code: private static byte[] expand(byte[] prk, byte[] previousRoundResult, byte[] info, byte counter) {
          Mac mac = initHmacSha256(prk);
          mac.update(previousRoundResult);
          mac.update(info);
          mac.update(counter);
          return mac.doFinal();
      }
docs: The "expand" stage of the extract-then-expand HKDF algorithm. All arguments are required (may not be null).
  
  @param prk the pseudo-random key generated by the extract() stage.
  @param previousRoundResult the output of the previous expand() round. For the first round, it should be a
  zero-length byte array.
  @param info the info parameter to hkdf().
  @param counter the counter for the current expand stage.
  @return output keying material of length SHA256_OUTPUT_BYTES.