I am trying to solve this very interesting challenge, with a couple of new ideas, and an interesting vulerability, which I am happy to share (privately).
Question: In the attached PDF (and in the referenced Master thesis), the example employs almost only code values which start with a leading 1 (e.g., a = 1001 , b = 10100, c = 1100, etc..), except for the letter 'r' which is mapped to 0. There are no codes longer than that starting with a leader 0 (e.g., '0011' or '01').
Is it safe to assume that this is also the case for the challenge here? This woud not make the challenge an easy one, but it would somehow help to reduce the size of the key space (from 62 code options, to only 32) and the workload of certain algorithms.
Thanks again for this very nice challenge.
PS: I appreciate this challenge is from quite a while ago, and it might be difficult to find the original material used to create it :-)