Function resolve 

pub fn resolve<'a, P: SignatureProvider<'a>>(
    provider: &'a P,
    signer: &'a SignerInfo<'a>,
    key: WorkingKey<'a>,
    parent: &Reader<'a>,
    scope: Scope<'a>,
) -> Result<Preparation<'a, P>, PrepareError<P::Error>>

Resolves a selected signer only after its complete binding syntax is checked.

This does not select a certificate, assert SID binding/trust, or discard retained conformance faults. The caller supplies an effective key selected through those independent facts. Unknown/invalid algorithms and provider failures remain distinct. Impossible fixed message-digest widths refuse before signature initialization or payload acquisition. RFC6211 §3 protected algorithms are resolved and compared through the same selected provider, including defaults; generic AlgorithmIdentifier::matches is not used. https://www.rfc-editor.org/rfc/rfc6211#section-3