# Basic Shift Cipher Decryption Algorithm HELP!

6 visualizzazioni (ultimi 30 giorni)
Jason il 11 Gen 2015
Risposto: Rahul Gulia il 22 Lug 2019
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?
##### 1 CommentoMostra -1 commenti meno recentiNascondi -1 commenti meno recenti
barjas abu matar il 3 Nov 2017
i need code basic shift cipher in decryption

Accedi per commentare.

### Risposta accettata

Mohammad Abouali il 11 Gen 2015
Modificato: Mohammad Abouali il 11 Gen 2015
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.
##### 2 CommentiMostra NessunoNascondi Nessuno
Jason il 11 Gen 2015
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 il 11 Gen 2015
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.

Accedi per commentare.

### Più risposte (2)

Aman Gupta il 27 Giu 2019
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);
##### 0 CommentiMostra -2 commenti meno recentiNascondi -2 commenti meno recenti

Accedi per commentare.

Rahul Gulia il 22 Lug 2019
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.
##### 0 CommentiMostra -2 commenti meno recentiNascondi -2 commenti meno recenti

Accedi per commentare.

### Categorie

Scopri di più su Characters and Strings in Help Center e File Exchange

### Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by