Entropy is a measure of complexity or disorder of a signal. The interesting part is that the disorder is with respect to the proper basis or dictionary. Something can look complex in one encoding but be low entropy in the right encoding. You need to know the right basis, or figure it out from the context, to accurately determine the entropy of a signal. A much stronger way of building a tool like the OP is to have a few pre-computed dictionaries for a range of typical source texts (source code, natural language), then encode the string against each dictionary, comparing the compressibility of the string. A high entropy string like a secret will compress poorly against all available dictionaries.