[tor-bugs] #23641 [Core Tor/Tor]: prop224: Fake client auth lines do not actually provide obfuscation
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Mon Sep 25 12:29:49 UTC 2017
#23641: prop224: Fake client auth lines do not actually provide obfuscation
------------------------------+--------------------------------
Reporter: asn | Owner: (none)
Type: defect | Status: new
Priority: Medium | Milestone: Tor: 0.3.2.x-final
Component: Core Tor/Tor | Version: Tor: 0.3.2.1-alpha
Severity: Normal | Keywords: prop224 tor-hs
Actual Points: | Parent ID:
Points: | Reviewer:
Sponsor: |
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prop224 spec says:
{{{
If client auth is disabled, fake data is placed in each of the fields
below
to obfuscate whether client authorization is enabled.
}}}
and
{{{
When client authorization is enabled, each "auth-client" line
contains
the descriptor cookie encrypted to each individual client. We assume
that
each authorized client possesses a pre-shared x25519 keypair which
is
used to decrypt the descriptor cookie.
}}}
However, after second thinking, those fake client auth lines don't
actually confuse the adversaries we care about:
a) HSDir adversary with no onion address: This adversary does not learn
whether client auth is enabled because of the desc padding anyway.
b) HSDir adversary with onion address: This adversary can decrypt the
first desc layer, and then attempt to decrypt the second layer with no
client auth info. If second layer decrypts, there is no client auth. If
decryption fails, client auth is enabled.
Hence for both adversaries (a) and (b), the fact that we add fake client
auth lines does not matter. Perhaps we could rip that feature out
(`get_fake_auth_client_str()` etc.)
The annoying thing here is that currently we actually ''require'' client
auth lines since: `T1N(str_desc_auth_client, R3_DESC_AUTH_CLIENT, GE(3),
NO_OBJ)`.
As a first step here it might be a good idea to change the `T1N()` to
`T0N()` and make sure that everything run ssmoothly when no client auth
lines are provided.
--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/23641>
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