The (or ) is a monoalphabetic substitution cipher, where each letter is replaced by another letter located a little further in the alphabet (therefore shifted but always the same for given cipher message).
The shift distance is chosen by a number called the offset, which can be right (A to B) or left (B to A).
For every shift to the right (of +N), there is an equivalent shift to the left (of 26-N) because the alphabet rotates on itself, the is therefore sometimes called a rotation cipher.
Encryption with is based on an alphabet shift. The most commonly used shift/offset is by 3 letters such that A becomes D .
| Plain Alphabet | ABCDEFGHIJKLMNOPQRSTUVWXYZ |
| Caesar Alphabet (🠜3) | DEFGHIJKLMNOPQRSTUVWXYZABC |
Example: Crypt DCODEX with a shift of 3 .
To encrypt D , take the alphabet and look 3 letters after: G . So D is encrypted with G .
To encrypt X , loop the alphabet: after X : Y , after Y : Z , after Z : A . So X is coded A .
DCODEX is coded GFRGHA
Another way to crypt, more mathematical, is to encode any letter x by (x+3) mod 26 .
Note A=0 , B=1 , …, Z=25 , and add a constant (the shift), then the result modulo 26 (alphabet length) is the coded text.
Example: To crypt D (of value 3 ), add the shift 3 : 3+3=6 and find the letter for 6 : 6=G , so D is crypted with G .
To encrypt X=23 , 23+3=26 and 26 mod 26 = 0 , 0=A , so X is encrypted with A , etc.
DCODEX is coded GFRGHA
decryption replaces a letter another with an inverse alphabet shift: a previous letter in the alphabet.
Example: Decrypt GFRGHA with a shift of 3.
To decrypt G , take the alphabet and look 3 letters before: D . So G is decrypted with D .
To decrypt X , loop the alphabet: before A : Z , before Z : Y , before Y : X . So A is decrypted X .
GFRGHA is decrypted DCODEX .
Another way to decrypt, more mathematical, note A=0 , B=1 , …, Z=25 , subtracts a constant (the shift), then the result modulo 26 (alphabet length) is the plain text.
Example: Take G=6 , subtract the shift 6-3=3 and 3=D , so G is decrypted with D
Take A=0 , 0-3=-3 and -3 mod 26 = 23 , 23=X , so A is decrypted with X , etc.
GFRGHA is decrypted DCODEX
A message encoded with the has constant shifts for each letter, so its frequency analysis diagram is shifted (by a number of letters equal to the shift).
The index of coincidence is equal to that of the plaintext (as with any substitution cipher).
Any reference to Caesar, general and emperor of Rome, his contemporaries (Cicero, Brutus, Cleopatra), or more generally to antiquity and the Roman Empire are clues.
Also, the presence of keywords like Julius (Iulius/Ivlivs), Ave, Augustus, or a (Caesar) salad can remind us of the Caesar imperator.
The English pronunciation of Caesar is close to seizure .
The easiest keyless/shiftless method consists of testing all shifts, if the alphabet has 26 letters, it takes only 25 tries (dCode does it automatically).
is best known with a shift of 3, all other shifts are possible. Some shifts are known with other cipher names.
Another variant changes the alphabet and introduces digits for example.
A with an offset of N corresponds to an Affine cipher Ax+B with A=1 and B=N .
Caesar is sometimes written Cesar (in French) or Ceaser (bad typography).
is applicable only to letters of the alphabet. There are, however, several solutions to crypt numbers:
— Write the numbers in Roman numerals, the numbers becoming letters, it is enough to encode them normally
Example: Nine becomes IX which becomes LA with a shift of 3.
— Shift the numbers with the same shift as the letters.
Example: 9 becomes 12 (shift of +3)
— Integrate numbers in the alphabet.
Example: With the alphabet ABCDEF123 , 21 becomes BA with an offset of 3.
Caesar (Caius Iulius Caesar) used this technique for some correspondences, especially military, for example with Cicerone (shift of 3).
However, it is possible that other civilizations also used it independently.
August Cipher is sometimes the name given to with a shift of 1.
is also known as Shift Cipher. This shifting property can be hidden in the name of Caesar variants, eg.:
CD code, C = D, the shift is 1
Hello (LO) code, L = O, the shift is 3
Jail (JL) code, J = L, the shift is 2
Ellen (LN) code, L = N, the shift is 2
Cutie (QT) code, Q = T, the shift is 3
Eiffel (FL) code, F = L, the shift is 6
WC code, W = C, the shift is 6
Empty (MT) code, M = T, the shift is 7
Baden Powell (scoutism founder), B = P, the shift is 14
Any (NE) code, N = E, the shift is 17
See You (CU) code, C = U, the shift is 18
I See (IC) code, I = C, the shift is 20
Easy (EZ) code, E = Z, the shift is 21
CEASAR (with a wrong spelling) where E=A or A=E, the shift is either +4 or -4 (=22)
Any 2-letter code that can give an association between a crypted char and the plain one (see gramograms)
ROT13 code, the shift is 13 and reversible
ROT5 code for digits, the shift is 5 and reversible
ROT47 code for ASCII printable characters, the shift is 47 and reversible
More generally ROT-N with N the shift, if N Ask a new question
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Caesar Cipher on dCode.fr [online website], retrieved on 2024-09-07,