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Thanks for your implementation. It was a helpful reference when I needed to implement Sig4 in CPP.
However there's a bug in your signature calculation code. When dealing with binary data, which can contain null characters, using strlen gives incomplete size. This was the source of a bug which took a lot of time to debug. I kept getting signature wrong responses from AWS.
Also, copy to c_string is unncessary. Strcpy is also slow. And conversion to hex can be done in a cleaner CPP style. If you don't want to incorporate these CPP style changes, I can remove them. Please let me know.
A reference for the null char issue https://stackoverflow.com/a/41601493/2382819