Add support for decryption with hidden recipients

pgpainless-core/src/test/java/org/pgpainless/decryption_verification/DecryptHiddenRecipientMessage.java

@@ -0,0 +1,64 @@
+/*
+ * 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.decryption_verification;
+
+import org.bouncycastle.openpgp.PGPException;
+import org.bouncycastle.openpgp.PGPSecretKeyRingCollection;
+import org.bouncycastle.util.io.Streams;
+import org.junit.jupiter.api.Test;
+import org.pgpainless.PGPainless;
+import org.pgpainless.key.TestKeys;
+import org.pgpainless.key.protection.UnprotectedKeysProtector;
+
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.nio.charset.StandardCharsets;
+
+public class DecryptHiddenRecipientMessage {
+
+    private static final String PLAINTEXT = "Hello WOrld";
+
+    /**
+     * Message was created using
+     * gpg --no-default-keyring --keyring ~/.pgpainless --no-default-recipient --armor --output ~/message.asc --encrypt --hidden-recipient emil@email.user ~/message.txt
+     * where ~/.pgpainless was a gnupg keyring containing all the test keys and message.asc contained the plaintext message.
+     */
+    private static final String CHIPERHTEXT = "-----BEGIN PGP MESSAGE-----\n" +
+            "\n" +
+            "hG4DAAAAAAAAAAASAgMEUif9JYa7qZedAFs0AUUJ3cxdc7tpsREZxhzzFpgWzKbH\n" +
+            "YB1i7rp3PM9zO88v3GRQY6TH5tR8CJkHRTWGu03c0SBTUbrzWngh0i2uR9DJnjLk\n" +
+            "B8HD4fgn9VP+Qj5Igg6KJdJSAVZlFqVh0H6PCEUaSre/KlyVVqikZKTm0hnWb6XZ\n" +
+            "/TrefNuUR6PDNyh/oMbfAQD46UZjfRrbNUdKjOyfwtBYkkcGKTF+HilQbjk1nKud\n" +
+            "pojSOQ==\n" +
+            "=nyvJ\n" +
+            "-----END PGP MESSAGE-----";
+
+    @Test
+    public void testDecryptionWithHiddenRecipient() throws IOException, PGPException {
+        PGPSecretKeyRingCollection emilSecret = TestKeys.getEmilSecretKeyRingCollection();
+        DecryptionStream decryptionStream = PGPainless.decryptAndOrVerify().onInputStream(new ByteArrayInputStream(CHIPERHTEXT.getBytes(StandardCharsets.UTF_8)))
+                .decryptWith(new UnprotectedKeysProtector(), emilSecret)
+                .doNotVerify()
+                .build();
+
+        ByteArrayOutputStream out = new ByteArrayOutputStream();
+        Streams.pipeAll(decryptionStream, out);
+        decryptionStream.close();
+
+        OpenPgpMetadata metadata = decryptionStream.getResult();
+    }
+}