Linear monoalphabetic substitution cipher
📐 Encryption Formula
E(x) = (a × x + b) mod 26
D(y) = a¹ × (y − b) mod 26
Where a is coprime with 26, and a¹ is the modular multiplicative inverse of a mod 26
✅ gcd(5, 26) = 1, coprime with 26 ✓
Valid a values coprime with 26:
a¹ = 21 (because 5 × 21 ≡ 1 mod 26)