1 Filename: 139-conditional-consensus-download.txt
2 Title: Download consensus documents only when it will be trusted
3 Author: Peter Palfrader
6 Implemented-In: 0.2.1.x
10 Servers only provide consensus documents to clients when it is known that
11 the client will trust it.
15 When clients[1] want a new network status consensus they request it
16 from a Tor server using the URL path /tor/status-vote/current/consensus.
17 Then after downloading the client checks if this consensus can be
18 trusted. Whether the client trusts the consensus depends on the
19 authorities that the client trusts and how many of those
20 authorities signed the consensus document.
22 If the client cannot trust the consensus document it is disregarded
23 and a new download is tried at a later time. Several hundred
24 kilobytes of server bandwidth were wasted by this single client's
27 With hundreds of thousands of clients this will have undesirable
28 consequences when the list of authorities has changed so much that a
29 large number of established clients no longer can trust any consensus
34 The objective of this proposal is to make clients not download
35 consensuses they will not trust.
39 The list of authorities that are trusted by a client are encoded in
40 the URL they send to the directory server when requesting a consensus
43 The directory server then only sends back the consensus when more than
44 half of the authorities listed in the request have signed the
45 consensus. If it is known that the consensus will not be trusted
46 a 404 error code is sent back to the client.
48 This proposal does not require directory caches to keep more than one
49 consensus document. This proposal also does not require authorities
50 to verify the signature on the consensus document of authorities they
53 The new URL scheme to download a consensus is
54 /tor/status-vote/current/consensus/<F> where F is a list of
55 fingerprints, sorted in ascending order, and concatenated using a +
58 Fingerprints are uppercase hexadecimal encodings of the authority
59 identity key's digest. Servers should also accept requests that
60 use lower case or mixed case hexadecimal encodings.
62 A .z URL for compressed versions of the consensus will be provided
63 similarly to existing resources and is the URL that usually should
68 The old location of the consensus should continue to work
69 indefinitely. Not only is it used by old clients, but it is a useful
70 resource for automated tools that do not particularly care which
71 authorities have signed the consensus.
73 Authorities that are known to the client a priori by being shipped
74 with the Tor code are assumed to handle this format.
76 When downloading a consensus document from caches that do not support this
77 new format they fall back to the old download location.
79 Caches support the new format starting with Tor version 0.2.1.1-alpha.
81 Anonymity Implications:
83 By supplying the list of authorities a client trusts to the directory
84 server we leak information (like likely version of Tor client) to the
85 directory server. In the current system we also leak that we are
86 very old - by re-downloading the consensus over and over again, but
87 only when we are so old that we no longer can trust the consensus.
92 1. For the purpose of this proposal a client can be any Tor instance
93 that downloads a consensus document. This includes relays,
94 directory caches as well as end users.