Conventional Cryptography - A Classical Introduction to Cryptography: Applications for Communications Security - page 41

k i

⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕

M M M M

c ⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕

M M M M

c

⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕

M M M M

Figure 2.26. One round of CS-CIPHER.

where AND is the bitwise logical AND and 55 is an hexadecimal constant which is
01010101 in binary. We notice that ϕ is linear, and actually an involution since

ϕ ( ϕ ( x )) = (ROTL( ϕ ( x )) AND 55 ) ⊕ ϕ ( x )

= x .

ϕ

Thus is a linear permutation. The permutation P is defined in order to be a nonlinear

involution:

P ( P ( x )) = x .

We can then finally define M . Fig. 2.27 represents M with the XOR with subkey bytes
at the input. It is easy to see that Fig. 2.28 represents the inverse transform where ϕ is

defined by

ϕ ( x )

= ⊕ .

(ROTL( x ) AND aa ) x

x l x r x l x r

k r k r

k l ⊕ ⊕ k l ⊕ ⊕

ROTL

ϕ

⊕ ⊕

M

P P

y l y r y l y r

Figure 2.27. The mixing box of CSC.