The value is the cipher key. For example, the code IBDLBEBZ is encrypted with a Caesar Cipher. It doesn’t take very much to see that a shift of “1” would reveal HACKADAY.

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So, for example, if you read an entire paragraph and notice that the letter D appears more often than any other, odds are that it used a Caesar cipher with a shift of 1 (making E a D in the code).