Perplexity: Unknown!
Lower perplexity is better.
Perplexity measures how many incorrect characters you guess for every correct one. n perplexity is equivalent to a dice with n sides, where each is a character, and only one is correct. In other words, it means you can narrow the number of possible characters down to about n, on average.
Credit to namespace for the idea. Code available here. MIT License. Feel free to modify!