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112 lines
4.1 KiB
Plaintext
112 lines
4.1 KiB
Plaintext
Filename: 106-less-tls-constraint.txt
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Title: Checking fewer things during TLS handshakes
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Author: Nick Mathewson
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Created: 9-Feb-2007
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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 document proposes that we relax our requirements on the context of
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X.509 certificates during initial TLS handshakes.
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Motivation:
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Later, we want to try harder to avoid protocol fingerprinting attacks.
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This means that we'll need to make our connection handshake look closer
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to a regular HTTPS connection: one certificate on the server side and
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zero certificates on the client side. For now, about the best we
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can do is to stop requiring things during handshake that we don't
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actually use.
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What we check now, and where we check it:
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tor_tls_check_lifetime:
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peer has certificate
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notBefore <= now <= notAfter
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tor_tls_verify:
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peer has at least one certificate
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There is at least one certificate in the chain
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At least one of the certificates in the chain is not the one used to
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negotiate the connection. (The "identity cert".)
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The certificate _not_ used to negotiate the connection has signed the
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link cert
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tor_tls_get_peer_cert_nickname:
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peer has a certificate.
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certificate has a subjectName.
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subjectName has a commonName.
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commonName consists only of characters in LEGAL_NICKNAME_CHARACTERS. [2]
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tor_tls_peer_has_cert:
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peer has a certificate.
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connection_or_check_valid_handshake:
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tor_tls_peer_has_cert [1]
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tor_tls_get_peer_cert_nickname [1]
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tor_tls_verify [1]
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If nickname in cert is a known, named router, then its identity digest
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must be as expected.
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If we initiated the connection, then we got the identity digest we
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expected.
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USEFUL THINGS WE COULD DO:
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[1] We could just not force clients to have any certificate at all, let alone
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an identity certificate. Internally to the code, we could assign the
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identity_digest field of these or_connections to a random number, or even
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not add them to the identity_digest->or_conn map.
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[so if somebody connects with no certs, we let them. and mark them as
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a client and don't treat them as a server. great. -rd]
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[2] Instead of using a restricted nickname character set that makes our
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commonName structure look unlike typical SSL certificates, we could treat
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the nickname as extending from the start of the commonName up to but not
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including the first non-nickname character.
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Alternatively, we could stop checking commonNames entirely. We don't
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actually _do_ anything based on the nickname in the certificate, so
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there's really no harm in letting every router have any commonName it
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wants.
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[this is the better choice -rd]
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[agreed. -nm]
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REMAINING WAYS TO RECOGNIZE CLIENT->SERVER CONNECTIONS:
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Assuming that we removed the above requirements, we could then (in a later
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release) have clients not send certificates, and sometimes and started
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making our DNs a little less formulaic, client->server OR connections would
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still be recognizable by:
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having a two-certificate chain sent by the server
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using a particular set of ciphersuites
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traffic patterns
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probing the server later
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OTHER IMPLICATIONS:
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If we stop verifying the above requirements:
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It will be slightly (but only slightly) more common to connect to a non-Tor
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server running TLS, and believe that you're talking to a Tor server (until
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you send the first cell).
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It will be far easier for non-Tor SSL clients to accidentally connect to
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Tor servers and speak HTTPS or whatever to them.
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If, in a later release, we have clients not send certificates, and we make
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DNs less recognizable:
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If clients don't send certs, servers don't need to verify them: win!
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If we remove these restrictions, it will be easier for people to write
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clients to fuzz our protocol: sorta win!
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If clients don't send certs, they look slightly less like servers.
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OTHER SPEC CHANGES:
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When a client doesn't give us an identity, we should never extend any
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circuits to it (duh), and we should allow it to set circuit ID however it
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wants.
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