Basic Shift Cipher Decryption Algorithm HELP!

Hello guys, I'm using matlab to make a function that basically decrypts a shift cipher by taking in the ciphertext string and key integer as parameters and returning the plaintext.

here is the code..

 function [ plainText ] = ccdt( c, k )

 s = double(c);

 for i = 1:numel(s)
     s(i) = s(i)-k;
 end

 plainText = char(s);

 return

 end

This works fine when the letters don't necessarily need to loop back around to the beginning of the alphabet. For example, if I decrypt the letter 'b' with key = 3, it should give me back 'y', but it's just returning whatever ascii code for 'b' minus 3 which isn't 'y'.

How can I fix this problem? also, how can i modify the code so that lower/ upper case letters don't really matter?

Thanks for your time!

[barjas abu matar]

i need code basic shift cipher in decryption

Answer 1

[Mohammad Abouali]

Use mode or reminder to loop over certain range of numbers. Something like this:

% A --> 65
% Z --> 90
% a --> 97
% z --> 122

shift=-3;

inputChar=char( [ (65:90)'; (97:122)'] );

for i=1:numel(inputChar)
  numericChar=double(inputChar(i));
  if ( numericChar>=65 && numericChar<=90 )
    numericChar=mod(numericChar-65+shift,26);
    outputChar(i,1)=char(numericChar+65);
  elseif ( numericChar>=97 && numericChar<=122 )
    numericChar=mod(numericChar-97+shift,26);
    outputChar(i,1)=char(numericChar+97);
  else
    error('just alphabetic chars are accepted')
  end

end

You can use both negative shift or positive shift, depending on the direction.

[Jason]

ah thanks for this. so basically, the only way to guarantee that the function shifts in between the ascii codes for the lower/upper case alphabet characters is to use multiple if/else to check that they are in between 65:90 and 97:122?

[Mohammad Abouali]

Not really. There are other approaches too.

You can prebuilt a lookup table. Or you can change everything to upper case and cycle there and then restore the caps (lower or upper).

I am sure we can find couple of other methods to get the same output.

Answer 2

[Aman Gupta]

function coded = caesar(x,n)
a = double(x);
a = a+n;
  for i = 1:length(a)
      while (a(i)>126 || a(i)<32)
          if a(i)>126
             a(i) = 31 + (a(i) - 126);
          elseif a(i)<32
              a(i) = 127 - (32 - a(i));
          end
      end
  end
coded = char(a);

Answer 3

[Rahul Gulia]

function coded = caesar(str,n)

num1 = double(str); %Converting string to double to make the defined shifts

for i = 1 : length(num1)

if num1(i) + n > 126 % If ASCII value goes beyond 126

m = num1(i)-126+n;

p = 31+m;

num1(i) = p;

elseif num1(i)+n < 32 % If ASCII value goes below 32

m = 32 - num1(i) + n;

p = 126 - m;

num1(i) = p;

else m = num1(i) + n; % In a normal condition

num1(i) = m;

end

code(i) = num1(i);

end

coded = char(code);

% I have written this code. Can anyone please expain as what is wrong in here? I know i have made a mistake. But i am not able to figure it out.

% Thanks in advance.