cppki: use appropriate digest algorithm during CMS signature creation

Mirror of github/smimesign#97

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.

Author: oncilla

pkg/scrypto/cms/protocol/protocol.go

@@ -648,7 +648,7 @@ func (sd *SignedData) AddSignerInfo(chain []*x509.Certificate, signer crypto.Sig
 	if err != nil {
 		return err
 	}
-	digestAlgorithmID := digestAlgorithmForPublicKey(pub)
+	digestAlgorithmID := digestAlgorithmForPublicKey(signer.Public())
 
 	signatureAlgorithmOID, ok := oid.X509PublicKeyAndDigestAlgorithmToSignatureAlgorithm[cert.PublicKeyAlgorithm][digestAlgorithmID.Algorithm.String()] // nolint:lll
 	if !ok {