Make sop decrypt throw for unencrypted data

pgpainless-sop/src/main/java/org/pgpainless/sop/DecryptImpl.java

@@ -149,6 +149,10 @@ public class DecryptImpl implements Decrypt {
                 decryptionStream.close();
                 OpenPgpMetadata metadata = decryptionStream.getResult();
 
+                if (!metadata.isEncrypted()) {
+                    throw new SOPGPException.BadData("Data is not encrypted.");
+                }
+
                 List<Verification> verificationList = new ArrayList<>();
                 for (SignatureVerification signatureVerification : metadata.getVerifiedInbandSignatures()) {
                     verificationList.add(map(signatureVerification));

pgpainless-sop/src/test/java/org/pgpainless/sop/EncryptDecryptRoundTripTest.java

@@ -515,4 +515,22 @@ public class EncryptDecryptRoundTripTest {
         assertThrows(SOPGPException.BadData.class, () ->
                 sop.decrypt().withSessionKey(wrongSessionKey).ciphertext(ciphertext));
     }
+
+    @Test
+    public void decryptNonEncryptedDataFailsBadData() throws IOException {
+        byte[] signed = sop.inlineSign()
+                .key(aliceKey)
+                .withKeyPassword(alicePassword)
+                .data(message)
+                .getBytes();
+
+        assertThrows(SOPGPException.BadData.class, () ->
+                sop.decrypt()
+                        .verifyWithCert(aliceCert)
+                        .withKey(aliceKey)
+                        .withKeyPassword(alicePassword)
+                        .ciphertext(signed)
+                        .toByteArrayAndResult()
+        );
+    }
 }