Have you considered Repurposing an existing keyboard ? The keys are generally connected in a grid so rather than having to read 100 keys you can just read the columns, the key is then to switch the rows on one by one, each key connects a unique combination of row and column if you know which row is switched on, its easy to know which keys are pressed.
This is my example but there are many other similar approaches, you can even combine this with shift in to get the required pin count down to 5 -
Duane B