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Caesar Cipher

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.

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Caesar Cipher -

Tag(s) : Cryptography, Substitution Cipher

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# Caesar Cipher

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## Caesar Encoder

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.

### How to encrypt using Caesar cipher?

Encryption with Caesar code is a simple substitution (one letter replaces another). Caesar code replaces each letter with an alphabet shift: a letter further in the alphabet.

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.

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: D=4, 4+3=7 and 7=G, so D is crypted with G.
X=23, 23+3=26 and 26 mod 26 = 0, 0=A, so X is crypted with A, etc.
DCODEX is coded GFRGHA

### How to decrypt Caesar cipher?

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: G=7, 7-3=4 and 4=D, so G is decrypted with D
A=0, 0-3=-3 and -3 mod 26 = 23, 23=X, so A is decrypted with X, etc.
GFRGHA is decrypted DCODEX

### How to recognize Caesar ciphertext?

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.

### How to decipher Caesar without knowing the shift?

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

### What are the variants of the Caesar cipher?

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.

### How to encrypt digits and numbers using Caesar cipher?

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.

### Why the name Caesar Cipher?

Caesar used this technique for some correspondences, especially military, for example with Cicerone (shift of 3).

### What is August Cipher?

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

### What are other Caesar Cipher names?

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.

### How to cipher CAESAR with the Caesar code?

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

### How to write Caesar Cipher in pseudo-code?

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

### When Caesar Cipher have been invented ?

Unknown, but Caesar is born in 100 before JC