LinkAuth method 1 is the one where we pull the TLS master secrets
out of the OpenSSL data structures and authenticate them with
RSA. Right now we list method 1 as required for clients and relays.
That's a problem, since we can't reasonably support it with NSS. So
let's remove it as a requirement and a recommendation.
As for method 3: I'd like to recommend it it, but that would make
0.2.9 start warning. Let's not do that till at least some time
after 0.3.5 (the next LTS) is stable.
Closes ticket 27286
Instead, count exits as usable if they have the exit flag, and
present if they also have a non-reject exit policy.
Requiring a threshold of usable descriptors avoids directories trickling
exit descriptors to clients to discover their ExitNodes settings.
Part of 27236.
Previously, Tor would only check the exit flag. In small networks, Tor
could bootstrap once it received a consensus with exits, without fetching
the new descriptors for those exits.
After bootstrap, Tor delays descriptor fetches, leading to failures in
fast networks like chutney.
Fixes 27236; bugfix on 0.2.6.3-alpha.
Bug description: For each descriptor, its revision counter is the OPE
ciphertext of the number of seconds since the start time of its SRV value.
This bug caused us to confuse the SRV start time in the middle of the lifetime
of a descriptor in some edge-cases, which caused descriptor rejects.
Bug cause: The bug occurs when we fetch a 23:00 consensus after
midnight (e.g. at 00:08 when not all dirauths have fetched the latest 00:00
consensus). In that case, the voting schedule (which was used for SRV start
time calculation) would return a valid-after past-midnight, whereas our
consensus would be pre-midnight, and that would confuse the SRV start time
computation which is used by HS revision counters (because we would reset the
start time of SRV, without rotating descriptors).
Bug fix: We now use our local consensus time to calculate the SRV start time,
instead of the voting schedule. The voting schedule does not work as originally
envisioned in this case, because it was created for voting by dirauths and not
for scheduling stuff on clients.
