Codes and Ciphers :: Onetime PadThe onetime pad is a long sequence of random letters. These letters are combined with the plaintext message to produce the ciphertext. To decipher the message, a person must have a copy of the onetime pad to reverse the process. A onetime pad should be used only once (hence the name) and then destroyed. This is the first and only encryption algorithm that has been proven to be unbreakable. To encipher a message, you take the first letter in the plaintext message and add it to the first random letter from the onetime pad. For example, suppose you are enciphering the letter S (the 19th letter of the alphabet) and the onetime pad gives you C (3rd letter of the alphabet). You add the two letters and subtract 1. When you add S and C and subtract 1, you get 21 which is U. Each letter is enciphered in this method, with the alphabet wrapping around to the begining if the addition results in a number beyond 26 (Z). To decipher a message, you take the first letter of the ciphertext and subtract the first random letter from the onetime pad. If the number is negative you wrap around to the end of the alphabet. Exampleplaintext : SECRETMESSAGE onetime pad: CIJTHUUHMLFRU ciphertext : UMLKLNGLEDFXY Table of Contents

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