[p2p-hackers] Homomorphic hashing and fountain codes - implementation question

[Nick Johnson]

If I can be forgiven a stupid question:
I'm reading in detail the paper 
(http://www.scs.cs.nyu.edu/~mfreed/docs/authcodes-ieee04.pdf) on 
homomorphic hash functions for use with Digital Fountain codes in 
preparation for implementing it. The problem I'm coming up against is in 
the description of the modifications to the Fountain code described on 
page 5. With their example settings, 256 bit long sub-blocks are now 
added modulo a 257 bit prime. This makes sense - what I don't get is how 
to encode the result in 256 bits! What is one supposed to do if the sum 
of the selected blocks overflows 256 bits?

Can anyone enlighten me?

-Nick Johnson