Shift Cipher

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.

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).

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

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.

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.

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.