Cryptography Reference
In-Depth Information
X
1
X
2
X
3
X
4
Z
1
Z
2
Z
3
Z
4
Z
5
Z
6
Y
1
Y
2
Y
3
Y
4
X
1
, ... , X
4
: input subblocks
Y
1
, ... , Y
4
: output subblocks
Z
1
, ... , Z
6
: round's key
bitwise XOR
addition
Multiplication modulo 2
16
+
1 (see text)
Figure 5.9:
Description of the IDEA algorithm.
Z
1
Z
2
Z
3
Z
4
Y
1
Y
2
Y
3
Y
4
Figure 5.10:
Output transformation in IDEA, using the same notation as in
Figure 5.9.
A closer look reveals that the structure of this network is surprisingly logical.
First of all, we will quickly notice that,
together
with the output transformation,
the network is symmetric. The reason is that, if we were to swap the plaintext
blocks with the ciphertext blocks, traversing the network from bottom to top,
