Re: [MLS] MLSPlaintext packets aren't authenticated using symmetric key schedule

Joel Alwen <jalwen@wickr.com> Thu, 20 August 2020 14:20 UTC

Return-Path: <jalwen@wickr.com>
X-Original-To: mls@ietfa.amsl.com
Delivered-To: mls@ietfa.amsl.com
Received: from localhost (localhost [127.0.0.1]) by ietfa.amsl.com (Postfix) with ESMTP id F39373A0B53 for <mls@ietfa.amsl.com>; Thu, 20 Aug 2020 07:20:55 -0700 (PDT)
X-Virus-Scanned: amavisd-new at amsl.com
X-Spam-Flag: NO
X-Spam-Score: -2.848
X-Spam-Level:
X-Spam-Status: No, score=-2.848 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, NICE_REPLY_A=-0.949, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, URIBL_BLOCKED=0.001] autolearn=ham autolearn_force=no
Authentication-Results: ietfa.amsl.com (amavisd-new); dkim=pass (2048-bit key) header.d=wickr-com.20150623.gappssmtp.com
Received: from mail.ietf.org ([4.31.198.44]) by localhost (ietfa.amsl.com [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id A2apxchFAt3o for <mls@ietfa.amsl.com>; Thu, 20 Aug 2020 07:20:53 -0700 (PDT)
Received: from mail-ed1-x52c.google.com (mail-ed1-x52c.google.com [IPv6:2a00:1450:4864:20::52c]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by ietfa.amsl.com (Postfix) with ESMTPS id 63C7B3A0B18 for <mls@ietf.org>; Thu, 20 Aug 2020 07:20:53 -0700 (PDT)
Received: by mail-ed1-x52c.google.com with SMTP id w2so1471484edv.7 for <mls@ietf.org>; Thu, 20 Aug 2020 07:20:53 -0700 (PDT)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=wickr-com.20150623.gappssmtp.com; s=20150623; h=subject:to:cc:references:from:autocrypt:message-id:date:user-agent :mime-version:in-reply-to:content-language:content-transfer-encoding; bh=GvDatw6Rm6yR5LuhDmxchI377lt2rkest2/yaYzyrpo=; b=GU93Mnh3LJh5qVhn9euZmns9chljhkluaSuELa1cSol5Oore769G5imGwQ2+kE2U+Z G1deMJhHS6KwHl/Zq9ZkotH+4oXGmtCFxkZhQVjXWyLroyfiEn7b3NPXUrWVlLXDy+s7 FRSPk2QHFHsyagLT2jhqUR1B6NlnTQSPGpos52Af2YTmANuI4BQp1Oypw4J527Z+jSIa KUnt9GtshIkAsPmeki9Ujfv3zqi4IGGaVRqBkA8rC20eUFZJ2tFAOGtqhW+CgQjfRH1M Gk1eaDqB7QXxyh3SPNV6XXZN+XVvR7qs85HP65olgPp3LWspzd9WwJqLE5sz8a2Vrq0+ lqNA==
X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:cc:references:from:autocrypt :message-id:date:user-agent:mime-version:in-reply-to :content-language:content-transfer-encoding; bh=GvDatw6Rm6yR5LuhDmxchI377lt2rkest2/yaYzyrpo=; b=aFDdUfk9FEnjsYsTpwOy7OzrIhD2nZkboTTbzIo2qE4APlvnGdaJvK50st/geGe0qd dezQvN0gz1YA9lEM0zCXUqNBX/k3U7EIWleyXk2rzfg+EpwsmHM2gx8F8KaWCZlDY1jf gN0DwYXW872gc8FsKoDY/NcJeQM6LtRr6iFRNS9yXlm8SGa2Ln+ARpAsUuwELOs2d+r9 bVIj7Pxx1RZxovQf0lPPaLYvBDiNN54jcVJ0HZwgCJE3n66I7ll2FBwPrTJIRgVCd5OS XmuQP2EjASVEPR2kKQpRijfyGvdQZkP1qeWuw8Vbxx7FJxS856bKO1v9mZP5rGfVaf1g xmiw==
X-Gm-Message-State: AOAM533YgzB1+ZGHaxyRaVricM1HCiiDxoPnaYW/uUdCDtnh8j1CCBku atl5T383TGbh2fMz1gwJluDQC58MdArl9A==
X-Google-Smtp-Source: ABdhPJwSrAEZjlmqln8fjL3XEah7LPvbMEFIHgzwqztDYY8yRWRZduZ9Sst4CPq6XS5L6KuIfuoRww==
X-Received: by 2002:a50:ee93:: with SMTP id f19mr3088103edr.31.1597933251346; Thu, 20 Aug 2020 07:20:51 -0700 (PDT)
Received: from [192.168.1.137] (84-114-27-5.cable.dynamic.surfer.at. [84.114.27.5]) by smtp.gmail.com with ESMTPSA id bz19sm484605ejc.72.2020.08.20.07.20.49 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 20 Aug 2020 07:20:50 -0700 (PDT)
To: Raphael Robert <raphael@wire.com>, Richard Barnes <rlb@ipv.sx>
Cc: Messaging Layer Security WG <mls@ietf.org>
References: <7d7283b6-8c70-d045-81c2-f552219869ad@wickr.com> <F5B1E029-D8B4-4BEA-BF7A-CDD531D662BD@wire.com> <CAL02cgRTtZp+gHKA0hXxxEn_L6SWRRTJa-U+bhQUhpvM8qZ+Cg@mail.gmail.com> <87d72ad5-dce9-18f7-f1c4-7a8317fd0739@wickr.com> <30DD617C-A8A8-4801-A62A-43A722B1B597@wire.com> <CAL02cgTo8CXNt26XKGrMo1vU-n6M88YtoJ4cqdxrvpyWaX1VNA@mail.gmail.com> <f4e7ea74-d368-154c-72ee-8d70b30235d2@wickr.com> <CAL02cgTbQ+cirgAGdpH4=c=Yv4X=vHqik1NQ3hHxeRC=L2Uc4Q@mail.gmail.com> <26970E9E-6EED-46D5-BD5D-8E1ADB40A93F@wire.com>
From: Joel Alwen <jalwen@wickr.com>
Autocrypt: addr=jalwen@wickr.com; keydata= mQENBFyIZvABCAC65JupY1w7gzhhNo41ftIk09n7Lid9p31jDR8Jefv9R5sWL+HZFGDeABAY 1J1JvV6vOaMsfdy9iUFfGS1GhMJ3+mh799SIsB3JSfPq/eq6Jut57D2yPtILmc7ZbuJyBHg0 xuYfKCQQAYikW+v2LJQU1Y+BUDbVldpzxSc8Z3PPSfunWdzhY6qAAhyCv+Y8EzJlQivMwD5B f6737krf8SoBsjsqCHQrRo/r+BSj5Wtd5/K3FkmWLOUAFoYK23+cpoFntGJKZfss27gDPhyS gX9ibXcBGQqBEF4qDPEzEHK8iQmXTxLul5Y7lQ6ADf69xH15WM4GmRBeCvR3Uanxcr2/ABEB AAG0HUpvZWwgQWx3ZW4gPGphbHdlbkB3aWNrci5jb20+iQFUBBMBCAA+FiEEYFNg9IH2SV6e 03O3FR5tDZv8eygFAlyIZvICGwMFCQHhM4AFCwkIBwIGFQoJCAsCBBYCAwECHgECF4AACgkQ FR5tDZv8eyjSywgApQNIRcL4IKTJ0I4XwcQRhICu1Bht3c2fUnG2YziJXjGf6DZ49uKKtuIu fk8mNS+vKRLoLZ7+u+Pv/Yjmk8jtrr6Saz1vnfsle3GgmXG5JaKOM5cOfeo5JnlNUP3QonR7 LMZwY1qVKg2mzNmwi0jG1zIGgQ5fiAwqe+YTNFli5bc/H1O9LcSmbrLV9OyucARq11DIiAvU fDknZ17OahQls+9mgfAXH5vZjzo296tYvzkOJQ2A6GPxdMHIXGbJM/vjuMe2QJl6C0zaqOtm JvFcx/HpNhmugYI9OsNAd7846HASDp8BKyfY5FYP7bn0/JBuCpg18Aykru6xyFjG3gv0Lw==
Message-ID: <1bd2268c-7f27-e24a-bcd7-3192afae13c9@wickr.com>
Date: Thu, 20 Aug 2020 16:20:49 +0200
User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.11.0
MIME-Version: 1.0
In-Reply-To: <26970E9E-6EED-46D5-BD5D-8E1ADB40A93F@wire.com>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 8bit
Archived-At: <https://mailarchive.ietf.org/arch/msg/mls/bnWqgVw-LYp0QCiZkTyVSzEZGI0>
Subject: Re: [MLS] MLSPlaintext packets aren't authenticated using symmetric key schedule
X-BeenThere: mls@ietf.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Messaging Layer Security <mls.ietf.org>
List-Unsubscribe: <https://www.ietf.org/mailman/options/mls>, <mailto:mls-request@ietf.org?subject=unsubscribe>
List-Archive: <https://mailarchive.ietf.org/arch/browse/mls/>
List-Post: <mailto:mls@ietf.org>
List-Help: <mailto:mls-request@ietf.org?subject=help>
List-Subscribe: <https://www.ietf.org/mailman/listinfo/mls>, <mailto:mls-request@ietf.org?subject=subscribe>
X-List-Received-Date: Thu, 20 Aug 2020 14:20:56 -0000

Yeah, good point. While I'm not sure every case of server inspection would
require checking sigs on packets I am sure it would be useful for some cases.

> The obvious fix is to include the membership_token in MLSPlaintext as well.

Yup, I'm all for it. (Or at least for making it an optional field in that struct.)

- Joël

On 19/08/2020 09:24, Raphael Robert wrote:
> I thought about this some more and now I’m not sure that the membership_token
> should only be implicit:
> 
> The whole reason for HS messages to not be encrypted is that they can be
> inspected along the way by an external party (e.g. the Delivery Service). If
> this is not a requirement, MLSCiphertext should really be used instead.
> Hiding the membership_token from external inspector means the inspector can not
> verify the signature anymore. I think that defies the point of HS messages being
> inspectable.
> 
> The obvious fix is to include the membership_token in MLSPlaintext as well.
> 
> Raphael
> 
>> On 18 Aug 2020, at 21:01, Richard Barnes <rlb@ipv.sx <mailto:rlb@ipv.sx>> wrote:
>>
>> Here ya go: https://github.com/mlswg/mls-protocol/pull/396
>>
>> On Tue, Aug 18, 2020 at 1:30 PM Joel Alwen <jalwen@wickr.com
>> <mailto:jalwen@wickr.com>> wrote:
>>
>>     > Joël, do you want to write a PR?  If not, I could probably get to it in the
>>     > next couple days.
>>
>>     Thanks. :-) If you could that would be really nice.
>>
>>     - Joël
>>
>>     On 18/08/2020 19:20, Richard Barnes wrote:
>>     > Sounds like we're converging here.  The only question in my mind is what
>>     goes in
>>     > the MAC -- seems like the easy option is probably "the remainder of the
>>     > MLSPlaintextTBS", i.e., everything from group_id to the end.  That seems
>>     like it
>>     > minimizes multiple serialization:
>>     >
>>     > tbs_content = serialize(group_id, ...)
>>     > membership_token = KDF.Extract(confirmation_key, tbs_content)
>>     > tbs = group_context || membership_token || tbs_content
>>     >
>>     > So the PR would be to basically pop the context off of MLSPlaintextTBS
>>     and add
>>     > the three lines above (with the switch for internal/external in prose).
>>     >
>>     > Joël, do you want to write a PR?  If not, I could probably get to it in
>>     the next
>>     > couple days.
>>     >
>>     > --Richard
>>     >
>>     > On Tue, Aug 18, 2020 at 12:24 PM Raphael Robert <raphael@wire.com
>>     <mailto:raphael@wire.com>
>>     > <mailto:raphael@wire.com <mailto:raphael@wire.com>>> wrote:
>>     >
>>     >     I think what you just described is indeed a combination of option 1 & 2.
>>     >     It’s a MAC over the payload we want to authenticate, but it’s
>>     implicit and
>>     >     we only include it in the MLSPlaintextTBS. Or in other words, we
>>     stripped it
>>     >     from MLSPlaintext because it is implicitly known to any valid member
>>     of the
>>     >     group.
>>     >
>>     >     Raphael
>>     >
>>     >>     On 18 Aug 2020, at 18:16, Joel Alwen <jalwen@wickr.com
>>     <mailto:jalwen@wickr.com>
>>     >>     <mailto:jalwen@wickr.com <mailto:jalwen@wickr.com>>> wrote:
>>     >>
>>     >>     Yeah, I like Option 2 here. I like that it avoids growing packet size.
>>     >>
>>     >>     One caveat though: I'd go for the MAC(...) version rather than the
>>     >>     confirmation_key. For starters thing including confirmation_key doesnt
>>     >>     authenticate the contents of the packet. But even if it did,
>>     signatures aren't
>>     >>     meant to hide the contents of what was signed. (Amend you favorite sig
>>     >>     scheme to
>>     >>     tack on the message at the end a signature and you've still got a
>>     secure
>>     >>     signature scheme. But clearly not a message hiding one.) That means
>>     that AFAIK
>>     >>     neither ECDSA nor EdDSA etc. were designed or analyzed for such a
>>     property. So
>>     >>     this would amount to a very non-standard use of a signature schemes by
>>     >>     MLS. Not
>>     >>     saying it doesn't work for the particular sig schemes in our
>>     ciphersuite. But
>>     >>     its def. not how signatures are "meant" to be used.
>>     >>
>>     >>     But that leaves Option 2 with, say, including tag = MAC(conf_key,
>>     >>     conf_trans_hash || MLSPlaintext.content) into whats being signed
>>     which I like
>>     >>     and think gets the job done. Both conf_* values are taken from the
>>     current
>>     >>     epoch. By MLSPlaintext.content I mean whats now called MLSPlaintextTBS.
>>     >>
>>     >>>     I would propose that we do need something additional on Commit
>>     messages as
>>     >>>     well as Proposals.
>>     >>
>>     >>     @Richard: For Proopsals I think this works. Is that about what you
>>     had in mind
>>     >>     for commits too?
>>     >>
>>     >>     - Joël
>>     >>
>>     >>
>>     >>     On 18/08/2020 17:26, Richard Barnes wrote:
>>     >>>     Thanks for pointing this out, Joël.  I agree that the attacks you're
>>     >>>     describing
>>     >>>     should work as things are currently specified.  And they're salient,
>>     >>>     especially
>>     >>>     the "replace Alice in the group" one.
>>     >>>
>>     >>>     Also agree with Raphael is correct that Commit is not affected by
>>     this, since
>>     >>>     someone who is not a member won't be able to generate the right
>>     confirmation
>>     >>>     value.  However, I don't think this is actually the right design
>>     to adopt
>>     >>>     for a
>>     >>>     general solution to this problem.  Confirmation verifies group
>>     membership
>>     >>>     *after* processing the handshake message; the point here is that we
>>     >>>     should also
>>     >>>     have a membership check *before* processing handshake messages.  In
>>     >>>     particular,
>>     >>>     I would propose that we do need something additional on Commit
>>     messages
>>     >>>     as well
>>     >>>     as Proposals.
>>     >>>
>>     >>>     Thinking about solutions here, a couple of options come to mind:
>>     >>>
>>     >>>     1. Use MLSCiphertext, but with an integrity-only encapsulation
>>     >>>     2. Incorporate in the signature something that is only known to
>>     the group
>>     >>>     (e.g.,
>>     >>>     confirmation_key or MAC(confirmation_key; confirmed_transcript_hash ||
>>     >>>     Proposal/Commit))
>>     >>>
>>     >>>     Option (1) has the appeal that you would only ever send
>>     MLSCiphertext, though
>>     >>>     switching between encrypted/not could be problematic.  Option (2)
>>     seems a lot
>>     >>>     more appealing: It doesn't add any overhead, since the
>>     group-secret value
>>     >>>     doesn't need to be sent.  And we already switch between the
>>     signature context
>>     >>>     that is added for group members vs. external.  In fact, I think option
>>     >>>     (2) would
>>     >>>     just amount to a one-line change to include an extra, secret value
>>     in the
>>     >>>     context at the top of the MLSPlaintextTBS struct.
>>     >>>   
>>      https://github.com/mlswg/mls-protocol/blob/master/draft-ietf-mls-protocol.md#content-signing-and-encryption
>>     >>>
>>     >>>     The only thing that seems odd about (2) is overloading signature
>>     >>>     verification in
>>     >>>     that way, i.e., using the ability to generate a signature over a
>>     secret
>>     >>>     thing to
>>     >>>     prove you know the secret thing.  That doesn't seem obviously
>>     flawed to
>>     >>>     me, but
>>     >>>     worth thinking about.
>>     >>>
>>     >>>     Does that make sense to folks?
>>     >>>
>>     >>>     --Richard
>>     >>>
>>     >>>
>>     >>>     On Tue, Aug 18, 2020 at 10:55 AM Raphael Robert
>>     >>>     <raphael=40wire.com@dmarc.ietf.org <mailto:40wire.com@dmarc.ietf.org>
>>     >>>     <mailto:raphael <mailto:raphael>=40wire.com@dmarc.ietf.org
>>     <mailto:40wire.com@dmarc.ietf.org>> <mailto:40wire.com@dmarc.ietf.org
>>     <mailto:40wire.com@dmarc.ietf.org>>>
>>     >>>     wrote:
>>     >>>
>>     >>>        Hi Joel,
>>     >>>
>>     >>>        For context: this would only apply when applications use cleartext
>>     >>>        MLSPlaintext for HS messages. The recommendation is still to
>>     encrypt them
>>     >>>        and send them around as MLSCiphertext.
>>     >>>        That being said, we said we would like to support scenarios
>>     where HS
>>     >>>        messages are not necessarily encrypted.
>>     >>>
>>     >>>        Question: would this attack work with Commit messages? I’m
>>     thinking that
>>     >>>        they would be rejected because the attacker cannot compute the
>>     >>>     confirmation_tag.
>>     >>>
>>     >>>        As you mention in the PS, the easy target would be Proposal
>>     messages.
>>     >>>
>>     >>>        I’d be interested to see what exactly you would propose as a
>>     mitigation
>>     >>>        mechanism.
>>     >>>
>>     >>>        Raphael
>>     >>>
>>     >>>>     On 18 Aug 2020, at 16:36, Joel Alwen <jalwen@wickr.com
>>     <mailto:jalwen@wickr.com>
>>     >>>>     <mailto:jalwen@wickr.com <mailto:jalwen@wickr.com>>
>>     >>>        <mailto:jalwen@wickr.com <mailto:jalwen@wickr.com>>> wrote:
>>     >>>>
>>     >>>>     Hey everyone,
>>     >>>>
>>     >>>>     Something thats been bugging Marta Mularczyk and Daniel Jost and
>>     me for
>>     >>>>     a bit
>>     >>>>     now is that handshake messages sent out as MLSPlaintext packets
>>     are only
>>     >>>>     authenticated using signatures, but not using the group's key
>>     schedule. For
>>     >>>>     non-members that makes sense but for group members that's weaker than
>>     >>>>     need be.
>>     >>>>
>>     >>>>     Suppose Alice is in a group using signing key pair (spk, ssk). I
>>     corrupt
>>     >>>        her to
>>     >>>>     learn ssk. Now I loose access to her device again. Later she
>>     generates a
>>     >>>>     fresh
>>     >>>>     key package with her same spk but a new HPKE key for her leaf.
>>     She sends
>>     >>>        out and
>>     >>>>     update proposal for her new key package and someone commits to
>>     the update.
>>     >>>>
>>     >>>>     Expected result: she (and the group at large) has achieved PCS again.
>>     >>>>
>>     >>>>     Actual result: using her stolen ssk I can still forge a new
>>     proposal's
>>     >>>        (sent as
>>     >>>>     MLSPlaintext packets) coming from Alice. Some things I could do
>>     with this
>>     >>>        power:
>>     >>>>     - I can generate a new key package kp for Alice using her spk and
>>     some
>>     >>>        HPKE key
>>     >>>>     she doesn't know. Then I forge an update proposal for Alice with
>>     kp. If it
>>     >>>        gets
>>     >>>>     committed I've effectively kicked her out of the group.
>>     >>>>     - I could forge Add's and Remove's coming from Alice, so I could
>>     trick the
>>     >>>>     group into thinking Alice is trying to Add my account to the
>>     group or remove
>>     >>>>     some other group member.
>>     >>>>
>>     >>>>     Lemme know if I've missed something here in that scenario...
>>     >>>>
>>     >>>>
>>     >>>>     If I didn't miss anything and the attacks really work as
>>     advertised then IMO
>>     >>>>     this is kinda weak sauce and worth avoiding if possible. So to
>>     that end, how
>>     >>>>     about we modify MLS such that MLSPlaintext packets coming from
>>     group members
>>     >>>>     must also be authenticated using something from the application key
>>     >>>>     schedule.
>>     >>>>     Now the above attacks fail. As soon as Alice's update is gets
>>     committed I no
>>     >>>>     longer know the group's key schedule and so can't forged packet from
>>     >>>        Alice. More
>>     >>>>     generally, this brings the PCS guarantees when using MLSPlaintexts
>>     >>>>     frameing in
>>     >>>>     line with what we're getting from MLSCiphertext packets.
>>     >>>>
>>     >>>>     Any thoughts?
>>     >>>>
>>     >>>>     - Joël
>>     >>>>
>>     >>>>
>>     >>>>
>>     >>>>     PS. For concreteness, we could probably extend the current
>>     mechanism for
>>     >>>        getting
>>     >>>>     concistancy (the confirmation_tag) to also provide symmetric key
>>     >>>        authentication.
>>     >>>>     E.g. include most of the MLSPlaintext content into whats being
>>     tagged by
>>     >>>>     confirmation_tag. That would cover the case of a commit packet
>>     and doesn't
>>     >>>        even
>>     >>>>     grow the size of MLSPlaintext packets over the current design.
>>     >>>>
>>     >>>>     For a proposal packet we could also have a confirmation_tag but
>>     this one is
>>     >>>>     computed using the *current* epoch's confirmation_key and
>>     >>>        confirmed_transcript_hash.
>>     >>>>
>>     >>>>     _______________________________________________
>>     >>>>     MLS mailing list
>>     >>>>     MLS@ietf.org <mailto:MLS@ietf.org> <mailto:MLS@ietf.org
>>     <mailto:MLS@ietf.org>> <mailto:MLS@ietf.org <mailto:MLS@ietf.org>>
>>     >>>>     https://www.ietf.org/mailman/listinfo/mls
>>     >>>
>>     >>>        _______________________________________________
>>     >>>        MLS mailing list
>>     >>>        MLS@ietf.org <mailto:MLS@ietf.org> <mailto:MLS@ietf.org
>>     <mailto:MLS@ietf.org>> <mailto:MLS@ietf.org <mailto:MLS@ietf.org>>
>>     >>>        https://www.ietf.org/mailman/listinfo/mls
>>     >
>>
>