Revisiting this code snippet a few years later, I'm so glad it's been useful to other AWS integrators!
My link to the AWS docs is broken, so it may be nonobvious that the stringToSign, targetSignature, and secretKey were all examples from AWS. You can find fresh examples in Task 2 and Task 3 of the current AWS signing docs: https://docs.aws.amazon.com/general/latest/gr/sigv4_signing.html. I'm keeping the code above as-is for posterity.
For anyone who'd like to use this code, I'm releasing it into the public domain with The Unlicense license:
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