x509-cert: support for computing certificate fingerprints

[secana]

Hi! I've got a Certitifacate and need the fingerprint, (SHA-1/SHA-256) of the certificates.

I've found the FingerprintBytes and corresponding implementation in the spki crate, but I have no idea how to get from one to another.

Can anybody here tell me: How to I get the fingerprint of a certificate?

Thx in advance!

[tarcieri]

We don't have anything built-in for this AFAICT, however you can look at how the spki crate computes a similar fingerprint and apply it to the whole certificate: https://github.com/RustCrypto/formats/blob/a0b4200/spki/src/fingerprint.rs#L17-L42

We should probably add such a feature so I will rename the issue title to reflect the potential new feature.

[secana]

Thanks. I'll see if I can hack something together that computes the hash of a certificate. An easy to use function directly in the Certificates structures, would be preferred in the future.

[carl-wallace]

Maybe it's a stretch, but I wonder if defining a trait that could be used for calculating cert fingerprints as well as key identifiers from SubjectPublicKeyInfo would be the thing to do. May also be useful for SignedAttributes in cms.

[tarcieri]

The logic for calculating fingerprints from spki could be extracted and generalized into der