Extend certcheck to cope with Android attestation extension

[daviddrysdale]

Android includes support for hardware-backed cryptographic keys. When asymmetric keys are generated in this system, the public key is exported in an X.509 certificate, which may be signed by hardware-protected device keys. The exported X.509 certificate may also include an Android-specific extension that attests to various features of the key and the device, described at https://developer.android.com/training/articles/security-key-attestation

This PR adds support for parsing this Android-specific extension, and along the way also improves support for dealing with certificates holding curve 25519 keys (Ed25519 or X25519).

[pav-kv]

@daviddrysdale Could you add some background/motivation for this PR in the description?

[pav-kv]

/gcbrun

[daviddrysdale]

Updated the text above.

[pav-kv]

A couple of style nits.

[pav-kv]

@daviddrysdale Are these certs related to CT? Or is the idea of x509/x509util packages that they are general purpose, so any possible "official" extension can be added in it?

[daviddrysdale]

Are these certs related to CT?

No, they're not in the Web PKI.

Or is the idea of x509/x509util packages that they are general purpose, so any possible "official" extension can be added in it?

The x509 package is definitely general, as it's a fork of the main Golang package. I tried to make x509util general too, although I imagine some CT-specific stuff might have crept in.

[pav-kv]

/gcbrun

[pav-kv]

There is some trouble with the CI build. Could you rebase to see if it helps to fix it?

Is this functional already in upstream x509? If not, any reason to have it in the fork first (except maybe convenience)? I'm happy to approve this PR, but I'm not sure who of this repo maintainers would follow further updates, particularly given they are defined in Andriod docs.

@AlCutter @getagit What is your view on this PR and the state of x509 fork? We previously discussed factoring it out to a separate repo, and/or finding an owner. I guess this discussion is relevant too.

[daviddrysdale]

Rebased on master

[pav-kv]

/gcbrun

[daviddrysdale]

Is this functional already in upstream x509? If not, any reason to have it in the fork first (except maybe convenience)?

x509util/ is entirely local to this repo, it's only x509/ that's a fork – adding it here is entirely convenience because I'm used to using certcheck (as I wrote it).

[pav-kv]

Is there a way to unit-test this file (and this PR in general)?

[daviddrysdale]

I normally just manually run certcheck --verbose over some cert files…

[roger2hk]

/gcbrun