Filename: 130-v2-conn-protocol.txt
Title: Version 2 Tor connection protocol
Author: Nick Mathewson
Created: 2007-10-25
Status: Closed
Implemented-In: 0.2.0.x
Overview:
This proposal describes the significant changes to be made in the v2
Tor connection protocol.
This proposal relates to other proposals as follows:
It refers to and supersedes:
Proposal 124: Blocking resistant TLS certificate usage
It refers to aspects of:
Proposal 105: Version negotiation for the Tor protocol
In summary, The Tor connection protocol has been in need of a redesign
for a while. This proposal describes how we can add to the Tor
protocol:
- A new TLS handshake (to achieve blocking resistance without
breaking backward compatibility)
- Version negotiation (so that future connection protocol changes
can happen without breaking compatibility)
- The actual changes in the v2 Tor connection protocol.
Motivation:
For motivation, see proposal 124.
Proposal:
0. Terminology
The version of the Tor connection protocol implemented up to now is
"version 1". This proposal describes "version 2".
"Old" or "Older" versions of Tor are ones not aware that version 2
of this protocol exists;
"New" or "Newer" versions are ones that are.
The connection initiator is referred to below as the Client; the
connection responder is referred to below as the Server.
1. The revised TLS handshake.
For motivation, see proposal 124. This is a simplified version of the
handshake that uses TLS's renegotiation capability in order to avoid
some of the extraneous steps in proposal 124.
The Client connects to the Server and, as in ordinary TLS, sends a
list of ciphers. Older versions of Tor will send only ciphers from
the list:
TLS_DHE_RSA_WITH_AES_256_CBC_SHA
TLS_DHE_RSA_WITH_AES_128_CBC_SHA
SSL_DHE_RSA_WITH_3DES_EDE_CBC_SHA
SSL_DHE_DSS_WITH_3DES_EDE_CBC_SHA
Clients that support the revised handshake will send the recommended
list of ciphers from proposal 124, in order to emulate the behavior of
a web browser.
If the server notices that the list of ciphers contains only ciphers
from this list, it proceeds with Tor's version 1 TLS handshake as
documented in tor-spec.txt.
(The server may also notice cipher lists used by other implementations
of the Tor protocol (in particular, the BouncyCastle default cipher
list as used by some Java-based implementations), and whitelist them.)
On the other hand, if the server sees a list of ciphers that could not
have been sent from an older implementation (because it includes other
ciphers, and does not match any known-old list), the server sends a
reply containing a single connection certificate, constructed as for
the link certificate in the v1 Tor protocol. The subject names in
this certificate SHOULD NOT have any strings to identify them as
coming from a Tor server. The server does not ask the client for
certificates.
Old Servers will (mostly) ignore the cipher list and respond as in the v1
protocol, sending back a two-certificate chain.
After the Client gets a response from the server, it checks for the
number of certificates it received. If there are two certificates,
the client assumes a V1 connection and proceeds as in tor-spec.txt.
But if there is only one certificate, the client assumes a V2 or later
protocol and continues.
At this point, the client has established a TLS connection with the
server, but the parties have not been authenticated: the server hasn't
sent its identity certificate, and the client hasn't sent any
certificates at all. To fix this, the client begins a TLS session
renegotiation. This time, the server continues with two certificates
as usual, and asks for certificates so that the client will send
certificates of its own. Because the TLS connection has been
established, all of this is encrypted. (The certificate sent by the
server in the renegotiated connection need not be the same that
as sentin the original connection.)
The server MUST NOT write any data until the client has renegotiated.
Once the renegotiation is finished, the server and client check one
another's certificates as in V1. Now they are mutually authenticated.
1.1. Revised TLS handshake: implementation notes.
It isn't so easy to adjust server behavior based on the client's
ciphersuite list. Here's how we can do it using OpenSSL. This is a
bit of an abuse of the OpenSSL APIs, but it's the best we can do, and
we won't have to do it forever.
We can use OpenSSL's SSL_set_info_callback() to register a function to
be called when the state changes. The type/state tuple of
SSL_CB_ACCEPT_LOOP/SSL3_ST_SW_SRVR_HELLO_A
happens when we have completely parsed the client hello, and are about
to send a response. From this callback, we can check the cipherlist
and act accordingly:
* If the ciphersuite list indicates a v1 protocol, we set the
verify mode to SSL_VERIFY_NONE with a callback (so we get
certificates).
* If the ciphersuite list indicates a v2 protocol, we set the
verify mode to SSL_VERIFY_NONE with no callback (so we get
no certificates) and set the SSL_MODE_NO_AUTO_CHAIN flag (so that
we send only 1 certificate in the response.
Once the handshake is done, the server clears the
SSL_MODE_NO_AUTO_CHAIN flag and sets the callback as for the V1
protocol. It then starts reading.
The other problem to take care of is missing ciphers and OpenSSL's
cipher sorting algorithms. The two main issues are a) OpenSSL doesn't
support some of the default ciphers that Firefox advertises, and b)
OpenSSL sorts the list of ciphers it offers in a different way than
Firefox sorts them, so unless we fix that Tor will still look different
than Firefox.
[XXXX more on this.]
1.2. Compatibility for clients using libraries less hackable than OpenSSL.
As discussed in proposal 105, servers advertise which protocol
versions they support in their router descriptors. Clients can simply
behave as v1 clients when connecting to servers that do not support
link version 2 or higher, and as v2 clients when connecting to servers
that do support link version 2 or higher.
(Servers can't use this strategy because we do not assume that servers
know one another's capabilities when connecting.)
2. Version negotiation.
Version negotiation proceeds as described in proposal 105, except as
follows:
* Version negotiation only happens if the TLS handshake as described
above completes.
* The TLS renegotiation must be finished before the client sends a
VERSIONS cell; the server sends its VERSIONS cell in response.
* The VERSIONS cell uses the following variable-width format:
Circuit [2 octets; set to 0]
Command [1 octet; set to 7 for VERSIONS]
Length [2 octets; big-endian]
Data [Length bytes]
The Data in the cell is a series of big-endian two-byte integers.
* It is not allowed to negotiate V1 connections once the v2 protocol
has been used. If this happens, Tor instances should close the
connection.
3. The rest of the "v2" protocol
Once a v2 protocol has been negotiated, NETINFO cells are exchanged
as in proposal 105, and communications begin as per tor-spec.txt.
Until NETINFO cells have been exchanged, the connection is not open.