is there a deadline for the bounty?
I think I found a weakness but I'm not sure I can prove it before tomorrow.
Is there a working source code that I can download and modify in order to test my theory?

I thought that I could use rainbow tables in order to store only a small part of the birthday/nonce pairs and still be able to find collisions.
But since the space of nonces is 2^26 and the space of birthdays is 2^50 it wouldn't work because it wuuld find too many artificial collisions.
IF the nonce and birhday spaces were of the same order, then it would work perfectly.... but it looks like you are safe from rainbow tables: I can't think of a way to make them work.

is there a deadline for the bounty?
I think I found a weakness but I'm not sure I can prove it before tomorrow.
Is there a working source code that I can download and modify in order to test my theory?