1 Filename: 130-v2-conn-protocol.txt
2 Title: Version 2 Tor connection protocol
6 Implemented-In: 0.2.0.x
10 This proposal describes the significant changes to be made in the v2
11 Tor connection protocol.
13 This proposal relates to other proposals as follows:
15 It refers to and supersedes:
16 Proposal 124: Blocking resistant TLS certificate usage
17 It refers to aspects of:
18 Proposal 105: Version negotiation for the Tor protocol
21 In summary, The Tor connection protocol has been in need of a redesign
22 for a while. This proposal describes how we can add to the Tor
25 - A new TLS handshake (to achieve blocking resistance without
26 breaking backward compatibility)
27 - Version negotiation (so that future connection protocol changes
28 can happen without breaking compatibility)
29 - The actual changes in the v2 Tor connection protocol.
33 For motivation, see proposal 124.
39 The version of the Tor connection protocol implemented up to now is
40 "version 1". This proposal describes "version 2".
42 "Old" or "Older" versions of Tor are ones not aware that version 2
43 of this protocol exists;
44 "New" or "Newer" versions are ones that are.
46 The connection initiator is referred to below as the Client; the
47 connection responder is referred to below as the Server.
49 1. The revised TLS handshake.
51 For motivation, see proposal 124. This is a simplified version of the
52 handshake that uses TLS's renegotiation capability in order to avoid
53 some of the extraneous steps in proposal 124.
55 The Client connects to the Server and, as in ordinary TLS, sends a
56 list of ciphers. Older versions of Tor will send only ciphers from
58 TLS_DHE_RSA_WITH_AES_256_CBC_SHA
59 TLS_DHE_RSA_WITH_AES_128_CBC_SHA
60 SSL_DHE_RSA_WITH_3DES_EDE_CBC_SHA
61 SSL_DHE_DSS_WITH_3DES_EDE_CBC_SHA
62 Clients that support the revised handshake will send the recommended
63 list of ciphers from proposal 124, in order to emulate the behavior of
66 If the server notices that the list of ciphers contains only ciphers
67 from this list, it proceeds with Tor's version 1 TLS handshake as
68 documented in tor-spec.txt.
70 (The server may also notice cipher lists used by other implementations
71 of the Tor protocol (in particular, the BouncyCastle default cipher
72 list as used by some Java-based implementations), and whitelist them.)
74 On the other hand, if the server sees a list of ciphers that could not
75 have been sent from an older implementation (because it includes other
76 ciphers, and does not match any known-old list), the server sends a
77 reply containing a single connection certificate, constructed as for
78 the link certificate in the v1 Tor protocol. The subject names in
79 this certificate SHOULD NOT have any strings to identify them as
80 coming from a Tor server. The server does not ask the client for
83 Old Servers will (mostly) ignore the cipher list and respond as in the v1
84 protocol, sending back a two-certificate chain.
86 After the Client gets a response from the server, it checks for the
87 number of certificates it received. If there are two certificates,
88 the client assumes a V1 connection and proceeds as in tor-spec.txt.
89 But if there is only one certificate, the client assumes a V2 or later
90 protocol and continues.
92 At this point, the client has established a TLS connection with the
93 server, but the parties have not been authenticated: the server hasn't
94 sent its identity certificate, and the client hasn't sent any
95 certificates at all. To fix this, the client begins a TLS session
96 renegotiation. This time, the server continues with two certificates
97 as usual, and asks for certificates so that the client will send
98 certificates of its own. Because the TLS connection has been
99 established, all of this is encrypted. (The certificate sent by the
100 server in the renegotiated connection need not be the same that
101 as sentin the original connection.)
103 The server MUST NOT write any data until the client has renegotiated.
105 Once the renegotiation is finished, the server and client check one
106 another's certificates as in V1. Now they are mutually authenticated.
108 1.1. Revised TLS handshake: implementation notes.
110 It isn't so easy to adjust server behavior based on the client's
111 ciphersuite list. Here's how we can do it using OpenSSL. This is a
112 bit of an abuse of the OpenSSL APIs, but it's the best we can do, and
113 we won't have to do it forever.
115 We can use OpenSSL's SSL_set_info_callback() to register a function to
116 be called when the state changes. The type/state tuple of
117 SSL_CB_ACCEPT_LOOP/SSL3_ST_SW_SRVR_HELLO_A
118 happens when we have completely parsed the client hello, and are about
119 to send a response. From this callback, we can check the cipherlist
122 * If the ciphersuite list indicates a v1 protocol, we set the
123 verify mode to SSL_VERIFY_NONE with a callback (so we get
126 * If the ciphersuite list indicates a v2 protocol, we set the
127 verify mode to SSL_VERIFY_NONE with no callback (so we get
128 no certificates) and set the SSL_MODE_NO_AUTO_CHAIN flag (so that
129 we send only 1 certificate in the response.
131 Once the handshake is done, the server clears the
132 SSL_MODE_NO_AUTO_CHAIN flag and sets the callback as for the V1
133 protocol. It then starts reading.
135 The other problem to take care of is missing ciphers and OpenSSL's
136 cipher sorting algorithms. The two main issues are a) OpenSSL doesn't
137 support some of the default ciphers that Firefox advertises, and b)
138 OpenSSL sorts the list of ciphers it offers in a different way than
139 Firefox sorts them, so unless we fix that Tor will still look different
144 1.2. Compatibility for clients using libraries less hackable than OpenSSL.
146 As discussed in proposal 105, servers advertise which protocol
147 versions they support in their router descriptors. Clients can simply
148 behave as v1 clients when connecting to servers that do not support
149 link version 2 or higher, and as v2 clients when connecting to servers
150 that do support link version 2 or higher.
152 (Servers can't use this strategy because we do not assume that servers
153 know one another's capabilities when connecting.)
155 2. Version negotiation.
157 Version negotiation proceeds as described in proposal 105, except as
160 * Version negotiation only happens if the TLS handshake as described
163 * The TLS renegotiation must be finished before the client sends a
164 VERSIONS cell; the server sends its VERSIONS cell in response.
166 * The VERSIONS cell uses the following variable-width format:
167 Circuit [2 octets; set to 0]
168 Command [1 octet; set to 7 for VERSIONS]
169 Length [2 octets; big-endian]
172 The Data in the cell is a series of big-endian two-byte integers.
174 * It is not allowed to negotiate V1 conections once the v2 protocol
175 has been used. If this happens, Tor instances should close the
178 3. The rest of the "v2" protocol
180 Once a v2 protocol has been negotiated, NETINFO cells are exchanged
181 as in proposal 105, and communications begin as per tor-spec.txt.
182 Until NETINFO cells have been exchanged, the connection is not open.