EMILE : Enseignement d'une Matière Intégrée à une Langue Étrangère

Shift Cipher – Substitution Cipher

 

CaesarCaesar 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.

 

Example:

Plain alphabet

A

B

C

D

E

F

G

Number

1

2

3

4

5

6

7

key=4

5

6

7

8

9

10

11

Caesar alphabet

E

F

G

H

I

J

K

WheelMessage #1 : key=4

GSRKVEXYPEXMSRW JMVWX QIWWEKI HIGVCTXIH!

How to crack a Caesar cipher or any shift cipher?

The Caesar cipher is one of the simplest encryption algorithms in which every Latin letter of a given string is simply shifted cyclically by a certain offset.

For cracking the encryption, we could iterate over all opportunities and as our alphabet uses just 26 Latin letters, we would obtain the decrypted string in at most 25 tries, which is quite trivial.

The main idea to crack the encryption is on base of an analysis of the quantity of every letter and to obtain the used key this way. This is possible, because our alphabet doesn't have a uniform distribution of every single letter.

Message #2

BGW KTASSB ZTEADT AF TVHTKM OF EKBHMGUKAHIB.

Table of frequencies of every single letter of our alphabet in English language:

 Frequencies Message
 Frequencies English

Message #3

Message final

 

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