Expose signature.Signer hash algorithm

[tri-adam]

Description

I'd like to obtain the hash algorithm used by a signature.Signer. I have a use case where the caller passes me a signature.Signer and I need to generate a signature and then wrap it in a descriptor that includes the hash algorithm. The implementation currently ignores the value in the signature.Signer to get around the fact that this value isn't exposed (sylabs/sif#249).

I appreciate that signature.Signer itself can't be extended without breaking v1 module compatibility, but I'm wondering whether concrete types that implement it such as ED25519Signer could expose the hash algorithm, by implementing crypto.SIgnerOpts or similar? If that would be acceptable, I'd be happy to submit a PR for that.

Thanks!

[znewman01]

I have no objections to a PR adding SignerOpts to the concrete Signer types.

CC @haydentherapper any thoughts?

EDIT: I forgot to how the signature.Signer API worked, please disregard

[haydentherapper]

Currently, the hash function is specified per signing (invocation of Sign or SignMessage), not per Signer. Is this proposing allowing you to set the hash function once for the Signer?

[znewman01]

Oh, that's a great point. There's a number of related features here:

• Signer.SignMessage takes a SignOption:

  sigstore/pkg/signature/signer.go

  Line 41 in abdf5cf

  SignMessage(message io.Reader, opts ...SignOption) ([]byte, error)

  
  which AFAICT doesn't directly let you change the hash function used. It does let you provide a digest directly instead of a message:

  sigstore/pkg/signature/options.go

  Line 41 in 83c4f71

  ApplyDigest(*[]byte)

  In some cases, Signers have only one specific hash function; ECDSASigner is a good example:

  sigstore/pkg/signature/ecdsa.go

  Lines 48 to 51 in f3ef3e6

  	type ECDSASigner struct {
  	hashFunc crypto.Hash
  	priv *ecdsa.PrivateKey
  	}

  
  In other cases, however, the Signers do not (e.g., kms/gcp.SignerVerifier):

  sigstore/pkg/signature/kms/gcp/signer.go

  Line 78 in 49a6f59

  signerOpts, err = g.client.getHashFunc()

  
  I see no reason why we couldn't implement SignerOpts in the former case.

• There's a struct signature.SignerOpts which implements crypto.SignerOpts but additionally lets you change the hash function used:

  sigstore/pkg/signature/signer.go

  Lines 51 to 54 in 01093f2

  	// HashFunc returns the hash function for this object
  	func (s SignerOpts) HashFunc() crypto.Hash {
  	return s.Hash
  	}

  There are a number of structs in sigstore/sigstore that implement crypto.Signer including the Sign(rand io.Reader, digest []byte, opts crypto.SignerOpts) interface. These can be used with signature.SignOpts to tweak the hash function. These structs don't have a defined hash function to use.

IIUC the FR, it's to expose the crypto.Hash used in cases where the hash function is hardcoded.