Caesar cipher (also referred to as Caesar algorithm, Caesar shift, shift cipher, or as Simple Caesar) is a simple symmetric encryption method based on monographic and monoalphabetic substitution.
In encryption, each letter of the plaintext is mapped to a ciphertext letter. This mapping is obtained by cyclically shifting (rotating) the characters of an ordered alphabet to the right by a certain number; cyclic means that when shifting beyond Z, one continues counting again starting at A. The number of characters shifted forms the ciphertext letter. The number of shifted characters forms the key, which remains unchanged for the entire encryption.
The shift of the alphabet by N characters (N = 1…25) represents the key. Instead of specifying the shift as a number, it can also be specified as a key letter. There are two common conventions here:
- A corresponds to no shift, B to a shift of 1, C to a shift of 2, etc.
- A corresponds to a shift of 1, B to a shift of 2, C to a shift of 3, etc.
The first convention is obtained by reading the key letter from the two superimposed alphabets (“a” on “D” is a shift of three characters). This is also common in the related Vigenère cipher. In addition, this convention corresponds to the mapping from letter to number, as is usually done for modulo arithmetic.
The second convention corresponds to the natural numbering of the displacements. Suetonius makes no historical reference to the fact that Caesar and Augustus each used their initial letter as a key letter.
As both conventions contradict each other, the indication of a key letter is misleading, whereas the indication as a number is unambiguous.