The Caesar / Rot-13 encryption algorithm (Menu Crypt/Decrypt \ Symmetric (classic))

This encryption algorithm invented by Caesar only shifts the letters of the alphabet.

The key for the Caesar encryption algorithm is entered in the Key entry Caesar / Rot-13 dialog.

Characters which are not part of the alphabet are normally left out of the encrypted text, so that after decryption these characters have to be inserted from the textual context. This applies to both punctuation characters and special characters (especially spaces between two words). The menu selection Options \ Text Options is used to specify whether punctuation and special characters should be retained during encryption and decryption. Which characters build up the alphabet can be selected in the Text Options dialog.

Under the Caesar encryption algorithm, one character from the alphabet is used as a key. Depending on the position of the letter, the letters of the plaintext are shifted in repeated cycles (the sequence of characters in the alphabet to be used is specified in the Text Options dialog). If the alphabet consists only of upper case letters (this is the classical case), a cyclical shift of one character takes place when "A" is entered, two characters for "B" ... 26 characters for "Z". To decrypt the text, the encrypted text is simply shifted in the other direction.

Example (classical case, alphabet of 26 letters, first letter A has the value 1, key is C):

If the key is "C", all the letters are cyclically shifted by three places.

Plaintext Encrypted Text

A D
B E
C F
... ...
W Z
X A
Y B
Z C

Remark:
The letter "Z" should not be used as the key, as in this case the "encrypted" text is identical to the plaintext.

A special case of Caesar is the so called Rot-13 encryption, which is used in chat rooms and news groups. Rot-13 shifts each character by 13 places, corresponding to a Caesar encryption with key "M". Repeated twice Rot-13 yields the plain text again.

An advancement of the Caesar cipher is the Vigenère cipher.

The Caesar encryption algorithm can be broken relatively easily with a Ciphertext Only attack (see Automatic analysis, Caesar encryption algorithm). An example of this will be found in the Examples chapter.