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Public Key Pinning is a security feature that tells a web browser to associate a public cryptographic key with a server or servers. When a web browser visits a website for the first time, it will read the HPKP header and store the hashes for the certificates that are provided. Each time the browser then revisits that website, the hash from the provided public key is compared against the stored keys, if the hashes do not match, the web browser should display a warning.

The HPKP header adds protection against man-in-the-middle (MITM) attacks but, if incorrectly configured can make your website display a TLS error for a long period of time.

Here’s a look at what this website publishes as it’s HKPK header.

Public-Key-Pins: pin-sha256="cYf9T3Il8DaCnaMaM0LatIAru1vqmcu2JSwS7uvyEB0=";
                 pin-sha256="u2q8QZ8Hjp3o/efZjsch9NKjnZmrISJQjwoi/rmsKLU=";
                 max-age=15768000; includeSubDomains

To explain it, the first pin-sha265 key is the hash of the public key that was used to generate the certificate this website uses, the second pin-sha256 key is the hash of the public key that I have as a backup for when I either need to generate a new certificate when the old one expires or if something happens and I need to revoke the old key. max-age tells the browser how long to store the pin-sha256 details, for me that is 182 days and finally includeSubDomains tells the browser that these hashes are valid for this domain and all sub domains.

Extracting the public key information

These commands will extract the public key information and encode it in base64.

openssl rsa -in KEYFILE -outform der -pubout | \
    openssl dgst -sha256 -binary | base64

The above command will extract the public key from a private key generated with openssl genrsa, you can replace rsa with dsa for DSA keys.

openssl req -in CSRFILE -pubkey -noout | \
    openssl rsa -pubin -outform der | \
    openssl dgst -sha256 -binary | base64

The above command will extract the public key from a CSR.

openssl x509 -in PEMFILE -pubkey -noout | \
    openssl rsa -pubin -outform der | \
    openssl dgst -sha256 -binary | base64

And the above command here will extract the public key from an existing x509 certificate.

All three of the above commands will generate something similar to below.

writing RSA key
cYf9T3Il8DaCnaMaM0LatIAru1vqmcu2JSwS7uvyEB0=

Backup key(s)

The backup key is actually really simple, you make any number of backups and store them for future use in case of problems or emergencies with the primary key file.

openssl genrsa -out backup1.key 4096

You can then use the command in the previous section to get the base64 encoded public key of this backup key.

The HPKP header

Public-Key-Pins: pin-sha256="PUBLIC_KEY";
                 max-age=EXPIRE_TIME [; includeSubdomains]
                 [; report-uri="REPORT_URI"]

As you can see the header is relatively simple, a definition of each option is below.

pin-sha256
The quoted string is the Base64 encoded fingerprint. You can specify this option multiple times.
max-age
The time, in seconds, that the browser should remember that this site is only to be accessed using one of the pinned keys.
includeSubdomains optional
If this optional parameter is specified, this rule applies to all of the website’s subdomains.
report-uri optional
If this optional parameter is specified, pin validation failures are reported to this URL. This won’t be covered here though.

HAProxy

In HAproxy you simply using the rspadd config option inside the frontend declaration.

rspadd Public-Key-Pins:\ pin-sha256="KEY=";\ pin-sha256="BACKUP_KEY";\ max-age=15768000;\ includeSubDomains

Kura

Anarchist. Pessimist. Bipolar. Hacker. Hyperpolyglot. Musician. Ex pro gamer. Cunt. They/Them.

Kura

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