// Copyright 2021 Google LLC
//
// 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.internal;

import java.security.InvalidKeyException;

/**
 * {@link InsecureNonceXChaCha20} stream cipher based on
 * https://download.libsodium.org/doc/advanced/xchacha20.html and
 * https://tools.ietf.org/html/draft-arciszewski-xchacha-01.
 *
 * <p>This cipher is meant to be used to construct an AEAD with Poly1305. Specifically, it is only
 * intended to be used for scenarios that require user-supplied nonces, which would be insecure if
 * the user-supplied nonce ever repeats. Therefore, most users should prefer {@link
 * com.google.crypto.tink.subtle.XChaCha20} instead, since it automatically supplies random nonce
 * inputs.
 */
public class InsecureNonceXChaCha20 extends InsecureNonceChaCha20Base {
  public static final int NONCE_SIZE_IN_BYTES = 24;

  /**
   * Constructs a new InsecureNonceXChaCha20 cipher with the supplied {@code key}.
   *
   * @throws IllegalArgumentException when {@code key} length is not {@link
   *     ChaCha20Util#KEY_SIZE_IN_BYTES}.
   */
  public InsecureNonceXChaCha20(byte[] key, int initialCounter) throws InvalidKeyException {
    super(key, initialCounter);
  }

  @Override
  int[] createInitialState(final int[] nonce, int counter) {
    if (nonce.length != nonceSizeInBytes() / 4) {
      throw new IllegalArgumentException(
          String.format(
              "XChaCha20 uses 192-bit nonces, but got a %d-bit nonce", nonce.length * 32));
    }
    // Set the initial state based on
    // https://tools.ietf.org/html/draft-arciszewski-xchacha-01#section-2.3.
    int[] state = new int[ChaCha20Util.BLOCK_SIZE_IN_INTS];
    ChaCha20Util.setSigmaAndKey(state, ChaCha20Util.hChaCha20(this.key, nonce));
    state[12] = counter;
    state[13] = 0;
    state[14] = nonce[4];
    state[15] = nonce[5];
    return state;
  }

  @Override
  int nonceSizeInBytes() {
    return NONCE_SIZE_IN_BYTES;
  }
}