Re: [MLS] UPKE for X25519/X448

Yevgeniy Dodis <dodis@cs.nyu.edu> Tue, 22 October 2019 17:45 UTC

Return-Path: <zaumka@gmail.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 D84251200CD for <mls@ietfa.amsl.com>; Tue, 22 Oct 2019 10:45:21 -0700 (PDT)
X-Virus-Scanned: amavisd-new at amsl.com
X-Spam-Flag: NO
X-Spam-Score: -1.423
X-Spam-Level:
X-Spam-Status: No, score=-1.423 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, FREEMAIL_FORGED_FROMDOMAIN=0.226, FREEMAIL_FROM=0.001, HEADER_FROM_DIFFERENT_DOMAINS=0.249, HTML_MESSAGE=0.001, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, URIBL_BLOCKED=0.001] autolearn=no autolearn_force=no
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 83U3IxOj6bvE for <mls@ietfa.amsl.com>; Tue, 22 Oct 2019 10:45:20 -0700 (PDT)
Received: from mail-io1-f46.google.com (mail-io1-f46.google.com [209.85.166.46]) (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 DFE161200F3 for <mls@ietf.org>; Tue, 22 Oct 2019 10:45:19 -0700 (PDT)
Received: by mail-io1-f46.google.com with SMTP id p6so13275483iod.7 for <mls@ietf.org>; Tue, 22 Oct 2019 10:45:19 -0700 (PDT)
X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=6M2QrLQvhdTxnmF2dKfPhQMxDDY16zfvpmzPAuX/B8o=; b=Rmz+OuCSekJkiVgmRawCq8rvKFkbbylhElGdYkKbA7SqP7esD+jiA268cI4CMmqH5b WgjMlpDNCQBc4ihUWOqu+NIyvc3c3MsF6E53lxMzL7u56CcipBmslc3RJW61ekGpwvKs BaRq23760AuA2e0+MSw7huwClZ//VGe3niscgy4LJN29z157NsiAfVS3/szWyMYy0Wdc 1lAcfKa+XIwipvWhU2XoaizWeoRRJszSZAAetZLkxR9cmJOV7aYAexCwuh5NFQ7AEF7/ BJzn9URsL39gsme5jvap19pVxuy3wF0oB5iSNE41Jbtuyuygtw/oFVFqWjvz1akBUV+V DqpQ==
X-Gm-Message-State: APjAAAUz/qRtjult4nWrKkr2E3SKlV1FkEFJKxr0ucWIasM6glxPd0Nr Aod/j8cfXi4/L9r2bBblRBMJewfldkFImqYm4yI=
X-Google-Smtp-Source: APXvYqwXmf/GcvXGzUr8qRuRqIsvTgr6UkSivIrz/Rt9TlwRrhWfnX9Mf1py0h0SO4Tm7Z+w6Ha010uwPq3pAT127Pk=
X-Received: by 2002:a05:6638:a0e:: with SMTP id 14mr4973938jan.4.1571766318700; Tue, 22 Oct 2019 10:45:18 -0700 (PDT)
MIME-Version: 1.0
References: <71e63449-abba-854d-2962-eac3a64a80d0@wickr.com> <398CD178-3DB6-4D70-B230-3362BE63A3BE@gmail.com> <44b5f5f7-79e1-c9e3-cde0-d75074168469@wickr.com> <5DAAF42C-C4CE-4631-A6E3-A5A5C1D0143A@gmail.com>
In-Reply-To: <5DAAF42C-C4CE-4631-A6E3-A5A5C1D0143A@gmail.com>
From: Yevgeniy Dodis <dodis@cs.nyu.edu>
Date: Tue, 22 Oct 2019 13:45:08 -0400
Message-ID: <CAMvzKsg0895RkaffJA7ZMQxKUr3uF3w-FZ3=T40YUDZd8TkisQ@mail.gmail.com>
To: Karthikeyan Bhargavan <karthik.bhargavan@gmail.com>
Cc: Joel Alwen <jalwen@wickr.com>, Messaging Layer Security WG <mls@ietf.org>
Content-Type: multipart/alternative; boundary="000000000000189a990595835d6f"
Archived-At: <https://mailarchive.ietf.org/arch/msg/mls/oupc_z4dsIkIFt-5i_SvP4HPVC4>
Subject: Re: [MLS] UPKE for X25519/X448
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: Tue, 22 Oct 2019 17:45:22 -0000

Good point, Karthik, we should definitely add this check (and possibly
something else).

As a small theoretical note, in our current model the users are considered
honest rather than malicious.
However, to model and prevent double-join attacks, the attacker is allowed
to request honest people not
to erase their local randomness (e.g., things like d' used above). So this
is why in our current description
we do not have to explicitly worry about d' being malicious.

We are pretty sure that with a check like the one you suggest we can also
withstand adversarial randomness
for UPKE. But we did not explore this yet - it is on the next to-do list.
In general, we don't consider too many insider
attacks (beside double join) for now, since TreeKEM and variants are not
secure against them anyway. Great
direction for the future.

Yevgeniy

On Tue, Oct 22, 2019 at 12:30 PM Karthikeyan Bhargavan <
karthik.bhargavan@gmail.com>; wrote:

> Yes, the sender would have to find a “d’” such that HKDF(sksize, d', "",
> "derive UPKE delta”) falls in a small set of values.
> This is a small risk, but it can be further reduced if we used epk in the
> derivation.
> E.g. why not define:
>
> d := HKDF(sksize, d’, epk, “derive UPKE delta”)
>
> This way, the attacker has to choose d’ based on the node’s old public
> key, which makes it harder for it to do malicious things.
>
> -Karthik
>
>
> > On 22 Oct 2019, at 17:05, Joel Alwen <jalwen@wickr.com>; wrote:
> >
> > Good question!
> >
> > I'll see if I can think of anything intelligent to say about it. :-)
> >
> > But one thing that comes to mind is that the sender gets to choose
> "only" d', not d. Instead d := HKDF(d') so to get d=0
> > you'd have to first invert HKDF.
> >
> > - Joel
> >
> > On 22/10/2019 17:02, Karthikeyan Bhargavan wrote:
> >> Sorry if this is already in the paper, but a question.
> >>
> >>> - UPKE-Decrypt(sk, (c1, c2)):
> >>> epk, context := HPKE.SetupBaseR(c1, sk, "")
> >>> d' || m := context.Open("", c2)
> >>> d := HKDF(sksize, d', "", "derive UPKE delta")
> >>> sk' := Mult(sk, d)
> >>> return (m, sk’)
> >>
> >> I believe it is important for the recipient to do some validation
> before returning from UPKE-Decrypt.
> >>
> >> For example, what if the (malicious) sender set d to “0” (whatever that
> means in the DH group).
> >> This would mean that the resulting key sk’ becomes “0” too, hence a
> non-member has been able to force the recipient group’s private key to a
> particular value, which is not ideal.
> >> What conditions should we add to avoid this kind of key-forcing attack
> from happening?
> >>
> >> -Karthik
> >>
> >>
> >>
> >>>
> >>>
> >>> References
> >>> ----------
> >>> [1] http:\\ia.cr\2019\1189.
> >>>
> >>> _______________________________________________
> >>> MLS mailing list
> >>> MLS@ietf.org
> >>> https://www.ietf.org/mailman/listinfo/mls
> >>
> >
> > _______________________________________________
> > MLS mailing list
> > MLS@ietf.org
> > https://www.ietf.org/mailman/listinfo/mls
>
> _______________________________________________
> MLS mailing list
> MLS@ietf.org
> https://www.ietf.org/mailman/listinfo/mls
>