Cryptography Reference
In-Depth Information
However, the IDEA algorithm has one big drawback: it is not scalable, which
means that it cannot benefit from the growing processing width of modern
computers (32 bits, 64 bits,
...
) and is particularly fixed on 64-bit blocks. We
know from the discussion of the CBC mode in Section 5.1.1 that too small
a block size theoretically represents a vulnerability for birthday attacks. In
fact, its small block size and its fixed key length were the only reasons IDEA
disqualified as a candidate for the AES algorithm, the successor of DES (see
Section 5.5).
Of course, IDEA has not been studied long enough to convince even the last
doubter of its security. Even the theoretically underpinned design, over which
one could go into raptures, doesn't exclude potential vulnerabilities. Crypt-
analysts traditionally look for sore points and don't normally let themselves
get carried away by the esthetic inner life of an algorithm. Anyhow, no suc-
cessful attack has become publicly known, although cryptologists have busied
themselves increasingly with IDEA.
5.3.6 Speed, Outlook
IDEA is about twice as fast in software as DES. Schneier [SchnCr, Section 13.9]
mentions 300 Kbytes/s on a 66-MHz PC-486 (compare this with RC5 in Sec-
tion 5.4). In hardware, however, IDEA is much harder to implement than DES,
which is mainly due to the '
' operation. An IDEA chip developed at the ETH
Zurich achieved about 22 Mbytes/s, but it is not produced in series. The major
obstacle appears to be reservations by the industry on account of the license
fees for the algorithm. Security has not yet reached the importance for an orga-
nization with 100 employees to pay 1000 dollars for encryption within their
internal networks. For example, a software vendor would have to pay 2 % of
their sales to Ascom Systec AG just for using IDEA in their products. This
is obviously too much even for an attractive name like IDEA. In addition to
its use in free software (for example, PGP and SSH; see Chapter 7) I know
that it is commercially used in the
Brokat
software package, which has been
deployed by at least four German banks since early 1997. Deutsche Telekom
presumably purchased a large number of IDEA licenses for their own products.
5.4 RC5: Yet Another Hope for DES Replacement
This section discusses perhaps the simplest and most flexible of modern algo-
rithms.
RC5
is a symmetric block algorithm introduced by Ron Rivest [RivRC5]
