mirror of
https://gitlab.torproject.org/tpo/core/tor.git
synced 2024-11-11 13:43:47 +01:00
100 lines
4.1 KiB
Plaintext
100 lines
4.1 KiB
Plaintext
Filename: 136-legacy-keys.txt
|
|
Title: Mass authority migration with legacy keys
|
|
Author: Nick Mathewson
|
|
Created: 13-May-2008
|
|
Status: Finished
|
|
|
|
Overview:
|
|
|
|
This document describes a mechanism to change the keys of more than
|
|
half of the directory servers at once without breaking old clients
|
|
and caches immediately.
|
|
|
|
Motivation:
|
|
|
|
If a single authority's identity key is believed to be compromised,
|
|
the solution is obvious: remove that authority from the list,
|
|
generate a new certificate, and treat the new cert as belonging to a
|
|
new authority. This approach works fine so long as less than 1/2 of
|
|
the authority identity keys are bad.
|
|
|
|
Unfortunately, the mass-compromise case is possible if there is a
|
|
sufficiently bad bug in Tor or in any OS used by a majority of v3
|
|
authorities. Let's be prepared for it!
|
|
|
|
We could simply stop using the old keys and start using new ones,
|
|
and tell all clients running insecure versions to upgrade.
|
|
Unfortunately, this breaks our cacheing system pretty badly, since
|
|
caches won't cache a consensus that they don't believe in. It would
|
|
be nice to have everybody become secure the moment they upgrade to a
|
|
version listing the new authority keys, _without_ breaking upgraded
|
|
clients until the caches upgrade.
|
|
|
|
So, let's come up with a way to provide a time window where the
|
|
consensuses are signed with the new keys and with the old.
|
|
|
|
Design:
|
|
|
|
We allow directory authorities to list a single "legacy key"
|
|
fingerprint in their votes. Each authority may add a single legacy
|
|
key. The format for this line is:
|
|
|
|
legacy-dir-key FINGERPRINT
|
|
|
|
We describe a new consensus method for generating directory
|
|
consensuses. This method is consensus method "3".
|
|
|
|
When the authorities decide to use method "3" (as described in 3.4.1
|
|
of dir-spec.txt), for every included vote with a legacy-dir-key line,
|
|
the consensus includes an extra dir-source line. The fingerprint in
|
|
this extra line is as in the legacy-dir-key line. The ports and
|
|
addresses are in the dir-source line. The nickname is as in the
|
|
dir-source line, with the string "-legacy" appended.
|
|
|
|
[We need to include this new dir-source line because the code
|
|
won't accept or preserve signatures from authorities not listed
|
|
as contributing to the consensus.]
|
|
|
|
Authorities using legacy dir keys include two signatures on their
|
|
consensuses: one generated with a signing key signed with their real
|
|
signing key, and another generated with a signing key signed with
|
|
another signing key attested to by their identity key. These
|
|
signing keys MUST be different. Authorities MUST serve both
|
|
certificates if asked.
|
|
|
|
Process:
|
|
|
|
In the event of a mass key failure, we'll follow the following
|
|
(ugly) procedure:
|
|
- All affected authorities generate new certificates and identity
|
|
keys, and circulate their new dirserver lines. They copy their old
|
|
certificates and old broken keys, but put them in new "legacy
|
|
key files".
|
|
- At the earliest time that can be arranged, the authorities
|
|
replace their signing keys, identity keys, and certificates
|
|
with the new uncompromised versions, and update to the new list
|
|
of dirserer lines.
|
|
- They add an "V3DirAdvertiseLegacyKey 1" option to their torrc.
|
|
- Now, new consensuses will be generated using the new keys, but
|
|
the results will also be signed with the old keys.
|
|
- Clients and caches are told they need to upgrade, and given a
|
|
time window to do so.
|
|
- At the end of the time window, authorities remove the
|
|
V3DirAdvertiseLegacyKey option.
|
|
|
|
Notes:
|
|
|
|
It might be good to get caches to cache consensuses that they do not
|
|
believe in. I'm not sure the best way of how to do this.
|
|
|
|
It's a superficially neat idea to have new signing keys and have
|
|
them signed by the new and by the old authority identity keys. This
|
|
breaks some code, though, and doesn't actually gain us anything,
|
|
since we'd still need to include each signature twice.
|
|
|
|
It's also a superficially neat idea, if identity keys and signing
|
|
keys are compromised, to at least replace all the signing keys.
|
|
I don't think this achieves us anything either, though.
|
|
|
|
|