Prevent plaintext encryption

pgpainless-core/src/main/java/org/pgpainless/encryption_signing/EncryptionOptions.java

@@ -124,6 +124,9 @@ public class EncryptionOptions {
     }
 
     public void overrideEncryptionAlgorithm(SymmetricKeyAlgorithm encryptionAlgorithm) {
+        if (encryptionAlgorithm == SymmetricKeyAlgorithm.NULL) {
+            throw new IllegalArgumentException("Plaintext encryption can only be used to denote unencrypted secret keys.");
+        }
         this.encryptionAlgorithmOverride = encryptionAlgorithm;
     }
 }

pgpainless-core/src/main/java/org/pgpainless/key/EvaluatedKeyRing.java

@@ -1,3 +1,18 @@
+/*
+ * Copyright 2021 Paul Schaub.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
 package org.pgpainless.key;
 
 import java.util.List;
@@ -29,8 +44,4 @@ public interface EvaluatedKeyRing {
         PGPSignature signature = getUserIdCertification(userId);
         return SignatureSubpacketsUtil.parseKeyFlags(signature);
     }
-
-
-
-
 }

pgpainless-core/src/test/java/org/bouncycastle/PGPPublicKeyRingTest.java

@@ -21,7 +21,6 @@ import static org.junit.jupiter.api.Assertions.assertTrue;
 
 import java.security.InvalidAlgorithmParameterException;
 import java.security.NoSuchAlgorithmException;
-import java.util.Collections;
 import java.util.Iterator;
 import java.util.List;