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0—1-1-21 Objective Upon completion of this section the learner will be able to describe the features and limitations of 3DES symmetric encryption algorithm. Introduction With advances in computer processing power, the original 56-bit DES key became too short (its keyspace is too small) to withstand even medium-budget attackers. One way of increasing DES effective key length, without changing the well-analyzed algorithm itself, is to use the same algorithms with different keys several times in a row.

DES Description DES is a symmetric encryption algorithm with a fixed key length of 56 bits. The algorithm is still very good, but because of the short key length is it susceptible to brute force attacks, with sufficient resources. Today, DES is generally considered obsolete because of its key length, and should almost never be used: relatively unsophisticated attackers can break it fairly easily. Note The literature discusses an experimental successful attack on DES that used 12 highperformance workstations and 50 days of computing time.

All rights reserved. 0—1-1-14 The DES Key DES has a fixed key length. The key is actually 64 bits long but only 56 bits are used for encryption, the remaining 8 bits being used for parity. The least significant bit of each key byte is used to indicate odd parity. Note A DES key is always 56 bits long. When even weaker encryption with 40-bit keys is used, it means that the encryption key is actually 40 secret bits + 16 known bits. Copyright © 2003, Cisco Systems, Inc. Encryption 1-1-19 DES in Action (Block Mode) © 2003, Cisco Systems, Inc.

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