Re: [MLS] Improving entropy in MLS

Konrad Kohbrok <konrad.kohbrok@datashrine.de> Mon, 19 April 2021 14:10 UTC

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Date: Mon, 19 Apr 2021 16:10:18 +0200 (CEST)
From: Konrad Kohbrok <konrad.kohbrok@datashrine.de>
To: Joel Alwen <jalwen@wickr.com>, mls@ietf.org
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Subject: Re: [MLS] Improving entropy in MLS
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Hi everyone,

I'm not sure what the authors and the chairs had in mind in terms of how we deal with the PRs and issue that are currently open (there are a couple of others beside the PR discussed in this thread), but I think it would be useful to have (virtual) interims again to make progress.

I know we are in a freeze, but I seem to remember that there is meant to be a "last call" soon and I feel that some of the issues are definitely worth discussing before that happens.

Cheers,
Konrad

> Joel Alwen <jalwen@wickr.com> hat am 05.04.2021 10:45 geschrieben:
> 
>  
> Hey People,
> 
> In principle modularity is definitely worth going for. The counter points though
> are "defense in depth" and "minimize your assumptions". Yes, the OS/cryptolib
> *should* provide good entropy. But we can still design protocols to minimize
> their dependce on that service.
> 
> Other's have faced this issue in the past. Compare the ECDSA's strong dependency
> on external entropy when signing to that of the newer sig scheme EdDSA which
> requires none. It is precisely this *unnecessary* vuln in ECDSA that was
> exploited to break the PS3's security. Lessons we're learnt and thats how we
> ended up with EdDSA. The goal of this PR is to not make the same mistake with MLS.
> 
> 
> 
> But modularity aside, its also not totally clear to me how the PR's mechanism
> would work as an external solution because it also feeds in entropy from ongoing
> MLs sessions. Here the solution benefits from ensuring that both A) entropy
> derived off the MLS session is never used for any other prupose and B) entropy
> is absorbed once for each epoch. (More often would be fine too but less
> efficient with no security improvement. Less would offer weaker security.)
> 
> To simulate the same security benefits with an external entropy pool (e.g. in
> the OS or cryptolib) you'd want to reproduce both A) and B) since they both help
> with security. A) is possible but potentially rather non-trivial if you want to
> use exporter keys (and i'm not sure there's any other way to do it). Meanwhile
> the PR, being an internal solution, has lower-level access to the key schedule
> so it can trivially insure A) by defining a unique prefix to the HKDF for this
> purpose e.g. automatically seperating the output away from all exported keys.
> Ensuring B) doesn't look completely straightforward to me either. A "polling"
> based solution seems pretty imperfect & less-efficient to me. An alternative
> would be to have clients notify external services whenever a new commit has just
> been processed but is that really what we want? Even commits containing only
> update (or even no) proposals? (Not to mention is that even really an "external"
> solution...)
> 
> 
> Any way, I'm not saying I think there is no *possible* external solution. Just
> that the mechanisms in the PR seem more aligned with an internal one built into
> the MLS implementation rather than bolted on after the fact, especially in some
> code that doesn't even "know" about MLS at all.
> 
> - Joël
> 
> 
> On 31/03/2021 09:55, Franziskus Kiefer wrote:
> > Hi Konrad, all
> >
> > I agree with Benjamin and Brendan that the protocol is not the right place for
> > something like this.
> > Since you already mention [1], don't you think it would be better to recommend
> > that implementations use RFC 8937 [2] and give an example of how to instantiate
> > G'(n)?
> >
> > Best,
> > Franziskus
> >
> > [1] https://eprint.iacr.org/2018/1057 <https://eprint.iacr.org/2018/1057>
> > [2] https://www.rfc-editor.org/rfc/rfc8937.html
> > <https://www.rfc-editor.org/rfc/rfc8937.html>
> >
> > On Wed, 31 Mar 2021 at 09:41, Konrad Kohbrok <konrad.kohbrok@datashrine.de
> > <mailto:konrad.kohbrok@datashrine.de>> wrote:
> >
> >     Hi Brendan,
> >
> >     Thanks for your feedback! You make two good arguments here, that I want to
> >     address independently.
> >
> >     The first is the maintenance of layers of abstraction. Yes, ultimately, we'd
> >     like to be able to rely on the OS's (CSP)RNG, but events in the past (see
> >     the introduction of [1] for a few examples) have shown that they are not
> >     always too reliable. As a consequence, protocols should be designed to be
> >     robust against bad sources of randomness. In particular, our design allows
> >     the inclusion of "remote" sources of randomness, which is something the OS's
> >     RNG can't provide. In addition, I'd like to mention that we're not intending
> >     to replace the OS's RNG, but rather complement it. Note, that the design
> >     ensures that no secret can be extracted from the pool without retrieving
> >     fresh randomness from the OS's RNG.
> >
> >     This brings me to the second point, where I agree that we should keep MLS
> >     free from unnecessary complexity that makes it more prone to bugs and you're
> >     right in that the source of randomness is a particularly fundamental and
> >     thus vulnerable part of MLS. However, the design is conceivably simple and
> >     uses the same, tried, tested and provably secure mechanism that is used in
> >     the key schedule of TLS and MLS: a simple extract-expand cycle. See [2] for
> >     a security proof of the MLS key schedule, which can easily be adapted to the
> >     design of the entropy pool.
> >
> >     To summarize, we're not proposing to replace the OS's RNG, but rather extend
> >     it such that additional sources of entropy can be included. The mechanism is
> >     simple and, in my opinion, reasonably easy to review, implement and prove
> >     secure without additional assumptions.
> >
> >     Cheers,
> >     Konrad
> >
> >     [1] https://eprint.iacr.org/2018/1057 <https://eprint.iacr.org/2018/1057>
> >     [2] https://eprint.iacr.org/2021/137 <https://eprint.iacr.org/2021/137>
> >
> >
> >     > Brendan McMillion <brendan@cloudflare.com <mailto:brendan@cloudflare.com>>
> >     hat am 30.03.2021 23:01 geschrieben:
> >     >
> >     >
> >     > Hey Konrad
> >     >
> >     > Thanks for writing this up. However, I feel pretty strongly opposed to
> >     this idea, just as a matter of maintaining proper layers of abstraction. MLS
> >     implementations aren't remotely the only things on a user's system that
> >     require strong randomness, which is why every OS has its own CSPRNG. Those
> >     CSPRNGs are going to be much more robust, better designed, and thoroughly
> >     audited than anything we'd implement. Instead, it's more likely we'd have
> >     MLS implementations do this incorrectly and introduce more
> >     randomness-related vulnerabilities than we prevented.
> >     >
> >     >
> >     >
> >     > On Tue, Mar 30, 2021 at 2:25 AM Konrad Kohbrok
> >     <konrad.kohbrok@datashrine.de <mailto:konrad.kohbrok@datashrine.de>> wrote:
> >     > > Great questions. I really should have included our thoughts on the
> >     threat model.
> >     > >
> >     > >  We don't assume the entropy pool to be more protected than any other
> >     secret in MLS.
> >     > >
> >     > >  Functionally, we want the pool to allow the following:
> >     > >  - a party should be able to accrue entropy over time using the OS's RNG
> >     > >  - a party should be able to inject entropy from other sources (in
> >     particular secrets derived from the epoch_secret of any of the party's
> >     groups whenever a fresh commit comes in)
> >     > >
> >     > >  Assuming the OS's RNG is really bad (e.g. it spits out only zeroes),
> >     then a party can still have decent entropy once it receives commits from
> >     groups that it is in. Any party that wants to predict the entropy in the
> >     pool would have to know every secret that is injected into the pool. That
> >     means that an adversary would have to compromise all of the party's groups
> >     to predict secrets extracted from the entropy pool.
> >     > >
> >     > >  Of course, the pool will take some time to accrue entropy from various
> >     sources, so it doesn't help if the adversary is present from the start, but
> >     it will help if the adversary compromises a party later on. If that happens,
> >     then the compromised party can recover if the adversary misses (i.e. doesn't
> >     intercept and decrypt) any commit from another party after the compromise.
> >     > >
> >     > >  Now we have to make sure that the design doesn't give us any
> >     disadvantages over just sampling from the OS's RNG every time we need fresh
> >     randomness.
> >     > >
> >     > >  Let's assume full state compromise. Then
> >     > >  1) the adversary can't obtain entropy extracted from the pool in the
> >     past (because it's essentially a KDF-chain, just as the key schedule)
> >     > >  2) the adversary can't predict what entropy the client will extract in
> >     the future if one of the following is true:
> >     > >  - the OS's RNG is good
> >     > >  - a commit from a group arrives, where the group itself is not
> compromised
> >     > >  - any other external entropy gets injected that the adversary doesn't
> >     have access to (e.g. a signature from a secret key that is kept secure in an
> >     HSM)
> >     > >
> >     > >  To summarize: It's no silver bullet against bad entropy in general, but
> >     it's cheap, no worse than just relying on the OS's RNG and provides a
> >     framework to accrue randomness over time from various sources even if the
> >     OS's RNG is bad/compromised and it allows parties to recover from compromise
> >     in that case.
> >     > >
> >     > >  Hope that shed some light on things.
> >     > >
> >     > >  Cheers,
> >     > >  Konrad
> >     > >
> >     > >
> >     > >
> >     > >  > Benjamin Beurdouche <benjamin.beurdouche@inria.fr
> >     <mailto:benjamin.beurdouche@inria.fr>> hat am 30.03.2021 09:53 geschrieben:
> >     > >  >
> >     > >  >
> >     > >  > Hi all,
> >     > >  >
> >     > >  > I like this idea. I am not fully clear about the threat model, though.
> >     > >  >
> >     > >  > From reading this, it looks like we assume that the adversary that
> >     compromised a
> >     > >  > principal doesn’t have complete control the entropy pool.
> >     > >  >
> >     > >  > This is in contrast over what we assume of the AEAD keys or even the
> >     KEM keys
> >     > >  > where a full state compromise will gain access to the values.
> >     > >  >
> >     > >  > However it looks very close to the assumptions about additional
> >     protections for signing keys.
> >     > >  > E.g., not directly accessible, and behind an interface that an
> >     adversary can query for certain operations
> >     > >  > through an API (e.g. when is stored in an HSM, co-processor or a
> >     functional secure enclave).
> >     > >  >
> >     > >  > Am I reading this properly?
> >     > >  > If yes, I can understand the value in this threat model, but we
> >     should make it clear that the
> >     > >  > entropy pool has to get these additional protections to provide these
> >     good properties.
> >     > >  >
> >     > >  > Seems to me like a good improvement in all cases anyway, so I think
> >     we should consider it.
> >     > >  >
> >     > >  > Thanks!
> >     > >  > Ben
> >     > >  >
> >     > >  > > On 30 Mar 2021, at 09:27, Konrad Kohbrok
> >     <konrad.kohbrok@datashrine.de <mailto:konrad.kohbrok@datashrine.de>> wrote:
> >     > >  > >
> >     > >  > > Hi everyone,
> >     > >  > >
> >     > >  > > MLS is a protocol that is very vulnerable to individual parties
> >     with bad randomness. For example, when a party joins a group, the secrecy of
> >     the group's key material relies on the quality of that party's key material.
> >     Similarly, when doing an external join, the groups entropy is completely
> >     replaced by that of the joining member.
> >     > >  > >
> >     > >  > > There are multiple ways to mitigate this threat and Joël and Sandro
> >     proposed a few of them in the following mail to the list:
> >     https://mailarchive.ietf.org/arch/msg/mls/ZR84smU5xeLrziNTk5W1P1Z1nQI/
> >     <https://mailarchive.ietf.org/arch/msg/mls/ZR84smU5xeLrziNTk5W1P1Z1nQI/>
> >     > >  > >
> >     > >  > > Concretely, there were two approaches: one that would be baked into
> >     the protocol (essentially using a secret derived from old path secrets to
> >     inject into new ones in addition to the current approach) and one that would
> >     mandate the use of an entropy pool.
> >     > >  > >
> >     > >  > > The ideas were discussed a bit at the time, but nothing has
> >     happened since then. Joël, Sandro and I have just opened a PR with a
> >     concrete design for an entropy pool that is modeled after the key schedule
> >     (https://github.com/mlswg/mls-protocol/pull/467
> >     <https://github.com/mlswg/mls-protocol/pull/467>). Concretely, it allows
> >     gathering entropy over time and for parties with a bad entropy source to
> >     profit from parties with a good one without compromising security.
> >     > >  > >
> >     > >  > > While for future iterations of MLS, we might want to consider a
> >     solution that is more integral to the protocol, we are aware that the
> >     authors want to avoid breaking changes at this point. With the entropy pool,
> >     we thus propose a solution that does not affect the protocol flow, but that
> >     still offers significant advantages over no mitigations at all.
> >     > >  > >
> >     > >  > > Cheers,
> >     > >  > > Konrad
> >     > >  > >
> >     > >  > > _______________________________________________
> >     > >  > > MLS mailing list
> >     > >  > > MLS@ietf.org <mailto:MLS@ietf.org>
> >     > >  > > https://www.ietf.org/mailman/listinfo/mls
> >     <https://www.ietf.org/mailman/listinfo/mls>
> >     > >
> >     > >  _______________________________________________
> >     > >  MLS mailing list
> >     > >  MLS@ietf.org <mailto:MLS@ietf.org>
> >     > >  https://www.ietf.org/mailman/listinfo/mls
> >     <https://www.ietf.org/mailman/listinfo/mls>
> >     > >
> >
> >     _______________________________________________
> >     MLS mailing list
> >     MLS@ietf.org <mailto:MLS@ietf.org>
> >     https://www.ietf.org/mailman/listinfo/mls
> >     <https://www.ietf.org/mailman/listinfo/mls>
> >
> >
> > _______________________________________________
> > MLS mailing list
> > MLS@ietf.org
> > https://www.ietf.org/mailman/listinfo/mls
> >
> 
> 
> On 31/03/2021 09:55, Franziskus Kiefer wrote:
> > Hi Konrad, all
> > 
> > I agree with Benjamin and Brendan that the protocol is not the right place for
> > something like this.
> > Since you already mention [1], don't you think it would be better to recommend
> > that implementations use RFC 8937 [2] and give an example of how to instantiate
> > G'(n)?
> > 
> > Best,
> > Franziskus
> > 
> > [1] https://eprint.iacr.org/2018/1057 <https://eprint.iacr.org/2018/1057>
> > [2] https://www.rfc-editor.org/rfc/rfc8937.html
> > <https://www.rfc-editor.org/rfc/rfc8937.html>
> > 
> > On Wed, 31 Mar 2021 at 09:41, Konrad Kohbrok <konrad.kohbrok@datashrine.de
> > <mailto:konrad.kohbrok@datashrine.de>> wrote:
> > 
> >     Hi Brendan,
> > 
> >     Thanks for your feedback! You make two good arguments here, that I want to
> >     address independently.
> > 
> >     The first is the maintenance of layers of abstraction. Yes, ultimately, we'd
> >     like to be able to rely on the OS's (CSP)RNG, but events in the past (see
> >     the introduction of [1] for a few examples) have shown that they are not
> >     always too reliable. As a consequence, protocols should be designed to be
> >     robust against bad sources of randomness. In particular, our design allows
> >     the inclusion of "remote" sources of randomness, which is something the OS's
> >     RNG can't provide. In addition, I'd like to mention that we're not intending
> >     to replace the OS's RNG, but rather complement it. Note, that the design
> >     ensures that no secret can be extracted from the pool without retrieving
> >     fresh randomness from the OS's RNG.
> > 
> >     This brings me to the second point, where I agree that we should keep MLS
> >     free from unnecessary complexity that makes it more prone to bugs and you're
> >     right in that the source of randomness is a particularly fundamental and
> >     thus vulnerable part of MLS. However, the design is conceivably simple and
> >     uses the same, tried, tested and provably secure mechanism that is used in
> >     the key schedule of TLS and MLS: a simple extract-expand cycle. See [2] for
> >     a security proof of the MLS key schedule, which can easily be adapted to the
> >     design of the entropy pool.
> > 
> >     To summarize, we're not proposing to replace the OS's RNG, but rather extend
> >     it such that additional sources of entropy can be included. The mechanism is
> >     simple and, in my opinion, reasonably easy to review, implement and prove
> >     secure without additional assumptions.
> > 
> >     Cheers,
> >     Konrad
> > 
> >     [1] https://eprint.iacr.org/2018/1057 <https://eprint.iacr.org/2018/1057>
> >     [2] https://eprint.iacr.org/2021/137 <https://eprint.iacr.org/2021/137>
> > 
> > 
> >     > Brendan McMillion <brendan@cloudflare.com <mailto:brendan@cloudflare.com>>
> >     hat am 30.03.2021 23:01 geschrieben:
> >     >
> >     >
> >     > Hey Konrad
> >     >
> >     > Thanks for writing this up. However, I feel pretty strongly opposed to
> >     this idea, just as a matter of maintaining proper layers of abstraction. MLS
> >     implementations aren't remotely the only things on a user's system that
> >     require strong randomness, which is why every OS has its own CSPRNG. Those
> >     CSPRNGs are going to be much more robust, better designed, and thoroughly
> >     audited than anything we'd implement. Instead, it's more likely we'd have
> >     MLS implementations do this incorrectly and introduce more
> >     randomness-related vulnerabilities than we prevented.
> >     >
> >     >
> >     >
> >     > On Tue, Mar 30, 2021 at 2:25 AM Konrad Kohbrok
> >     <konrad.kohbrok@datashrine.de <mailto:konrad.kohbrok@datashrine.de>> wrote:
> >     > > Great questions. I really should have included our thoughts on the
> >     threat model.
> >     > > 
> >     > >  We don't assume the entropy pool to be more protected than any other
> >     secret in MLS.
> >     > > 
> >     > >  Functionally, we want the pool to allow the following:
> >     > >  - a party should be able to accrue entropy over time using the OS's RNG
> >     > >  - a party should be able to inject entropy from other sources (in
> >     particular secrets derived from the epoch_secret of any of the party's
> >     groups whenever a fresh commit comes in)
> >     > > 
> >     > >  Assuming the OS's RNG is really bad (e.g. it spits out only zeroes),
> >     then a party can still have decent entropy once it receives commits from
> >     groups that it is in. Any party that wants to predict the entropy in the
> >     pool would have to know every secret that is injected into the pool. That
> >     means that an adversary would have to compromise all of the party's groups
> >     to predict secrets extracted from the entropy pool.
> >     > > 
> >     > >  Of course, the pool will take some time to accrue entropy from various
> >     sources, so it doesn't help if the adversary is present from the start, but
> >     it will help if the adversary compromises a party later on. If that happens,
> >     then the compromised party can recover if the adversary misses (i.e. doesn't
> >     intercept and decrypt) any commit from another party after the compromise.
> >     > > 
> >     > >  Now we have to make sure that the design doesn't give us any
> >     disadvantages over just sampling from the OS's RNG every time we need fresh
> >     randomness.
> >     > > 
> >     > >  Let's assume full state compromise. Then
> >     > >  1) the adversary can't obtain entropy extracted from the pool in the
> >     past (because it's essentially a KDF-chain, just as the key schedule)
> >     > >  2) the adversary can't predict what entropy the client will extract in
> >     the future if one of the following is true:
> >     > >  - the OS's RNG is good
> >     > >  - a commit from a group arrives, where the group itself is not compromised
> >     > >  - any other external entropy gets injected that the adversary doesn't
> >     have access to (e.g. a signature from a secret key that is kept secure in an
> >     HSM)
> >     > > 
> >     > >  To summarize: It's no silver bullet against bad entropy in general, but
> >     it's cheap, no worse than just relying on the OS's RNG and provides a
> >     framework to accrue randomness over time from various sources even if the
> >     OS's RNG is bad/compromised and it allows parties to recover from compromise
> >     in that case.
> >     > > 
> >     > >  Hope that shed some light on things.
> >     > > 
> >     > >  Cheers,
> >     > >  Konrad
> >     > > 
> >     > > 
> >     > > 
> >     > >  > Benjamin Beurdouche <benjamin.beurdouche@inria.fr
> >     <mailto:benjamin.beurdouche@inria.fr>> hat am 30.03.2021 09:53 geschrieben:
> >     > >  >
> >     > >  >
> >     > >  > Hi all,
> >     > >  >
> >     > >  > I like this idea. I am not fully clear about the threat model, though.
> >     > >  >
> >     > >  > From reading this, it looks like we assume that the adversary that
> >     compromised a
> >     > >  > principal doesn’t have complete control the entropy pool.
> >     > >  >
> >     > >  > This is in contrast over what we assume of the AEAD keys or even the
> >     KEM keys
> >     > >  > where a full state compromise will gain access to the values.
> >     > >  >
> >     > >  > However it looks very close to the assumptions about additional
> >     protections for signing keys.
> >     > >  > E.g., not directly accessible, and behind an interface that an
> >     adversary can query for certain operations
> >     > >  > through an API (e.g. when is stored in an HSM, co-processor or a
> >     functional secure enclave).
> >     > >  >
> >     > >  > Am I reading this properly?
> >     > >  > If yes, I can understand the value in this threat model, but we
> >     should make it clear that the
> >     > >  > entropy pool has to get these additional protections to provide these
> >     good properties.
> >     > >  >
> >     > >  > Seems to me like a good improvement in all cases anyway, so I think
> >     we should consider it.
> >     > >  >
> >     > >  > Thanks!
> >     > >  > Ben
> >     > >  >
> >     > >  > > On 30 Mar 2021, at 09:27, Konrad Kohbrok
> >     <konrad.kohbrok@datashrine.de <mailto:konrad.kohbrok@datashrine.de>> wrote:
> >     > >  > >
> >     > >  > > Hi everyone,
> >     > >  > >
> >     > >  > > MLS is a protocol that is very vulnerable to individual parties
> >     with bad randomness. For example, when a party joins a group, the secrecy of
> >     the group's key material relies on the quality of that party's key material.
> >     Similarly, when doing an external join, the groups entropy is completely
> >     replaced by that of the joining member.
> >     > >  > >
> >     > >  > > There are multiple ways to mitigate this threat and Joël and Sandro
> >     proposed a few of them in the following mail to the list:
> >     https://mailarchive.ietf.org/arch/msg/mls/ZR84smU5xeLrziNTk5W1P1Z1nQI/
> >     <https://mailarchive.ietf.org/arch/msg/mls/ZR84smU5xeLrziNTk5W1P1Z1nQI/>
> >     > >  > >
> >     > >  > > Concretely, there were two approaches: one that would be baked into
> >     the protocol (essentially using a secret derived from old path secrets to
> >     inject into new ones in addition to the current approach) and one that would
> >     mandate the use of an entropy pool.
> >     > >  > >
> >     > >  > > The ideas were discussed a bit at the time, but nothing has
> >     happened since then. Joël, Sandro and I have just opened a PR with a
> >     concrete design for an entropy pool that is modeled after the key schedule
> >     (https://github.com/mlswg/mls-protocol/pull/467
> >     <https://github.com/mlswg/mls-protocol/pull/467>). Concretely, it allows
> >     gathering entropy over time and for parties with a bad entropy source to
> >     profit from parties with a good one without compromising security.
> >     > >  > >
> >     > >  > > While for future iterations of MLS, we might want to consider a
> >     solution that is more integral to the protocol, we are aware that the
> >     authors want to avoid breaking changes at this point. With the entropy pool,
> >     we thus propose a solution that does not affect the protocol flow, but that
> >     still offers significant advantages over no mitigations at all.
> >     > >  > >
> >     > >  > > Cheers,
> >     > >  > > Konrad
> >     > >  > >
> >     > >  > > _______________________________________________
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> >     > > 
> >     > >  _______________________________________________
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> >     > >
> > 
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