Caesar Cipher

The Caesar Cipher is a classical substitution cipher named after Julius Caesar, who reportedly used it to encrypt private correspondence. It shifts each letter in the plaintext by a fixed number of positions down the alphabet. This simplicity makes it easy to understand and implement, but also vulnerable to frequency analysis and brute-force attacks due to its limited keyspace.

The Caesar Cipher is a foundational example of substitution ciphers, conceptually related to more complex ciphers such as the Simple Substitution Cipher and the Affine Cipher, both of which generalize the idea of letter replacement.

Caesar Cipher: Encoding

Each plaintext letter is shifted by a fixed number of positions modulo 26. For example, with a shift of 3, encoding “HELLO”:

Plaintext: H  E  L  L  O
Shift:     +3 +3 +3 +3 +3
Ciphertext: K  H  O  O  R

Encoded Message: KHOOR

Caesar Cipher: Decoding

Decryption reverses the shift by subtracting the key modulo 26:

Ciphertext: K  H  O  O  R
Shift:      -3 -3 -3 -3 -3
Plaintext:   H  E  L  L  O

Decoded Message: HELLO

Caesar Cipher: Notes

Despite its simplicity, the Caesar Cipher illustrates the core concept of substitution. It can be combined with other systems or extended into polyalphabetic methods such as the Vigenère Cipher or numeric variations like the Cadenus–Gronsfeld Cipher. It remains a valuable educational tool for understanding classical encryption principles and the history of cryptography.