Cryptography Reference
In-Depth Information
K
1
⊕
F
π
σ
K
2
+
G
π
σ
K
3
∗
F
Figure 2.13.
Variants of the Feistel scheme with two branches.
2.5.2 Lai-Massey Scheme
A famous block cipher which is not based on the Feistel scheme is the IDEA cipher.
IDEA stands for International Data Encryption Algorithm. It follows two previous
versions called PES (Proposed Encryption Standard) and IPES (Improved Proposed
Encryption Standard). It was developed during the PhD studies of Xuejia Lai under
the supervision of James Massey at the ETH Zurich. IDEA was published in Lai's
thesis (Ref. [110]) in 1992. It is patented by Ascom and made freely available for
noncommercial use.
5
Like DES, IDEA is a block cipher for 64-bit blocks. IDEA uses much longer keys
than DES as it allows for 128-bit keys. In the same way that DES was dedicated to
hardware, IDEA was dedicated to software implementation on 16-bit microprocessors
(which used to be a luxurious architecture in the early nineties). It makes an extensive
use of the XOR, the addition modulo 2
16
, and the product of nonzero residues modulo
2
16
+
1.
IDEA uses a structure similar to the Feistel scheme which can be called the
Lai-Massey scheme. It also enables making a permutation from a function. It however
requires a two-branch balanced structure and a commutative and associative law like the
XOR operation. As depicted in Fig. 2.14, it simply consists of adding to both branches
the output of a round function whose input is the difference of the two branches: a
5
The commercial development of IDEA is currently managed by the company MediaCrypt.
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