Public Key Pinning Extension for HTTPGoogle, Inc.1600 Amphitheatre PkwyMountain ViewCA94043UScevans@google.comGoogle, Inc.1600 Amphitheatre PkwyMountain ViewCA94043USpalmer@google.comGoogle, Inc.1600 Amphitheatre PkwyMountain ViewCA94043USsleevi@google.com
General
Web SecurityI-DInternet-DraftCertificatePublic keyX.509Certification authorityPublic key pinningHTTPSTLSSSLThis document describes an extension to the HTTP protocol allowing web
host operators to instruct user agents to remember ("pin") the hosts'
cryptographic identities for a given period of time. During that time, UAs
will require that the host present a certificate chain including at least
one Subject Public Key Info structure whose fingerprint matches one of the
pinned fingerprints for that host. By effectively reducing the number of
authorities who can authenticate the domain during the lifetime of the pin,
pinning may reduce the incidence of man-in-the-middle attacks due to
compromised Certification Authorities.This document defines a new HTTP header that enables a web host to
express to user agents (UAs) which Subject Public Key Info (SPKI)
structure(s) UAs SHOULD expect to be present in the host's certificate chain
in future connections using TLS . We call this
"public key pinning" (PKP). At least one UA (Google Chrome) has experimented
with the idea by shipping with a user-extensible embedded set of Pins.
Although effective, this does not scale. This proposal addresses the scale
problem.Deploying PKP safely will require operational and organizational maturity
due to the risk that hosts may make themselves unavailable by pinning to a
(set of) SPKI(s) that becomes invalid (see ). With care, host operators can greatly
reduce the risk of main-in-the-middle (MITM) attacks and other
false-authentication problems for their users without incurring undue
risk.PKP is meant to be used toegether with HTTP Strict Transport Security
(HSTS) , but is possible to pin keys without
requiring HSTS.The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC
2119.The "Public-Key-Pins" and "Public-Key-Pins-Report-Only" header fields,
also referred to within this specification as the PKP and PKP-RO header
fields, respectively, are new response headers defined in this
specification. They are used by a server to indicate that a UA should
perform Pin Validation () for
the host emitting the response message, and to provide the necessary
information for the UA to do so. describes the syntax (Augmented Backus-Naur
Form) of the header fields, using the grammar defined in
and the rules defined in Section 3.2 of
. The field values of both header fields conform to the
same rules.
Public-Key-Directives = [ directive ] *( OWS ";" OWS [ directive ] )
directive = simple-directive
/ pin-directive
simple-directive = directive-name [ "=" directive-value ]
directive-name = token
directive-value = token
/ quoted-string
pin-directive = "pin-" token "=" quoted-string
OWS is used as defined in Section 3.2.3 of . token
and quoted-string are used as defined in Section 3.2.6 of
.The directives defined in this specification are described below. The
overall requirements for directives are:The order of appearance of directives is not significant.A given simple-directive MUST NOT appear more than once in a given header
field. Directives are either optional or required, as stipulated in their
definitions.Directive names are case-insensitive.UAs MUST ignore any header fields containing directives, or other
header field value data, that do not conform to the syntax defined in this
specification. In particular, UAs must not attempt to fix malformed header
fields.If a header field contains any directive(s) the UA does not
recognize, the UA MUST ignore those directives.If the PKP or PKP-RO header field otherwise satisfies the above requirements
(1 through 5), the UA MUST process the directives it recognizes.Additional directives extending the semantic functionality of the header
fields can be defined in other specifications. The first such specification
will need to define a reistry for such directives. Such future directives
will be ignored by UAs implementing only this specification, as well as by
generally non-conforming UAs.In the pin-directive, the token is the name of a cryptographic hash
algorithm. The only algorithm allowed at this time is "sha256"; additional
algorithms may be defined in the future. The quoted-string is a sequence of
base 64 digits: the base 64-encoded SPKI Fingerprint (see ).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.According to rule 5, above, the UA MUST ignore pin-directives with tokens
naming hahs algorithms it does not 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 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 Pinned Host.The "max-age" directive specifies the number of seconds after the
reception of the PKP header field during which the UA SHOULD regard the host
(from whom the message was received) as a Known Pinned Host.The "max-age" directive is REQUIRED to be present within a
"Public-Key-Pins" header field, and is OPTIONAL within a
"Public-Key-Pins-Report-Only" header field.If present, the max-age directive is REQUIRED to have a directive value,
for which the the syntax (after quoted-string unescaping, if necessary) is
defined as:
max-age-value = delta-seconds
delta-seconds = 1*DIGIT
delta-seconds is used as defined in , Section
1.2.1.The OPTIONAL includeSubDomains directive is a valueless directive that,
if present (i.e., it is "asserted"), signals to the UA that the Pinning
Policy applies to this Pinned Host as well as any subdomains of the host's
domain name.The OPTIONAL report-uri directive indicates the URI to which the UA
SHOULD report Pin Validation failures (). The UA POSTs the reports to the
given URI as described in .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 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 connection (as
described in ).Hosts may set report-uris that use HTTP, HTTPS, or other schemes. If the
scheme in the report-uri is one that uses TLS (e.g. HTTPS or WSS), UAs MUST
perform 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 a
Known HSTS Host.Note that the report-uri need not necessarily be in the same Internet
domain or web origin as the host being reported about.UAs SHOULD make their best effort to report Pin Validation failures to
the report-uri, but may fail to report in exceptional conditions. For
example, if connecting the report-uri itself incurs a Pinning Validation
failure or other certificate validation failure, the UA MUST cancel the
connection. Similarly, if Known Pinned Host A sets a report-uri referring to
Known Pinned Host B, and if B sets a report-uri referring to A, and if both
hosts fail Pin Validation, the UA SHOULD detect and break the loop by
failing to send reports to and about those hosts.In any case of report failure, the UA MAY attempt to re-send the report
later.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 than
once per distinct set of declared Pins. shows some example PKP and PKP-RO response
header fields. (Lines are folded to fit.)
Public-Key-Pins: max-age=3000;
pin-sha256="d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM=";
pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g=";
Public-Key-Pins: max-age=2592000;
pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g=";
pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ="
Public-Key-Pins: max-age=2592000;
pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g=";
pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ=";
report-uri="http://example.com/pkp-report"
Public-Key-Pins-Report-Only: max-age=2592000;
pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g=";
pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ=";
report-uri="https://other.example.net/pkp-report"
Public-Key-Pins:
pin-sha256="d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM=";
pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ=";
max-age=259200
Public-Key-Pins:
pin-sha256="d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM=";
pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g=";
pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ=";
max-age=10000; includeSubDomains
This section describes the processing model that Pinned Hosts implement.
The model has 2 parts: (1) the processing rules for HTTP request messages
received over a secure transport (e.g. TLS); and (2) the processing rules
for HTTP request messages received over non-secure transports, such as
TCP.When replying to an HTTP request that was conveyed over a secure
transport, a Pinned Host SHOULD include in its response exactly one PKP
header field, exactly one PKP-RO header field, or one of each. Each instance
of either header field MUST satisfy the grammar specified in .Establishing a given host as a Known Pinned Host, in the context of a
given UA, is accomplished as follows:Over the HTTP protocol running over secure transport, by correctly
returning (per this specification) at least one valid PKP header field to
the UA.Through other mechanisms, such as a client-side pre-loaded Known Pinned
Host List.Pinned Hosts SHOULD NOT include the PKP header field in HTTP responses
conveyed over non-secure transport. UAs MUST ignore any PKP header received
in an HTTP response conveyed over non-secure transport.The UA processing model relies on parsing domain names. Note that
internationalized domain names SHALL be canonicalized according to the
scheme in Section 10 of .If the UA receives, over a secure transport, an HTTP response that
includes a PKP header field conforming to the grammar specified in , and there are no underlying secure transport
errors or warnings (see ), the UA MUST
either:Note the host as a Known Pinned Host if it is not already so noted (see
),or,Update the UA's cached information for the Known Pinned Host if any of of
the max-age, includeSubDomains, or report-uri header field value directives
convey information different from that already maintained by the UA.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 max-age
header field value token has a value of 0, the UA MUST remove its cached
Pinning Policy information (including the includeSubDomains directive, if
asserted) if the Pinned Host is 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-RO
header fieled in an HTTP response message over secure transport, then the UA
MUST process only the first PKP header field (if present) and only the first
PKP-RO header field (if present).If the UA receives the HTTP response over insecure transport, or if the
PKP header is not a Valid Pinning Header (see ),
the UA MUST ignore any present PKP header field(s). Similarly, if the UA
receives the HTTP response over insecure transport, the UA MUST 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 in .A server MAY set both the "Public-Key-Pins" and
"Public-Key-Pins-Report-Only" headers simultaneously. The headers do not
interact with one another but the UA MUST process the PKP header and SHOULD
process both.The headers are processed according to .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, 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 header, the UA performs
Pin Validation only to check whether or not it should POST a report, but not
for causing connection failure.Note: There is no purpose to using the PKP-RO header without the
report-uri directive. User Agents MAY discard such headers without
interpreting them further.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 well as
enforcing Pin Validation.If a host sets the PKP-RO header, the UA SHOULD note the Pins and
directives given in the PKP-RO header as specified by the max-age 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 would-be Pin Validation
failures that would occur if the report-only policy were enforced.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 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 it SHOULD evaluate the
specified policy and SHOULD report any would-be Pin Validation failures that
would occur if the report-only policy were enforced.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 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 Expiration Date refers to a date
in the past. The UA MUST ignore any expired Known Pinned Hosts in its
cache.For example, if a UA is beginning to perform Pin Validation for a Known
Pinned Host and finds that the cached pinning information for the host
indicates an Effective Expiration Date in the past, the UA MUST NOT continue
with Pin Validation for the host, and must consider the host to no longer be
a Known Pinned Host.Known Pinned Hosts are identified only by domain names, and never IP
addresses. If the substring matching the host production from the
Request-URI (of the message to which the host responded) syntactically
matches the IP-literal or IPv4address productions from Section 3.2.2 of
, then the UA MUST NOT note this host as a Known
Pinned Host.Otherwise, if the substring does not congruently match an existing Known
Pinned Host's domain name, per the matching procedure specified in Section
8.2 of [RFC6797], then the UA MUST add this host to the Known Pinned Host
cache. The UA caches:the Pinned Host's domain name,the Effective Expiration Date, or enough information to calculate it (the
Effective Pin Date and the value of the max-age directive),whether or not the includeSubDomains directive is asserted, andthe value of the report-uri directive, if present.If any other metadata from optional or future PKP header directives are
present in the Valid Pinning Header, and the UA understands them, the UA MAY
note them as well.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 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 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-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 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 UA implementation guidance on how to select a maximum max-age, see .The UA MUST NOT modify any pinning metadata of any superdomain matched
Known Pinned Host.UAs MUST NOT heed http-equiv="Public-Key-Pins" or
http-equiv="Public-Key-Pins-Report-Only" attribute settings on <meta>
elements in received content.An SPKI Fingerprint is defined as the output of a known cryptographic
hash algorithm whose input is the DER-encoded ASN.1 representation of the
subjectPublicKeyInfo (SPKI) field of an X.509 certificate. A Pin is defined
as the combination of the known algorithm identifier and the SPKI
Fingerprint computed using that algorithm.The SPKI Fingerprint is encoded in base 64 for use in an HTTP header
.In this version of the specification, the known cryptographic hash
algorithm is SHA-256, identified as "sha256" .
(Future specifications may add new algorithms and deprecate old ones.) UAs
MUST ignore Pins for which they do not recognize the algorithm identifier.
UAs MUST continue to process the rest of a PKP response header field and
note Pins for algorithms they do recognize; UAs MUST recognize "sha256". reproduces the definition of
the SubjectPublicKeyInfo structure in .
SubjectPublicKeyInfo ::= SEQUENCE {
algorithm AlgorithmIdentifier,
subjectPublicKey BIT STRING }
AlgorithmIdentifier ::= SEQUENCE {
algorithm OBJECT IDENTIFIER,
parameters ANY DEFINED BY algorithm OPTIONAL }
If the certificate's subjectPublicKeyInfo is incomplete when taken in
isolation, such as when holding a DSA key without domain parameters, a
public key pin cannot be formed.We pin public keys, rather than entire certificates, to enable operators
to generate new certificates containing old public keys (see ).See for an example non-normative
program that generates SPKI Fingerprints from certificates.Upon receipt of the PKP response header field, the UA notes the host as a
Known Pinned Host, storing the Pins and their associated directives in
non-volatile storage (for example, along with the HSTS metadata). The Pins
and their associated directives are collectively known as Pinning
Metadata.The UA MUST note the Pins for a Host if and only if all three of the
following conditions hold:It received the PKP response header field over an error-free TLS
connection. If the host is a Pinned Host, this includes the validation added
in .The TLS connection was authenticated with a certificate chain containing
at least one of the SPKI structures indicated by at least one of the given
SPKI Fingerprints (see ).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 must set a Backup Pin; see
.)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 response
header field that meets all these critera is known as a Valid Pinning
Header.Whenever a UA receives a Valid Pinning Header, it MUST set its Pinning
Metadata to the exact Pins, Effective Expiration Date (computed from
max-age), and (if any) report-uri given in the most recently received Valid
Pinning Header.For forward compatibility, the UA MUST ignore any unrecognized PKP and
PKP-RO header directives, while still processing those directives it does
recognize. specifies the directives max-age,
Pins, includeSubDomains, and report-uri but future specifications and
implementations might use additional directives.Upon receipt of a PKP-RO response header field, the UA SHOULD evaluate
the policy expressed in the field, and SHOULD generate and send a report
(see ). However, failure to
validate the Pins 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 Known Pinned Host.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 has
already sent a report for a given policy. UAs SHOULD make a best effort not
to inundate report-uris with redundant reports.When a UA connects to a Pinned Host, if the TLS connection has errors,
the UA MUST terminate the connection without allowing the user to proceed
anyway. (This behavior is the same as that required by .)If the connection has no errors, then the UA will determine whether to
apply a new, additional correctness check: Pin Validation. A UA SHOULD
perform Pin Validation whenever connecting to a Known Pinned Host. It is
acceptable to allow Pin Validation to be disabled for some Hosts according
to local policy. For example, a UA may disable Pin Validation for Pinned
Hosts whose validated certificate chain terminates at a user-defined trust
anchor, rather than a trust anchor built-in to the UA.To perform Pin Validation, the UA will compute the SPKI Fingerprints for
each certificate in the Pinned Host's validated certificate chain, using
each supported hash algorithm for each certificate. (As described in , certificates whose SPKI cannot be taken in
isolation cannot be pinned.) The UA MUST ignore superfluous certificates in
the chain that do not form part of the validating chain. The UA will then
check that the set of these SPKI Fingerprints intersects the set of SPKI
Fingerprints in that Pinned Host's Pinning Metadata. If there is set
intersection, the UA continues with the connection as normal. Otherwise, the
UA MUST treat this Pin Validation Failure as a non-recoverable error. Any
procedure that matches the results of this Pin Validation procedure is
considered equivalent.Note that the UA MUST perform Pin Validation when setting up the TLS
session, before beginning an HTTP conversation over the TLS channel.UAs send validation failure reports only when Pin Validation is actually
in effect. Pin Validation might not be in effect e.g. because the user has
elected to disable it, or because a presented certificate chain chains up to
a locally-installed anchor. In such cases, UAs SHOULD NOT send reports.UAs MAY choose to implement additional sources of pinning information,
such as through built-in lists of pinning information. Such UAs SHOULD allow
users to override such additional sources, including disabling them from
consideration.The effective policy for a Known Pinned Host that has both built-in Pins
and Pins from previously observed PKP header response fields is
implementation-defined.If UAs accept hosts that authenticate themselves with self-signed end
entity certificates, they MAY also allow hosts to pin the public keys in
such certificates. The usability and security implications of this practice
are outside the scope of this specification.When a Known Pinned Host has set the report-uri directive, the UA SHOULD
report Pin Validation failures to the indicated URI. The UA does this by
POSTing a JSON message to the URI; the JSON message
takes this form:
{
"date-time": date-time,
"hostname": hostname,
"port": port,
"effective-expiration-date": expiration-date,
"include-subdomains": include-subdomains,
"served-certificate-chain": [
pem1, ... pemN
],
"validated-certificate-chain": [
pem1, ... pemN
],
"known-pins": [
known-pin1, ... known-pinN
]
}
Whitespace outside of quoted strings is not significant. The key/value
pairs may appear in any order, but each MUST appear only once.The date-time indicates the time the UA observed the Pin Validation
failure. It is provided as a string formatted according to Section 5.6,
"Internet Date/Time Format", of .The hostname is the hostname to which the UA made the original request
that failed Pin Validation. It is provided as a string.The port is the port to which the UA made the original request that
failed Pin Validation. It is provided as an integer.The effective-expiration-date is the Effective Expiration Date for the
noted Pins. It is provided as a string formatted according to Section 5.6,
"Internet Date/Time Format", of .include-subdomains indicates whether or not the UA has noted the
includeSubDomains directive for the Known Pinned Host. It is provided as one
of the JSON identifiers "true" or "false".The served-certificate-chain is the certificate chain, as served by the
Known Pinned Host during TLS session setup. It is provided as an array of
strings; each string pem1, ... pemN is the PEM representation of each X.509
certificate as described in .The validated-certificate-chain is the certificate chain, as constructed
by the UA during certificate chain verification. (This may differ from the
served-certificate-chain.) It is provided as an array of strings; each
string pem1, ... pemN is the PEM representation of each X.509 certificate as
described in . For UAs that build
certificate chains in more than one way during the validation process, they
SHOULD send the last chain built. In this way they can avoid keeping too
much state during the validation process.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 syntax:
known-pin = token "=" quoted-string
As in , the token refers to the algorithm
name, and the quoted-string refers to the base 64 encoding of the SPKI
Fingerprint. When formulating the JSON POST body, the UA MUST either use
single-quoted JSON strings, or use double-quoted JSON strings and \-escape
the embedded double quotes in the quoted-string part of the known-pin. shows an example
of a Pin Validation failure report. (PEM strings are shown on multiple lines
for readability.)
{
"date-time": "2014-04-06T13:00:50Z",
"hostname": "www.example.com",
"port": 443,
"effective-expiration-date": "2014-05-01T12:40:50Z"
"include-subdomains": false,
"served-certificate-chain": [
"-----BEGIN CERTIFICATE-----\n
MIIEBDCCAuygAwIBAgIDAjppMA0GCSqGSIb3DQEBBQUAMEIxCzAJBgNVBAYTAlVT\n
...
HFa9llF7b1cq26KqltyMdMKVvvBulRP/F/A8rLIQjcxz++iPAsbw+zOzlTvjwsto\n
WHPbqCRiOwY1nQ2pM714A5AuTHhdUDqB1O6gyHA43LL5Z/qHQF1hwFGPa4NrzQU6\n
yuGnBXj8ytqU0CwIPX4WecigUCAkVDNx\n
-----END CERTIFICATE-----",
...
],
"validated-certificate-chain": [
"-----BEGIN CERTIFICATE-----\n
MIIEBDCCAuygAwIBAgIDAjppMA0GCSqGSIb3DQEBBQUAMEIxCzAJBgNVBAYTAlVT\n
...
HFa9llF7b1cq26KqltyMdMKVvvBulRP/F/A8rLIQjcxz++iPAsbw+zOzlTvjwsto\n
WHPbqCRiOwY1nQ2pM714A5AuTHhdUDqB1O6gyHA43LL5Z/qHQF1hwFGPa4NrzQU6\n
yuGnBXj8ytqU0CwIPX4WecigUCAkVDNx\n
-----END CERTIFICATE-----",
...
],
"known-pins": [
'pin-sha256="d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM="',
"pin-sha256=\"E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g=\""
]
}
Pinning public keys helps hosts strongly assert their cryptographic
identity even in the face of issuer error, malfeasance or compromise. But
there is some risk that a host operator could lose or lose control of their
host's private key (such as by operator error or host compromise). If the
operator had pinned only the key of the host's end entity certificate, the
operator would not be able to serve their web site or application in a way
that UAs would trust for the duration of their pin's max-age. (Recall that
UAs MUST close the connection to a host upon Pin Failure.)Therefore, there is a necessary trade-off between two competing goods:
pin specificity and maximal reduction of the scope of issuers on the one
hand; and flexibility and resilience of the host's 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) of the host's end entity
certificate(s). Often, a valid certificate chain will have at least two
certificates above the end entity certificate: the intermediate issuer, and
the trust anchor. Operators can pin any one or more of the public keys in
this chain, and indeed could pin to issuers not in the chain (as, for
example, a Backup Pin). Pinning to an intermediate issuer, or even to a
trust anchor or root, still significantly reduces the number of issuers who
can issue end entity certificates for the Known Pinned Host, while still
giving that host flexibility to change keys without a disruption of
service.As mentioned in , UAs MAY cap the max-age
value at some upper limit. There is a security trade-off in that low maximum
values provide a narrow window of protection for users who visit the Known
Pinned Host only infrequently, while high maximum values might potentially
result in a UA's inability to successfully perform Pin Validation for a
Known Pinned Host if the UA's noted Pins and the host's true Pins
diverge.Such divergence could occur for several reasons, including: UA error;
host operator error; network attack; or a Known Pinned Host that
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 observe the
new true Pins for the host. (This last example underscores the importance
for host operators to phase in new keys gradually, and to set the max-age
value in accordance with their planned key migration schedule.)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 days
(5,184,000 seconds) may be considered a balance between the two competing
security concerns.It may happen that Pinned Hosts whose hostnames share a parent domain use
different Valid Pinning Headers. If a host whose hostname is a parent domain
for another host sets the includeSubDomains directive, the two hosts' Pins may
conflict with each other. For example, consider two Known Pinned Hosts,
example.com and subdomain.example.com. Assume example.com sets a Valid Pinning
Header such as this:
Public-Key-Pins: max-age=12000; pin-sha256="ABC..."; pin-sha256="DEF...";
includeSubDomains
Assume subdomain.example.com sets a Valid Pinning Header such as
this:
Public-Key-Pins: pin-sha256="GHI..."; pin-sha256="JKL..."
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 note the Pins
in the Valid Pinning Header, and perform Pin Validation as normal on
subsequent conections. If the UA then contacts example.com, again it will
note the Pins and perform Pin Validation on future connections.However, if the UA happened to visit example.com before
subdomain.example.com, the UA would, due to example.com's use of the
includeSubDomains directive, attempt to perform Pin Validation for
subdomain.example.com using the SPKI hashes ABC... and DEF..., which are not
valid for the certificate chains subdomain.example.com (which uses
certificates with SPKIs GHI... and JLK...). Thus, depending on the order in
which the UA observes the Valid Pinning Headers for hosts example.com and
subdomain.example.com, Pin Validation might or might not fail for
subdomain.example.com, even if the certificate chain the UA receives for
subdomain.example.com is perfectly valid.Thus, Pinned Host operators must use the includeSubDomains directive with
care. For example, they may choose to use overlapping pin sets for hosts under
a parent domain that uses includeSubDomains, or to not use the
includeSubDomains directive in their effective-second-level domains, or to
simply use the same pin set for all hosts under a given parent domain.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 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 the PKP header. Then, in
case the operator loses control of their 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 Valid Pinning Headers), can connect to the host
without error.Because having a backup key pair is so important to recovery, UAs MUST
require that hosts set a Backup Pin (see ). The
down 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 attack
without being discovered. Operators must take care to avoid leaking the key
such as keeping it offline.HTTP cookies 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 client to send the cookies to a phony subdomain of the host. To
prevent this, hosts SHOULD set the "secure" attribute and omit the "domain"
attribute on all security-sensitive cookies, such as session cookies. These
settings tell the browser that the cookie should only be sent back to the
originating host (not its subdomains), and should only be sent over HTTPS
(not HTTP).Hosts can use HSTS or HPKP as a "super-cookie", by setting distinct
policies for a number of subdomains. For example, assume example.com wishes
to track distinct UAs without explicitly setting a cookie, or if a
previously-set cookie is deleted from the UA's cookie store. Here are two
attack scenarios.example.com can use report-uri and the ability to pin arbitrary
identifiers to distinguish UAs.
example.com sets a Valid Pinning Header in its response to requests. The
header asserts the includeSubDomains directive, and specifies a report-uri
directive as well. Pages served by the host also include references to
subresource https://bad.example.com/foo.png.The Valid Pinning Header includes a "pin" that is not really the hash of
an SPKI, but is instead an arbitrary distinguishing string sent only in
response to a particular request. For each request, the host creates a new,
distinct distinguishing string and sets it as if it were a pin.The certificate chain served by bad.example.com does not pass Pin
Validation given the pin set the host asserted in (1). The HPKP-conforming
UA attempts to report the Pin Validation failure to the specified
report-uri, including the certificate chain it observed and the SPKI hashes
it expected to see. Among the SPKI hashes is the distinguishing string in
step (2).Different site operators/origins can optionally collaborate by setting
the report-uri to be in an origin they share administrative control of. UAs
MAY, therefore, refuse to send reports outside of the origin that set the
PKP or PKP-RO header.example.com can use SNI and subdomains to distinguish UAs.
example.com sets a Valid Pinning Header in its response to requests. The
header asserts the includeSubDomains directive.On a subsequent page view, the host responds with a page including the
subresource https://0.fingerprint.example.com/foo.png, and the server
responds using a certificate chain that does not pass Pin Validation for the
pin-set defined in the Valid Pinning Header in step (1). The HPKP-conforming
UA will close the connection, never completing the request to
0.fingerprint.example.com. The host may thus note that this particular UA
had noted the (good) Pins for that subdomain.example.com can distinguish 2^N UAs by serving Valid Pinning Headers from
an arbitrary number N distinct subdomains, giving some UAs Valid Pinning
Headers for some, but not all subdomains (causing subsequent requests for
n.fingerprint.example.com to fail), and giving some UAs no Valid Pinning
Header for other subdomains (causing subsequent requests for
m.fingerprint.example.com to succeed).Conforming implementations (as well as implementations conforming to
) must store state about which domains have set
policies, hence which domains the UA has contacted. A forensic attacker
might find this information useful, even if the user has cleared other parts
of the UA's state.IANA is requested to register the header described in this document in
the "Message Headers" registry, with the following parameters:Header Field Names should be "Public-Key-Pins" and
"Public-Key-Pins-Report-Only".Protocol should be "http"Status should be "standard"Reference should be this documentWhen pinning works to detect impostor Pinned Hosts, users will experience
denial of service. It is advisable for UAs to explain the reason why, i.e.
that it was impossible to verify the confirmed cryptographic identity of the
host.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 Pinned
Hosts.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, Tom Ritter, and Yan Zhu for suggestions and edits
that clarified the text.[RFC EDITOR: PLEASE REMOVE THIS SECTION]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
.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 . Now using max-age purely as
specified in .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 ().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.Public Key PinningThis POSIX shell program generates SPKI Fingerprints, suitable for use in
pinning, from PEM-encoded certificates. It is non-normative.
openssl x509 -noout -in certificate.pem -pubkey | \
openssl asn1parse -noout -inform pem -out public.key
openssl dgst -sha256 -binary public.key | base64
This section is non-normative guidance which may smooth the adoption of
public key pinning.Operators should get the backup public key signed by a different (root
and/or intermediary) CA than their primary certificate, and store the backup
key pair safely offline. The semantics of an SPKI Fingerprint do not require
the issuance of a certificate to construct a valid Pin. However, in many
deployment scenarios, in order to make a Backup Pin operational the server
operator will need to have a certificate to deploy TLS on the host. Failure
to obtain a certificate through prior arrangement will leave clients that
recognize the site as a Known Pinned Host unable to successfully perform Pin
Validation until such a time as the operator can obtain a new certificate
from their desired certificate issuer.It is most economical to have the backup certificate signed by a
completely different signature chain than the live certificate, to maximize
recoverability in the event of either root or intermediary signer
compromise.Operators should periodically exercise their Backup Pin plan — an
untested backup is no backup at all.Operators should start small. Operators should first deploy public key
pinning by using the report-only mode together with a report-uri directive
that points to a reliable report collection endpoint. When moving out of
report-only mode, operators should start by setting a max-age of minutes or
a few hours, and gradually increase max-age as they gain confidence in their
operational capability.