Block Ciphers - Modern Cryptanalysis: Techniques for Advanced Code Breaking - page 94

Listing 4-8 The FEAL round function, f .
##########################################
# Feal round function, f
##########################################
def f(alpha, beta):
# Split alpha and beta
a = demux(alpha)
b = demux(beta)
# Make the output four 8-bit values
fs = [0,0,0,0]
# Calculate each 8-bit value
fs[1] = a[1] ^ b[0] ^ a[0]
fs[2] = a[2] ^ b[1] ^ a[3]
fs[1] = sbox(fs[1], fs[2], 1)
fs[2] = sbox(fs[2], fs[1], 0)
fs[0] = sbox(a[0], fs[1], 0)
fs[3] = sbox(a[3], fs[2], 1)
# Return the 32-bit result
return mux(fs)
4.7.4 Key Scheduling
The key scheduling algorithm for FEAL is meant to split up a 64-bit key into various derived subparts (based
on the f K key scheduling function), for use in the main round function of FEAL.
1. Let K = ( A 0 , B 0 ) and D 0 = 0.
2. For eight rounds, r = 1, ... , 8:
(a) D r = A r -1 .
(b) A r = B r -1 .
(c) B r = f K ( A r -1 , B r -1 ⊕ D r -1 ).
(d)
(e)
In Python, we can implement this fairly easily, as shown in Listing 4-9 .