One of the methods used to encrypt a message is to use the message itself. We begin by providing a secret number that is used for the offset of the first letter and is known only to sender and receiver. This starting value is called a seed. For example, if we encode the word, CAT, we begin by finding the alphabetic value of each letter: 3, 1 and 20; and then use the value of the previous letter as an offset, with the first letter using the seed value.
Suppose the seed is 5:
C (3) (3 + 5[seed] = 8) H
A (1) (1 + 3[1st letter] = 4) D
T (20) (20 + 1[2nd letter] = 21) U
So the encoded message is HDU. It should be clear how, using the seed 5, it is possible to decode it easily.
Decode this message, without being told the seed:
X B M W W N B G C U A O O M W N W H C V R Y L S W A Q