decrypt.m

I try to create GUI using Matlab to code hills polygraph cipher

function cipher = decrypt(plain,rk,a,b,c,d,k1,k2)
l = size(plain);
n = mod((mod(a*d,26)-mod(b*c,26)),26);
%fprintf('n: %d\n',n);

for i = 1 : 26
    if(mod(i*n,26) == 1)
        n = i;
        break;
    end
end
%fprintf('n: %d\n',n);

%ubah teks ke angka  
  for i = 1 : 2 : l(1,2)
      %fprintf('%d\n',l);
      for j = 1 : 26
          if plain(i) == rk(j)
              y1=j-1;%fprintf('y1: %d\n',y1);
              if plain(i) == plain(i+1)
                  y2=j-1;
              end
          elseif (plain(i+1) == rk(j))
              y2=j-1;%fprintf('y2: %d\n',y2);
          end
      end
      x1 = mod(n*(mod(d*(y1-k1),26)-mod(b*(y2-k2),26)),26);
      x2 = mod(n*(mod(a*(y2-k2),26)-mod(c*(y1-k1),26)),26);
      %fprintf('x1 dan x2: %d %d\n',x1,x2);
      
      cipher(i)=char(rk(x1+1));
      cipher(i+1)=char(rk(x2+1));
            
      %fprintf('%d\n',i);
  end
return