Tool to decrypt/encrypt with Caesar. Caesar cipher (or Caesar code) is a shift cipher, one of the most easy and most famous encryption systems. It uses the substitution of a letter by another one further in the alphabet.

Caesar Cipher - dCode

Tag(s) : Substitution Cipher

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Encryption with **Caesar code** is based on an alphabet shift (move of letters further in the alphabet), it is a monoalphabetic substitution cipher, ie. a same letter is replaced with only one other (always the same for given cipher message). The most commonly used shift/offset is by 3 letters.

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, 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`

**Caesar code** 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 **Caesar cipher** has a shift in its frequency analysis diagram (equal to the selected shift) and a coincidence index similar to the one of the plain text.

Any reference to Caesar, emperor of Rome, or more generally to antiquity and the Roman Empire are clues.

Also, the presence of keywords like Julius (Iulius/Ivlivs), Brutus, Augustus, or a(Caesar) salad can remind us of the Caesar imperator.

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).

**Caesar cipher** 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 **Caesar cipher** 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) ou Ceaser (bad typography).

**Caesar cipher** 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).

August Cipher is sometimes the name given to **Caesar Cipher** with a shift of 1.

**Caesar cipher** 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

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 < 26 then the Latin alphabet is used, else it can be any other custom alphabet.

The 25 ways to cipher Caesar by itself: DBFTBS, ECGUCT, FDHVDU, GEIWEV, HFJXFW, IGKYGX, JHLZHY, KIMAIZ, LJNBJA, MKOCKB, NLPDLC, OMQEMD, PNRFNE, QOSGOF, RPTHPG, SQUIQH, TRVJRI, USWKSJ, VTXLTK, WUYMUL, XVZNVM, YWAOWN, ZXBPXO, AYCQYP, BZDRZQ

For N from 1 to Text Length Do

Take C = Nth character of Text

Calculate R = the rank of C in the alphabet

Calculate R2 = (R + Shift) Modulo 26

Write the letter with rank R2 in the alphabet

End For Loop

The code was named after Julius Caesar who was born in 100 BCE the first man which has testimonies (like Suetonius) proving that he used this type of substitution to protect his military communications. The exact date of creation and its real author are unknown.

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NB: for encrypted messages, test our automatic cipher identifier!

- Caesar Cipher Decoder
- Caesar Encoder
- How to encrypt using Caesar cipher?
- How to decrypt Caesar cipher?
- How to recognize Caesar ciphertext?
- How to decipher Caesar without knowing the shift?
- What are the variants of the Caesar cipher?
- How to encrypt digits and numbers using Caesar cipher?
- Why the name Caesar Cipher?
- What is August Cipher?
- What are other Caesar Cipher names?
- How to cipher CAESAR with the Caesar code?
- How to write Caesar Cipher in pseudo-code?
- When Caesar Cipher was invented?

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Source : https://www.dcode.fr/caesar-cipher

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