Algorithm

        ################  encoding #################

msg='Enter the message to encode it :';
plainText=input( msg , 's')


for i =1 : length(plainText)
      if ( plainText(i)>="a" && plainText(i)<="z")
          continue ;
      elseif (plainText(i) !=" " )
          disp('can not be encode this message ')
          return ;
      end
  end

k= input('Enter the value of K :')

for i =1 : length(plainText)
    if (plainText(i) !=" " )
        plainText(i)= mod(plainText(i)-'a' + k  , 26) +'A';

    end
end

  disp('the encoded message : ');
  disp( plainText);





        ################  decoding #################

msg='Enter the message to decode it :';
plainText=input( msg , 's')


for i =1 : length(plainText)
      if ( plainText(i)>="A" && plainText(i)<="Z")
          continue ;
      elseif (plainText(i) !=" " )
          disp('can not be decode this message ')
          return ;
      end
  end

k= input('Enter the value of K :')

for i =1 : length(plainText)
    if (plainText(i) !=" " )
        plainText(i)= mod(plainText(i)-'A' - k  , 26) +'a';

    end
end

  disp('the decoded message : ');
  disp(plainText);