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Certificate coupling

Purpose

The coupling plugin provides a mechanism to couple a peer certificate permanently. While uncoupled, any peer certificate is accepted if it can be verified using a trustchain. The certificate is then added to a coupling list. Once the coupling list reaches a predefined count of entries, no new certificates are accepted. Only certificates listed in the couplings file now can authenticate successfully.

There are different scenarios where such a coupling might make sense, but of course only if a peer talks to an exclusive peer list. Once coupled, only listed certificates are accepted: Newly issued certificates issued by i.e. a compromised CA don't get a chance to authenticate successfully.

Implementation

The coupling functionality is implemented in the coupling plugin, add

--enable-coupling
to the ./configure arguments.

The coupling plugin uses a text file to store the list, each line consists of:

hex-encoded-hash-of-full-cert unix-timestamp-of-coupling cert-subject-dn
The hash itself is sufficient to validate a coupling entry, the other values are for informative purposes only.
You can manually alter the coupling file, but make sure the hex encoded hash begins with the first character of a line.

Configuration

The plugin is configured using the following strongswan.conf options:

Key Default Description
charon.plugins.coupling.max 1 Maximum number of coupling entries to create
charon.plugins.coupling.hash sha1 Hashing algorithm to fingerprint coupled certificates (md5/sha1/sha256/sha384/sha512)
charon.plugins.coupling.file null File to store coupling list to

The default value for max of 1 allows only a single certificate to couple. Once coupled, no other certificates get accepted. Using a higher max value accepts more than one certificate. But be aware that until the list of coupled certificates reaches max, any certificate (verifiable by a trustchain) gets accepted and inserted to the coupling list.

The hash is created using the specified hashing algorithm over the full DER encoded certificate.

The plugin does not load if no file is specified.