// 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.subtle;

import com.google.crypto.tink.aead.internal.InsecureNonceChaCha20;
import java.nio.ByteBuffer;
import java.security.GeneralSecurityException;
import java.security.InvalidKeyException;
import java.util.Arrays;

/**
 * A stream cipher, as described in RFC 8439 https://tools.ietf.org/html/rfc8439, section 2.4.
 *
 * <p>This cipher is meant to be used to construct an AEAD with Poly1305.
 */
class ChaCha20 implements IndCpaCipher {
  static final int NONCE_LENGTH_IN_BYTES = 12;

  private final InsecureNonceChaCha20 cipher;

  ChaCha20(final byte[] key, int initialCounter) throws InvalidKeyException {
    cipher = new InsecureNonceChaCha20(key, initialCounter);
  }

  @Override
  public byte[] encrypt(final byte[] plaintext) throws GeneralSecurityException {
    ByteBuffer output = ByteBuffer.allocate(NONCE_LENGTH_IN_BYTES + plaintext.length);
    byte[] nonce = Random.randBytes(NONCE_LENGTH_IN_BYTES);
    output.put(nonce); // Prepend nonce to ciphertext output.
    cipher.encrypt(output, nonce, plaintext);
    return output.array();
  }

  @Override
  public byte[] decrypt(final byte[] ciphertext) throws GeneralSecurityException {
    if (ciphertext.length < NONCE_LENGTH_IN_BYTES) {
      throw new GeneralSecurityException("ciphertext too short");
    }
    byte[] nonce = Arrays.copyOf(ciphertext, NONCE_LENGTH_IN_BYTES);
    ByteBuffer rawCiphertext =
        ByteBuffer.wrap(
            ciphertext, NONCE_LENGTH_IN_BYTES, ciphertext.length - NONCE_LENGTH_IN_BYTES);
    return cipher.decrypt(nonce, rawCiphertext);
  }
}