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[tor-dev] Proposal 189: AUTHORIZE and AUTHORIZED cells
Title: AUTHORIZE and AUTHORIZED cells
Author: George Kadianakis
Created: 04 Nov 2011
Proposal 187 introduced the concept of the AUTHORIZE cell, a cell
whose purpose is to make Tor bridges resistant to scanning attacks.
This is achieved by having the bridge and the client share a secret
out-of-band and then use AUTHORIZE cells to validate that the
client indeed knows that secret before proceeding with the Tor
This proposal specifies the format of the AUTHORIZE cell and also
introduces the AUTHORIZED cell, a way for bridges to announce to
clients that the authorization process is complete and successful.
AUTHORIZE cells should be able to perform a variety of
authorization protocols based on a variety of shared secrets. This
forces the AUTHORIZE cell to have a dynamic format based on the
authorization method used.
AUTHORIZED cells are used by bridges to signal the end of a
successful bridge client authorization and the beginning of the
actual link handshake. AUTHORIZED cells have no other use and for
this reason their format is very simple.
Both AUTHORIZE and AUTHORIZED cells are to be used under censorship
conditions and they should look innocuous to any adversary capable
of monitoring network traffic.
As an attack example, an adversary could passively monitor the
traffic of a bridge host, looking at the packets directly after the
TLS handshake and trying to deduce from their packet size if they
are AUTHORIZE and AUTHORIZED cells. For this reason, AUTHORIZE and
AUTHORIZED cells are padded with a random amount of padding before
3.1. AUTHORIZE cell
The AUTHORIZE cell is a variable-sized cell.
The generic AUTHORIZE cell format is:
AuthMethod [1 octet]
PadLen [2 octets]
Padding ['PadLen' octets]
'AuthMethod', is the authorization method to be used.
'MethodFields', is dependent on the authorization Method used. It's
a meta-field hosting an arbitrary amount of fields.
'PadLen', specifies the amount of padding in octets.
'Padding', is 'PadLen' octets of random content.
3.2. AUTHORIZED cell format
The AUTHORIZED cell is a variable-sized cell.
The AUTHORIZED cell format is:
'AuthMethod' [1 octet]
'PadLen' [2 octets]
'Padding' ['PadLen' octets]
where all fields have the same meaning as in section 3.1.
3.3. Cell parsing
Implementations MUST ignore the contents of 'Padding'.
Implementations MUST reject an AUTHORIZE or AUTHORIZED cell where
the 'Padding' field is not 'PadLen' octets long.
Implementations MUST reject an AUTHORIZE cell with an 'AuthMethod'
they don't recognize.
4.1. Why not let the pluggable transports do the padding, like they
are supposed to do for the rest of the Tor protocol?
The arguments of section "Alternative design: Just use pluggable
transports" of proposal 187, apply here as well:
All bridges who use client authorization will also need camouflaged
4.2. How should multiple round-trip authorization protocols be handled?
Protocols that require multiple round-trips between the client and
the bridge should use AUTHORIZE cells for communication.
The format of the AUTHORIZE cell is flexible enough to support
messages from the client to the bridge and the inverse.
In the end of a successful multiple round-trip protocol, an
AUTHORIZED cell must be issued from the bridge to the client.
4.3. AUTHORIZED seems useless. Why not use VPADDING instead?
As noted in proposal 187, the Tor protocol uses VPADDING cells for
padding; any other use of VPADDING makes the Tor protocol kludgy.
In the future, and in the example case of a v3 handshake, a client
can optimistically send a VERSIONS cell along with the final
AUTHORIZE cell of an authorization protocol. That allows the
bridge, in the case of successful authorization, to also process
the VERSIONS cell and begin the v3 handshake promptly.
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