Re: [MLS] Re-randomized TreeKEM
Richard Barnes <rlb@ipv.sx> Tue, 22 October 2019 21:20 UTC
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From: Richard Barnes <rlb@ipv.sx>
Date: Tue, 22 Oct 2019 17:20:19 -0400
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To: Yevgeniy Dodis <dodis@cs.nyu.edu>
Cc: Karthikeyan Bhargavan <karthik.bhargavan@gmail.com>, Messaging Layer Security WG <mls@ietf.org>, Joel Alwen <jalwen@wickr.com>
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Subject: Re: [MLS] Re-randomized TreeKEM
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Just so we're clear, I would be *strongly* opposed to putting anything like this naïve UPKE scheme into MLS. For the simple reason that it expands the size of Welcome message by a factor of K. RTreeKEM or nothing :) --Richard On Mon, Oct 21, 2019 at 6:53 PM Yevgeniy Dodis <dodis@cs.nyu.edu> wrote: > Great summary, Karthik! > > As I put in the other thread, comparison with the "naive" UPKE (for K=100 > or less) might be a good idea. > > Pros of Naive Scheme: > - more general, uses any PKE > - using stream ciphers to generate K key pairs as needed makes the > efficiency hit noticeably less than a factor of K, > and perhaps closer to a factor of 2 (see below), but unclear a-priori. > - for K>1, offers non-trivial (but still sub-optimal) security enhancement > over basic TreeKEM (K=1) > > Cons (pros of RTreeKEM): > - Public key storage increases by at least factor of K > - While the naive use increases secret storage and computation by a factor > of K, using stream cipher, after i uses one can only store the > current seed to generate last (K-i) keys. However, K public keys should be > published right away, so we must > lose at least factor of 2 compared to TreeKEM to generate all keys twice > (but possibly factor of K if people update too frequently, so all but > 1 of the K keys gets used). So overall efficiency hit between 2 and K, > which might already be comparable or worse than RTreeKEM. > - Still much worse security than RTreeKEM, but possibly more expensive > too, already for small K! > > Please let us know if you think this should be explored further. > Yevgeniy > > On Mon, Oct 21, 2019 at 8:03 AM Karthikeyan Bhargavan < > karthik.bhargavan@gmail.com> wrote: > >> I see. So, here’s how I read the improvements proposed in RTreeKEM. >> >> Currently, in TreeKEM (like in ART before it) we rely on each member to >> regularly *send* updates in order to get both PCS and FS for the group >> secrets. >> The informal secrecy guarantees we get are that: >> - (FS) if member A sends an update in epoch N (moving the epoch to N+1), >> and if A gets compromised in epoch N+1, the messages sent in epoch N remain >> secret >> - (PCS) if member A sends an update in epoch N (moving the epoch to N+1), >> and if A was (passively) compromised in epoch N, the messages sent in epoch >> N+1 remain secret >> In other words, each member who sends an update gets local protection >> against compromise, encouraging vulnerable members to keep sending updates. >> >> However, as Joel, Sandro, Yevgeniy, and Yiannis note in their paper, we >> could do better, at least for FS, if we use one-time decryption keys. >> If each recipient deletes the old decryption key after processing an >> update, then even just by *processing* an update, we get an additional >> guarantee: >> - (FS’) if member A processes an update in epoch N (moving the epoch to >> N+1), and if A gets compromised in epoch N+1, the messages sent in epoch N >> remain secret >> >> It is also worth remembering that Signal also has a notion of one-time >> prekeys that work similarly for new messaging sessions. >> Although the following would be a bit ridiculous to use in large dynamic >> groups, here is a sketch to achieve the receiver FS guarantee without the >> need for new crypto. >> - Every time a member A sends an update, it generates fresh node secrets >> for nodes on the path from A to the root >> - From each node secret, A generates a large number K (= 100) >> private-public encryption keypairs and sends the public keys with the >> update. >> - On receiving the update, each member B stores all K public keys for >> each node in its co-path >> - Each of these public keys can be used only once for sending an update, >> after which the private key is deleted from all recipients. >> - The last public key at each node is not deleted; it can only be >> replaced when one of the members under that node sends a new update (with a >> fresh batch of public keys).. >> >> As far as I understand, the above scheme can be seen as an (inefficient) >> implementation of UPKE, right? >> Of course, it increases the size of each update by K, and only provides >> FS for K updates, after which some member has to send an update. >> Conversely, it does not require any new crypto algorithm. Is this a good >> baseline to compare UPKE schemes against? >> >> If I am mis-reading something, do let me know! >> >> -Karthik >> >> >> >> >> >> >> On 17 Oct 2019, at 17:18, Joel Alwen <jalwen@wickr.com> wrote: >> >> I think the challenge with the hash-forward approach is how to do that >> homomorphically. I.e. what we need are two algorithms; one to refresh >> the PK without knowing the SK (but possibly knowing a secret >> rerandomizer delta if needed) and one to update SK (again possibly using >> delta). So to use a hash-forward approach their must be: >> >> 1) a way to evolve PK forward to PK' and >> 2) a *one-way* method to evolve SK forward to SK' compatible PK'. >> >> One-wayness is what gives us Forward Secrecy and "compatibility" between >> the two key evolution methods is what allows for asynchronous (i.e. 1 >> packet) updates. >> >> Currently we use a secret re-randomizer delta to ensure the SK update >> method is one-way. That is, without the delta you cant "undo" the >> update. But that would break if we (at least naively) used some public >> delta, say hash(ciphertext). So I think this is the challenge that we'd >> have to overcome.. Basically, make sure we SK evolution is one-way but >> also compatible the public evolution of PK. >> >> >> >> Now one way sweet way to get all this (and more) would be to use a HIBE. >> >> Initial, PK for a ratchet tree node (i.e. its "identity" since this is a >> HIBE now) is simply the empty vector PK := () while the secret key is >> the master public key for a fresh HIBE instance SK := MSK. We also >> include, as a second component of the nodes PK, the master public key >> PK_0 = MPK. To "hash forward" / "re-randomize" when sending a ciphertext >> C to that node we can do: >> >> PK' := (PK, hash(C)). >> SK' := DeriveHIBEKey(PK, SK). >> >> So simply append hash(C) to the identity for that node and derive the >> corresponding HIBE key. >> >> Ignoring the problems with using HIBE for a second, this is a very cool >> solution. We don't need to send out the updated PK since everyone in the >> group (and even the adversary) can compute it for themselves. We also >> dont need a re-randomize delta as part of the plaintext because we're >> using delta := hash(ciphertext) so the plaintext is shorter again. >> Moreover, HIBE security means that learning SK' doesn't tell you >> anything interesting about SK. In particular, we have forward security. >> (In fact, FS will hold even if hash(C) were chosen *completely* >> adversarially, say, as part of a malicious update in an insider attack!) >> >> Of course, the problem with this solution is that we're using HIBE. >> Worse, with unbounded depth because each new ciphertext sent to a node >> results in going one depth further into the hierarchy. AFAIK all HIBE >> constructions have pretty horrible (read exponential) efficiency as a >> function of their depth. (And I won't mention the state of >> standardization and open implementations for HIBE.) >> >> Now there could be a totally different approach that entirly avoids >> HIBE. But even with this approach there's at least some glimer of hope >> to improve on it because, if we don't wory about insider attacks we can >> assume C is honestly generated which means hash(C) really has a ton of >> entropy. So we dont seem to need the full expresivity HIBE identities >> allow us. Rather we only need HIBE for "random" identities. Still, that >> seems like a pretty slim hope for major efficiency improvement. It also >> doesn't do anything to address the lack of implementations and standards. >> >> - Joël >> >> On 17/10/2019 16:37, Karthik Bhargavan wrote: >> >> Thanks Yevgeniy, >> >> This helps a lot. >> >> To further my understanding, another question: >> >> Intuitively, the sender will not only encrypt the message, but also a >> random Delta value. It will change public key using homomorthism by >> multiplying with g^Delta (in specific DH based scheme), while the >> recipient will decrypt Delta (using old secret key), and add it to the >> old secret key to get there new one. So now corrupting (old sk plus >> Delta) will not help decrypting the ciphertext just decepted, emailing >> forward secrecy. >> >> >> I see that in the DH-based scheme, this Delta needs to be private, >> otherwise the adversary can compute sk once it knows sk+Delta. >> But, in general, is it possible to conceive of a UPKE scheme where the >> recipient effectively “hashes forward” its symmetric key, >> where this one way hash-forward function does not have to rely on an >> externally chosen secret value? >> >> Best, >> Karthik >> >> This is the high level, hope it makes sense. >> Thanks for your question, >> Yevgeniy >> >> On Thu, Oct 17, 2019, 1:43 AM Karthik Bhargavan >> <karthikeyan.bhargavan@inria.fr >> <mailto:karthikeyan.bhargavan@inria.fr <karthikeyan.bhargavan@inria.fr>>> >> wrote: >> >> Hi Joel, >> >> This looks very interesting. It is new to me since I was not at >> the interim. >> After reading the paper and the slides, I am still a bit fuzzy >> about what the recipient of an update needs to do. >> >> For example, for the running example in your slide deck, it would >> help if I could see: >> - what secret keys does each leaf need to keep >> - how do these secrets change when an update from some other node >> is received. >> Just working this out for one update is enough. >> >> I know that this is made precise in the eprint, but it would be >> faster if you could help us understand it :) >> >> Best, >> Karthik >> >> On 16 Oct 2019, at 23:51, Joel Alwen <jalwen@wickr.com >> >> <mailto:jalwen@wickr.com <jalwen@wickr.com>>> wrote: >> >> >> <FS-TreeKEM.pdf> >> >> >> _______________________________________________ >> MLS mailing list >> MLS@ietf.org <mailto:MLS@ietf.org <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 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] Re-randomized TreeKEM Joel Alwen
- Re: [MLS] Re-randomized TreeKEM Karthik Bhargavan
- Re: [MLS] Re-randomized TreeKEM Yevgeniy Dodis
- Re: [MLS] Re-randomized TreeKEM Karthik Bhargavan
- Re: [MLS] Re-randomized TreeKEM Karthik Bhargavan
- Re: [MLS] Re-randomized TreeKEM Joel Alwen
- Re: [MLS] Re-randomized TreeKEM Brendan McMillion
- Re: [MLS] Re-randomized TreeKEM Karthikeyan Bhargavan
- Re: [MLS] Re-randomized TreeKEM Konrad Kohbrok
- Re: [MLS] Re-randomized TreeKEM Benjamin Beurdouche
- Re: [MLS] Re-randomized TreeKEM Konrad Kohbrok
- Re: [MLS] Re-randomized TreeKEM Joel Alwen
- Re: [MLS] Re-randomized TreeKEM Joel Alwen
- Re: [MLS] Re-randomized TreeKEM Yevgeniy Dodis
- Re: [MLS] Re-randomized TreeKEM Yevgeniy Dodis
- Re: [MLS] Re-randomized TreeKEM Brendan McMillion
- Re: [MLS] Re-randomized TreeKEM Joel Alwen
- Re: [MLS] Re-randomized TreeKEM Richard Barnes
- Re: [MLS] Re-randomized TreeKEM Karthikeyan Bhargavan
- Re: [MLS] Re-randomized TreeKEM Richard Barnes
- Re: [MLS] Re-randomized TreeKEM Benjamin Beurdouche
- Re: [MLS] Re-randomized TreeKEM Brendan McMillion
- Re: [MLS] Re-randomized TreeKEM Dennis Jackson
- Re: [MLS] Re-randomized TreeKEM Konrad Kohbrok
- Re: [MLS] Re-randomized TreeKEM Yevgeniy Dodis
- Re: [MLS] Re-randomized TreeKEM Brendan McMillion
- Re: [MLS] Re-randomized TreeKEM Dennis Jackson
- Re: [MLS] Re-randomized TreeKEM Yevgeniy Dodis
- Re: [MLS] Re-randomized TreeKEM Yevgeniy Dodis
- Re: [MLS] Re-randomized TreeKEM Brendan McMillion
- Re: [MLS] Re-randomized TreeKEM Konrad Kohbrok
- Re: [MLS] Re-randomized TreeKEM Raphael Robert
- Re: [MLS] Re-randomized TreeKEM Brendan McMillion
- Re: [MLS] Re-randomized TreeKEM Dennis Jackson
- Re: [MLS] Re-randomized TreeKEM Brendan McMillion
- Re: [MLS] Re-randomized TreeKEM Dennis Jackson
- Re: [MLS] Re-randomized TreeKEM Joel Alwen
- Re: [MLS] Re-randomized TreeKEM Konrad Kohbrok
- Re: [MLS] Re-randomized TreeKEM Richard Barnes