Move PGP object factory code into a dedicated Utils class

Ghostryde.java has a lot of duplicate code with RydeEncoder and the future
RydeDecoder - the encryption/decryption, compression/decompression, file
encoding/decoding. The "de-XXX" part of each of these pairs needs to read a PGP
object from a stream using PGPObjectFactory.

Since we want to move the duplicate code into their own files, we will need to
move the "read PGP objects from stream" functions to a common utility class.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=205092800

java/google/registry/rde/PgpUtils.java

@@ -0,0 +1,65 @@
+// Copyright 2018 The Nomulus Authors. All Rights Reserved.
+//
+// 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 google.registry.rde;
+
+import static com.google.common.base.Preconditions.checkNotNull;
+import static com.google.common.base.Preconditions.checkState;
+
+import com.google.common.collect.Streams;
+import java.io.IOException;
+import java.io.InputStream;
+import java.util.stream.Stream;
+import javax.annotation.Nullable;
+import org.bouncycastle.openpgp.PGPEncryptedDataList;
+import org.bouncycastle.openpgp.PGPObjectFactory;
+import org.bouncycastle.openpgp.jcajce.JcaPGPObjectFactory;
+
+/** Utilities that help us deal with Pgp objects. */
+final class PgpUtils {
+
+  private PgpUtils() {}
+
+  /** Safely extracts an object from an OpenPGP message. */
+  static <T> T pgpCast(@Nullable Object object, Class<T> expect) {
+    checkNotNull(object, "PGP error: expected %s but out of objects", expect.getSimpleName());
+    checkState(
+        expect.isAssignableFrom(object.getClass()),
+        "PGP error: expected %s but got %s",
+        expect.getSimpleName(),
+        object.getClass().getSimpleName());
+    return expect.cast(object);
+  }
+
+  static <T> T readSinglePgpObject(InputStream input, Class<T> expect) {
+    try {
+      PGPObjectFactory fact = new JcaPGPObjectFactory(input);
+      return PgpUtils.pgpCast(fact.nextObject(), expect);
+    } catch (IOException e) {
+      throw new RuntimeException(e);
+    }
+  }
+
+  /**
+   * Stream the "list" that is PGPEncryptedDataList.
+   *
+   * <p>PGPEncryptedDataList is apparently very "legacy". It's not actually a list, and although it
+   * implements "Iterable", there's no explicit type given. This requires multiple casts, which is
+   * ugly. Moving the casts to a dedicated function makes it all clearer.
+   */
+  static <T> Stream<T> stream(PGPEncryptedDataList ciphertexts, Class<T> expect) {
+    return Streams.stream((Iterable<?>) ciphertexts)
+        .map(ciphertext -> PgpUtils.pgpCast(ciphertext, expect));
+  }
+}