@Override
    public byte[] computeMac(byte[] data) throws GeneralSecurityException {
      return prfHmac.compute(data, outputLength);
    }
@param serializedKeyFormat serialized {@code HmacKeyFormat} proto
@return {@code KeyData} with a new {@code HmacKey} proto
java_src/src/main/java/com/google/crypto/tink/mac/internal/LegacyHmacTestKeyManager.java