HTTP Public Key Pinning (HPKP) is a mechanism to make sure a HTTP client (e.g. web browser) only trusts a pre-defined set of certificates when establishing a TLS (“https”) connection. If the certificate presented by the server does not (or no longer) match the previously defined hashes, the connection does not get established. This is useful to counter man-in-the-middle attacks where someone intercepts the connection and in cases where DNS records or the certificate gets compromised. HPKP information is provided as a HTTP header with a list of hash values of certificate fingerprints as content. The client then checks the certificate used for the TLS connection against this list and will only trust the specified list of certificates for future connections to the domain. This highlights a important detail: Since HPKP operates on HTTP level and certificates are exchange on the (lower) TLS protocol level, the first connection to the host will not check for certificate fingerprints since the client does not yet know about them or the fact that the server offers HPKP. This also means there is no way to tell the client that a HPKP setting has changed prior to establishing a TLS connection and talking HTTP.
While HPKP is useful to enhance security, it’s a two-sided sword as well. Incorrect configuration may render your domain useless if clients look for the wrong certificate fingerprint. HPKP information is usually preserved by the client for two months or longer before getting refreshed. While clearing a clients history or settings will force a reset, there is no way to communicate this to users since the client will not even try to establish a HTTP connection. Worst case, the domain won’t be valid to serve content for the defined refresh interval. That being said, careful selection about what gets “pinned” at the header information helps to avoid most of the trouble.
The header is designed to contain a number of base64 encoded hash values which are valid for the specific domain and includes an obligation to pin “backup” hashes in case the “primary” hashes do not match anymore. A client will just iterate through the full trust-chain for provided certificate and look if any certificates fingerprint matches any of the hashes provided by HPKP headers. If any certificate matches, the check passes and the TLS connection is established. Now, what should be pinned at HPKP headers? The obvious answer would be “the certificate!” but wait - certificates may change quite often, especially when using short-living validity like Let’s Encrypt does. In this case the new certificates hash is not yet part of HPKP information stored with the client but the old one is expired, as a result the client would not be able to connect until the client re-requests HPKP information. More information can be checked at RFC 7469.
Lets start with the “primary” hashes. Adding the hash of the domains certificate will certainly not harm, however that means the header information needs to be updated as soon as a new certificate gets issued for the domain. Next should be the Intermediate certificate(s) of the CA in case the CA uses them, this means any other certificate for the domain issued by the CA and signed with the same intermediate certificate will be valid for the host. Third is the Root certificate of the CA itself, which means that any certificate issued directly or via any Intermediate certificate by the CA would be trusted as well. Taking Let’s Encrypt for example, the following certificate hashes would get pinned:
- DST Root CA X3
- Let’s Encrypt Authority X3
- Domain certificate
Great, now what happens if your domain certificate expires and “Let’s Encrypt Authority X3” or the “DST Root CA X3” go out of business or get banned from the clients trust stores (hello WoSign, hello DigiNotar…)? The client would not accept any of the certificates because they are either removed from its trust store (Root, Intermediate) or not covered by HPKP information (new domain certificate). This is where pinned “backup” hashes come into play.
Since there is no limitation in hashes, except the 8192 bytes limit for HTTP headers, it’s possible to pin hashes of Intermediate and Root certificates of other CAs which may be an option in case your primary CA gets into trouble. The next best thing to Let’s Encrypt might be Comodo, which means pinning Comodos Intermediate certificates or their Root CA in addition to Let’s Encrypt would flag their certificates to be valid as well.
There is another way though. Since what we’re pinning is not the hash of the actual certificate but the “SPKI Fingerprint”, we can also pin fingerprints of one or more Certificate Signing Request (CSR) which are not yet issued as a certificate. With this, a future certificate issued for this CSR is already pinned, regardless which CA signs the certificate. So in case of a problem with the current CA, that CSR is used to create a certificate at an arbitrary other CA. The certificates fingerprint would then already be part of the HPKP information since it matches the CSR fingerprint.
HPKP transfers the SHA256 hash of the SPKI fingerprint of a certificate or CSR, generated with OpenSSL and no other hash algorithms are supported.
When pinning a CAs root or intermediate certificate, the first step is to acquire the correct public key. Usually CAs offer support pages where those can be obtained. The information about which root or intermediate certificates are used, can easily be looked up from the domain certificate, for example by inspecting it at a browser.
In this case the “DST Root CA X3” is the CAs root certificate and “Let’s Encrypt Authority X3” is a intermediate certificate. Searching for those names leads to the download page https://letsencrypt.org/certificates/ where the public keys can be downloaded as PEM format.
Creating the SHA256 hashes for the CAs root or intermediate certificates SPKI fingerprint is quite straight forward:
The same command can be applied for the actual domains certificate:
To create a CSR and a hash of its SPKI fingerprint, a private key is required to start with. Both the private key and the created CSR must be stored at a safe place for future use. The hash of the CSRs fingerprint can be used immediately for HPKP though.
Note that CAs may require or check the provided data of a CSR, for example the legitimization of the specified organisation or address. At the very least however, the “Common Name” is critical since it must contain the domain name for which the certificate will be created for.
In case our certificate should contain multiple domains at its Server Name Indication (“SNI”) information, the default OpenSSL configuration needs to be tweaked a bit. The following additional parameters are required for the OpenSSL configuration file which comes with Debian GNU/Linux.
The the configuration file gets included when creating the CSR.
With that done, a SHA256 hash of the CSRs fingerprint gets created to be used as HPKP information. The command is equal for SNI and non-SNI CSRs.
HQZ03DioNrXVV7/zEuQONyO8cwUo3ncA71fzLO+o/d8= is the base64 encoded SPKI fingerprint of the CSR.
The header is a quite straight forward list of hashes and adds
max-age information to define the maximum time a client (e.g. browser) will cache HPKP information once it got provided. This sample pins the CAs root certificate, two intermediate certificates and several CSRs as backup. The client will validate the presented certificate against all entries at this list, regardless of its order. Maximum age should be set to two months (expressed in seconds) or longer.
HTTP Servers like nginx allow to simply add those headers at the respective sites configuration:
General availability of the header can be checked by using a browsers development console and look for response headers. There are some more sophisticated sites that check syntax, content and validity of that headers information
Before deploying to production, HPKP settings need to be double-checked since they may lead to unavailability of the site in case their content is incorrect. If the syntax is wrong, a client may just ignore the header and not add any security.