Caesarcipher

The Caesar cipher is a simple substitution cipher in which each letter of the plaintext is shifted a fixed number of places down or up the alphabet. It is a monoalphabetic cipher, meaning each plaintext symbol maps to a single ciphertext symbol using a constant shift, called the key.

Historically attributed to Julius Caesar, it is often described as using a shift of three, though any value from 1 to 25 could be used. The method can be applied to the standard 26-letter Latin alphabet, with non-letter characters typically left unchanged or treated separately. To encrypt, every letter is replaced by the letter a fixed number of positions ahead; to decrypt, the process is reversed.

Mathematically, if A=0, B=1, ..., Z=25, the encryption is E(x) = (x + k) mod 26 and the decryption is D(y) = (y − k) mod 26, where k is the shift and wraparound is allowed.

An example with k = 3: the plaintext ATTACK AT DAWN becomes DWWDFN DW GDZQ. This illustrates both the simple rule and how spaces are preserved for readability in many implementations.

Security and limitations: the Caesar cipher offers only 26 possible keys, making brute-force decryption trivial. It is also vulnerable to frequency analysis and crib-based attacks because the same substitution applies throughout. Consequently, it provides no real security for modern communications and is mainly used for educational purposes and in puzzles.

Variants include ROT-n, where n is any fixed shift, and other fixed-shift schemes on different alphabets. The Caesar cipher remains a foundational example in the study of cryptography.