[tor-dev] Proposal 274: Rotate onion keys less frequently
nickm at torproject.org
Fri Feb 24 16:24:51 UTC 2017
Title: Rotate onion keys less frequently.
Author: Nick Mathewson
This document proposes that, in order to limit the bandwidth needed
for microdescriptor listing and transmission, we reduce the onion key
rotation rate from the current value (7 days) to something closer to
Doing this will reduce the total microdescriptor download volume
by approximately 70%.
Currently, clients must download a networkstatus consensus document
once an hour, and must download every unfamiliar microdescriptor
listed in that document. Therefore, we can reduce client directory
bandwidth if we can cause microdescriptors to change less often.
Furthermore, we are planning (in proposal 140) to implement a
diff-based mechanism for clients to download only the parts of each
consensus that have changed. If we do that, then by having the
microdescriptor for each router change less often, we can make these
consensus diffs smaller as well.
I analyzed microdescriptor changes over the month of January
2017, and found that 94.5% of all microdescriptor transitions
were changes in onion key alone.
Therefore, we could reduce the number of changed "m" lines in
consensus diffs by approximately 94.5% * (3/4) =~ 70%,
if we were to rotate onion keys one-fourth as often.
The number of microdescriptors to actually download should
decrease by a similar number.
This amount to a significant reduction: currently, by
back-of-the-envelope estimates, an always-on client that downloads
all the directory info in a month downloads about 449MB of compressed
consensuses and something around 97 MB of compressed
microdescriptors. This proposal would save that user about 12% of
their total directory bandwidth.
If we assume that consensus diffs are implemented (see proposal 140),
then the user's compressed consensus downloads fall to something
closer to 27 MB. Under that analysis, the microdescriptors will
dominate again at 97 MB -- so lowering the number of microdescriptors
to fetch would save more like 55% of the remaining bandwidth.
[Back-of-the-envelope technique: assume every consensus is
downloaded, and every microdesc is downloaded, and microdescs are
downloaded in groups of 61, which works out to a constant rate.]
We'll need to do more analysis to assess the impact on clients that
connect to the network infrequently enough to miss microdescriptors:
nonetheless, the 70% figure above ought to apply to clients that connect
at least weekly.
(XXXX Better results pending feedback from ahf's analysis.)
4. Security analysis
The onion key is used to authenticate a relay to a client when the
client is building a circuit through that relay. The only reason to
limit their lifetime is to limit the impact if an attacker steals an
onion key without being detected.
If an attacker steals an onion key and is detected, the relay can
issue a new onion key ahead of schedule, with little disruption.
But if the onion key theft is _not_ detected, then the attacker
can use that onion key to impersonate the relay until clients
start using the relay's next key. In order to do so, the
attacker must also impersonate the target relay at the link
layer: either by stealing the relay's link keys, which rotate
more frequently, or by compromising the previous relay in the
Therefore, onion key rotation provides a small amount of
protection only against an attacker who can compromise relay keys
very intermittently, and who controls only a small portion of the
network. Against an attacker who can steal keys regularly it
does little, and an attacker who controls a lot of the network
can already mount other attacks.
I propose that we move the default onion key rotation interval
from 7 days to 28 days, as follows.
There should be a new consensus parameter, "onion-key-rotation-days",
measuring the key lifetime in days. Its minimum should be 1, its
maximum should be 90, and its default should be 28.
There should also be a new consensus parameter,
"onion-key-grace-period-days", measuring the interval for which
older onion keys should still be accepted. Its minimum should be
1, its maximum should be onion-key-rotation-days, and its default
should be 7.
Every relay should list each onion key it generates for
onion-key-rotation-days days after generating it, and then
replace it. Relays should continue to accept their most recent
previous onion key for an additional onion-key-rotation-days days
after it is replaced.
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