Added ConvertToED25519 to ED25519phSignerVerifier

Signed-off-by: Riccardo Schirone <riccardo.schirone@trailofbits.com>

pkg/signature/ed25519ph.go

@@ -46,6 +46,16 @@ func LoadED25519phSigner(priv ed25519.PrivateKey) (*ED25519phSigner, error) {
 	}, nil
 }
 
+// ToED25519SignerVerifier creates a ED25519SignerVerifier from a ED25519phSignerVerifier
+//
+// Clients that use ED25519phSignerVerifier should use this method to get a
+// SignerVerifier that uses the same ED25519 private key, but with the Pure
+// Ed25519 algorithm. This might be necessary to interact with Fulcio, which
+// only supports the Pure Ed25519 algorithm.
+func (e ED25519phSignerVerifier) ToED25519SignerVerifier() (*ED25519SignerVerifier, error) {
+	return LoadED25519SignerVerifier(e.priv)
+}
+
 // SignMessage signs the provided message. If the message is provided,
 // this method will compute the digest according to the hash function specified
 // when the ED25519phSigner was created.

pkg/signature/signerverifier_test.go

@@ -17,6 +17,7 @@ package signature
 import (
 	"bytes"
 	"crypto"
+	"crypto/ed25519"
 	"crypto/rsa"
 	"encoding/base64"
 	"testing"
@@ -53,3 +54,29 @@ func TestLoadRSAPSSSignerVerifier(t *testing.T) {
 		t.Fatalf("unexpected error verifying expected signature: %v", err)
 	}
 }
+
+func TestConvertED25519ph(t *testing.T) {
+	privateKey, err := cryptoutils.UnmarshalPEMToPrivateKey([]byte(ed25519Priv), cryptoutils.SkipPassword)
+	if err != nil {
+		t.Fatalf("unexpected error unmarshalling public key: %v", err)
+	}
+	edPriv, ok := privateKey.(ed25519.PrivateKey)
+	if !ok {
+		t.Fatalf("expected ed25519.PrivateKey")
+	}
+
+	sv, err := LoadED25519phSignerVerifier(edPriv)
+	if err != nil {
+		t.Fatalf("unexpected error creating signer/verifier: %v", err)
+	}
+
+	newSV, err := sv.ToED25519SignerVerifier()
+	if err != nil {
+		t.Fatalf("unexpected error converting to ed25519: %v", err)
+	}
+
+	message := []byte("sign me")
+	sig, _ := base64.StdEncoding.DecodeString("cnafwd8DKq2nQ564eN66ckYV8anVFGFi5vaYiQg2aal7ej/J0/OE0PPdKHLHe9wdzWRMFy5MpurRD/2cGXGLBQ==")
+	testingSigner(t, newSV, "ed25519", crypto.SHA256, message)
+	testingVerifier(t, newSV, "ed25519", crypto.SHA256, sig, message)
+}