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function y = vigenere(txt, key); % This function performs the Vigenere cipher on the plaintext in % the variable txt, using key (we assume the key is in numerical % representation). xln=length(txt); kln=length(key); for j=1:xln, jmod=mod(j,kln); if jmod==0, jmod=kln; end; %map an index of 0 into an index of kln y(j)=shift(txt(j),key(jmod)); end;