Shift Cipher

The shift cipher encryption uses an alphabet and 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.

It is thus possible to define different type 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), it is called Vigenere Cipher.

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

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 coded with the date 2020/10/10 or (2,0,2,0,1,0,1,0) becomes FAVEDOEE