Caesar cipher matlab code

Can anyone write a very basic code for caesar cipher that shifts input characters by 5 place?

[James Tursa]

What have you done so far? What specific problems are you having with your code?

Answer 1

[Md. Atiqur Rahman]

%% To whom it may concern
Plaintext= input('Plaintext: ','s')
Shift= input('Shift: ')
Output = Plaintext+Shift;
r=Output-26;

for i = 1:length(Plaintext)
    
    if Output(i) <= 90
        Output(i) = char(Output(i));
        
    else
        Output(i) = char(r(i));
    end
end
fprintf('Ciphertext: ','s')
disp(char(Output))

[Walter Roberson]

Let's see:

Plaintext = 'To whom it may concern';
O1 = CC(Plaintext, 5);
[Plaintext;
 O1;
 char(Plaintext+5)]

ans = 3×22 char array
    'To whom it may concern'
    'YZ%bSZX%T_%XLd%NZYNP]Y'
    'Yt%|mtr%ny%rf~%htshjws'

CC(O1, -5)

ans = 'TU CNUS OZ SGE IUTIKXT'

CC(Plaintext, 96)

ans = 'µf½®µ³f¯ºf³§¿f©µ´©«¸´'

CC(Plaintext, 5120)

ans = 'ᐺᑕᐆᑝᑎᑕᑓᐆᑏᑚᐆᑓᑇᑟᐆᑉᑕᑔᑉᑋᑘᑔ'

function Output = CC(Plaintext, Shift)
    Output = Plaintext+Shift;
    r=Output-26;
    for i = 1:length(Plaintext)
        
        if Output(i) <= 90
            Output(i) = char(Output(i));
            
        else
            Output(i) = char(r(i));
        end
    end
    Output = char(Output);
end

With a shift of 5, you move 'T' to 'Y' and you move 'o' to 'Z'. But 'Y' and 'Z' are adjacent to each other, so in order for them to appear together as output, the two input characters would have had to be adjacent to each other in the alphabet.

You can see that when you shift your first output back, you do not get the original input.

You can see that you are not handling large shifts properly.

[Md. Atiqur Rahman]

According to my code U appears as Z and T appears as Y for a shift of 5 places in both cases. Besides, caesar cipher limited upto 25 place shift. I think this works fine for any shift (<25) if you try again. Now, if I need to shift small letters then i can do this by inserting one more 'if' statement. Hope this explanation helps you.

[Walter Roberson]

"To whom it may concern" is the input suggested by your own comment. Considering there is no documentation in your code that would limit it to upper case, it would be expected that the code should work for your suggested input.

Notice that with the shift of +5, that letters at the end of the upper-case range are not reversed with a shift of -5

Plaintext = upper('To whom it may concern')

Plaintext = 'TO WHOM IT MAY CONCERN'

O1 = CC(Plaintext, 5);
[Plaintext;
 O1;
 char(Plaintext+5)]

ans = 3×22 char array
    'TO WHOM IT MAY CONCERN'
    'YT%BMTR%NY%RFD%HTSHJWS'
    'YT%\MTR%NY%RF^%HTSHJWS'

CC(O1, -5)

ans = 'TO =HOM IT MA? CONCERN'

O2 = CC(Plaintext, -24);
[Plaintext;
 O2;
 char(Plaintext-24)]

ans = 3×22 char array
    'TO WHOM IT MAY CONCERN'
    '<?0715)+76+-:6'
    '<?0715)+76+-:6'

CC(O2, 24)

ans = 'TO WHOM IT MAY CONCERN'

function Output = CC(Plaintext, Shift)
    Output = Plaintext+Shift;
    r=Output-26;
    for i = 1:length(Plaintext)
        
        if Output(i) <= 90
            Output(i) = char(Output(i));
            
        else
            Output(i) = char(r(i));
        end
    end
    Output = char(Output);
end

[Md. Atiqur Rahman]

%%Decryption

Ciphertext= input('Ciphertext: ','s')
Shift= input('Shift: ')
Output = Ciphertext-Shift
r=Output+26;

for i = 1:length(Ciphertext)
    
    if Output(i) >= 65
        Output(i) = char(Output(i));
        
    else
        Output(i) = char(r(i));
    end
end
fprintf('Plaintext: ','s')
disp(char(Output))

[Walter Roberson]

No, notice how the spaces do not map back to spaces when you reverse the shift.

Plaintext = ('To whom it may concern')

Plaintext = 'To whom it may concern'

O1 = CC(Plaintext, 5);
[Plaintext;
 O1;
 char(Plaintext-5)]

ans = 3×22 char array
    'To whom it may concern'
    'Oj5rcjh5do5h\t5^ji^`mi'
    'Ojrcjhdoh\t^ji^`mi'

CC(O1, -5)

ans = 'ToTwhomTitTmayTconcern'

O2 = CC(Plaintext, -24);
[Plaintext;
 O2;
 char(Plaintext+24)]

ans = 3×22 char array
    'To whom it may concern'
    'lRRRyR{{}'
    'l888y8{{}'

CC(O2, 24)

ans = 'ToTwhomTitTmayTconcern'

function Output = CC(Ciphertext, Shift)
    Output = Ciphertext-Shift;
    r=Output+26;
    for i = 1:length(Ciphertext)
        
        if Output(i) >= 65
            Output(i) = char(Output(i));
            
        else
            Output(i) = char(r(i));
        end
    end
    Output = char(Output);
end

[Md. Atiqur Rahman]

Spaces are not my concern as long message can be extracted. If you have a better solution post it here. which must work upto 25 shifts.

Thanks for constructive criticism.

[Walter Roberson]

There have been many answers posted

https://www.mathworks.com/matlabcentral/answers/466636-caesar-cyphor-encryption-problem

https://www.mathworks.com/matlabcentral/answers/517454-caesar-s-cypher-is-the-simplest-encryption-algorithm-it-adds-a-fixed-value-to-the-ascii-unicode?s_tid=srchtitle

[Md. Atiqur Rahman]

% Caesar cipher Encryption

Plaintext= input('Plaintext: ','s');

Shift= input('Shift: ');

Output = Plaintext+Shift;

r=Output-26;

for i = 1:length(Plaintext)

if Output(i) <= 90

Output(i) = char(Output(i));

Output(Output<=58)=32;

else

Output(i) = char(r(i));

end

end

fprintf('Ciphertext: ','s');

disp(char(Output))

%Caesar cipher Decryption

Ciphertext= input('Ciphertext: ','s');

Shift= input('Shift: ');

Output = Ciphertext-Shift;

r=Output+26;

for i = 1:length(Ciphertext)

if Output(i) >= 65

Output(i) = char(Output(i));

else

Output(i) = char(r(i));

Output(Output<=58)=32;

end

end

fprintf('Plaintext: ','s');

disp(char(Output))

[Md. Atiqur Rahman]

I think I've done it better 😎

[Walter Roberson]

        Output(i) = char(Output(i));

Why are you doing that redundant operation? Yes, you convert to char but you store it in the same array so it would convert back to the same data type.

        Output(Output<=58)=32;

Why are you mixing logical indexing of the entire array inside a loop?

Answer 2

[Walter Roberson]

cc5('HELLO')

ans = 'MJQQT'

cc5('ABCXYZ')

ans = 'FGHCDE'

function out = cc5(in)
  TT('A':'Z') = ['F':'Z', 'A':'E'];
  out = TT(in); 
end

[Md. Atiqur Rahman]

Plaintext= input('Plaintext: ','s')
Shift= input('Shift: ')
if Plaintext+Shift <= 90
    Ciphertext = (char(Plaintext+Shift))
else
    Ciphertext = (char(Plaintext+Shift-26))
end
%Can you fix this?

[Walter Roberson]

Your Plaintext is a vector of characters. Plaintext+Shift <= 90 would be a vector. When you if a vector, the result is considered true only if all of the values in the vector are non-zero. So your Plaintext+Shift <= 90 test would be true only if all of the characters where no more than 'U'

You should investigate logical indexing.