function encrypt(inputFile, outputFile, key, mode)
%Implements a random shift cipher
% input: inputFile - the file to be encrypted or decrypted
% input: outputFile - where the result of the operations are stored
% input: key - the encryption key, can be any positive integer
% input: mode - 0: encrypt, 1: decrypt

% step 1: read the input data
% step 2: compute the encrypted text
% step 3: write the output data

% open the input file
[inputFID, message] = fopen(inputFile, 'r');
if inputFID==-1
    %error
    fprintf(2,'Error, could not open the input file: %s\n', message);
    return;
end


[outputFID, message] = fopen(outputFile, 'w');
if outputFID==-1
    %error
    fprintf(2, 'Error, could not open the output file: %s\n', message);
    fclose(inputFID);
    return;
end

% seed the pseudo-random number generator
rng(key);

data = fread(inputFID,1);

while length(data)>0

    % ASCII characters go between 0 and 127    
    offset = randi(128) - 1; % gives a random number between 0 and 127
    
    if mode==0 %encrypt mode
        output = data + offset;
    else %decrypt mode
        output = data - offset;
    end
    
    % wrap-around 
    if output > 127
        output = output - 128;
    end
    if output < 0
        output = output + 128;
    end

    fprintf(outputFID, '%c', output);

    data = fread(inputFID, 1);
end

fclose(inputFID);
fclose(outputFID);

end