I know how to calculate the entropy of a key that relates to its selection process. For example if the key space is 1000, entropy of a randomly chosen key is 1000. Suppose now you have two keys $X$ ...
Given an entrophy of 1.5 bits per character, how long does a passphrase have the be to guarantee a key strength of a 128 bit random key? Assuming an alphabet of 26, one character takes log2(26)=4.7 ...