HTTP Public Key Pinning (HPKP)

HTTP Public Key Pinning (HPKP) was a security feature that used to tell a web client to associate a specific cryptographic public key with a certain web server to decrease the risk of MITM attacks with forged certificates. It has been removed in modern browsers and is no longer supported.

To ensure the authenticity of a server's public key used in TLS sessions, this public key is wrapped into an X.509 certificate which is usually signed by a certificate authority (CA). Web clients such as browsers trust a lot of these CAs, which can all create certificates for arbitrary domain names. If an attacker is able to compromise a single CA, they can perform MITM attacks on various TLS connections. HPKP can circumvent this threat for the HTTPS protocol by telling the client which public key belongs to a certain web server.

HPKP is a Trust on First Use (TOFU) technique. The first time a web server tells a client via a special HTTP header which public keys belong to it, the client stores this information for a given period of time. When the client visits the server again, it expects at least one certificate in the certificate chain to contain a public key whose fingerprint is already known via HPKP. If the server delivers an unknown public key, the client should present a warning to the user.

To enable this feature for your site, you need to return the Public-Key-Pins HTTP header when your site is accessed over HTTPS:

Public-Key-Pins: pin-sha256="base64=="; max-age=expireTime [; includeSubDomains][; report-uri="reportURI"]

pin-sha256

The quoted string is the Base64 encoded Subject Public Key Information (SPKI) fingerprint. It is possible to specify multiple pins for different public keys. Some browsers might allow other hashing algorithms than SHA-256 in the future. See below on how to extract this information out of a certificate or key file.

max-age

The time, in seconds, that the browser should remember that this site is only to be accessed using one of the defined keys.

includeSubDomains Optional

If this optional parameter is specified, this rule applies to all of the site's subdomains as well.

report-uri Optional

If this optional parameter is specified, pin validation failures are reported to the given URL.

First you need to extract the public key information from your certificate or key file and encode them using Base64.

The following commands will help you extract the Base64 encoded information from a key file, a certificate signing request, or a certificate.

openssl rsa -in my-rsa-key-file.key -outform der -pubout | openssl dgst -sha256 -binary | openssl enc -base64

openssl ec -in my-ecc-key-file.key -outform der -pubout | openssl dgst -sha256 -binary | openssl enc -base64

openssl req -in my-signing-request.csr -pubkey -noout | openssl pkey -pubin -outform der | openssl dgst -sha256 -binary | openssl enc -base64

openssl x509 -in my-certificate.crt -pubkey -noout | openssl pkey -pubin -outform der | openssl dgst -sha256 -binary | openssl enc -base64

The following command will extract the Base64 encoded information for a website.

openssl s_client -servername www.example.com -connect www.example.com:443 | openssl x509 -pubkey -noout | openssl pkey -pubin -outform der | openssl dgst -sha256 -binary | openssl enc -base64

Public-Key-Pins:
  pin-sha256="cUPcTAZWKaASuYWhhneDttWpY3oBAkE3h2+soZS7sWs=";
  pin-sha256="M8HztCzM3elUxkcjR2S5P4hhyBNf6lHkmjAHKhpGPWE=";
  max-age=5184000; includeSubDomains;
  report-uri="https://www.example.org/hpkp-report"

In this example, pin-sha256="cUPcTAZWKaASuYWhhneDttWpY3oBAkE3h2+soZS7sWs=" pins the server's public key used in production. The second pin declaration pin-sha256="M8HztCzM3elUxkcjR2S5P4hhyBNf6lHkmjAHKhpGPWE=" also pins the backup key. max-age=5184000 tells the client to store this information for two months, which is a reasonable time limit according to the IETF RFC. This key pinning is also valid for all subdomains, which is told by the includeSubDomains declaration. Finally, report-uri="https://www.example.net/hpkp-report" explains where to report pin validation failures.

Instead of using a Public-Key-Pins header you can also use a Public-Key-Pins-Report-Only header. This header only sends reports to the report-uri specified in the header and does still allow browsers to connect to the webserver even if the pinning is violated.

The concrete steps necessary to deliver the HPKP header depend on the web server you use.

Apache

Adding a line similar to the following to your webserver's config will enable HPKP on your Apache. This requires mod_headers enabled.

Header always set Public-Key-Pins "pin-sha256=\"base64+primary==\"; pin-sha256=\"base64+backup==\"; max-age=5184000; includeSubDomains"

Nginx

Adding the following line and inserting the appropriate pin-sha256="..." values will enable HPKP on your Nginx. This requires the ngx_http_headers_module.

add_header Public-Key-Pins 'pin-sha256="base64+primary=="; pin-sha256="base64+backup=="; max-age=5184000; includeSubDomains' always;

Lighttpd

The following line with your relevant key information (pin-sha256="..." fields) will enable HPKP on lighttpd.

setenv.add-response-header  = ( "Public-Key-Pins" => "pin-sha256=\"base64+primary==\"; pin-sha256=\"base64+backup==\"; max-age=5184000; includeSubDomains")

server.modules += ( "mod_setenv" )

IIS

Add the following line to the Web.config file to send the Public-Key-Pins header:

<system.webServer>
  ...

  <httpProtocol>
    <customHeaders>
      <add name="Public-Key-Pins" value="pin-sha256=&quot;base64+primary==&quot;; pin-sha256=&quot;base64+backup==&quot;; max-age=5184000; includeSubDomains" />
    </customHeaders>
  </httpProtocol>

  ...
</system.webServer>

• Public-Key-Pins
• Public-Key-Pins-Report-Only
• Browser test site: HSTS and HPKP test
• Expect-CT