Caesar cipher

The Caesar cipher is the earliest documented encryption technique. Named after Julius Caesar, who used it to correspond with his generals by shifting every letter of his message three positions forward in the alphabet, it sits in the family of monoalphabetic substitution ciphers — and happens to be the simplest one to communicate: a single integer is all you need.

Principle

Every plaintext letter is replaced by the one sitting N positions later in the alphabet, where N is the key (also called the shift). The alphabet wraps around: after Z, you come back to A. Decrypting is applying a shift of −N.

Formally, for a 26-letter alphabet indexed from 0 to 25:

Encrypt : C(x) = (x + N) mod 26
Decrypt : D(y) = (y − N) mod 26

With N = 3 (the historical Caesar shift), the mapping table is:

plain : A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
cipher : D E F G H I J K L M N O P Q R S T U V W X Y Z A B C

Example

With N = 3, encrypting CIPHERCHRONICLE:

C → F    H → K    R → U    I → L
I → L    E → H    O → R    C → F
P → S    R → U    N → Q    L → O
H → K    C → F    I → L    E → H

Result: FLSKHUFKURQLFOH. The demo below replays this encryption step by step in a 5×5 grid.

Close variants

• ROT-13 — a special case with N = 13. Since 26 / 2 = 13, the cipher is involutive: encrypting and decrypting are the same operation.
• ROT-5, ROT-18, ROT-47 — extensions covering digits or the full printable ASCII range.
• Progressive Caesar — the shift grows at every letter (N, N+1, N+2…). A direct ancestor of the Trithemius cipher.
• Keyword shift — the shift is no longer constant but driven by a word, which leads straight to Vigenère.

Strengths and weaknesses

Strengths

• Extremely easy to memorize and apply by hand.
• Requires no equipment — a paper strip or a cipher disk is enough.
• Pedagogically powerful: it’s the first cipher we teach because it cleanly illustrates the concept of a key.

Weaknesses

• Only 25 useful keys (shift 0 means no encryption). Brute-forcing all of them takes seconds.
• Frequency analysis cracks it even without brute force: the most common ciphertext letter almost certainly stands for E in English or French.
• Zero resilience against a modern attacker. Reserve it for games, puzzles and teaching.

How to break it by hand

1. Count how often each letter appears in the ciphertext.
2. Spot the most frequent letter — it very likely encodes E (or A in Latin).
3. Compute the distance between that letter and E. That’s a candidate key N.
4. Apply the inverse shift and check whether the output reads as natural text.

On short texts you can also try all 25 shifts and keep the one that yields an intelligible result — instant by hand.

In CipherChronicle

The Caesar cipher is the pedagogical entry point of the platform. The first public grids use N = 3 (the classic) or N = 13 (ROT-13) to ease readers into the idea of a key without giving the whole game away at once.