17. A computer system comprising: a computer processor; and a computer-readable storage medium comprising instructions executable by the processor and when executed performing actions comprising: identifying sub-string repetition in a pass phrase; generating a sub-string index; determining the number of bits required to represent each character in the pass phrase; assigning an entropy value to each character in the pass phrase in accordance with a probability function that assigns highest probabilities at boundaries of an interval for the entropy of a character; and computing a total entropy value for the pass phrase.
17. A computer system comprising: a computer processor; and a computer-readable storage medium comprising instructions executable by the processor and when executed performing actions comprising: identifying sub-string repetition in a pass phrase; generating a sub-string index; determining the number of bits required to represent each character in the pass phrase; assigning an entropy value to each character in the pass phrase in accordance with a probability function that assigns highest probabilities at boundaries of an interval for the entropy of a character; and computing a total entropy value for the pass phrase. 18. The computer system of claim 17 , wherein the generating comprises replacing duplicate sub-strings in the sub-string index with sub-string references.