code: private static byte[] extract(byte[] salt, byte[] inputKeyingMaterial) {
          return initHmacSha256(salt == null ? NULL_SALT : salt).doFinal(inputKeyingMaterial);
      }
docs: The "extract" stage of the extract-then-expand HKDF algorithm.
  
  @param salt an optional salt value. If null, will use the all-zeros constant NULL_SALT.
  @param inputKeyingMaterial input keying material. Must not be null.
  @return a pseudo-random key of length SHA256_OUTPUT_BYTES to be used by the expand() stage.