title

private-key encryption scheme used by Julius Caesar to communicate with his generals, achieved by shifting each alphabet by some fixed amount (the key)
message space $M = {A, B, ..., Z} *$ and key space $K = {0, 1, ..., 25}$ :
Gen outputs a uniformly random key k from $K = {0, 1, ..., 25}$ .
Encryption shifts the alphabet of each letter in the plain-text by k: $E n c_{k} (m_{1} m_{2} ... m_{n}) = c_{1} c_{2} ... c_{n}, w h ere c_{i} = m_{i} + k mod 26$
Decryption shifts each letter back: $De c_{k} (c_{1} c_{2} ... c_{n}) = m_{1} m_{2} ... m_{n}, w h ere m_{i} = c_{i} + k mod 26$

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