Caesar Cipher

The Caesar cipher (or Caesar code) 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 Caesar code is therefore sometimes called a rotation cipher.

Encryption with Caesar code is based on an alphabet shift. The most commonly used shift/offset is by 3 letters such that A becomes D.

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.

Caesar code decryption replaces a letter another with an inverse alphabet shift: a previous letter in the alphabet.

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.

A message encoded with the Caesar cipher 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).

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) or 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

— Shift the numbers with the same shift as the letters.

— Integrate numbers in the alphabet.

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 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

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 < 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.