% ex4-14.m
% exercise 4.14 encryption
A = [0 0 1 0 0; 0 0 1 0 1; 1 0 0 1 1; 1 0 1 0 0 ;1 0 0 1 0; 0 1 1 1 1;
1 1 0 0 1; 1 1 1 1 1; 0 1 1 0 1; 0 1 0 0 1; 0 0 0 1 1; 1 0 0 1 0; 
0 1 1 1 1; 1 0 0 1 1; 0 1 1 1 1; 0 0 1 1 0; 1 0 1 0 0]

[m,n] = size(A)
b = [3,-1,6,7,-5]
for i=1:m
for j=1:n
 sum = 0;
 for p = 1:n
  sum =  sum + b(p)*A(i,mod(j-p,n));
  end;
  C(i,j) = sum;
end; end;
save C  'message.mat'
%
% check
%
bhat = fft(b);
for i=1:m
 Chat(i,:) = fft(C(i,:);
 Xhat(i,:) = Chat./bhat;
 X(i,:) = ifft(Xhat(i,:);
end;