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