UnrecognizedExtension: APIs for (basic) DER unpacking

[woodruffw]

Opening to gather interest/appetite before I send a patch.

Background

Sigstore generates and uses X.509v3 certificates that contain custom extensions; these custom extensions contain values that are (generally) encoded as UTF8Strings. It would be nice to be able to consume these extension values directly via Cryptography's APIs, without having to use pyasn1 or another dependency to unpeel the DER encoding on the extension values.

Status quo

At the moment, UnrecognizedExtension provides value as a raw view into the (usually) DER-encoded extension value.

Proposal

Add some APIs for common (primitive) extension value retrievals: UTF8String is the main one I'm personally interested in, but there are probably a few others that would be generally useful (UTCTime, GeneralizedTime, IA5String, etc.).

My first thought was to have these as value_as_X APIs, e.g.:

# property: str
ext.value_as_utf8string

...but that's pretty ugly.

Alternatives considered

• Support the Sigstore extension types directly in Cryptography? I'm not sure if this is of interest to you as an upstream.
• Do this in a third-party library that re-uses rust-asn1. I'll probably do this if neither of the above ideas is appealing to you.

[reaperhulk]

We’ve had some past discussions about exposing the imperative DER API for consumers. I’m generally positive on that effort (as it’d remove another reason to use pyopenssl and/or pyasn1). I think that hypothetical API would cover this reasonably well too?

[woodruffw]

We’ve had some past discussions about exposing the imperative DER API for consumers. I’m generally positive on that effort (as it’d remove another reason to use pyopenssl and/or pyasn1). I think that hypothetical API would cover this reasonably well too?

Yep, I think it would!

[alex]

There'll be a substantial design effort involved in figuring out a public DER API, but it can be done.

[Nothing4You]

just leaving OtherName here as search keyword

[reaperhulk]

@woodruffw are you still interested in this? 😄

[woodruffw]

@woodruffw are you still interested in this? 😄

I am, but it's pretty far down on the priority set at the moment (I still need it for sigstore-python, but it won't become an urgent need until they deprecate the previous thing I rely on) 😅