groups of 5
Warning : this is a digression
pretty sure I've mentioned it before - groups of five are a classic way to transmit cipher. Back in the day, many ciphers were sent by unreliable methods and the grouping made it easier to detect and fix errors. As for why 5 in particular, I'm sure it has something to do with the facts that we use a base 10 counting system yet 10 is beyond most people's short term memory, while 5 is not.
That stated, there may be a particular reason this cipher is in groups of 5 so you cannot totally discount that. Especially as in classic systems you'd pad out the last one so that you sent all groups of 5.
As for this cipher, I really haven't looked at it. Had you presented it as a puzzle, I would have but I'm afraid I'm of the belief your homework is your homework and you must sink or swim accordingly. Asking your professor and fellow students for help may be fair game, but trying to enlist outside experts seems....