Chiffrement affine (Affine cipher)

$\EuScript{P}$= $\EuScript{C}$= $\frac{\mathbb{Z}}{26\mathbb{Z}}$

$\EuScript{K} = \frac{\mathbb{Z}}{26\mathbb{Z}}^{*} \times \frac{\mathbb{Z}}{26\mathbb{Z}}$

$\forall K = (a, b) \in \EuScript{K},\; x\in \EuScript{P},\; E_{K}(x) = ax + b = y,\; D_{K}(y) = a^{-1}(x - b) = x$

vérifions (1) : $D_{K}oE_{K}(x) = a^{-1}((ax + b) - b) = x$