I'd be ok making as_der() public, I don't think it makes sense to do AsRef<> though.

I don't think it makes sense to do AsRef<> though.

There are quite a lot of functions/methods expecting T where T: AsRef<[u8]> or similar, e.g. https://docs.rs/sha1/latest/sha1/trait.Digest.html, https://doc.rust-lang.org/std/fs/fn.write.html. I was considering making as_der public, which is indeed more explicit, but the use of .as_der() everywhere feels a bit unnecessary because AsRef<[u8]> already narrows it done to some binary representation and there isn’t really another one which isn’t rather obscure. IIRC, a DER encoded OID is valid BER and CER encoding.

I don't think Digest is a good analogy here. Code should be clear and readable, and explicitly indicating that what you want is DER contributes to that.

I changed it as suggested (public as_der), impl AsRef<[u8]> might be subject of another PR, but it’s fine for my use case.