code: protected final byte[] computeSignature(String stringToSign,
              byte[] signingKey, AWS4SignerRequestParams signerRequestParams) {
          return sign(stringToSign.getBytes(Charset.forName("UTF-8")), signingKey,
                  SigningAlgorithm.HmacSHA256);
      }
docs: Step 3 of the AWS Signature version 4 calculation. It involves deriving
  the signing key and computing the signature. Refer to
  http://docs.aws.amazon
  .com/general/latest/gr/sigv4-calculate-signature.html