFC6962]).
5. Privacy Considerations 5. Privacy Considerations
Hosts can use HSTS or HPKP as a "super-cookie", by setting distinct Hosts can use HSTS or HPKP as a "super-cookie", by setting distinct
policies for a number of subdomains. For example, assume example.com policies for a number of subdomains. For example, assume example.com
wishes to track distinct UAs without explicitly setting a cookie, or wishes to track distinct UAs without explicitly setting a cookie, or
if a previously-set cookie is deleted from the UA's cookie store. that a previously set cookie is deleted from the UA's cookie store.
Here are two attack scenarios. Here are two attack scenarios.
o example.com can use report-uri and the ability to pin arbitrary o example.com can use report-uri and the ability to pin arbitrary
identifiers to distinguish UAs. identifiers to distinguish UAs.
1. example.com sets a Valid Pinning Header in its response to 1. example.com sets a Valid Pinning Header in its response to
requests. The header asserts the includeSubDomains directive, requests. The header asserts the includeSubDomains directive
and specifies a report-uri directive as well. Pages served by and specifies a report-uri directive as well. Pages served by
the host also include references to subresource the host also include references to subresource
https://bad.example.com/foo.png. https://bad.example.com/foo.png.
2. The Valid Pinning Header includes a "pin" that is not really 2. The Valid Pinning Header includes a "pin" that is not really
the hash of an SPKI, but is instead an arbitrary the hash of an SPKI but is instead an arbitrary distinguishing
distinguishing string sent only in response to a particular string sent only in response to a particular request. For
request. For each request, the host creates a new, distinct each request, the host creates a new, distinct distinguishing
distinguishing string and sets it as if it were a pin. string and sets it as if it were a pin.
3. The certificate chain served by bad.example.com does not pass 3. The certificate chain served by bad.example.com does not pass
Pin Validation given the pin set the host asserted in (1). Pin Validation given the pin set the host asserted in step
The HPKP-conforming UA attempts to report the Pin Validation (1). The HPKP-conforming UA attempts to report the Pin
failure to the specified report-uri, including the certificate Validation failure to the specified report-uri, including the
chain it observed and the SPKI hashes it expected to see. certificate chain it observed and the SPKI hashes it expected
Among the SPKI hashes is the distinguishing string in step to see. Among the SPKI hashes is the distinguishing string in
(2). step (2).
o Different site operators/origins can optionally collaborate by o Different site operators/origins can optionally collaborate by
setting the report-uri to be in an origin they share setting the report-uri to be in an origin they share
administrative control of. UAs MAY, therefore, refuse to send administrative control of. UAs MAY, therefore, refuse to send
reports outside of the origin that set the PKP or PKP-RO header. reports outside of the origin that set the PKP or PKP-RO header.
o example.com can use server name indication (SNI; [RFC3546]) and o example.com can use server name indication (SNI; [RFC3546]) and
subdomains to distinguish UAs. subdomains to distinguish UAs.
1. example.com sets a Valid Pinning Header in its response to 1. example.com sets a Valid Pinning Header in its response to
skipping to change at page 23, line 11 skipping to change at page 23, line 29
configuration for n.fingerprint.example.com. If the UA fails configuration for n.fingerprint.example.com. If the UA fails
to connect, it was in the set of UAs that were pinned, which to connect, it was in the set of UAs that were pinned, which
can be distinguished from the UAs that were not pinned, as can be distinguished from the UAs that were not pinned, as
they will succeed in connecting. The host may repeat this for they will succeed in connecting. The host may repeat this for
a sufficient number of subdomains necessary to distinguish a sufficient number of subdomains necessary to distinguish
individual UAs. individual UAs.
o Conforming implementations (as well as implementations conforming o Conforming implementations (as well as implementations conforming
to [RFC6797]) must store state about which domains have set to [RFC6797]) must store state about which domains have set
policies, hence which domains the UA has contacted. Because these policies, hence which domains the UA has contacted. Because these
policies cause remotely-detectable behaviours, it is advisable policies cause remotely detectable behaviors, it is advisable that
that UAs have a way for privacy-sensitive users to clear current UAs have a way for privacy-sensitive users to clear current Pins
Pins for Pinned Hosts, and to allow users to query the current for Pinned Hosts and that UAs allow users to query the current
state of Pinned Hosts. In addition, note that because Pinning a state of Pinned Hosts. In addition, note that because pinning a
Host implies a degree of persistent state, an attacker with host implies a degree of persistent state, an attacker with
physical access to a device may be able to recover information physical access to a device may be able to recover information
about hosts a user has visited, even if the user has cleared other about hosts a user has visited, even if the user has cleared other
parts of the UA's state. parts of the UA's state.
o Pin reports, as noted in Section 3, contains information about the o Pin reports, as noted in Section 3, contains information about the
certificate chain that has failed pin validation. In some cases, certificate chain that has failed pin validation. In some cases,
such as organization-wide compromise of the end-to-end security of such as organization-wide compromise of the end-to-end security of
TLS, this may include information about the interception tools and TLS, this may include information about the interception tools and
design used by the organization that the organization would design used by the organization that the organization would
otherwise prefer not be disclosed. otherwise prefer not be disclosed.
6. IANA Considerations 6. IANA Considerations
IANA is requested to register the response headers described in this IANA has registered the response headers described in this document
document in the "Message Headers" registry ([permanent-headers] with under "Permanent Message Header Field Names" in the "Message Headers"
the following parameters: registry [message-headers] with the following parameters:
o Header Field Names should be "Public-Key-Pins" and "Public-Key- o Header Field Names: Public-Key-Pins and Public-Key-Pins-Report-
Pins-Report-Only". Only
o Protocol should be "http" o Protocol: http
o Status should be "standard" o Status: standard
o Reference should be this document o Reference: RFC 7469
7. Usability Considerations 7. Usability Considerations
When pinning works to detect impostor Pinned Hosts, users will When pinning works to detect impostor Pinned Hosts, users will
experience denial of service. It is advisable for UAs to explain the experience denial of service. It is advisable for UAs to explain the
reason why, i.e. that it was impossible to verify the confirmed reason why, i.e., that it was impossible to verify the confirmed
cryptographic identity of the host. cryptographic identity of the host.
It is advisable that UAs have a way for users to clear current Pins It is advisable that UAs have a way for users to clear current Pins
for Pinned Hosts, and to allow users to query the current state of for Pinned Hosts and that UAs allow users to query the current state
Pinned Hosts. of Pinned Hosts.
8. Acknowledgements
Thanks to Tobias Gondrom, Jeff Hodges, Paul Hoffman, Ivan Krstic,
Adam Langley, Barry Leiba, Nicolas Lidzborski, SM, James Manger, Yoav
Nir, Trevor Perrin, Eric Rescorla, Pete Resnick, Tom Ritter, and Yan
Zhu for suggestions and edits that clarified the text.
9. What's Changed
[RFC EDITOR: PLEASE REMOVE THIS SECTION]
Added attack scenario for hostile pinning, as well as mitigations.
Added privacy considerations for the report-uri processing.
Moved pin-directive into its own directive section, leaving the
header syntax to only define directive-name and directive-value.
Updated Privacy Considerations to note that UAs should offer ways to
clear data, and in doing so, clarified the term 'forensic attacker'
to indicate an attacker with physical access.
Updated ABNF for Public-Key-Directives to indicate that at least one
directive is required, and that a directive is required between each
semi-colon.
Clarified that max-age is REQUIRED for PKP, but OPTIONAL for PKP-RO
(where it has no effect.
Updated header field syntax and description to match that in
[RFC7230].
Updated normative references to current documents.
Removed the strict directive.
Removed the requirement that the server set the Valid Pinning Header
on every response.
Added normative references for SHA, JSON, and base-64.
Added the Privacy Considerations section.
Changed non-normative pin generation code from Go to POSIX shell
script using openssl.
Changed max-max-age from SHOULD to MAY, and used the example of 60
days instead of 30.
Removed the section "Pin Validity Times", which was intended to be in
harmony with [I-D.perrin-tls-tack]. Now using max-age purely as
specified in [RFC6797].
Added new directives: includeSubDomains, report-uri and strict.
Added a new variant of the PKP Header: Public-Key-Pins-Report-Only.
Removed the section on pin break codes and verifiers, in favor the of
most-recently-received policy (Section 2.5).
Now using a new header field, Public-Key-Pins, separate from HSTS.
This allows hosts to use pinning separately from Strict Transport
Security.
Explicitly requiring that UAs perform Pin Validation before the HTTP
conversation begins.
Backup Pins are now required.
Separated normative from non-normative material. Removed tangential
and out-of-scope non-normative discussion.
10. References
10.1. Normative References 8. References
[I-D.josefsson-pkix-textual] 8.1. Normative References
Josefsson, S. and S. Leonard, "Textual Encodings of PKIX,
PKCS, and CMS Structures", draft-josefsson-pkix-textual-07
(work in progress), September 2014.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3339] Klyne, G., Ed. and C. Newman, "Date and Time on the [RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
Internet: Timestamps", RFC 3339, July 2002. Timestamps", RFC 3339, July 2002,
<http://www.rfc-editor.org/info/rfc3339>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, January 2005. 3986, January 2005,
<http://www.rfc-editor.org/info/rfc3986>.
[RFC4634] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and HMAC-SHA)", RFC 4634, July 2006.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006. Encodings", RFC 4648, October 2006,
<http://www.rfc-editor.org/info/rfc4648>.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008. Specifications: ABNF", STD 68, RFC 5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008. (TLS) Protocol Version 1.2", RFC 5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008. (CRL) Profile", RFC 5280, May 2008,
<http://www.rfc-editor.org/info/rfc5280>.
[RFC6234] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms [RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234, May 2011. (SHA and SHA-based HMAC and HKDF)", RFC 6234, May 2011,
<http://www.rfc-editor.org/info/rfc6234>.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265, [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
April 2011. April 2011, <http://www.rfc-editor.org/info/rfc6265>.
[RFC6797] Hodges, J., Jackson, C., and A. Barth, "HTTP Strict [RFC6797] Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
Transport Security (HSTS)", RFC 6797, November 2012. Transport Security (HSTS)", RFC 6797, November 2012,
<http://www.rfc-editor.org/info/rfc6797>.
[RFC7159] Bray, T., "The JavaScript Object Notation (JSON) Data [RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, March 2014. Interchange Format", RFC 7159, March 2014,
<http://www.rfc-editor.org/info/rfc7159>.
[RFC7230] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol [RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
(HTTP/1.1): Message Syntax and Routing", RFC 7230, June Protocol (HTTP/1.1): Message Syntax and Routing", RFC
2014. 7230, June 2014, <http://www.rfc-editor.org/info/rfc7230>.
[RFC7234] Fielding, R., Nottingham, M., and J. Reschke, "Hypertext [RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Transfer Protocol (HTTP/1.1): Caching", RFC 7234, June Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
2014. RFC 7234, June 2014,
<http://www.rfc-editor.org/info/rfc7234>.
[RFC7468] Josefsson, S. and S. Leonard, "Textual Encodings of PKIX,
PKCS, and CMS Structures", RFC 7468, April 2015,
<http://www.rfc-editor.org/info/rfc7468>.
[W3C.REC-html401-19991224] [W3C.REC-html401-19991224]
Raggett, D., Hors, A., and I. Jacobs, "HTML 4.01 Raggett, D., Hors, A., and I. Jacobs, "HTML 4.01
Specification", World Wide Web Consortium Recommendation Specification", World Wide Web Consortium Recommendation
REC-html401-19991224, December 1999, REC-html401-19991224, December 1999,
<http://www.w3.org/TR/1999/REC-html401-19991224>. <http://www.w3.org/TR/1999/REC-html401-19991224>.
[permanent-headers] [message-headers]
Klyne, G., "Permanent Message Header Field Names", July IANA, "Message Headers",
2014, <http://www.iana.org/assignments/message-headers/ <http://www.iana.org/assignments/message-headers/>.
message-headers.xml#perm-headers/>.
10.2. Informative References
[I-D.perrin-tls-tack] 8.2. Informative References
Marlinspike, M., "Trust Assertions for Certificate Keys",
draft-perrin-tls-tack-02 (work in progress), January 2013.
[RFC3546] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., [RFC3546] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
and T. Wright, "Transport Layer Security (TLS) and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 3546, June 2003. Extensions", RFC 3546, June 2003,
<http://www.rfc-editor.org/info/rfc3546>.
[RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate [RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
Transparency", RFC 6962, June 2013. Transparency", RFC 6962, June 2013,
<http://www.rfc-editor.org/info/rfc6962>.
[TACK] Marlinspike, M., "Trust Assertions for Certificate Keys",
Work in Progress, draft-perrin-tls-tack-02, January 2013.
[why-pin-key] [why-pin-key]
Langley, A., "Public Key Pinning", May 2011, Langley, A., "Public Key Pinning", Imperial Violet: Adam
<http://www.imperialviolet.org/2011/05/04/pinning.html>. Langley's Weblog, May 2011,
<https://www.imperialviolet.org/2011/05/04/pinning.html>.
Appendix A. Fingerprint Generation Appendix A. Fingerprint Generation
This POSIX shell program generates SPKI Fingerprints, suitable for This Portable Operating System Interface (POSIX) shell program
use in pinning, from PEM-encoded certificates. It is non-normative. generates SPKI Fingerprints, suitable for use in pinning, from PEM-
encoded certificates. It is non-normative.
openssl x509 -noout -in certificate.pem -pubkey | \ openssl x509 -noout -in certificate.pem -pubkey | \
openssl asn1parse -noout -inform pem -out public.key openssl asn1parse -noout -inform pem -out public.key
openssl dgst -sha256 -binary public.key | openssl enc -base64 openssl dgst -sha256 -binary public.key | openssl enc -base64
Figure 11: Example SPKI Fingerprint Generation Code Figure 11: Example SPKI Fingerprint Generation Code
Appendix B. Deployment Guidance Appendix B. Deployment Guidance
This section is non-normative guidance which may smooth the adoption This section is non-normative guidance that may smooth the adoption
of public key pinning. of public key pinning.
o Operators should get the backup public key signed by a different o Operators should get the backup public key signed by a different
(root and/or intermediary) CA than their primary certificate, and (root and/or intermediary) CA than their primary certificate, and
store the backup key pair safely offline. The semantics of an store the backup key pair safely offline. The semantics of an
SPKI Fingerprint do not require the issuance of a certificate to SPKI Fingerprint do not require the issuance of a certificate to
construct a valid Pin. However, in many deployment scenarios, in construct a valid Pin. However, in many deployment scenarios, in
order to make a Backup Pin operational the server operator will order to make a Backup Pin operational, the server operator will
need to have a certificate to deploy TLS on the host. Failure to need to have a certificate to deploy TLS on the host. Failure to
obtain a certificate through prior arrangement will leave clients obtain a certificate through prior arrangement will leave clients
that recognize the site as a Known Pinned Host unable to that recognize the site as a Known Pinned Host unable to
successfully perform Pin Validation until such a time as the successfully perform Pin Validation until such a time as the
operator can obtain a new certificate from their desired operator can obtain a new certificate from their desired
certificate issuer. certificate issuer.
o It is most economical to have the backup certificate signed by a o It is most economical to have the backup certificate signed by a
completely different signature chain than the live certificate, to completely different signature chain than the live certificate, to
maximize recoverability in the event of either root or maximize recoverability in the event of compromise of either the
intermediary signer compromise. root or intermediary signer.
o Operators should periodically exercise their Backup Pin plan -- an o Operators should periodically exercise their Backup Pin plan -- an
untested backup is no backup at all. untested backup is no backup at all.
o Operators should start small. Operators should first deploy o Operators should start small. Operators should first deploy
public key pinning by using the report-only mode together with a public key pinning by using the report-only mode together with a
report-uri directive that points to a reliable report collection report-uri directive that points to a reliable report collection
endpoint. When moving out of report-only mode, operators should endpoint. When moving out of report-only mode, operators should
start by setting a max-age of minutes or a few hours, and start by setting a max-age of minutes or a few hours and gradually
gradually increase max-age as they gain confidence in their increase max-age as they gain confidence in their operational
operational capability. capability.
Acknowledgements
Thanks to Tobias Gondrom, Jeff Hodges, Paul Hoffman, Ivan Krstic,
Adam Langley, Barry Leiba, Nicolas Lidzborski, SM, James Manger, Yoav
Nir, Trevor Perrin, Eric Rescorla, Pete Resnick, Tom Ritter, and Yan
Zhu for suggestions and edits that clarified the text.
TACK [TACK] is a fruitful source of alternative design
considerations.
Authors' Addresses Authors' Addresses
Chris Evans Chris Evans
Google, Inc. Google, Inc.
1600 Amphitheatre Pkwy 1600 Amphitheatre Pkwy
Mountain View, CA 94043 Mountain View, CA 94043
US United States
Email: cevans@google.com EMail: cevans@google.com
Chris Palmer Chris Palmer
Google, Inc. Google, Inc.
1600 Amphitheatre Pkwy 1600 Amphitheatre Pkwy
Mountain View, CA 94043 Mountain View, CA 94043
US United States
Email: palmer@google.com EMail: palmer@google.com
Ryan Sleevi Ryan Sleevi
Google, Inc. Google, Inc.
1600 Amphitheatre Pkwy 1600 Amphitheatre Pkwy
Mountain View, CA 94043 Mountain View, CA 94043
US United States
Email: sleevi@google.com EMail: sleevi@google.com
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