Consider adding a new `Error` variant to express `AlgorithmIdentifier` parameters must be absent

[mzohreva]

Currently there is no way to communicate that parameters field in spki::AlgorithmIdentifier must not be specified. This is useful for e.g. Ed25519 keys where the algorithm OID contains all information needed and the RFC specifies that:

the parameters MUST be absent.

Source: https://datatracker.ietf.org/doc/html/rfc8410#section-3

[tarcieri]

FWIW, the ed25519 crate uses the ParametersMalformed variant for this:

https://github.com/RustCrypto/signatures/blob/0ed65a1dffea36bda55afdbeb586b3d6ad970dde/ed25519/src/pkcs8.rs#L74

(it could probably use a comment with that RFC citation, though)

[mzohreva]

I think that implementation is using pkcs8::Error which has the variant you mentioned. In my case I was trying to implement spki::DecodePublicKey trait which requires TryFrom<SubjectPublicKeyInfo<'a>, Error = Error> where Error is spki::Error. Is there a better way to achieve that (to ultimately be able to parse an Ed25519 public key from DER)?

[tarcieri]

There's unfortunately not support for public keys in the ed25519 crate itself. That should probably get added.

[tarcieri]

I've added ed25519::pkcs8::PublicKeyBytes in RustCrypto/signatures#455 which can be used to parse SPKI-encoded Ed25519 public keys.

It returns spki::Error::ParametersMalformed in the event there are any parameters present.