Hacking Secret Ciphers with Python

196 http://inventwithpython.com/hacking

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MODULAR ARITHMETIC WITH

THE MULTIPLICATIVE AND

AFFINE CIPHERS

Topics Covered In This Chapter:
 Modular Arithmetic
 “Mod” is “Remainder Of”(Sort Of)
 GCD: Greatest Common Divisor (aka Greatest Common Factor)
 Multiple Assignment Trick
 Euclid’s Algorithm for Finding the GCD of Two Numbers
 “Relatively Prime”
 The Multiplicative Cipher
 Finding Modular Inverses
 The cryptomath Module

“People have been defending their own privacy for centuries

with whispers, darkness, envelopes, closed doors, secret

handshakes, and couriers. The technologies of the past did

not allow for strong privacy, but electronic technologies do.”

Eric Hughes, “A Cypherpunk's Manifesto”, 1993