mahnoor - the problem is with
k=repmat((key),1,length(plain_text))
You don't want to repeat the key for each letter of plain_text (unless the key is a single character) but you want to repeat it enough times so that the "extended" key has the same characters as the plain text. For example, if
key = 'logan';
plainText = 'superheromovies2017';
then the extended key would be
extendedKey = 'loganloganloganloga';
So your code will have to determine this. I suppose you could do something like
extendedKey = '';
if length(key) < length(plainText)
keyReps = floor(length(plainText)/length(key));
extendedKey = [repmat(key,1,keyReps) key(1:rem(length(plainText),length(key)))];
else
extendedKey = key(1:length(plainText));
end
This should give you the correct key, but you may want to step through the remainder of the code to ensure that it is doing what you expect.
