Function prepare_digest_data_for_openpgp 

fn prepare_digest_data_for_openpgp(
    signature_type: SignatureType,
    oid: AlgorithmIdentifier,
    digest: &[u8],
) -> Result<Cow<'_, [u8]>, Error>

Transforms the raw digest data for cryptographic signing with OpenPGP.

Raw cryptographic signing primitives have special provisions that need to be taken care of when using certain combinations of signing schemes and hashing algorithms.

This function transforms the digest into bytes that are ready to be passed to raw cryptographic functions. The exact specifics of the transformations are documented inside the function.

Errors

Returns an error if

• the signature_type is SignatureType::Pkcs1 and the encoding of the digest data fails,
• or the signature_type is the unsupported SignatureType::PssSha1, SignatureType::PssSha224, SignatureType::PssSha256, SignatureType::PssSha384, or SignatureType::PssSha512.