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