code: public boolean isValidSignature(final Key key) {
          final byte[] computedHmac = key.sign(getVersion(), getTimestamp(), getInitializationVector(),
                  getCipherText());
          return Arrays.equals(getHmac(), computedHmac);
      }
docs: Recompute the HMAC signature of the token with the stored shared secret key.
  
  @param key
  the shared secret key against which to validate the token
  @return true if and only if the signature on the token was generated using the supplied key