@Override
  public void readFields(DataInput in) throws IOException {
    id = WritableUtils.readVInt(in);
    expirationDate = WritableUtils.readVLong(in);
    int keyLength = WritableUtils.readVInt(in);
    if (keyLength < 0) {
      secret = null;
    } else {
      byte[] keyBytes = new byte[keyLength];
      in.readFully(keyBytes);
      secret = AuthenticationTokenSecretManager.createSecretKey(keyBytes);
    }
  }
Represents a secret key used for signing and verifying authentication tokens by
{@link AuthenticationTokenSecretManager}.
hbase-server/src/main/java/org/apache/hadoop/hbase/security/token/AuthenticationKey.java