Re: [TLS] network-based security solution use cases

Flemming Andreasen <> Tue, 07 November 2017 23:11 UTC

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To: Eric Rescorla <>
Cc: "Nancy Cam-Winget (ncamwing)" <>, "" <>
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From: Flemming Andreasen <>
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Date: Tue, 7 Nov 2017 18:11:49 -0500
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Subject: Re: [TLS] network-based security solution use cases
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On 11/7/17 12:23 PM, Eric Rescorla wrote:
> On Tue, Nov 7, 2017 at 7:56 AM, Flemming Andreasen < 
> <>> wrote:
>     Thank you for the feedback Ekr - please see below for responses
>     On 11/6/17 12:43 PM, Eric Rescorla wrote:
>>     I took a look at this.
>>     Without getting into the question of whether the types of middleboxes
>>     you describe here provide a security benefit, there are several
>>     points
>>     in the document that are either wrong or at least
>>     misleading/confusing.
>>     - Key Synchronization
>>     This document notes that in TLS 1.2, it is possible for a middlebox
>>     that traffic keys match on both sides of the connection it is
>>     proxying,
>>     but in TLS 1.3 it is not:
>>        There are several techniques that can be utilized.  Those
>>     techniques
>>        function with TLS 1.2 and earlier versions but not with TLS 1.3.
>>        One technique is for the middlebox to negotiate the same master
>>        secret with the original TLS client and server, as if the two
>>        endpoints handshake directly.  This is also known as "Triple
>>        Handshake" used by legitimate middleboxes.  [BreakTLS]
>>     describes the
>>        methods with RSA and DH key exchanges. When the proxy session keys
>>        are the same as for a direct handshake, the middlebox is able to
>>        "cut-through" the two TLS proxy sessions when it finishes the
>>        security inspection.
>>        This technique is not functional with TLS 1.3 since the transcript
>>        hash over the handshake messages is part of the key derivation
>>        procedure.
>>     First, I would note that this property of TLS 1.2 is bad, as it leads
>>     to Unknown Key Share (UKS) attacks, which can be the basis of real
>>     attacks on TLS 1.2 as fielded. It is for this reason that the TLS
>>     WG published RFC 7627.
>     Thanks for the bringing that one up - we will make sure to include
>     that in the next version.
>     Understood - it is nevertheless a change from TLS 1.2. In TLS 1.3,
>     it's an integral part of the protocol that cannot be disabled. The
>     TLS 1.2 extension does not share that property.
> It seems like an odd complaint that we are making protection against 
> Unknown Key Shares an integral part of the protocol

I understand your point. What I am saying is that there are some side 
effects of doing so that impacts use case scenarios that exist today. If 
you cannot drop out, then you have two choices:
1) Always be a MITM, which you may not want to (e.g. consider sessions 
with financial institutions)
2) Never be a MITM, which means you cannot do any of the network-based 
security use cases we have highlighted

What you really want is the ability to do it more selectively.

>>     Finally, you note several times in the document (e.g., S 4.5), that
>>     with key synchronization, the middlebox can perform the handshake and
>>     then disengage from the connection. However, the cases you mention
>>     (e.g.., detecting exploit attempts) ultimately require examining
>>     all traffic in the connection.
>     We have several different use case scenarios in there, and I agree
>     with the observation that they cannot all be satisfied at the same
>     time. For example, we cannot scan for malware in an encrypted
>     stream to a financial institution that we are not allowed to
>     decrypt. That doesn't mean either use case is invalid. Customers
>     do both in real-life (just not at the same time).
> I'm finding it a bit puzzling what you claim customers do. 
> Specifically, you seem to be saying that customers examine the 
> beginning of the connection and then stop. What are they scanning for 
> and what makes them stop?
It's the handshake process, which typically governs the associated 
policy for the session (e.g. decrypt-and-inspect or do-not-decrypt). You 
will try to determine whether the session should undergo inspection at 
the time the ClientHello passes through the box, however sometimes, you 
end up making a final (and different) decision when the ServerHello is 
received. The decision is guided by policies configured for the 
middlebox which may determine it based on various criteria (e.g. 
category of the destination such as "Financial Services", geography, etc.)

>>     - PSK and resumption
>>     You write:
>>        In TLS 1.3, the above mechanism is replaced by Pre-Shared Keys
>>     (PSK),
>>        which can be negotiated as part of an initial handshake and
>>     then used
>>        in a subsequent handshake to perform resumption using the
>>     PSK.  TLS
>>        1.3 states that the client SHOULD include a "key_share"
>>     extension to
>>        enable the server to decline resumption and fall back to a full
>>        handshake, however it is not an absolute requirement.
>>        Example scenarios that are impacted by this are middleboxes
>>     that were
>>        not part of the initial handshake, and hence do not know the
>>     PSK.  If
>>        the client does not include the "key_share" extension, the
>>     middlebox
>>        cannot force a fallback to the full handshake.  If the middlebox
>>        policy requires it to inspect the session, it will have to
>>     fail the
>>        connection instead.
>>     In TLS 1.3, PSKs and session resumption are basically the same and
>>     have the same properties. While I concede that the document does
>>     not require the client to offer key_shares, as a practical matter
>>     it is very unlikely that a client will act in such a way that
>>     failure to retain the PSK causes a hard failure, for two reasons:
>>     (a) In every version of TLS, the ticket lifetime is just a hint, and
>>         the server can forget it at any time
>>     (
>>     <>).
>>     Thus,
>>         a client which does not allow a full handshake will often
>>         find itself unable to connect.
>>     (b) The relevant question is not whether the client offers a key
>>     share
>>         extension but whether it advertises any DH groups. If the
>>     client sends
>>         a group but not a key share, the server can send
>>     HelloRetryRequest
>>         to force the client to send a key share.
>>     For these reasons, as far as I know, every client (and at least every
>>     browser client) should allow a full handshake even when trying to
>>     resume.
>     If that is indeed the case, then I would suggest changing the spec
>     language to a MUST here instead so we can guarantee it.
> I don't believe this is necessary. There are certainly reasons why you 
> would want to do PSK w/o (EC)DHE (e.g., you are really PSK only).
Agree with the example, but is that really a resumption scenario ?

>>     - Server Certificate Concealment
>>     You note that in TLS 1.2 the middlebox can examine both the SNI
>>     and the
>>     server's certificate in order to decide whether to MITM the
>>     connection,
>>     but that in TLS 1.3, the middlebox cannot guarantee that the SNI in
>>     uses is correct:
>>        In TLS 1.2, the ClientHello, ServerHello and Certificate
>>     messages are
>>        all sent in clear-text, however in TLS 1.3, the Certificate
>>     message
>>        is encrypted thereby hiding the server identity from any
>>        intermediary.  Note that even _if_ the SNI is provided (in
>>     cleartext)
>>        by the client, there is no guarantee that the actual server
>>        responding is the one indicated in the SNI from the client.
>>     In this case, it's important to distinguish between conformant and
>>     nonconformant clients. In the former case -- as when the user is
>>     not attempting to evade inspection -- the SNI will reflect the
>>     identity that the client expects and therefore will compare the
>>     certificate against.
>>     Of course, in the case where the client is attempting to evade
>>     inspection, the certificate might not match. However, in this
>>     scenario, the client and server are colluding and so there are
>>     a variety of ways to avoid inspection, even when doing TLS 1.2.
>>     For example:
>>     1. The client and server can share a prearranged public key and then
>>     negotiate static RSA. The client sends an innocuous SNI and the
>>     server
>>     can simply send the certificate of the corresponding server, captured
>>     by connecting to that server. The client then enciphers the PMS
>>     under the server's prearranged key and continues with TLS as usual.
>>     This cannot be detected by the middlebox without decrypting the EPMS.
>>     2. If an (EC)DHE cipher suite must be negotiated, then the attack
>>     described above does not work directly because the server's signature
>>     over the ServerKeyExchange can be validated, and of course the
>>     attacker server does not have the legitimate server's key. However,
>>     the server can forward the ClientHello to the legitimate server,
>>     capture the Certificate and ServerKeyExchange and the proxy those to
>>     the client. This looks legitimate to the inspection device and then
>>     the client can simply send the Encrypted PMS in the first chunk of
>>     data that is apparently encrypted under the server's key.
>>     For these reasons, being able to see the server's certificate
>>     provides
>>     a false sense of security, as this check is easily bypassed.
>     I agree we should distinguish between between conformant and
>     non-comformant.
>     For the conformant case, I do not know to what extent we can rely
>     on the client always sending the SNI, but even if it is always
>     sent, there are still use case scenarios around certificate audit,
>     etc. that cannot be satisified in TLS 1.3 without becoming an
>     active MITM.
> You can do certificate audit by making an independent connection and 
> eliciting the certificate.
Not always. The certificate audit may be for an external server you 
connected to (outbound scenario). Even for inbound scenarios, you would 
have to ensure you actually find the local servers and check their 
certificates, which would imply a need for continuous scanning of all of 
your infrastructure. I guess it's technically doable (similar to 
vulnerability scanners), but it's difficult on an IPv6 enabled network 
due to the size of the address space.

>     With the recently adopted proposal around SNI encryption, there
>     are more use cases that cannot be satisfied without becoming an
>     active MITM.
> Yes, I agree with that.
>     For the non-conformant case, you bring up some good points. We can
>     reduce the set of bypass scenarios there, but we cannot eliminate
>     them.
> I'm not persuaded that you can meaningfully reduce them. What 
> technical approach do you believe would do so?
Let me rephrase that: A determined attacker will be able to bypass the 
above mechanism, however what we see in practice today is that malware 
is increasingly using TLS to hide command and control traffic (not to 
mention that it often gets downloaded over a TLS protected session to 
begin with). I am not aware of specific examples of malware that try to 
bypass inspection using techniques as described above (nor have I 
looked), but it's certainly possible. There are other discrepancies that 
will show up though when you start looking at the actual destination 
reached and possibly do other analysis over the (encrypted) session.


-- Flemming

> -Ekr
>     Thanks again for the feedback - we will update accordingly in the
>     next version of the draft.
>     -- Flemming
>>     -Ekr
>>     [0] Note: the use of static DH is very rare.
>>     On Fri, Nov 3, 2017 at 6:49 PM, Nancy Cam-Winget (ncamwing)
>>     < <>> wrote:
>>         All,
>>         @IETF99, awareness was raised to some of the security WGs
>>         (thanks Kathleen ☺) that TLS 1.3 will obscure visibility
>>         currently afforded in TLS 1.2 and asked what the implications
>>         would be for the security solutions today.
>>         <>is
>>         an initial draft to describe some of the impacts relating to
>>         current network security solutions.  The goal of the draft is
>>         NOT to propose any solution as a few have been proposed, but
>>         rather to raise awareness to how current network-based
>>         security solutions work today and their impact on them based
>>         on the current TLS 1.3 specification.
>>         Regards, Nancy, Flemming and Eric
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