10. A computer-implemented method comprising: identifying sub-string repetition in a pass phrase; generating a sub-string index; determining a number of bits required to represent each character in the pass phrase; assigning, using a processor, an entropy value to each character in the pass phrase in accordance with a probability function that assigns lowest probabilities when a ratio of a number of pass phrase characters to pass phrase length is equal to a ratio of a size the interval to a sum of interval sizes; and computing a total entropy value for the pass phrase.
15. The computer-implemented method of claim 10 , further comprising executable instructions to provide an indication of pass phrase quality as an expression of an entropy percentage value.