P is a finite set of possible plaintexts;

E is a finite set of possible ciphertexts;

K is a finite set of possible keys;

For each k ∈ K, there is an encryption rule ek ∈ ε corresponding decryption rule dk ∈ D. each ek: P→ E and dk: E → P are functions such that: d_k(e_k(x)) = x for every element plain text x ∈ P.

P = E = K = [0:26]

for k in K define: e_k(x) = (x + k) % 26 and d_k(y) = (y - k) % 26

example: wewillmeetatmidnight and k = 11 encrypt to hphtwwxppelextoytrse