written 8.3 years ago by
amatul
• 0


Structure of cipher is similar to SPN.
Keylength=32
, Blocklength=32
Assuming Kr is the round key used in the round
We divide the input into 8 blocks of 4 bits each,the following are done
in 1 round of cipher
1.Substitution:
4x4 sbox is used here.
The operations in substitution are:
a.Finding inverse in field$(2^4)$
If input is 0000 ,then ouput is 0,inverse of input infield$(2^4)$ otherwise .
The irreducible polynomial used here is
$x^4+x^3+1$.
But this may ensure fixed points i.e
s(0000)=0000
s(0001)=0001
b.Xoring with an appropriate 4 bit number
c=0011
Xoring with 0011 after obtaining inverse ensures no fixed points
Substitution layer is the confusion layer.
2.Permutation layer:
This has 2 operations:
a.Linear Transformation
For ensuring permutation we use MDS matrix as they ensure high
branch number
MDS matrix choosen M is
$$
\begin{pmatrix}
2&3&1&1\\
1&2&3&1\\
1&1&2&3\\
3&1&1&2\\
\end{pmatrix}
$$
We divide the input say a1a2a3a4a5a6a7a8(where ai corresponds to 1 block of 4 bits) and apply the transformation
M*$$
\begin{pmatrix}
a1&a2\\
a3&a4\\
a5&a6\\
a7&a8\\
\end{pmatrix}
$$
b.Shifting
Let the input be b1b2b3b4b5b6b7b8 after applying linear transformation to given input,then it would result in b5b6b7b8b1b2b3b4 after applying shifting
Permutation layer is the diffusion layer in the cipher
3.Xoring with round key
This ensures mixing between round input and round key