[tor-dev] Proposal 198: Restore semantics of TLS ClientHello

Nick Mathewson nickm at freehaven.net
Tue Mar 20 15:33:21 UTC 2012

Filename: 198-restore-clienthello-semantics.txt
Title: Restore semantics of TLS ClientHello
Author: Nick Mathewson
Created: 19-Mar-2012
Status: Open


   Currently, all supported Tor versions try to imitate an older version
   of Firefox when advertising ciphers in their TLS ClientHello.  This
   feature is intended to make it harder for a censor to distinguish a
   Tor client from other TLS traffic.  Unfortunately, it makes the
   contents of the ClientHello unreliable: a server cannot conclude that
   a cipher is really supported by a Tor client simply because it is
   advertised in the ClientHello.

   This proposal suggests an approach for restoring sanity to our use of
   ClientHello, so that we still avoid ciphersuite-based fingerprinting,
   but allow nodes to negotiate better ciphersuites than they are
   allowed to negotiate today.

Background reading:

   Section 2 of tor-spec.txt 2 describes our current baroque link
   negotiation scheme.  Proposals 176 and 184 describe more information
   about how it got that way.

   Bug 4744 is a big part of the motivation for this proposal: we want
   to allow Tors to advertise even more ciphers, some of which we would
   actually prefer to the ones we are using now.

   What you need to know about the TLS handshake is that the client
   sends a list of all the ciphersuites that it supports in its
   ClientHello message, and then the server chooses one and tells the
   client which one it picked.

Motivation and constraints:

   We'd like to use some of the ECDHE TLS ciphersuites, since they allow
   us to get better forward-secrecy at lower cost than our current
   DH-1024 usage.  But right now, we can't ever use them, since Tor will
   advertise them whether or not it has a version of OpenSSL that
   supports them.

   (OpenSSL before 1.0.0 did not support ECDHE ciphersuites; OpenSSL
   before 1.0.0e or so had some security issues with them.)

   We cannot have the rule be "Tors must only advertise ciphersuites
   that they can use", since current Tors will advertise such
   ciphersuites anyway.

   We cannot have the rule be "Tors must support every ECDHE ciphersuite
   on the following list", since current Tors don't do all that, and
   since one prominent Linux distribution builds OpenSSL without ECC
   support because of patent/freedom fears.

   Fortunately, nearly every ciphersuite that we would like to advertise
   to imitate FF8 (see bug 4744) is currently supported by OpenSSL 1.0.0
   and later.  This enables the following proposal to work:

Proposed spec changes:

   I propose that the rules for handling ciphersuites at the server side
   become the following:

   If the ciphersuites in the ClientHello contains no ciphers other than
   the following[*], they indicate that the Tor v1 link protocol is in use.


   If the advertised ciphersuites in the ClientHello are _exactly_[*]
   the following, they indicate that the Tor v2+ link protocol is in
   use, AND that the ClientHello may have unsupported ciphers.  In this
   case, the server may choose DHE_RSA_WITH_AES_128_CBC_SHA  or
   DHE_RSA_WITH_AES_256_SHA, but may not choose any other cipher.


  [*] The "extended renegotiation is supported" ciphersuite, 0x00ff, is
      not counted when checking the list of ciphersuites.

  Otherwise, the ClientHello has these semantics: The inclusion of any
  cipher supported by OpenSSL 1.0.0 means that the client supports it,
  with the exception of
  which is never supported. Clients MUST advertise support for at least one of

  The server MUST choose a ciphersuite with ephemeral keys for forward
  secrecy; MUST NOT choose a weak or null ciphersuite; and SHOULD NOT
  choose any cipher other than AES or 3DES.

Discussion and consequences:

  Currently, OpenSSL 1.0.0 (in its default configuration) supports every
  cipher that we would need in order to give the same list as Firefox
  versions 8 through 11 give in their default configuration, with the
  exception of the FIPS ciphersuite above.  Therefore, we will be able
  to fake the new ciphersuite list correctly in all of our bundles that
  include OpenSSL, and on every version of Unix that keeps up-to-date.

  However, versions of Tor compiled to use older versions of OpenSSL, or
  versions of OpenSSL with some ciphersuites disabled, will no
  longer give the same ciphersuite lists as other versions of Tor.  On
  these platforms, Tor clients will no longer impersonate Firefox.
  Users who need to do so will have to download one of our bundles, or
  use a (non-system) OpenSSL.

  The proposed spec change above tries to future-proof ourselves by not
  declaring that we support every declared cipher, in case we someday
  need to handle a new Firefox version.  If a new Firefox version
  comes out that uses ciphers not supported by OpenSSL 1.0.0, we will
  need to define whether clients may advertise its ciphers without
  supporting them; but existing servers will continue working whether
  we decide yes or no.

  The restriction to "servers SHOULD only pick AES or 3DES" is meant to
  reflect our current behavior, not to represent a permanent refusal to
  support other ciphers.  We can revisit it later as appropriate, if for
  some bizarre reason Camellia or Seed or Aria becomes a better bet than

Open questions:

  Should the client drop connections if the server chooses a bad
  cipher, or a suite without forward secrecy?

  Can we get OpenSSL to support the dubious FIPS suite excluded above,
  in order to remove a distinguishing opportunity?  It is not so simple
  as just editing the SSL_CIPHER list in s3_lib.c, since the nonstandard
  SSL_RSA_FIPS_WITH_3DES_EDE_CBC_SHA cipher is (IIUC) defined to use the
  TLS1 KDF, while declaring itself to be an SSL cipher (!).

  Can we do anything to eventually allow the IE7+[**] cipher list as
  well?  IE does not support TLS_DHE_RSA_WITH_AES_{256,128}_SHA or
  SSL_DHE_RSA_WITH_3DES_EDE_CBC_SHA, and so wouldn't work with current
  Tor servers, which _only_ support those.  It looks like the only
  forward-secure ciphersuites that IE7+ *does* support are ECDHE ones,
  and DHE+DSS ones.  So if we want this flexibility, we could mandate
  server-side ECDHE, or somehow get DHE+DSS support (which would play
  havoc with our current certificate generation code IIUC), or say that
  it is sometimes acceptable to have a non-forward-secure link
  protocol[***].  None of these answers seems like a great one.  Is one
  best?  Are there other options?

  [**] Actually, I think it's the Windows SChannel cipher list we
  should be looking at here.
  [***] If we did _that_, we'd want to specify that CREATE_FAST could
  never be used on a non-forward-secure link.  Even so, I don't like the
  implications of leaking cell types and circuit IDs to a future

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