Réponse :
Explications étape par étape
Bonjour
A => 0 => 7 * 0 + 5 = 5 => F
B => 1 => 7 * 1 + 5 = 12 => M
C => 2 => 7 * 2 + 5 = 19 => T
D => 3 => 7 * 3 + 5 = 26 = 1 x 26 + 0 => 0 => A
E => 4 => 7 * 4 + 5 = 33 = 26 + 7 => 7 => H
F => 5 => 7 * 5 + 5 = 40 = 26 + 14 => 14 => O
G => 6 => 7 * 6 + 5 = 47 = 26 + 21 => 21 => V
H => 7 => 7 * 7 + 5 = 54 = 2 x 26 + 2 => 2 => C
I => 8 => 7 * 8 + 5 = 61 = 2 x 26 + 9 => 9 => J
J => 9 => 7 * 9 + 5 = 68 = 2 x 26 + 16 => 16 => Q
K => 10 => 7 * 10 + 5 = 75 = 2 x 26 + 23 => 23 => X
L => 11 => 7 * 11 + 5 = 82 = 3 x 26 + 4 => 4 => E
M => 12 => 7 * 12 + 5 = 89 = 26 x 3 + 11 => 11 => L
N => 13 => 7 * 13 + 5 = 96 = 3 x 26 + 18 => 18 => S
O => 14 => 7 * 14 + 5 = 103 = 3 x 26 + 25 => 25 => Z
P => 15 => 7 * 15 + 5 = 110 = 4 x 26 + 6 => 6 => G
Q => 16 => 7 * 16 + 5 = 117 = 4 x 26 + 13 => 13 => N
R => 17 => 7 * 17 + 5 = 124 = 4 x 26 + 20 => 20 => U
S => 18 => 7 * 18 + 5 = 131 = 5 x 26 + 1 => 1 => B
T => 19 => 7 * 19 + 5 = 138 = 5 x 26 + 8 => 8 => I
U => 20 => 7 * 20 + 5 = 145 = 5 x 26 + 15 => 15 => P
V => 21 => 7 * 21 + 5 = 152 = 5 x 26 + 22 => 22 => W
W => 22 => 7 * 22 + 5 = 159 = 6 x 26 + 3 => 3 => D
X => 23 => 7 * 23 + 5 = 166 = 6 x 26 + 10 => 10 => K
Y => 24 => 7 * 24 + 5 = 173 = 6 x 26 + 17 => 17 => R
Z => 25 => 7 * 25 + 5 = 180 = 6 x 26 + 24 => 24 => Y
b) crypter le mot CODE :
TZAH
c) décrypter :
OZSTIJZS FOOJSH
FONCTION AFFINE
2) g(x) = 2x + 3
a) coder le mot ANNÉE :
2 * 0 + 3 = 3 => D
2 * 13 + 3 = 26 + 3 => 3 => D
2 * 13 + 3 = 26 + 3 => 3 => D
2 * 4 + 3 = 11 => L
2 * 4 + 3 = 11 => L
On remarque que À et N représente là même lettre cryptée donc incorrecte
Donc il est impossible d’utiliser cette fonction affine pour crypter un code
