Re: [MLS] KDF instead of hashing up the tree
Konrad Kohbrok <konrad.kohbrok@datashrine.de> Thu, 07 February 2019 07:37 UTC
Return-Path: <konrad.kohbrok@datashrine.de>
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 74E121310F5 for <mls@ietfa.amsl.com>; Wed, 6 Feb 2019 23:37:33 -0800 (PST)
X-Virus-Scanned: amavisd-new at amsl.com
X-Spam-Flag: NO
X-Spam-Score: -2.6
X-Spam-Level:
X-Spam-Status: No, score=-2.6 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-0.001, URIBL_BLOCKED=0.001] autolearn=ham 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 dChB02s_HIGe for <mls@ietfa.amsl.com>; Wed, 6 Feb 2019 23:37:29 -0800 (PST)
Received: from mx2.mailbox.org (mx2.mailbox.org [80.241.60.215]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ietfa.amsl.com (Postfix) with ESMTPS id 041CF131129 for <mls@ietf.org>; Wed, 6 Feb 2019 23:37:28 -0800 (PST)
Received: from smtp1.mailbox.org (smtp1.mailbox.org [IPv6:2001:67c:2050:105:465:1:1:0]) (using TLSv1.2 with cipher ECDHE-RSA-CHACHA20-POLY1305 (256/256 bits)) (No client certificate requested) by mx2.mailbox.org (Postfix) with ESMTPS id 49074A12D5; Thu, 7 Feb 2019 08:37:26 +0100 (CET)
X-Virus-Scanned: amavisd-new at heinlein-support.de
Received: from smtp1.mailbox.org ([80.241.60.240]) by spamfilter06.heinlein-hosting.de (spamfilter06.heinlein-hosting.de [80.241.56.125]) (amavisd-new, port 10030) with ESMTP id 975R1m-rKImt; Thu, 7 Feb 2019 08:37:20 +0100 (CET)
To: Benjamin Beurdouche <benjamin.beurdouche@inria.fr>
Cc: ML Messaging Layer Security <mls@ietf.org>
References: <dc702cea-d780-216b-ab8e-1eba99a2bace@datashrine.de> <CAL02cgTdx7_=t9jfZj2iFULFK4x-RSL+J5LrqRN=3co1nSKS7A@mail.gmail.com> <DB120F33-B500-42F2-8117-8883B396B278@gmail.com> <7507c820-d574-a570-6aba-c469366cc9c5@datashrine.de> <CAL02cgSsoi5JiEpLf4PCP0MufS2qAJQugW7WOVFVkH0ffLURfA@mail.gmail.com> <321d21c2-22ab-def4-7014-8948eeaa0dea@datashrine.de> <1AF700AD-243F-4BDD-AC44-F57CAF115E80@inria.fr>
From: Konrad Kohbrok <konrad.kohbrok@datashrine.de>
Message-ID: <05a6578c-2cc9-5338-e5df-3f13b9eb216f@datashrine.de>
Date: Thu, 07 Feb 2019 09:37:19 +0200
MIME-Version: 1.0
In-Reply-To: <1AF700AD-243F-4BDD-AC44-F57CAF115E80@inria.fr>
Content-Type: text/plain; charset="utf-8"
Content-Language: en-GB
Content-Transfer-Encoding: 8bit
Archived-At: <https://mailarchive.ietf.org/arch/msg/mls/IB3MEFfPcyTXhnLS21EfeJx4fUs>
Subject: Re: [MLS] KDF instead of hashing up the tree
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, 07 Feb 2019 07:37:34 -0000
Hey Benjamin, sure, that sounds good. I was just wondering if we want to keep the notion of a "Derive-Key-Pair" function, e.g., for a case where a private key is not necessarily a (pseudo-)random string you get out of a KDF. But if everyone is ok with that assumption and the additional notion of a "Derive-Public-Key" function, then I'm absolutely happy with that. Konrad On 06/02/2019 13:37, Benjamin Beurdouche wrote: > Hi Konrad, > > I think I got the idea, but let me try to reformulate with the following tree: > > AB > / \ > A B > > What we currently want to do: > > For A: > x_a <-$- {0,1}^n // This is A's new secret octet string > a <-- KDF(x_a,"kem_key") // This is A's private KEM-key > A <-- Derive-Public-Key(a) // This is A's public KEM-key > > For AB: > OLD = x_ab <-- Hash(x_a) // This is AB's new secret octet string > NEW = x_ab <-- KDF(x_a,”parent_key") // This is AB's new secret octet string > ab <-- KDF(x_ab,"kem_key") // This is AB's new private KEM-key > AB <-- Derive-Public-Key(ab) // This is AB's public KEM-key > > What we need for a computational security proof is a (“technical") key separation > between A and AB’s secrets (a & ab) hence moving the draft towards using a KDF. > (I think I remember the TLS story which was expand one and split versus > expand twice). I completely agree with that and that doesn’t cost anything : ) > > Regarding the multiple options: to me option 1 and 3 are really the same as I > consider the following relationship: > (y,Y) <-- Derive-Key-Pair(X) > := > y <-- KDF(X,”kem_key”); > Y <-- Derive-Public-Key(y); > > Am I wrong in saying that I believe option 1 already gives us everything we need ? > I really like that design actually which fits the derivation scheme for the message protection. > > Best, > B. > >> On Feb 6, 2019, at 11:07 AM, Konrad Kohbrok <konrad.kohbrok@datashrine.de> wrote: >> >> A question I ran into while working on the PR: Currently, there is the notion of >> "Derive-Key-Pair" function that produces a key pair from an octet string. >> >> Do we want to keep that function for modularity/composability purposes at the >> possible price of an additional KDF expand operation per key derivation? This is >> probably just a formal detail, but I wasn't quite sure what the best way forward >> would be. >> >> The options as I see them are as follows: >> >> Consider a node A that wants to update its secret. >> >> Option 1, derive the private KEM key directly, no Derive-Key-Pair function. >> Instead a Derive-Public-Key function: >> >> x_a <-$- {0,1}^n // This is A's new secret octet string >> a <-- KDF(x_a,"kem_key") // This is A's private KEM-key >> A <-- Derive-Public-Key(a) // This is A's public KEM-key >> x_b <-- KDF(x_a,"parent_secret") // This is B's new secret octet string >> b <-- KDF(x_b,"kem_key") // This is B's new private KEM-key >> B <-- Derive-Public-Key(b) // This is B's public KEM-key >> >> >> Option 2, stick with a Derive-Key-Pair function, but add additional KDF expand >> to ensure key separation: >> >> x_a <-$- {0,1}^n // This is B's new seed octet string >> x_a' <-- KDF(x_a,"secret_seed") // This is A's new secret octet string >> (a,A) <-- Derive-Key-Pair(x_a') // This is A's new private/public key pair >> x_b <-- KDF(x_a,"parent_secret") // This is B's new seed octet string >> x_b' <-- KDF(x_b,"secret_seed") // This is B's new secret octet string >> (b,A) <-- Derive-Key-Pair(x_b) // This is B's new private/public key pair >> >> >> Option 3, use Derive-Key-Pair function directly on seed. This would add the >> requirement to the Derive-Key-Pair notion, that it (internally) uses a KDF to >> derive the private key. >> >> x_a <-$- {0,1}^n >> (a,A) <-- Derive-Key-Pair(x_a) // This is A's new private/public key pair >> x_b <-- KDF(x_a,"parent_secret") // This is B's new secret octet string >> (b,B) <-- Derive-Key-Pair(x_b) // This is B's new private/public key pair >> >> >> My guess is that in most cases the Derive-Key-Pair function will in the end call >> a KDF to get the private key anyway, which means that we have one superfluous >> KDF expand operation. >> >> Konrad >> >> >> >> On 23/01/2019 17:47, Richard Barnes wrote: >>> Ah, ok, I get it. I misunderstood "new secret" as "fresh entropy". >>> >>> In that case, this falls into the "sure, if it makes the analysis better" >>> bucket. We've been treating hashes/HKDF invocations as basically free. At some >>> point we might need to worry about that, but I suspect that today is not that day. >>> >>> Want to make a PR? >>> >>> --Richard >>> >>> >>> On Wed, Jan 23, 2019 at 10:19 AM Konrad Kohbrok <konrad.kohbrok@datashrine.de >>> <mailto:konrad.kohbrok@datashrine.de>> wrote: >>> >>> Exactly, thanks Karthik! >>> >>> Say we have the same tree as in the example in 5.4: >>> >>> G >>> / \ >>> / \ >>> E _ >>> / \ / \ >>> A B C D >>> >>> Then A generates a fresh secret X_1secret and derives the following new secrets: >>> X_1kemkey=HKDF(X_1secret,"kemkey") >>> X_2secret=HKDF(X_1secret,"parent") >>> >>> X_2kemkey=HKDF(X_2secret,"kemkey") >>> X_3secret=HKDF(X_2secret,"parent") >>> >>> X_3kemkey=HKDF(X_3secret,"kemkey") >>> >>> A then sends E(pk(B), X_2secret) to B, E(pk(C),X_3secret) to C and >>> E(pk(D),X_3secret) to D. >>> >>> Hopefully that makes the idea a little clearer. Sorry for the terrible notation. >>> >>> Konrad >>> >>> On 23/01/2019 16:59, Karthikeyan Bhargavan wrote: >>>> If I understand correctly, Chris and Konrad are not suggesting changing >>> the secrets. >>>> Instead, they are suggesting that H(.) be implemented as something like: >>>> >>>> H(x) = HKDF(x,label=”parent”) >>>> >>>> where x is the tree secret for the current node. >>>> >>>> Similarly, when generating the KEM private key for a node, we use >>>> >>>> KGEN(x) = HKDF(x,label=“kem key”) >>>> >>>> This would be a good way of making sure that each key in the protocol is >>>> independent, but at no additional cost. >>>> Am I understanding this correctly, Konrad? >>>> >>>>> On 23 Jan 2019, at 15:29, Richard Barnes <rlb@ipv.sx <mailto:rlb@ipv.sx >>> <mailto:rlb@ipv.sx>>> wrote: >>>>> >>>>> Note this is a little bit expensive in terms of message size; it changes the >>>>> size of an update from log(N) to log(N)^2. It does not change the number of >>>>> DH operations >>>>> >>>>> This is because you have to send the fresh secret for each intermediate node >>>>> in the tree to all its descendants. Comparing the secrets you generate in >>>>> each case, from leaf to root, along a path of depth 3 with S0 at the leaf: >>>>> >>>>> Current: S0, S1 = H(S0), S2 = H^2(S0), S3 = H^3(S0) >>>>> Proposed: S0, S1 = KDF(T0, S0), S2 = KDF(T1, S1), S3 = KDF(T2, S2) >>>>> >>>>> Where T* are the fresh secrets called for here. This doesn't change to whom >>>>> you encrypt things, but changes what you encrypt to each copath node: >>>>> >>>>> Current -> Proposed >>>>> S1 -> S1, T1, T2 >>>>> S2 -> S2, T2 >>>>> S3 -> S3 >>>>> >>>>> (This is of course because you need to enable each recipient to compute >>> up the >>>>> tree.) So there's your log->log^2. >>>>> >>>>> This discussion is not to say that I'm opposed to this idea. It just looks >>>>> like it has some non-negligible cost, so we should make sure we know what >>>>> we're getting for that cost. >>>>> >>>>> --Ricahrd >>>>> >>>>> >>>>> >>>>> On Wed, Jan 23, 2019 at 8:58 AM Konrad Kohbrok <konrad.kohbrok@datashrine..de >>>>> <mailto:konrad.kohbrok@datashrine.de >>> <mailto:konrad.kohbrok@datashrine.de>>> wrote: >>>>> >>>>> Hey everyone, >>>>> >>>>> I just discussed the current draft with my advisor Chris Brzuska and he >>>>> came up >>>>> with a suggestion that I thought I'd just quickly relay here. As I >>> have only >>>>> started following the discussion recently, I apologize if this was >>> already >>>>> brought up in the past. >>>>> >>>>> In terms of key separation, wouldn't it make for a cleaner design, if we >>>>> used a >>>>> KDF instead of a hash function? Instead of generating a new >>> leaf-node secret >>>>> and then hashing it to compute the new secret for the parent node, it >>> would be >>>>> better to generate a new secret and then from that secret >>> independently (i.e. >>>>> with different labels) compute the new leaf secret and the new secret >>> for the >>>>> parent node. This key independence would also make the proof easier. In >>>>> terms of >>>>> overhead, this would mean two KDF operations instead of one hashing >>> operation. >>>>> >>>>> Cheers, >>>>> Konrad >>>>> >>>>> _______________________________________________ >>>>> MLS mailing list >>>>> 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>> >>>>> 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 >
- [MLS] KDF instead of hashing up the tree Konrad Kohbrok
- Re: [MLS] KDF instead of hashing up the tree Richard Barnes
- Re: [MLS] KDF instead of hashing up the tree Karthikeyan Bhargavan
- Re: [MLS] KDF instead of hashing up the tree Konrad Kohbrok
- Re: [MLS] KDF instead of hashing up the tree Richard Barnes
- Re: [MLS] KDF instead of hashing up the tree Konrad Kohbrok
- Re: [MLS] KDF instead of hashing up the tree Konrad Kohbrok
- Re: [MLS] KDF instead of hashing up the tree Benjamin Beurdouche
- Re: [MLS] KDF instead of hashing up the tree Konrad Kohbrok
- Re: [MLS] KDF instead of hashing up the tree Konrad Kohbrok
- Re: [MLS] KDF instead of hashing up the tree Benjamin Beurdouche