SubjectPublicKeyInfo  ::=  SEQUENCE  {
    algorithm            AlgorithmIdentifier,
    subjectPublicKey     BIT STRING  }

AlgorithmIdentifier  ::=  SEQUENCE  {
    algorithm            OBJECT IDENTIFIER,
    parameters           ANY DEFINED BY algorithm OPTIONAL  }

Figure 4: SPKI Definition

If the SubjectPublicKeyInfo of a certificate 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 [why-pin-key]).

See Appendix A for an example non-normative program that generates SPKI Fingerprints from SubjectPublicKeyInfo fields in certificates.

2.5. Noting Pins

Upon receipt of the Public-Key-Pins response header field, the UA notes the host as a 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 observe these conditions when noting a Host:

• The UA MUST note the Pins if and only if it received the Public-Key-Pins response header field over an error-free TLS connection. If the host is a Pinned Host, this includes the validation added in Section 2.6.
• The UA MUST note the Pins if and only if 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 Section 2.6.)
• The UA MUST note the Pins if and only if 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 Section 4.3.)

If the Public-Key-Pins response header field does not meet all three of these criteria, the UA MUST NOT note the host as a Pinned Host. A Public-Key-Pins response header field that meets all these critera is known as a Valid Pinning Header.

The UA MUST ignore Public-Key-Pins response header fields received on connections that do not meet the first criterion.

Whenever a UA receives a Valid Pinning Header, it MUST set its Pinning Metadata to the exact Pins, max-age, and (if any) report-uri and strict mode given in the most recently received Valid Pinning Header.

For forward compatibility, the UA MUST ignore any unrecognized Public-Key-Pins header directives, while still processing those directives it does recognize. Section 2.1 specifies the directives max.age, pins, includeSubDomains, report-uri, and strict, but future specifications and implementations might use additional directives.

2.6. Validating Pinned Connections

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 [RFC6797].)

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, but MAY allow Pin Validation to be disabled for 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. However, if the Pinned Host Metadata indicates that the Pinned Host is operating in "strict mode" (see Section 2.1.4), then the UA MUST perform Pin Validation.

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. (For the purposes of Pin Validation, the UA MUST ignore certificates whose SPKI cannot be taken in isolation, and 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 Failure as a non-recoverable error. Any procedure that matches the results of this Pin Validation procedure is considered equivalent.

Note that, although the UA has previously received Pins at the HTTP layer, it can and MUST perform Pin Validation at the TLS layer, before beginning an HTTP conversation over the TLS channel. The TLS layer thus evaluates TLS connections with pinning information the UA received previously, regardless of mechanism: statically preloaded, via HTTP header, or some other means (possibly in the TLS layer itself).

2.7. Interactions With Preloaded Pin Lists

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.

UAs that support additional sources of pinning information MUST use the most recently observed pinning information when performing Pin Validation for a host. The most recently observed pinning information is determined based upon the most recent Effective Pin Date, as described in Section 2.3.2.

If the result of noting a Valid Pinning Header is to disable pinning for the host, such as through supplying a max-age directive with a value of 0, UAs MUST allow this new information to override any other pinning data. That is, a host must be able to un-pin itself, even in the presence of built-in pins.

Example: A UA may ship with a pre-configured list of pins that are collected from past observations of Valid Pinning Headers supplied by hosts. In such a solution, the pre-configured list should track when the Valid Pinning Header was last observed, in order to permit site operators to later update the value by supplying a new Valid Pinning Header. Updates to such a pre-configured list should not update the Effective Pin Dates for each host unless the list vendor has actually observed a more recent header. This is to prevent situations where updating the Effective Pin Date on a pre-configured list of pins may effectively extend the max-age beyond the site operator's stated policy.

Example: A UA may ship with a pre-configured list of pins that are collected through out-of-band means, such as direct contact with the site operator. In such a solution, the site operator accepts responsibility for keeping the configured Valid Pinning Header in sync with the vendor's list, allowing the UA vendor to have each update to the list be treated as as an update of the Effective Pin Date.

2.8. Pinning Self-Signed End Entities

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.

3. Reporting Pin Validation Failure

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,
  "certificate-chain": [
    pem1, ... pemN
  ],
  "known-pins": [
    known-pin1, ... known-pinN
  ]
}

known-pin = token "=" quoted-string

Public-Key-Pins: pin-sha1="ABC..."; pin-sha1="DEF..."; includeSubDomains

Public-Key-Pins: pin-sha1="GHI..."; pin-sha1="JKL..."

openssl x509 -noout -in certificate.pem -pubkey | \
    openssl asn1parse -inform pem -out public.key
openssl dgst -sha1 -binary public.key | base64