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```
Filename: 136-legacy-keys.txt
Title: Mass authority migration with legacy keys
Author: Nick Mathewson
Created: 13-May-2008
Status: Closed
Implemented-In: 0.2.0.x
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.
```
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