Fix signature status when verifying unsigned message

Return `SIGNATURE_NOT_SIGNED` instead of `SIGNATURE_NO_VERIFIER` when
verifying a messages with no embedded signatures.

CHANGELOG.md

@@ -6,6 +6,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+### Changed
+
+- Changed the returned `SignatureVerificationError.Status` when trying to verify a message with no embedded signature. It used to return `constants.SIGNATURE_NO_VERIFIER` and now returns `constants.SIGNATURE_NOT_SIGNED`.
+This change impacts : 
+	- `func (sk *SessionKey) DecryptAndVerify(...)`
+	- `func (msg *PlainMessageReader) VerifySignature(...)`
+	- `func (keyRing *KeyRing) Decrypt(...)`
+
 ### Added 
 - Helper to access the SignatureVerificationError explicitly when decrypting streams  in mobile apps: 
 	```go

crypto/signature.go

@@ -99,8 +99,10 @@ func processSignatureExpiration(md *openpgp.MessageDetails, verifyTime int64) {
 
 // verifyDetailsSignature verifies signature from message details.
 func verifyDetailsSignature(md *openpgp.MessageDetails, verifierKey *KeyRing) error {
-	if !md.IsSigned ||
-		md.SignedBy == nil ||
+	if !md.IsSigned {
+		return newSignatureNotSigned()
+	}
+	if md.SignedBy == nil ||
 		len(verifierKey.entities) == 0 ||
 		len(verifierKey.entities.KeysById(md.SignedByKeyId)) == 0 {
 		return newSignatureNoVerifier()

helper/mobile_test.go

@@ -85,7 +85,7 @@ func TestMobileSignedMessageDecryptionWithSessionKey(t *testing.T) {
 		t.Fatal("Expected no error when decrypting, got:", err)
 	}
 
-	assert.Exactly(t, constants.SIGNATURE_NO_VERIFIER, decrypted.SignatureVerificationError.Status)
+	assert.Exactly(t, constants.SIGNATURE_NOT_SIGNED, decrypted.SignatureVerificationError.Status)
 	assert.Exactly(t, message.GetString(), decrypted.Message.GetString())
 
 	publicKey, _ = crypto.NewKeyFromArmored(readTestFile("keyring_publicKey", false))