Exponentiation Using Canonical Recoding

The canonical bit recoding technique can be used to reduce the average numb e r o f m ultiplications required to compute X E provided that X ,1 is supplied along with X. We model the generation of the digits of the canonical recoding D of an n-bit long exponent E as a Markov c hain, and show that binary, quaternary, and octal methods applied to D require 4 3 n, 4 3 n, and 23 18 n multiplications, compared to 3 2 n, 11 8 n, and 31 24 n required by these methods applied to E. We show that in general the canonically recoded m-ary method for constant m requires fewer multiplications than the standard m-ary method. However, when m is picked optimally for each method for a given n, then the average number of multiplications required by the standard method is fewer than those required by the recoded version.