[dlc-dev] CET collision attacks

Thibaut Le Guilly thibaut at cryptogarage.co.jp
Tue Mar 30 08:49:03 CEST 2021


Hi Lloyd,

Thanks for this analysis. I'm trying to properly understand it, but I'm a
bit stuck and was hoping you could help me clarify things.

> On average how many nonces and outcomes per nonce do we need before there
is a collision?
> Each term like H(R_0 || X || o_a0) - H(R_0 || X || o_b0) can take on m *
(m - 1) possible values where m is the number of possible outcomes.
> For n nonces we have (m^2 - m) ^ n possible CET encryption keys. Let's
say we have m = 2^10 = 1024 outcomes per nonce this means to get at least
2^256 possible CETs we need roughly n = 13 nonces to be involved on average
to have a collision.

I don’t understand why you relate the number of possible “CET encryption
keys” with the subtraction above. It seems to me that the number of CET
encryption keys would be m^n if you have m outcome per nonces and n nonces.
Also your problem statement says “on average”, but if you reach 2^256 CET
encryption keys, it seems to me that the probability of having a collision
reaches 1 (from the pigeonhole principle). So I’m not sure what you meant
by average.

Cheers,

Thibaut

On Mon, Mar 29, 2021 at 2:04 PM Lloyd Fournier via dlc-dev <
dlc-dev at mailmanlists.org> wrote:

> Hi all,
>
> A few days ago Nadav pointed out that when you add attestation points from
> the same oracle in the "no hash"[2] scheme you will get the same point even
> if the individual points were different.
> This means you can get a CET encryption key that is the same for a
> different set of outcomes. This is bad.
>
> Observe a bet on a number 0-3 decomposed into two binary nonces (R_0,
> R_1). The encryption keys for each outcome would be:
>
> 0: R_0 + 0*X + R_1 + 0*X
> 1: R_0 + 1*X + R_1 + 0*X
> 2: R_0 + 0*X + R_1 + 1*X
> 3: R_0 + 1*X + R_1 + 1*X
>
> Note that CETs 1 and 2 are the equal? I call this a CET collision.
>
> ## How did you miss this Lloyd?
>
> I actually have not studied the case of combining points from the same
> oracle. I assumed it would be simpler than the case of having different
> oracles and so security would follow on from that. It turns out this was a
> very very wrong assumption.
>
> I also never considered an adversary's ability to "collide" CET encryption
> keys so that they are the same. I only found that if a CET is used then all
> the oracle's whose attestation points were combined to create it did indeed
> participate in the attestation (their contribution cannot be canceled out
> as long as you have these proofs of knowledge on each R).
> But if the decryption key for a CET is the same as another one then (from
> the point of view of the protocol) they have actually attested to two
> contradictory things.
>
> In this case the oracles are not to blame for the equivocation so this
> breaks fraud proofs in the case of multiple nonces from the same oracle.
> Clearly in the no hash scheme these collisions happen without any
> malicious behavior at all but that doesn't mean they can't happen in the
> hash scheme. This is what this post is about.
>
> ## Collision attacks without oracle participation fundamentals
>
> Consider an adversary who is going to find a magic combination of
> events/nonces to combine to create a CETs such that if the oracle(s) attest
> to one thing the adversary is going to be able to decrypt more than one CET
> from the information available to it. The oracles are behaving completely
> honestly in this scenario. What are the constraints on this attack?
>
> Conjecture   #1 The two CETs will have exactly the same encryption key.
>
> If they didn't then the adversary would have created a combination of
> attention points where they know the scalar offset between the two groups
> that caused the collision. They don't know the discrete logarithms of the
> component points so this should be impossible..
>
> Conjecture #2 The points that make up the two CETs will be derived from
> the same nonces.
>
> First let's denote sum(R + c * X) = R_0 + c_0 * X + R_1 + c_1 * X ... R_n
> + c_n * X for n possible nonces.
>
> If you know sum(R_a + c_b*X) = sum(R_b + c_b*X) where not every R_a on the
> left is the same as every R_b on the right then you have figured out an
> equality between two groups of independent points which means you can find
> discrete logarithms (same reason as Conjecture #1).
>
> The two conjectures above allow us to hone in on what the adversary must
> be doing. Since they are using the same nonces then we can remove those
> from the equation:
>
> 1. sum(R_a + c_a*X) = sum(R_b + c_b*X)
> 2. sum(c_a*X) = sum(c_b*X)                      # Every R_a = R_b so
> subtract from both sides
> 3. sum(c_a) = sum(c_b)                             # Divide by X
>
> Ok so the adversary MUST be creating the collision by having two sets of
> c's (of equal size) that are different but sum to the same value. Clearly
> in the no hash scheme this is very very easy to create such sums but what
> about the hash scheme?
>
> ## Collision attacks on original DLC scheme
>
> In the original proposal "c"s were computed as H(R || X || o) where o is
> the outcome (the no-hash hash scheme can be thought of as just c = o).
> So to cause a collision the attacker has to find two sets of o's (items in
> the set denoted as o_a and o_b) against a fixed set of Rs  such that
> sum(H(R || X || o_a)) = sum(H(R || X || o_b)) which can be simplified to:
>
> sum(H(R || X || o_a) - H(R || X || o_b)) =  0
>
> Just to be clear this means finding o_a0, ..., o_an and o_b0, ..., o_bn
> such that:
>
> H(R_0 || X || o_a0) - H(R_0 || X || o_b0) + H(R_1 || X || o_a1) - H(R_1 ||
> X || o_b1) + ... + H(R_0 || X || o_an) - H(R_0 || X || o_bn) = 0
>
> On average how many nonces and outcomes per nonce do we need before there
> is a collision?
>
> Each term like H(R_0 || X || o_a0) - H(R_0 || X || o_b0) can take on m *
> (m - 1) possible values where m is the number of possible outcomes.
> For n nonces we have (m^2 - m) ^ n possible CET encryption keys. Let's say
> we have m = 2^10 = 1024 outcomes per nonce this means to get at least 2^256
> possible CETs we need roughly n = 13 nonces to be involved on average to
> have a collision.
>
> But how hard would it be to find a collision if one were to exist? Well it
> looks a bit like Wagner's k-sum problem [1] where k is the number of nonces
> but I think it's best to leave out estimating this for now since it
> requires a more in-depth study.
>
> ## Don't hash the outcomes
>
> What if instead of H(R || X || o) we computed c as o * H(R || X). A bit of
> a mid-way between the hash and no-hash schemes. On the surface this would
> seem to fix the original problem with c = o because the H(R || X) term
> delinearizes it so collisions don't happen naturally. It also preserves
> most of the speedup that the no hash scheme offers since each CET
> encryption key can be produced by a single addition from the previous one.
> The collision problem for this scheme becomes to find:
>
> 1. sum(H(R || X) *o_a) = sum(H(R|| X) * o_b)
> 2. sum(H(R || X)* o_a - H(R || X) * o_b) = 0
> 3. sum(H(R || X) * (o_a - o_b) ) = 0
>
> So what are the possible values of (o_a - o_b)? If there are m outcomes
> then it should be 2 * (m-1). e.g. if we have 1024 outcomes the values could
> be -1023..1023 (not including 0). This is far fewer than the m^2 - m we
> have in the original.
> For m = 2^10 outcomes per nonce we would need around n = 24 nonces on
> average to have a collision or roughly double what the original needs for a
> collision to exist.
>
> To actually find a collision it looks like a subset sum kind of problem
> except with being able to choose coefficients from -m..m. I am not going to
> guess how difficult this is yet.
>
> ## What about malicious oracles
>
> A malicious oracle may choose their R values such that there is a
> collision. I *think* this would be much more efficient than a malicious
> user just finding a collision on an existing set of nonces. In order for
> fraud proofs to work we would have to identify what threshold of nonces is
> it likely to be a malicious user who created the collision by combining a
> bunch of nonces together or whether they had help from a malicious oracle.
>
> Of course we really want to avoid this whole game altogether. I am hopeful
> that with the "nonce-only-hash" H(R ||X) * o scheme it might be hard for
> the oracle to find collisions no matter how many Rs they try. But showing
> this to be the case will take more work. It feels like the full-hash scheme
> will struggle to do this.
>
> --
>
> Please review my claims here and happy to discuss at the next meeting.
> Sorry for the awful notation.
>
> Cheers,
>
> LL
>
> [1] https://joinmarket.me/blog/blog/avoiding-wagnerian-tragedies/
> [2] https://github.com/LLFourn/dlc-sec/blob/master/main.pdf (section 4)
>
> dlc-dev mailing list
> dlc-dev at mailmanlists.org
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>
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