Use appropriate digest algorithm during signature creation

[oncilla]

Pass the public key instead of the marshalled public key to
digestAlgorithmForPublicKey in SignedData.AddSignerInfo.

Previously, the marshalled public key was passed instead of the actual
public key. The result is that always SHA256 was being selected, even
for ECDSA where the hash algorithm should be selected based on the curve.

[oncilla]

Hi @lgarron

What can I do to move this forward?

[lgarron]

I'm not qualified to review this, but @vcsjones may be!

[vcsjones]

I see:

smimesign/ietf-cms/protocol/protocol.go

Lines 755 to 766 in e650daf

	func digestAlgorithmForPublicKey(pub crypto.PublicKey) pkix.AlgorithmIdentifier {
	if ecPub, ok := pub.(*ecdsa.PublicKey); ok {
	switch ecPub.Curve {
	case elliptic.P384():
	return pkix.AlgorithmIdentifier{Algorithm: oid.DigestAlgorithmSHA384}
	case elliptic.P521():
	return pkix.AlgorithmIdentifier{Algorithm: oid.DigestAlgorithmSHA512}
	}
	}
	return pkix.AlgorithmIdentifier{Algorithm: oid.DigestAlgorithmSHA256}
	}

Since we're passing in something that is not an ecdsa.PublicKey, none of the if checks match and it defaults to SHA256.

I think this change looks good, but it would be great to get some test coverage that indeed the right digest algorithm is used for the curve. Would you be able to add some test coverage for this, @oncilla?

[vcsjones]

Changes look reasonable as I previously mentioned, but hoping we can get some test coverage for this.

[oncilla]

@vcsjones sure. I will have look tomorrow