First, we consider a particularly simple implementation of a simple substitution
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First, we consider a particularly simple implementation of a simple substitution cipher. In the simplest case, the message is encrypted by substituting the letter of the alphabet n places ahead of the current letter. For example, with n = 3, the substitution-which acts as the key-is plaintext: a b c d e f g h i j k l m n o p q r s t u v W x y z ciphertext: D E F G H I J K L M N O P Q R S T U V W X Y Z A B C where we've followed the convention that the plaintext is lowercase, and the ciphertext is uppercase. In this example, the key could be given succinctly as "3" since the amount of the shift is, in effect, the key. Using the key 3, we can encrypt the plaintext message fourscoreandsevenyearsago by looking up each plaintext letter in the table above and then substituting the corresponding letter in the ciphertext row, or by simply replacing each letter by the letter that is three positions ahead of it in the alphabet. For the particular plaintext in, the resulting ciphertext is IRXUVFRUHDAGVHYHABHDUVDIR. To decrypt this simple substitution, we look up the ciphertext letter in the ciphertext row and replace it with the corresponding letter in the plaintext row, or we can shift each ciphertext letter backward by three. The simple substitution with a shift of three is known as the Caesar's cipher. Recover the plaintext and key for the simple substitution example that appears in (2.2) on page 24. PBFPVYFBQXZTYFPBFEQJHDXXQVAPTPQJKTOYQWIPBVWLXTOXBTFXQWA XBVCXQWAXFQJVWLEQNTOZQGGQLFXQWAKVWLXqWAEBIPBFXFQVXGTVJV WLBTPQWAEBFPBFHCVLXBQUFEVWLXGDPEQVPQGVPPBFTIXPFHXZHVFAG FOTHFEFBQUFTDHZBQPOTHXTYFTODXQHFTDPTOGHFQPBQWAQJJTODXQH FOQPwithDHHIXQVAPBFZQHCFWPFHPBFIPBQWKFABVYYDZBOTHPBQPQJT QOTOGHFQAPBFEQJHDXXQVAVXEBQPEFZBVFOJIWFFACFCCFHQWAUVWFL QHGFXVAFXQHFUFHILTTAVWAFFAWTEVOITDHFHFQAITIXPFHXAFQHEFZ QWGFLVWPTOFFAExplanation / Answer
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