HashAgile: expose the algorithm and the content

[baloo]

This allows to verify an hmac signature emitted by a TPM.

[baloo]

missing clippy lints are fixed in #657, i'll rebase once that merges.

[Superhepper]

Question: Is there any way to reverse the dependency, so that the abstraction stuff remains in the abstraction module, without making things very awkward?

[baloo]

I've moved the implementation in tss-esapi/src/abstraction/hashing.rs, I had to massage the visibility of the fields a bit, but let me know if that's an acceptable alternative.

[Superhepper]

Yeah, I think its fine. How would the use statements end up looking if I would want to use the functionality?

use tss_esapi::{structures::HashAgile, abstractions::*};

?

Or can it be narrowed down? I mean if this is the case I still think it is reasonable.

[baloo]

You don't need any use imports.

    let tss_esapi::structures::Signature::Hmac(hmac_sig) = signature else {
        return Err(io::Error::other("expected HMAC signature").into());
    };

    let sig_slice = hmac_sig
        .to_output::<sha2::Sha256>()
        .map_err(|e| io::Error::other(format!("failed to convert HMAC signature: {e}")))?;

You will need to enable abstractions, and rustcrypto features though.

[Copilot]

Pull request overview

This PR extends HashAgile to better support validating TPM-produced HMACs by exposing the hashing algorithm and providing a conversion helper for the digest when using RustCrypto.

Changes:

• Made HashAgile’s internal fields crate-visible and added a public hashing_algorithm() accessor.
• Added a RustCrypto-only HashAgile::to_output() helper to convert the stored digest into a digest::Output<H> for a selected RustCrypto hash type.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
tss-esapi/src/structures/hash/agile.rs	Exposes the stored hashing algorithm via a new accessor and relaxes field visibility for internal use.
tss-esapi/src/abstraction/hashing.rs	Adds RustCrypto interoperability (to_output) for extracting HashAgile digest content as a typed RustCrypto output.

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