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  package org.apache.cayenne.crypto.transformer.bytes;
  
  import java.security.Key;
  import java.util.Arrays;
  
  import org.apache.cayenne.crypto.CayenneCryptoException;
  
  /**
   * This class not only parse HMAC but also verifies it
   * and throws {@link org.apache.cayenne.crypto.CayenneCryptoException} in case it is invalid.
   *
   * @since 4.0
   */
  class HmacDecryptor extends HmacCreator implements BytesDecryptor {
  
      BytesDecryptor delegate;
  
      HmacDecryptor(BytesDecryptor delegate, Header header, Key key) {
          super(header, key);
          this.delegate = delegate;
      }
  
      @Override
      public byte[] decrypt(byte[] input, int inputOffset, Key key) {
          byte hmacLength = input[inputOffset++];
          if(hmacLength <= 0) {
              throw new CayenneCryptoException("Input is corrupted: invalid HMAC length.");
          }
  
          byte[] receivedHmac = new byte[hmacLength];
          byte[] decrypted = delegate.decrypt(input, inputOffset + hmacLength, key);
          byte[] realHmac = createHmac(decrypted);
  
          System.arraycopy(input, inputOffset, receivedHmac, 0, hmacLength);
          if(!Arrays.equals(receivedHmac, realHmac)) {
              throw new CayenneCryptoException("Input is corrupted: wrong HMAC.");
          }
          return decrypted;
      }
  }
  