I've seen implementation of Rijndael cipher with 16-bit blocksize. And I have also tried to implement 16-bit Rijndael cipher by myself with only a little sneak peeking at the example of the implementation.
I think this was a very good way to learn how Rijndael or AES cipher works, by simplify the cipher and reducing the block size to small numbers that is possible to calculate with your brain.
I would love to learn how twofish works, and I was hoping to use same method as with Rijndael cipher. But I was not able to find anyone who had already implemented a 16-bits Twofish cipher, so I started to wonder if this was possible. After looking at the flowchart of the Twofish cipher:
K0 to K7 should be possible to reduce to 2-bit size, same with the S-boxes, MDS and PHT. But what about the rotary bitshifting and K2r+8? And are there other problems with reducing blocksize of Twofish cipher? Or the first question I should have asked, is it possible?
This is only for learning purpose, I know very well that 16-bits cipher is «unsecure» and highly probability for collision.