// Copyright 2017 Google Inc.
//
// 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 com.google.crypto.tink.aead;

import com.google.crypto.tink.Aead;
import com.google.crypto.tink.CryptoFormat;
import com.google.crypto.tink.PrimitiveSet;
import com.google.crypto.tink.PrimitiveWrapper;
import com.google.crypto.tink.Registry;
import com.google.crypto.tink.subtle.Bytes;
import java.security.GeneralSecurityException;
import java.util.Arrays;
import java.util.List;
import java.util.logging.Logger;

AeadWrapper is the implementation of SetWrapper for the Aead primitive.
Key rotation works as follows: each ciphertext is prefixed with the keyId. When decrypting, we
first try all primitives whose keyId starts with the prefix of the ciphertext. If none of these
succeed, we try the raw primitives. If any succeeds, we return the ciphertext, otherwise we
simply throw a GeneralSecurityException.
/**
 * AeadWrapper is the implementation of SetWrapper for the Aead primitive.
 *
 * <p>Key rotation works as follows: each ciphertext is prefixed with the keyId. When decrypting, we
 * first try all primitives whose keyId starts with the prefix of the ciphertext. If none of these
 * succeed, we try the raw primitives. If any succeeds, we return the ciphertext, otherwise we
 * simply throw a GeneralSecurityException.
 */
public class AeadWrapper implements PrimitiveWrapper<Aead> {
  private static final Logger logger = Logger.getLogger(AeadWrapper.class.getName());

  private static class WrappedAead implements Aead {
    private final PrimitiveSet<Aead> pSet;
    private WrappedAead(PrimitiveSet<Aead> pSet) {
      this.pSet = pSet;
    }

    @Override
    public byte[] encrypt(final byte[] plaintext, final byte[] associatedData)
        throws GeneralSecurityException {
      return Bytes.concat(
          pSet.getPrimary().getIdentifier(),
          pSet.getPrimary().getPrimitive().encrypt(plaintext, associatedData));
    }

    @Override
    public byte[] decrypt(final byte[] ciphertext, final byte[] associatedData)
        throws GeneralSecurityException {
      if (ciphertext.length > CryptoFormat.NON_RAW_PREFIX_SIZE) {
        byte[] prefix = Arrays.copyOfRange(ciphertext, 0, CryptoFormat.NON_RAW_PREFIX_SIZE);
        byte[] ciphertextNoPrefix =
            Arrays.copyOfRange(ciphertext, CryptoFormat.NON_RAW_PREFIX_SIZE, ciphertext.length);
        List<PrimitiveSet.Entry<Aead>> entries = pSet.getPrimitive(prefix);
        for (PrimitiveSet.Entry<Aead> entry : entries) {
          try {
            return entry.getPrimitive().decrypt(ciphertextNoPrefix, associatedData);
          } catch (GeneralSecurityException e) {
            logger.info("ciphertext prefix matches a key, but cannot decrypt: " + e.toString());
            continue;
          }
        }
      }

      // Let's try all RAW keys.
      List<PrimitiveSet.Entry<Aead>> entries = pSet.getRawPrimitives();
      for (PrimitiveSet.Entry<Aead> entry : entries) {
        try {
          return entry.getPrimitive().decrypt(ciphertext, associatedData);
        } catch (GeneralSecurityException e) {
          continue;
        }
      }
      // nothing works.
      throw new GeneralSecurityException("decryption failed");
    }
  }

  AeadWrapper() {}

  @Override
  public Aead wrap(final PrimitiveSet<Aead> pset) throws GeneralSecurityException {
    return new WrappedAead(pset);
  }

  @Override
  public Class<Aead> getPrimitiveClass() {
    return Aead.class;
  }

  public static void register() throws GeneralSecurityException {
    Registry.registerPrimitiveWrapper(new AeadWrapper());
  }
}