code: protected byte[] newSigningKey(AWSCredentials credentials,
              String dateStamp, String regionName, String serviceName) {
          byte[] kSecret = ("AWS4" + credentials.getAWSSecretKey())
                  .getBytes(Charset.forName("UTF-8"));
          byte[] kDate = sign(dateStamp, kSecret, SigningAlgorithm.HmacSHA256);
          byte[] kRegion = sign(regionName, kDate, SigningAlgorithm.HmacSHA256);
          byte[] kService = sign(serviceName, kRegion,
                  SigningAlgorithm.HmacSHA256);
          return sign(AWS4_TERMINATOR, kService, SigningAlgorithm.HmacSHA256);
      }
docs: Generates a new signing key from the given parameters and returns it.