DES is a secret key crypto algorithm created to use 56-bit keys. 3-DES uses DES three times with one, two or three keys, effectively giving you a larger key size, while not changing the algorithm. DES works only with 56-bit keys. (What about 40-bit DES, you ask? It is 56-bit DES but the key is padded effectively giving you only 40 bits... really weak.) With crypto -- assuming the algorithm is a good algorithm -- we are concerned with the effectiveness of a "brute force attack" -- how long would it take for someone to guess the key. For comparison shopping, consider a really, really fast computer -- as yet non-existent as far as we know -- that could guess a 56-bit key in 3.5 hours. That same computer would take 2 million million million years to guess a 128-bit key, such as IDEA and AES use.
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Foundations: Cryptography 101 Smith, Richard, Internet Cryptography, Addison-Wesley Pub Co; ISBN: 0201924803 Schneier, Bruce, Applied Cryptography: Protocols, Algorithms and Source Code in C, Second Edition, John Wiley & Sons; ISBN: 0471117099.
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