code: private String createSignature(Credentials credentials, String secretKey) {
          return new SignatureGenerator().generate(
                  secretKey,
                  credentials.getMethod(),
                  credentials.getTimestamp(),
                  credentials.getPath(),
                  credentials.getContent());
      }
docs: Create a signature given the set of request credentials and a secret key.
  
  @param credentials the credentials specified on the request
  @param secretKey the secret key that will be used to generate the signature
  @return the signature