/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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 org.apache.hadoop.security.token;

import java.io.IOException;
import java.security.InvalidKeyException;
import java.security.NoSuchAlgorithmException;

import javax.crypto.KeyGenerator;
import javax.crypto.Mac;
import javax.crypto.SecretKey;
import javax.crypto.spec.SecretKeySpec;

import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.classification.InterfaceStability;
import org.apache.hadoop.ipc.StandbyException;


/**
 * The server-side secret manager for each token type.
 * @param <T> The type of the token identifier
 */
@InterfaceAudience.LimitedPrivate({"HDFS", "MapReduce"})
@InterfaceStability.Evolving
public abstract class SecretManager<T extends TokenIdentifier> {
  /**
   * The token was invalid and the message explains why.
   */
  @SuppressWarnings("serial")
  @InterfaceStability.Evolving
  public static class InvalidToken extends IOException {
    public InvalidToken(String msg) { 
      super(msg);
    }
  }
  
  /**
   * Create the password for the given identifier.
   * identifier may be modified inside this method.
   * @param identifier the identifier to use
   * @return the new password
   */
  protected abstract byte[] createPassword(T identifier);
  
  /**
   * Retrieve the password for the given token identifier. Should check the date
   * or registry to make sure the token hasn't expired or been revoked. Returns 
   * the relevant password.
   * @param identifier the identifier to validate
   * @return the password to use
   * @throws InvalidToken the token was invalid
   */
  public abstract byte[] retrievePassword(T identifier) throws InvalidToken;
  
  /**
   * Create an empty token identifier.
   * @return the newly created empty token identifier
   */
  public abstract T createIdentifier();

  /**
   * No-op if the secret manager is available for reading tokens, throw a
   * StandbyException otherwise.
   * 
   * @throws StandbyException if the secret manager is not available to read
   *         tokens
   */
  public void checkAvailableForRead() throws StandbyException {
    // Default to being available for read.
  }
  
  /**
   * The name of the hashing algorithm.
   */
  private static final String DEFAULT_HMAC_ALGORITHM = "HmacSHA1";

  /**
   * The length of the random keys to use.
   */
  private static final int KEY_LENGTH = 20;

  /**
   * A thread local store for the Macs.
   */
  private static final ThreadLocal<Mac> threadLocalMac =
    new ThreadLocal<Mac>(){
    @Override
    protected Mac initialValue() {
      try {
        return Mac.getInstance(DEFAULT_HMAC_ALGORITHM);
      } catch (NoSuchAlgorithmException nsa) {
        throw new IllegalArgumentException("Can't find " + DEFAULT_HMAC_ALGORITHM +
                                           " algorithm.");
      }
    }
  };

  /**
   * Key generator to use.
   */
  private final KeyGenerator keyGen;
  {
    try {
      keyGen = KeyGenerator.getInstance(DEFAULT_HMAC_ALGORITHM);
      keyGen.init(KEY_LENGTH);
    } catch (NoSuchAlgorithmException nsa) {
      throw new IllegalArgumentException("Can't find " + DEFAULT_HMAC_ALGORITHM +
      " algorithm.");
    }
  }

  /**
   * Generate a new random secret key.
   * @return the new key
   */
  protected SecretKey generateSecret() {
    SecretKey key;
    synchronized (keyGen) {
      key = keyGen.generateKey();
    }
    return key;
  }

  /**
   * Compute HMAC of the identifier using the secret key and return the 
   * output as password
   * @param identifier the bytes of the identifier
   * @param key the secret key
   * @return the bytes of the generated password
   */
  protected static byte[] createPassword(byte[] identifier, 
                                         SecretKey key) {
    Mac mac = threadLocalMac.get();
    try {
      mac.init(key);
    } catch (InvalidKeyException ike) {
      throw new IllegalArgumentException("Invalid key to HMAC computation", 
                                         ike);
    }
    return mac.doFinal(identifier);
  }
  
  /**
   * Convert the byte[] to a secret key
   * @param key the byte[] to create a secret key from
   * @return the secret key
   */
  protected static SecretKey createSecretKey(byte[] key) {
    return new SecretKeySpec(key, DEFAULT_HMAC_ALGORITHM);
  }
}
