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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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.

Encryption with Caesar code is a monoalphabetical substitution, ie. a same letter is replaced with only one other. Caesar code is defined on an alphabet shift: a letter further in the alphabet.

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 crypted 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 crypt X=23, 23+3=26 and 26 mod 26 = 0, 0=A, so X is crypted 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 de crypt, 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 shit) and a coincidence index similar to the one of the plain text.

The easiest method consist in testing all shifts, if the alphabet has 26 letters, it takes only 25 tries.

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 introduce 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 cipher is appliable 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 used this technique for some correspondences, especially military, for example with Cicerone (shift of 3).

August Cipher is 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 bc. the first man which has testimonys (like Suetonius) proving that he used this type of subtitution to protect his military communications. The exact date of creation and its real author are unknown.

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- 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 have been invented ?

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

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