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

Nick Mathewson nickm at alum.mit.edu
Mon Mar 26 14:23:15 UTC 2012

On Mon, Mar 26, 2012 at 3:17 AM, Robert Ransom <rransom.8774 at gmail.com> wrote:
>>    (OpenSSL before 1.0.0 did not support ECDHE ciphersuites; OpenSSL
>>    before 1.0.0e or so had some security issues with them.)
> Can Tor detect that it is running with a version of OpenSSL with those
> security issues and refuse to support the broken ciphersuites?

We can detect if the version number is for a broken version, but I
don't know a good way to detect if the version number is old but the
issues are fixed (for example, if it's one of those Fedora versions
that lock the openssl version to something older so that they don't
run into spurious ABI incompatibility).

I need to find out more about what the security issues actually were:
when I took a quick look, the only one I was a problem with doing
multithreaded access to SSL data structures when using ECC.  That
wouldn't be a problem for us, but if there are other issues, we should
know about them.

>>   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
> I'm no longer comfortable with 128-bit symmetric keys.  An attacker
> with many messages encrypted with a 128-bit symmetric cipher can
> attempt a brute-force search on many messages at once, and is likely
> to succeed in finding keys for some messages.  (See
> http://cr.yp.to/papers.html#bruteforce .)

Hm. We'd need to check whether all the servers today support an AES256
ciphersuite.  Also, wasn't there some dodgy issue in the AES256 key
schedule?  Or is that basically irrelevant?

>>   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.
> Why standardize on OpenSSL 1.0.0, rather than OpenSSL 1.0.1?

1.0.0 is good enough to get everything we need for ff8+.  Also, when I
wrote the document, 1.0.0 was pretty ubiquitous but 1.0.1 had only
been out for a few days.  We could do 1.0.1, I guess.

>>   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 (!).
> Would that FIPS ciphersuite provide forward secrecy?  If not, then
> there is no point in having clients or servers implement it.

The idea would be that, so long as we advertise ciphers we can't
support, an MITM adversary could make a Tor detector by forging
ServerHello responses to choose the FIPS suite, and then seeing
whether the client can finish the handshake to the point where they
realize that the ServerHello was forged.

This is probably not the best MITM Tor-detection attack, but it might
be nice to stomp them as we find them.

>>   [**] 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
>>   compromise.
> A relay whose link protocol implementations can't provide forward
> secrecy to its clients cannot be used as an entry guard -- it would be
> overloaded with CREATE cells very quickly.

Why is that?  It shouldn't be facing more than 2x the number of create
cells that a relay faces, and with the ntor handshake, create cell
processing ought to get much faster.

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