(This is discussed in https://en.wikipedia.org/wiki/Tap_code cited downthread by 'dmckeon.)
We broke the knock code into two "byte" pairs. So HELL would be an order of 2315 Beef noodles and 3131 Chicken noodles.
To access the orders between cellblocks it was necessary for the person in solitary to write their PIN (their username was their jail ID#) on a piece of toilet paper and pass it to someone outside solitary on a day they were in court. Then both sender and recipient could log into the same account from different cellblocks and read the order. Once read the order would be deleted and another order placed to reply to the message.
At first I was out of solitary, but then I got sent for extortion (charge dropped after 10 days, story of my life) and while I was in solitary the staff went through my stuff and found some pieces of paper where I had sat encoding and decoding messages. The staff didn't have the table though and through the FOIA I managed to get access to their working out, but they never successfully decoded any of the undecoded messages they obtained from the kiosk. After that we rotated the encoding table 90 degrees in our heads to act as a very simple form of encryption in case they tried again. It took the authorities another 6 months to persuade the kiosk developers to add a piece of code which checked the jail ID# that was logging in against the cellblock they were in to make sure you couldn't log into an account if you weren't in the right cellblock.