Sam,
Thanks for posting this. It's great to see people doing real formal
analysis of the TLS 1.3 draft; this is really helpful in guiding the design.
As you say, the current draft-10 doesn't permit certificate-based client
authentication when PSK is used [0], but it's clear that we want to at least
be able to have this in the post-handshake case (since otherwise you
wouldn't be able to do on-the-fly client auth with a resumed handshake),
and this shows that we have to be very careful when we do that.
Note: It's less clear we want to allow this in the initial handshake, if
only
because it complicates the state machine.
This result motivates and confirms the need to modify the handshake hashes
to contain the server Finished when we add post-handshake authentication
as is done in PR#316, which of course we'll be discussing in Yokohama.
I'd be very interested in learning of the results you get when you model
that.
Thanks again for your contribution here. It's really important to have
this kind of analysis, especially at this stage before the design is
completely
hardened.
Best,
-Ekr
[0] Client auth doesn't appear in the PSK diagram in Figure 4
and Section 6.3.5 requires that CertificateRequest appear after a
Certificate
which clearly isn't possible with PSK:
A non-anonymous server can optionally request a certificate from
the client, if appropriate for the selected cipher suite. This
message, if sent, will immediately follow the server's Certificate
message.
https://tools.ietf.org/html/draft-ietf-tls-tls13-10#section-6.3.5
On Sat, Oct 31, 2015 at 8:19 PM, Sam Scott <[email protected]> wrote:
> Dear all,
>
> We [1] are in the process of performing an automated symbolic analysis
> of the TLS 1.3 specification draft (revision 10) using the Tamarin
> prover [2], which is a tool for automated security protocol analysis.
>
> While revision 10 does not yet appear to permit certificate-based client
> authentication in PSK (and in particular resumption using PSK), we modelled
> what we believe is the intended functionality. By enabling client
> authentication either in the initial handshake, or with a post- handshake
> signature over the handshake hash, our Tamarin analysis finds an attack.
> The
> result is a complete breakage of client authentication, as the attacker can
> impersonate a client when communicating with a server:
>
> Suppose a client Alice performs an initial handshake with Charlie. Charlie,
> masquerading as Alice, subsequently performs a handshake with Bob.
> Following a
> PSK resumption, Bob requests authentication from Charlie (impersonating
> Alice).
> Charlie then requests authentication from Alice, and the returned signature
> will also be a valid signature for the session with Bob.
>
> Initial h/s Initial h/s
> |<-------------->| |<-------------->|
> | exchange PSK | | exchange PSK |
> | | | |
> |Start PSK resume| |Start PSK resume|
> |--------------->| |--------------->|
> |client_random nc| |client_random nc|
> | | | |
> | Accept resume | | Accept resume |
> Alice|<---------------|(as Charlie) Charlie (as Alice)|<---------------|Bob
> |server_random ns| |server_random ns|
> | | | |
> | | | |
> |Client auth req | |Client auth req |
> |<---------------| |<---------------|
> | | | |
> | Client auth | | Client auth |
> |--------------->| |--------------->|
> sign nc,ns,... relay signature
>
>
>
> This attack is possible because the client signature is over the handshake
> hash, which only covers the nonces and other easily duplicated information,
> and in particular does not contain the server certificate (because none is
> presented in PSK mode [3]) or the server Finished.
>
> While the modifications proposed in PR#316 [4] explicitly allow client
> authentication in these contexts, the PR also redefines the client
> signature
> based on a new "Handshake Context" value which includes the server Finished
> message. Intuitively, this new definition appears to address the attack
> because the attacker cannot transplant the Finished message between
> connections. We are currently working towards a Tamarin proof that PR#316
> indeed prevents our attack.
>
> Therefore we would like to support the inclusion of Finished as
> part of the handshake context, in order to address this problem.
>
> Many thanks,
>
> Sam Scott
>
> [1] Cas Cremers, Marko Horvat - University of Oxford;
> Thyla van der Merwe, Sam Scott - Royal Holloway, University of London.
> [2] http://www.infsec.ethz.ch/research/software/tamarin.html
> [3] Except in 0-RTT where the server's Certificate is explicitly
> included.
> [4] https://github.com/tlswg/tls13-spec/pull/316/
>
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>
>
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