Unformatted text preview: Red with the plaintext shifted the length of the key. Since English has 1.3 bits of real information per byte (see Section 11.1), there is plenty of redundancy for determining a unique decryption. Despite this, the list of software vendors that tout this toy algorithm as being “almost as secure as DES” is staggering [1387]. It is the algorithm (with a 160bit repeated “key”) that the NSA finally allowed the U.S. digital cellular phone industry to use for voice privacy. An XOR might keep your kid sister from reading your files, but it won’t stop a cryptanalyst for more than a few minutes. 1.5 OneTime Pads
Believe it or not, there is a perfect encryption scheme. It’s called a onetime pad, and was invented in 1917 by Major Joseph Mauborgne and AT&T’s Gilbert Vernam [794]. (Actually, a onetime pad is a special case of a threshold scheme; see Section 3.7.) Classically, a onetime pad is nothing more than a large nonrepeating set of truly random key letters, written on sheets of paper, and glued together in a pad. In its original form, it was a onetime tape for teletypewriters. The sender uses each key letter on the pad to encrypt exactly one plaintext character. Encryption is the addition modulo 26 of the plaintext character and the onetime pad key character. Each key letter is used exactly once, for only one message. The sender encrypts the message and then destroys the used pages of the pad or used section of the tape. The receiver has an identical pad and uses each key on the pad, in turn, to decrypt each letter of the ciphertext. The receiver destroys the same pad pages or tape section after decrypting the message. New message—new key letters. For example, if the message is: ONETIMEPAD and the key sequence from the pad is TBFRGFARFM then the ciphertext is IPKLPSFHGQ because O + T mod 26 = I N + B mod 26 = P E + F mod 26 = K etc. Previous Table of Contents Next Products  Contact Us  About Us  Privacy  Ad Info  Home Use of this site is subject to certain Terms & Conditions, Copyright © 19962000 EarthWeb Inc. All rights reserved. Reproduction whole or in part in any for...
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 Fall '10
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 Cryptography, Bruce Schneier, Applied Cryptography, EarthWeb, Search Search Tips

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