User defined function encryption

Hello,

I am working on problem 2 in the attached PDF. I am trying to understand what they are saying in (b) as far as creating the double array. Does that mean I concatenate strIn with keyL? Also, how do I remove the last few characters in keyL so that numel(keyL) == numel(strIn)

Here is what I have so far, it's far from complete but I don't know if I should use a while statement.

function strOut = Convert(strIn,key,mode)
keyL = [key key];
if mode == 1
    while (numel(keyL) >= numel(strIn)) == 1
        numel(keyL) == numel(strIn)
        for i = 1:numel(keyL)