Our design for limiting the predictability of HSDir upload locations relies on a shared random value (SRV) that isn't predictable in advance or too influenceable by an attacker. The authorities must run a protocol to generate such a value at least once per hsdir period. Here we describe how they publish these values; the procedure they use to generate them can change independently of the rest of this specification. For more information see [SHAREDRANDOM-REFS].
According to proposal 250, we add two new lines in consensuses:
"shared-rand-previous-value" SP NUM_REVEALS SP VALUE NL "shared-rand-current-value" SP NUM_REVEALS SP VALUE NL
If the previous or current shared random value cannot be found in a consensus, then Tor clients and services need to generate their own random value for use when choosing HSDirs.
To do so, Tor clients and services use:
SRV = H("shared-random-disaster" | INT_8(period_length) | INT_8(period_num))
where period_length is the length of a time period in minutes, rounded down; period_num is calculated as specified in [TIME-PERIODS] for the wanted shared random value that could not be found originally.
It's theoretically possible that the consensus shared random values will change or disappear in the middle of a time period because of directory authorities dropping offline or misbehaving.
To avoid client reachability issues in this rare event, hidden services should use the new shared random values to find the new responsible HSDirs and upload their descriptors there.
XXX How long should they upload descriptors there for?