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Shift Cipher

Tool for automatic Shift Cipher that consists in replacing a letter by another one in the alphabet using a shifting system, a basic or complex sequence of numbers.

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Shift Cipher -

Tag(s) : Substitution Cipher

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# Shift Cipher

## Successive Shifts Encoder

### What is a Shift cipher? (Definition)

The shift cipher is a cryptographic substitution cipher where each letter in the plaintext is replaced by a letter a certain number of positions further down the alphabet. This number of positions is sometimes called a key.

The Caesar code is the most well-known shift cipher, usually presented with a shift key of value 3.

### How to encrypt using the shift cipher?

The shift cipher encryption uses an alphabet and a key (made up of one or more values) that shifts the position of its letters.

A letter in position $N$ in the alphabet, can be shifted by $X$ into the letter located at position $N+X$ (This is equivalent to using a substitution with a shifted alphabet).

Example: Take the letter E in position 5 in the alphabet ABCDEFGHIJKLMNOPQRSTUVWXYZ, it will be encrypted by a shift of 3 in position 8 or H.

If the shifted position exceeds the number of letters in the alphabet, then take it at the beginning (imagine the alphabet as cyclical)

Example: Z shifted by 1 gives A.

Traditional shift encryption only applies to the 26 letters of the alphabet. For numbers or special characters, it is possible to change the alphabet or leave them unchanged.

### What are the different possible shifts?

It is possible to define different types of shifts, some shifts correspond to known encryption algorithms:

A single shift (all letters are shifted by the same value) is called Caesar Code.

A multiple shift, according to a sequence or a key that is repeated (the letters are shifted from each of the key values), is called Vigenere Cipher.

A mathematical shift, the easier is progressive, shifting the nth letter of the value n is the Trithemius Cipher or if the shift is more complex Affine Cipher or even Hill Cipher.

In addition, each offset can be applied to a single or several letters, to a single or several words, etc.

### How to decrypt using the shift cipher?

Decryption requires knowing the shift used and the alphabet.

Take a letter in position N in the alphabet that has been encrypted by a shift of X, it must be shifted by -X to return to its original position N-X.

Example: The letter H in position 8 in the alphabet ABCDEFGHIJKLMNOPQRSTUVWXYZ, will be decrypted from a shift of 3 in position 8-3=5 or E.

Example: The word TIJGU is decoded with an offset of 1 as SHIFT

### How to use a date as the key shift?

The shift cipher can take a date as key (called date shift cipher), generally in the formats YYYYMMDD is used as it contains a series of 8 digits which can be used as the shift key.

Example: DATECODE encrypted with the date 2020/10/10 or (2,0,2,0,1,0,1,0) becomes FAVEDOEE

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