World 1 · Substitution · 7 minAffine: multiply, then shift
E(x) = a·x + b (mod 26)
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Number the letters A=0 … Z=25. The affine cipher multiplies each number by a, adds b, and wraps round 26:
E(x) = (a·x + b) mod 26
The Caesar shift is the special case a = 1. Multiplying scatters the letters, so neighbours in the alphabet end up far apart.
Check yourself
How many values of a can be undone?
Check yourself
Why is a = 13 forbidden?
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