#!/usr/bin/env perl
use 5.020;
use warnings;
use autodie;
use PDL;
use PDL::NiceSlice;

# This code attempts to find all differential characteristics in the
# FEAL-4 cipher round subfunction 'G'.
#
# Reference: http://theamazingking.com/crypto-feal.php
#
#
# 'G' function is addition of a, b and x, then bitwise rotate left
# by 2 bits
# a, b, x and the final value are all 8 bits.
# For our purposes, x can be ignored, as it's constant 0 or 1
#
#        a
#        |
#  x -> [+] <- b
#        |
#      [<<<]
#        |
#       OUT
#

# Perform addition
my $G = sequence( byte, 256 ) + sequence( byte, 256 )->transpose;

# Bitwise rotation
$G = ( $G << 2 ) | ( $G >> 6 );

# At this point, $G contains all possible inputs for a and b, and
# the associated output value
#
# Now we wish to find all differentials throughout this function
#
# To do this, we need to find differentials between each possible
# inputs to 'a', and 'b' and observe the differential in the result
#
# There are two known differentials for this function.  A differential
# value of 0 and 0x80 (128) for 'a' will always return a constant
# differential output (0 and 2) respectively.

# Calculate the differential table
my $diffs = $G ^ $G ( (0) );

# Find the minimum and maximum value for each differential
my ( $min, $max ) = minmaxover($diffs);

# Print index of differentials where minimum and maximum value are
# equal.  As the index is also in the input value, this returns the
# actual differential:
print "Contant differentials for input differentials of: ",
  which( $min == $max ), "\n";