Entropy test vs frequency analysis I'm currently doing the cryptopals challenges and for it's first set of problems it tells you to use frequency analysis of occurring letters to determine the likelihood of a key guess being correct and yielding legible english. 

I was wondering, would calculating the entropy of the resulting plaintext not serve the same purpose, if not to a better degree seeing as it's not confined to the english language? Are there any benefits or drawbacks that one approach has over the other I'm missing?
I was wondering, would calculating the entropy of the resulting plaintext not serve the same purpose, if not to a better degree seeing as it's not confined to the english language?


First note that a cipher is susceptible to frequency analysis if it doesn't change the frequency at which characters occur. That is the "probability" of each substituted letter occuring is the same as the probability of the cleartext language. Now remember how the entropy is usually computed:

$$H(X)=-\sum_{x_i\in X} \Pr[X=x_i]\cdot \log_2(\Pr[X=x_i])$$

while replacing the probability terms for the measured approximation. Now as you can see entropy doesn't care where each probability value sits, if the same set of probabilities occurs you'll get the same entropy. And the set of probabilities is the same for each key for a given ciphertext susceptible to frequency analysis.

So "no", entropy will not help you here.