Caesar's cypher is the simplest encryption algorithm. It adds a fixed value to the ASCII (unicode) value of each character of a text. In other words, it shifts the characters. Decrypting a text is simply shifting it back by the same amount, that is,

function coded = caesar(a,b)

coded = char(a+b);

c = length(coded);

for ii = 1:c

if coded(ii) > 126;

d = coded(ii) - 126;

coded(ii) = (32+d);

elseif coded(ii)<32;

d = coded(ii) - 32;

coded(ii) = char(126-d);

end

end

end

[muhammad usman]

what is wrong with my code. i am unable to understand how is my logic wrong

[David Hill]

What is a? Isn't 'a' already a character array like: 'my name is Dave'? Is b the shift number?

[muhammad usman]

a is the vector input and b is shift number.

i have modified function but still not accurate

function coded = caesar(a,b)
while b>126 % b must be 126 or less so that only 1 cycle for each number
    b= b-1;
end
code = a+b;
c = length(code);
for ii = 1:c
    if code(ii) > 126;
        d = code(ii) - 126;
        code(ii) = (31+d);
    elseif code(ii)<32;
        d = code(ii) - 32;
        code(ii) = (127+d);
    end
end
coded = char(code);
end 

Answer 1

[Walter Roberson]

You start by doing

coded = char(a+b);

Suppose b is negative enough that a+b would be less than zero. Because of the char() that would be char() of a negative value and because char cannot be negative that would be changed to 0.

You need to do your work in signed integer or in floating-point and convert to char afterwards

[muhammad usman]

i have modified function and included your sugestion. but still not working

function coded = caesar(a,b)
while b>126 % b must be 126 or less so that only 1 cycle for each number
    b= b-1;
end
code = a+b;
c = length(code);
for ii = 1:c
    if code(ii) > 126;
        d = code(ii) - 126;
        code(ii) = (31+d);
    elseif code(ii)<32;
        d = code(ii) - 32;
        code(ii) = (127+d);
    end
end
coded = char(code);
end 

[Walter Roberson]

if b were 256 then that should probably be the same outcome as if it were 0, but you treat it as 126. You also do not raise negatives into the positive range at that stage.

Answer 2

[David Hill]

Are you using the full range from 0-255 for ascii? or do you want the output to have only letters (a-zA-Z) and no special characters?

function coded = caesar(a,b)
coded=char(mod(unicode2native(a)+b,256));
end 

Answer 3

[KaMATLAB]

function txt = caesar(txt,key)
    txt = double(txt) + key;
    first = double(' ');
    last = double('~');
    % use mod to shift the characters - notice the + 1
    % this is a common error and results in shifts 
    % being off by 1
    txt = char(mod(txt - first,last - first + 1) + first);
end

[KaMATLAB]

This is the solution to the question.

Answer 4

[Prerona Dey]

function y = caesar2(ch, key)
    v = ' ' : '~';
    [~, loc] = ismember(ch, v);
    v2 = circshift(v, -key);
    y = v2(loc);
end

Can u please explain me this soultion I found.

Answer 5

[Zia Ur Rehman]

Hi folks,

I write this code, this is working fine with the problem.

Need further improvement if any from seniors as I'm very novice in coding and MATLAB.

function coded = caesar(a,b)
% removing the ';' so you can see how it works in output
c = double(a) % to convert the given char(string) into double(numeric)
d=c+b % adding the shift smount to encrypt
l= length(d) % measuring the length as we need to traverse every element to check if it lies in the limit(32:126)
for e = 1:l % applying loop to check each element if it lies in the limit
    while d(e) > 126 % using while as if we use 'if' statement it will only execute once but we need execution untill the value comes in the limit
        d(e) = d(e)-95 % if number is greater than 126 so wrap around  by adding (126-32+1=95) we use +1 as we need next number not the same number
    end
    while d(e) < 32 % using while as if we use if statement it will only execute once but we need execution untill the value comes in the limit
        d(e) = d(e) + 95 % if number is less than 32 so wrap around by subtracting (126-32+1=95) we use +1 as we need next number not the same number
    end
end
coded=char(d) % d is now updated according to the limit
%coded = char(double(a) + b)
end

[John D'Errico]

This is not an answer to the question, instead a request for some coding adivce on how to improve your code.

Does your code work? Is there a problem with the code?

[Walter Roberson]

Let us test:

in = 'To whom it may concern'

in = 'To whom it may concern'

O1 = caesar(in, 5)

O1 = 'Yt%|mtr%ny%rf~%htshjws'

O2 = caesar(O1, -5)

O2 = 'To whom it may concern'

O3 = caesar(in, 5120)

O3 = 'Jeum^ecu_jucWouYedY[hd'

O4 = caesar(O3, -5120)

O4 = 'To whom it may concern'

function coded = caesar(a,b)
% removing the ';' so you can see how it works in output
c = double(a); % to convert the given char(string) into double(numeric)
d=c+b; % adding the shift smount to encrypt
l= length(d); % measuring the length as we need to traverse every element to check if it lies in the limit(32:126)
for e = 1:l % applying loop to check each element if it lies in the limit
    while d(e) > 126 % using while as if we use 'if' statement it will only execute once but we need execution untill the value comes in the limit
        d(e) = d(e)-95; % if number is greater than 126 so wrap around  by adding (126-32+1=95) we use +1 as we need next number not the same number
    end
    while d(e) < 32 % using while as if we use if statement it will only execute once but we need execution untill the value comes in the limit
        d(e) = d(e) + 95; % if number is less than 32 so wrap around by subtracting (126-32+1=95) we use +1 as we need next number not the same number
    end
end
coded=char(d); % d is now updated according to the limit
%coded = char(double(a) + b)
end

That seems to work.

Notices, though, that I added semi-colons everywhere, to prevent it from repeatedly outputing results as it goes.