The Code Book: The Science of Secrecy from Ancient Egypt to Quantum Cryptography

by Simon Singh

However, there is a neat trick for calculating exponentials in modular arithmetic. We know that, since 7 = 4 + 2 + 1, 887 (mod 187) = [884 (mod 187) × 882 (mod 187) × 881 (mod 187)] (mod 187)