Re: [kitten] [TLS] Fwd: Last Call: <draft-ietf-kitten-tls-channel-bindings-for-tls13-09.txt> (Channel Bindings for TLS 1.3) to Proposed Standard

[Jonathan Hoyland <[e-mail]>]

Hi Simo,

As I said in my email to Sam, my language here was sloppy, I meant that the
channel binding is included in the key derivation process, and thus the
output keys will be related to each other.
The term channel bindings has two different meanings using in very similar
contexts, which leads to great confusion and is why I prefer the term
compound authentication (see Chapter 4.4 of my thesis)
<https://pure.royalholloway.ac.uk/portal/files/33074422/2018HoylandJGPhD.pdf>.

A channel binding is a "name" of a channel as you say, but it is also used
to mean using one channel to authenticate another.
Very specifically if channel `a` is bound to channel `b` then that means
that if channel `b` is securely authenticated then channel `a` was formed
honestly (i.e. even if all of the long term key material for channel `a`
has been leaked, I can be sure that my channel `a` peer is the same actor
as my channel `b` peer.)

Channel bindings that aren't bound to a specific session are less secure
than ones that are, so yes, you can use them, but that doesn't mean they
have the security properties you need.
The purpose of binding a protocol to an underlying TLS session is to
leverage some of the security guarantees offered by the TLS layer.
If those guarantees are never necessary because the over-the-top protocol
is always perfectly secure, just run the protocol on the wire and drop the
TLS complication.
If those guarantees are at least sometimes necessary then you need to
securely bind the two channels together.

Channel binding does more than simply guarantee that the messages were the
same, it also guarantees that the identity of the actors at each layer
remains the same, and that the keys derived in each layer match.
With an appropriately designed channel binding you can use TLS's protection
against MITM attacks to protect insecure protocols run over the top.
Having now looked at SCRAM a little bit, using the TLS channel binding
guarantees that some malicious server isn't passing the client's messages
off to another server and performing a malicious authentication there.
Using a unique string from SCRAM means that messages from some other
protocol won't be compatible.
If you don't require a unique string I could write a new protocol SCRAM++
with cross-compatible messages, but different security guarantees, and use
that to compromise SCRAM clients.
Obviously that's a patently malicious example, but it's perfectly possible
that some new version of SCRAM produces messages that an old version of
SCRAM misinterprets.
It's even highly likely that a server that supports old versions of SCRAM
for backwards compatibility would use the same key material for all
versions.

Requiring a unique string makes the messages totally unrelated.

I don't find the arguments "It's always been insecure in the past" or "We
have bad APIs" especially compelling.

Regards,

Jonathan


On Fri, 29 Oct 2021 at 14:55, Simo Sorce <[e-mail]> wrote:

> Hi Jonathan,
>
> On Thu, 2021-10-28 at 18:46 +0100, Jonathan Hoyland wrote:
> > Hi Ruslan,
> >
> > Yes, two distinct TLS connections having the same exporter key would be
> > really bad, but I'm specifically talking about two runs of some protocol
> > bound to a single TLS session.
> > A single TLS session will return the same key (modulo rekeying,
> resumption
> > etc.) if you call the Exporter API with the same label.
> > This means that the channel bindings it produces, which will then be used
> > to derive the keys of the higher layer protocol, will produce related
> keys.
> >
> > The key reason for making the binding retrieval context-specific is to
> make
> > sure that different protocols run in the same TLS session produce
> disjoint
> > keys.
>
> Note, Channel Bindings are *not* a key and should never used as such,
> so I think you are using a false premise here.
> Channel Bindings are, at most, a unique value associated with a session
> (keep in mind that often Channel Bindings do not even bind to a
> specific session, see 'tls-server-end-point' which is used in
> preference in many cases within HTTPS endpoints because of the way
> server termination is built in some scalable architectures).
>
> > When someone tries to copy a message from a SCRAM handshake into some
> > GSS-API run on a single TLS connection I want to be sure that it will be
> > rejected, without having to understand exactly how every version of SCRAM
> > and GSS-API ever (including ones that will be drafted in the future)
> works
> > (not to mention every other protocol past, present, and future that uses
> > the same string.)
>
> Both SCRAM and GSS-API are authentication mechanisms that use shared
> keys for authentication. Both protocols are inherently resistant to
> message swapping because the chances of producing a message on one
> protocol that can be successfully decrypted/HMACed on the other are
> astronomically low (they must be or the protocols are broken).
>
> Channel bindings are just a way to tie those authentications to the
> specific channel used underneath. I can't see how using the same value
> for both protocols can lead to any compromise (see again 'tls-server-
> end-point', where there isn't even a "same" channel guarantee, or
> previous CBs where the binding was done to an IP address or other clear
> text session value).
>
> If passing an arbitrary channel binding values could suddenly (and
> predictably?) allow one protocol to be swapped for the other or have
> one protocol generate derived session keys that are usable with the
> other, it would mean those authentication mechanisms (SCRAM,GSS-
> API,...) are completely broken, as that would mean the keys and
> derivations used in those protocols are not providing the necessary
> security guarantees.
>
> > In the same way that we include strings in the TLS key schedule to ensure
> > that handshake messages can't be confused with each other or with earlier
> > versions of TLS, or even with Exported Authenticators, or MLS, or any
> other
> > protocol that uses keys, we should use channel bindings to separate
> > protocols run over the top of TLS.
>
> Yes, this is done because those messages are not further confounded
> with additional private keys, but the situation is substantially
> different when you talk about authentication mechanism that already use
> additional key material, key derivation and random challenges.
>
> HTH,
> Simo.
>
> > Regards,
> >
> > Jonathan
> >
> > On Wed, 27 Oct 2021 at 19:39, Ruslan N. Marchenko <[e-mail]>
> > wrote:
> >
> > > Hi Jonathan,
> > >
> > > Am Mittwoch, dem 27.10.2021 um 18:02 +0100 schrieb Jonathan Hoyland:
> > > > Hi Ruslan,
> > > >
> > > > Without digging into the guts of GSS-API and SCRAM I can't give you a
> > > > concrete attack, the issue is that all our assumptions about protocol
> > > > security assume that different protocols use totally separate key
> > > > material.
> > > > For example if I have one protocol that signs arbitrary blobs and
> > > > another that requires me to sign a challenge to prove I know a
> private
> > > > key then even if they're both perfectly secure on their own they are
> > > > totally broken in composition.
> > > > This separation of keys is one of the core reasons for the use of
> > > > HKDFs, it lets us take some source randomness and use it to generate
> > > > independent keys.
> > > >
> > > > Designing a channel binding that explicitly suggests people use the
> > > > same key material for different things is bad practice and poor
> > > > protocol design.
> > > > Pretty much every attack is on the table if you have multiple
> protocols
> > > > using the same keys, message forgery, improper authentication, key
> > > > leakage, etc.
> > > > The reason for separation of keys is to simply rule out this entire
> > > > class of vulnerability.
> > >
> > > Sorry maybe I'm missing something but why would two unrelated protocols
> > > have same key? The binding is derived from TLS session key, if two
> > > unrelated TLS sessions are having the same key this is the problem by
> > > itself, and binding material is not amplifying it, just falling to the
> > > same problem bucket. And if they are related (eg resumed) then protocol
> > > muxing should perhaps be part of applicaiton stack and it should still
> > > be happy with a common security context.
> > >
> > > > If you don't separate the keys then whenever you make any change to
> any
> > > > protocol you have to consider its security in composition with every
> > > > other protocol, including ones that you don't know about or that
> > > > haven't been written yet.
> > > >
> > > > Using different labels for different purposes shouldn't require
> > > > anything complicated, as it's just changing the input literals to the
> > > > API call.
> > > >
> > > There's a difference in telling TLS stack "give me binding of that
> > > type" - which is pretty much high level abstract call, and asking TLS
> > > to perform key derivation (export) for specific label - which requires
> > > speaking native TLS implementation api. Now that we have a bunch of TLS
> > > implementations and several abstractions and all other binding types
> > > are independent of the higher protocol - it may be challenging changing
> > > the API to make the binding retrieval context-specific. Not that it is
> > > impossible to make such change, but why?
> > >
> > >
> > >
> > >
> > _______________________________________________
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> > [e-mail]
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>
> --
> Simo Sorce
> RHEL Crypto Team
> Red Hat, Inc
>
>
>
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