Triple DES with Three Keys
Although the attacks just described appear impractical, anyone
using two-key 3DES may feel some concern. Thus, many researchers now feel that
three-key 3DES is the preferred alternative (e.g., [KALI96a]). Three-key 3DES has an
effective key length of 168 bits and is defined as follows:
C = E(K3, D(K2, E(K1, P)))
Backward compatibility with DES is provided by putting K3 = K2 or K1 = K2.
A number of Internet-based applications have adopted three-key
3DES, including PGP and S/MIME, both discussed in Chapter 15.
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