// 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.mac;

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

MacWrapper is the implementation of PrimitiveWrapper for the Mac primitive.
The returned primitive works with a keyset (rather than a single key). To compute a MAC tag, it uses the primary key in the keyset, and prepends to the tag a certain prefix associated with the primary key. To verify a tag, the primitive uses the prefix of the tag to efficiently select the right key in the set. If the keys associated with the prefix do not validate the tag, the primitive tries all keys with OutputPrefixType.RAW. /**
 * MacWrapper is the implementation of PrimitiveWrapper for the Mac primitive.
 *
 * <p>The returned primitive works with a keyset (rather than a single key). To compute a MAC tag,
 * it uses the primary key in the keyset, and prepends to the tag a certain prefix associated with
 * the primary key. To verify a tag, the primitive uses the prefix of the tag to efficiently select
 * the right key in the set. If the keys associated with the prefix do not validate the tag, the
 * primitive tries all keys with {@link com.google.crypto.tink.proto.OutputPrefixType#RAW}.
 */
class MacWrapper implements PrimitiveWrapper<Mac> {
  private static final Logger logger = Logger.getLogger(MacWrapper.class.getName());

  private static class WrappedMac implements Mac {
    private final PrimitiveSet<Mac> primitives;
    private final byte[] formatVersion = new byte[] {CryptoFormat.LEGACY_START_BYTE};

    private WrappedMac(PrimitiveSet<Mac> primitives) {
      this.primitives = primitives;
    }

    @Override
    public byte[] computeMac(final byte[] data) throws GeneralSecurityException {
      if (primitives.getPrimary().getOutputPrefixType().equals(OutputPrefixType.LEGACY)) {
        return Bytes.concat(
            primitives.getPrimary().getIdentifier(),
            primitives.getPrimary().getPrimitive().computeMac(Bytes.concat(data, formatVersion)));
      }
      return Bytes.concat(
          primitives.getPrimary().getIdentifier(),
          primitives.getPrimary().getPrimitive().computeMac(data));
    }

    @Override
    public void verifyMac(final byte[] mac, final byte[] data) throws GeneralSecurityException {
      if (mac.length <= CryptoFormat.NON_RAW_PREFIX_SIZE) {
        // This also rejects raw MAC with size of 4 bytes or fewer. Those MACs are
        // clearly insecure, thus should be discouraged.
        throw new GeneralSecurityException("tag too short");
      }
      byte[] prefix = Arrays.copyOfRange(mac, 0, CryptoFormat.NON_RAW_PREFIX_SIZE);
      byte[] macNoPrefix = Arrays.copyOfRange(mac, CryptoFormat.NON_RAW_PREFIX_SIZE, mac.length);
      List<PrimitiveSet.Entry<Mac>> entries = primitives.getPrimitive(prefix);
      for (PrimitiveSet.Entry<Mac> entry : entries) {
        try {
          if (entry.getOutputPrefixType().equals(OutputPrefixType.LEGACY)) {
            entry.getPrimitive().verifyMac(macNoPrefix, Bytes.concat(data, formatVersion));
          } else {
            entry.getPrimitive().verifyMac(macNoPrefix, data);
          }
          // If there is no exception, the MAC is valid and we can return.
          return;
        } catch (GeneralSecurityException e) {
          logger.info("tag prefix matches a key, but cannot verify: " + e.toString());
          // Ignored as we want to continue verification with the remaining keys.
        }
      }

      // None "non-raw" key matched, so let's try the raw keys (if any exist).
      entries = primitives.getRawPrimitives();
      for (PrimitiveSet.Entry<Mac> entry : entries) {
        try {
          entry.getPrimitive().verifyMac(mac, data);
          // If there is no exception, the MAC is valid and we can return.
          return;
        } catch (GeneralSecurityException ignored) {
          // Ignored as we want to continue verification with other raw keys.
        }
      }
      // nothing works.
      throw new GeneralSecurityException("invalid MAC");
    }
  }

  MacWrapper() {}

  @Override
  public Mac wrap(final PrimitiveSet<Mac> primitives) throws GeneralSecurityException {
    return new WrappedMac(primitives);
  }

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

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