Perform key sanity check when verifying signatures

[frasertweedale]

Currently if a key is too small, the verification fails with InvalidSignature instead
of KeySizeTooSmall. Add a key sanity check step before attempting to validate
the signature so that the appropriate error gets returned to the caller.

Thanks to @begriffs for reporting.

[frasertweedale]

@begriffs have a look at: 59ca5e6

Note that this step is not automatically performed during verification for a couple of reasons:

1. An offending JWK in a JWK store might not actually be used for a particular verification; checking all JWKs in a JWKStore is too aggressive and may result in spurious verification failures.

2. Passing the per-signature verification failures up to the caller is... quite awkward, to say the least. A single signature may be checked against multiple keys. If the all fail, we'd need to pass all the errors up, presumably with a mapping of each error the JWK that caused it, then let the user sort it out.

Instead of presenting the above problems to library users, I've decided to just implement the checkJWK sanity check function. Library users can call it themselves for each JWK that intend to use for verification, and can fail / report / omit offending keys according to their needs. It is an optional extra step. it can also aid debugging (e.g. could be used for post facto diagnostics).

Let me know what you think of the approach and its usability.

Cheers!

[begriffs]

Thanks for the helper function. I will try it out and see how it goes!

[frasertweedale]

@begriffs did you have any feedback re checkJWK ?

[begriffs]

Thanks, I haven't gotten around to using this function, but I filed an issue in my project so that I or someone else will use it to improve error messages for users.

[frasertweedale]

@begriffs righto, I'll merge it soon.

[ecthiender]

Why can't verifyClaims function verify the size of the key and fail with KeySizeTooSmall ?

Much better UX for library users IMHO.

[frasertweedale]

@ecthiender it's because there can be multiple keys that could be used to verify some signature, and there is not yet a way to propagate complete information about the verification results for particular keys.

I'll think about how the API could be enhanced to provide that. If you have ideas, please share.