Re: [TLS] MITM Attacks on Client Authentication after Resumption

"Paterson, Kenny" <> Mon, 03 March 2014 16:54 UTC

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From: "Paterson, Kenny" <>
To: Karthikeyan Bhargavan <>
Thread-Topic: [TLS] MITM Attacks on Client Authentication after Resumption
Date: Mon, 3 Mar 2014 16:54:41 +0000
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Subject: Re: [TLS] MITM Attacks on Client Authentication after Resumption
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Sorry, I was just trying to clarify the impact of your attack for
non-experts - the suggestion that it is a MITM attack might make some
readers think that M ends up knowing the keys at the end of the attack.

I totally agree that it's unclear as to what "bindings" should be assumed
by the protocol participants at the end of step 3.

As I said, and I'll reiterate - great work guys!



On 03/03/2014 16:46, "Karthikeyan Bhargavan"
<> wrote:

>Rather, an application running at C might end up using TLS-protected data
>that was supplied by M in step 2 of the attack as if it came from S (or
>vice-versa). This is a comparable attack to the original renegotiation
>attack of Ray&Dispensa and Rex.
>Yes, indeed, the impact of our renegotiation attack is comparable to the
>2009 renegotiation attacks.
>In fact, the setup of our attack is closer to Rex’s version than Ray’s
>as far as I can see - for the 3rd step of the attack, it
>seems that M just passes messages backwards and forwards and ends up NOT
>knowing the keys shared by C and S. Any authentication protocol is
>vulnerable to a "message passing" attack of this type in which the
>adversary simply acts as a wire, and it's not considered an attack on such
>I would note that the renegotiation handshake we are speaking about is
>not exactly a standalone authentication protocol. It is the second
>handshake on a connection, with different client or server identity than
>the first handshake. This leaves room for
> confusion: should an implementation associate the connection to the
>first set of principals or the second?
>Indeed, it's correct that C *is* connected to S and S *is* connected to C
>after step 3! (not just that they "think" they are so-connected).
>Kenny, you seem to suggest that only the second should be considered.
>Common web browsers and HTTPS libraries only consider the first. It is
>interesting to try and understand which is the right answer here.
>What matters here is how a server application running at S interprets data
>that may have been sent under TLS protection in the first 2 steps in the
>attack: it may assume it came from C when in fact it came from M.
>The more accurate description from Karthik's text is this:
>Step 3. (Renegotiation C-M-S)
>Both handshakes complete with new mutually-authenticated sessions and
>record keys.
>C now thinks it is connected to S and S thinks it is connected to C.
>(M does not know the new record keys but its previous messages to S on
>the same connection
>may be treated as authenticated by C.)
>Indeed, it's correct that C *is* connected to S and S *is* connected to C
>after step 3! (not just that they "think" they are so-connected).
>On 03/03/2014 15:20, "Karthikeyan Bhargavan"
><> wrote:
>We are going to present some new man-in-the-middle attacks on TLS
>applications in Tuesday¹s working group meeting. These attacks were
>found as part of our research on trying to prove cryptographic
>security for a TLS implementation [1].  We presenting some of the
>materials here to kick-start some discussion and get early comments on
>our proposed countermeasure. Some people on the list already know of
>these attacks but this is the first public disclosure.
>More details are at
><> [2]
>Consider a client C that normally authenticates to a server S using a
>client certificate.  If C uses the same certificate to authenticate to
>a malicious server M, then we show that M can use C¹s certificate to
>authenticate its own connection to S.
>The attack relies on the combination of an initial RSA or DHE
>handshake, followed by session resumption on a new connection,
>followed by a client-authenticated renegotiation. During the first two
>handshakes, C has a connection to M and M has a connection to
>S. During the third handshake, M is able to authenticate as C to S and
>as S to C.
>This server-based man-in-the-middle attack should normally have been
>prevented by the Renegotiation Indication (RI) extension [3] but by
>injecting session resumption between the two full handshakes, we are
>able to bypass the renegotiation countermeasure.
>Triple Handshake Attack
>I¹ll briefly summarise the attack below for an initial RSA key
>exchange.  The webpage [2] has diagrams that will be easier to follow,
>describes more attack variants, and provides some disclosure status.
>The attack proceeds in three steps:
>Step 1. (Initial Handshakes C-M, M-S)
>- C connects to M and M connects to S, both handshakes use RSA.
>- M forwards C¹s and S¹s client hellos to each other.
>- M receives an encrypted PMS from C and reencrypts it towards S.
>- Both handshakes complete with new sessions and record keys.
> Both sessions have the same master secret, random nonces, and session id.
> (M knows the master secret and record keys since it participated in both
>Step 2. (Session Resumption C-M, M-S)
>- C resumes its session with M on a new connection.
>- M resumes its session with S on a new connection.
>- M forwards all the abbreviated handshake messages unchanged between C
>and S.
>- Note that the RI extensions on both handshakes are empty,
> since it is the first handshake on the connection
>- Both handshakes complete with new record keys (and reuse old sessions)
> Both connections have the same record keys and handshake logs (verify
> (M still knows the record keys and can send messages in either
>Step 3. (Renegotiation C-M-S)
>- S requests M for renegotiation with client certificate.
> M requests C for renegotiation with client certificate.
>- M forwards all renegotiation messages unchanged between C and S
>- Note that since the handshake logs in the preceding handshake were the
> the RI extensions on both handshakes will be the same.
>- Both handshakes complete with new mutually-authenticated sessions and
>record keys.
> C now thinks it is connected to S and S thinks it is connected to C.
> (M does not know the new record keys but its previous messages to S on
>the same connection
> may be treated as authenticated by C.)
>At the end of Step 3, S has an incoming connection on which it
>initially received data from an anonymous client (M) and later
>received data from an authenticated client (C). This breaks the
>intended guarantees of the RI extension.
>During Step 3, C has a connection on which it first received M¹s
>certificate and later S¹s certificate. If C refuses to accept this
>change of server identity, then it can prevent Step 3 of the
>attack. Indeed, we recommend mainstream web browsers and HTTPS
>libraries should systematically forbid the change of server identities
>during renegotiation.
>However, already at the end of Step 2, a number of connection and
>session parameters, such as the tls-unique channel binding for the two
>connections are the same. So any application-level mechanism that
>relies on the TLS master secret [4] or channel bindings [5] or exports
>TLS keying material [6] is vulnerable to a similar man-in-the-middle
>We argue that the core vulnerability here is that the TLS master
>secret is not bound to enough elements of the TLS session. We propose
>a new TLS extension that binds the master secret to the hash of the
>all relevant handshake messages in the initial handshake.
>The proposed draft is available at:
>The key idea is that each full handshake is associated with a session
>hash, computed as
>session_hash = Hash(handshake_messages)
>where handshake_messages consist of all messages up to and including
>the ClientKeyExchange.  The extended master secret computation enabled
>by the extension is then computed as
>master_secret = PRF(pre_master_secret,
>                                       "extended master secret",
>                                        session_hash) [0..47];
>We¹ve implemented this extension in OpenSSL without much difficulty.
>Changing the master secret derivation may seem radical, but we believe
>it is the main way to counter future attacks that may rely on the
>session synchronization (step 1) that we exploit here.
>An alternative countermeasure would be an extension (along the lines
>of [3]) that includes the session hash as defined above in the
>ClientHello and ServerHello messages of the abbreviated
>handshake. This would provide an explicit link between the resumption
>handshake and its original full handshake, and hence prevent the
>renegotiation attack described above.
>We welcome comments and suggestions.
>-Karthik Bhargavan, Antoine Delignat-Lavaud, and Alfredo Pironti
>[1] <>
>[2] <>
>[3] RFC5746: Transport Layer Security Renegotiation Indication Extension
>[4] The Compound Authentication Binding Problem
>[5] RFC5929: Channel Bindings for TLS
>[6] RFC5705: Keying Material Exporters for Transport Layer Security