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Algorithm. Fast exponentiation modulo m.

Input:	an integer b and natural numbers n and m greater than 1
Output:	bn mod m

1. let a := 1
2. let c := b
3. repeat
   1. let r := n mod 2
   2. if r=1 then let a := (a ⋅ c) mod m
   3. let n := n div 2
   4. let c := (c ⋅ c) mod m
4. until n = 0
5. return a