content: /*
   * Copyright (C) 2018 The Android Open Source Project
   *
   * 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.android.server.locksettings;
  
  import java.nio.ByteBuffer;
  import java.security.InvalidKeyException;
  import java.security.NoSuchAlgorithmException;
  
  import javax.crypto.Mac;
  import javax.crypto.spec.SecretKeySpec;
  
  /**
   * Implementation of NIST SP800-108
   * "Recommendation for Key Derivation Using Pseudorandom Functions"
   * Hardcoded:
   * [PRF=HMAC_SHA256]
   * [CTRLOCATION=BEFORE_FIXED]
   * [RLEN=32_BITS]
   * L = 256
   * L suffix: 32 bits
   */
  class SP800Derive {
      private final byte[] mKeyBytes;
  
      SP800Derive(byte[] keyBytes) {
          mKeyBytes = keyBytes;
      }
  
      private Mac getMac() {
          try {
              final Mac m = Mac.getInstance("HmacSHA256");
              m.init(new SecretKeySpec(mKeyBytes, m.getAlgorithm()));
              return m;
          } catch (InvalidKeyException | NoSuchAlgorithmException e) {
              throw new RuntimeException(e);
          }
      }
  
      private static void update32(Mac m, int v) {
          m.update(ByteBuffer.allocate(Integer.BYTES).putInt(v).array());
      }
  
      /**
       *  Generate output from a single, fixed input.
       */
      public byte[] fixedInput(byte[] fixedInput) {
          final Mac m = getMac();
          update32(m, 1); // Hardwired counter value
          m.update(fixedInput);
          return m.doFinal();
      }
  
      /**
       * Generate output from a label and context. We add a length field at the end of the context to
       * disambiguate it from the length even in the presence of zero bytes.
       */
      public byte[] withContext(byte[] label, byte[] context) {
          final Mac m = getMac();
          // Hardwired counter value: 1
          update32(m, 1); // Hardwired counter value
          m.update(label);
          m.update((byte) 0);
          m.update(context);
          update32(m, context.length * 8); // Disambiguate context
          update32(m, 256); // Hardwired output length
          return m.doFinal();
      }
  }
  