Hello there,
we are happy to tell you that we finished coding proposal 250 and our first attempt at implementation is ready for review.
You can find the final specification here: https://gitweb.torproject.org/user/dgoulet/torspec.git/log/?h=prop250_final_... and the corresponding code here: https://gitweb.torproject.org/user/dgoulet/tor.git/log/?h=prop250_final_v1
Trac ticket #16943 (https://trac.torproject.org/projects/tor/ticket/16943) will be used for code review. Please use this mailing list to discuss any issues with the spec.
Some notes: We've separated this in 7 commits prefixed with prop250: except first one that adds a needed tor_htonll/ntohll function to tor utils. This code is mostly contained in two *new* files (with their headers) that are shared-random.{c|h} and shared-random-state.{c|h}.
For what it's worth, we expect this code to run for a long time before the shared random values generated by the authorities are used for anything (e.g. HSDir scrambling).
Cheers!