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  package org.apache.cayenne.crypto.transformer.bytes;
  
  import java.security.InvalidKeyException;
  import java.security.Key;
  import java.security.NoSuchAlgorithmException;
  import javax.crypto.Mac;
  
  import org.apache.cayenne.CayenneRuntimeException;
  
  /**
   * Actual authentication code generation logic that is used
   * by both {@link HmacEncryptor} and {@link HmacDecryptor}.
   *
   * @since 4.0
   */
  abstract class HmacCreator {
  
      /**
       * Default algorithm for authentication code creation.
       */
      public static final String DEFAULT_HMAC_ALGORITHM = "HmacSHA256";
  
      private Header header;
      private Mac mac;
  
      HmacCreator(Header header, Key key) {
          this.header = header;
          try {
              // Currently algorithm is hardcoded, but can be easily transformed into configurable parameter
              mac = Mac.getInstance(DEFAULT_HMAC_ALGORITHM);
              mac.init(key);
          } catch (NoSuchAlgorithmException nsae) {
              throw new CayenneRuntimeException("Algorithm %s not supported for HMAC generation", nsae, DEFAULT_HMAC_ALGORITHM);
          } catch (InvalidKeyException ike) {
              throw new CayenneRuntimeException("Invalid key for HMAC generation", ike);
          }
      }
  
      byte[] createHmac(byte[] input) {
          byte[] rawHeader = new byte[header.size()];
          header.store(rawHeader, 0, header.getFlags());
          mac.update(rawHeader);
          return mac.doFinal(input);
      }
  }
  