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Modular arithmetic is arithmetic with wrap-around at a fixed modulus m, like numbers on a clock.
A modular inverse of a modulo m is a number a_inv such that a ร a_inv โก 1 (mod m).
Eulerโs Theorem says that if a and n are coprime, then a raised to the power ฯ(n) is congruent to 1 modulo n.
Fast exponentiation (binary exponentiation) computes a^n using repeated squaring in O(log n) multiplications.