`Application` tags for `Choice` in der

[yguenduez]

Hello dear all,

sorry for my newb question. I wanted to know, if there is a possibility to decode (and possibly encode) something like this for der:

Identifier ::= [APPLICATION 328] CHOICE {
    hash_over_key OCTET STRING (SIZE(8))
}

I tried with deriving Choice:

#[derive(Choice)]
pub enum Identifier<'a> {
    #[asn1(type = "OCTET STRING")]
    HashOverKey(OctetStringRef<'a>),
}

// This is currently not possible due to conflicting trait implementations
impl FixedTag for Identifier<'_> {
    const TAG: Tag = Tag::Application {
        constructed: true,
        number: TagNumber(328),
    };
}

But sadly FixedTag is already implemented by a blank implementation of Choice. I've seen a similar solved issue with Sequence, which was solved by deriving DecodeValue and EncodeValue and then manully implementing FixedTag: see here

Do you see any possibility to achieve something similar with Choice?

[tarcieri]

You need to put Identifier inside of an Application, as opposed to trying to impl FixedTag on it: https://docs.rs/der/0.8.0-rc.7/der/asn1/struct.Application.html

Note that this is only supported in the v0.8.0 prerelease series.

[yguenduez]

Thanks @tarcieri ! This helped me a lot.

Just in case someone else needs it:

let identifier =
    Identifier::HashOverKey(OctetStringRef::try_from("asdf".as_bytes()).unwrap());
let app_tagged = Application {
    tag_number: der::TagNumber(328),
    tag_mode: der::TagMode::Explicit,
    value: identifier.clone(),
};

let buf = &mut [0u8; 1024];
let encoded_slice = app_tagged.encode_to_slice(buf).unwrap();

let parsed: Application<Identifier> = Application::from_der(encoded_slice).unwrap();

assert_eq!(parsed.tag_number, der::TagNumber(328));
assert_eq!(parsed.tag_mode, der::TagMode::Explicit);
assert_eq!(parsed.value, identifier);

[dishmaker]

It would've been simpler with ApplicationExplicit<328, Identifier> from PR #1945 , but i need to refactor it.