public byte[] computePrimary(byte[] input, int outputLength) throws GeneralSecurityException {
    return getPrfs().get(getPrimaryId()).compute(input, outputLength);
  }
Convenience method to compute the primary PRF on a given input. See PRF.compute for details of
the parameters.
java_src/src/main/java/com/google/crypto/tink/prf/PrfSet.java