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

Karthikeyan Bhargavan <> Mon, 03 March 2014 16:47 UTC

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From: Karthikeyan Bhargavan <>
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Date: Mon, 3 Mar 2014 16:46:51 +0000
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To: "Paterson, Kenny" <>
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Subject: Re: [TLS] MITM Attacks on Client Authentication after Resumption
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> 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 version:

> 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
> protocols.

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).
> Cheers
> Kenny
> 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]
> Scenario
> ======
> 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
> handshakes)
> 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
> data)
>  (M still knows the record keys and can send messages in either
> direction.)
> 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
> same, 
>  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.
> Countermeasures 
> ===========
> 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
> attack.
> 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
> (draft-puthenkulam-eap-binding-04)
> [5] RFC5929: Channel Bindings for TLS
> [6] RFC5705: Keying Material Exporters for Transport Layer Security