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In The Beginning

Problem

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) maps (3 + 5[seed] = 8) H
A (1) maps (1 + 3[1st letter] = 4) D
T (20) maps (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.

However, your task is to decode the following 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

Good luck!

Problem ID: 5 (Aug 2000)     Difficulty: 1 Star

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