We had a similar initial thought, but I was concerned about something like morse code being both too slow and vulnerable to "timing attacks" (professor noticing

We had a similar initial thought, but I was concerned about something like morse code being both too slow and vulnerable to "timing attacks" (professor noticing that the amount of time it takes to recite any digit is directly correlated with what that digit is). I also assumed that this would be an interactive exercise wherein the professor might stop you to chat midway through and/or drop a few "your audio broke up there; can you back up and repeat the last N digits?"s to throw you off. That, and it's more interesting if the solution also has a shot of working in person, which is why I didn't go for anything like "hang the numbers around the room".