Hi, An quick update on progress with our statechain implementation which we are pressing ahead with - we have started work on a version in Rust ( https://github.com/commerceblock/mercury) that is based on the 2P ECDSA gotham-city wallet from KZen (https://github.com/KZen-networks/gotham-city), and using their implementation of Lindel's 2P ECDSA protocol, which is very fast (we can always swap to a different protocol later). Also, we are planning on using a sparse Merkle tree attested to a Mainstay slot ( mainstay.xyz) for the proof-of-publication/proof-of-ownership - using the protocol described here: https://github.com/commerceblock/mercury/blob/master/doc/statechains.md and https://github.com/thyeem/monotree. Any comments on these choices or on anything else are highly appreciated.
Tom On Sun, Apr 5, 2020 at 10:25 PM Tom Trevethan <t...@commerceblock.com> wrote: > Hi Bob and Nadav, > > There seems to be no way to prevent a malicious SE from stealing an > output from the current owner by either colluding with (or being) a > previous owner. But with a proof-of-publication (i.e. the statechain) it is > possible for the current owner to have a proof that the SE has stolen from > them. It seems to me that the statechain itself provides two functions: 1. > Proof that an output has only a single owner at any time (preventing the SE > from double-spending) and 2. a way for the current owner to prove their > ownership, and require their permission to change ownership. 1. can just be > a publication by the SE, but 2. requires that the output is transferred to > a public key of the owner, and only via a signature of the previous owner > (in this way the SE cannot re-assign ownership unilaterally). Therefore I > think Nadav is right, and this needs to be a key that the SE can never know > (even if they are malicious), but which can be used to prove ownership, and > in turn prove fraud on the part of the SE. > > I don't think that this should be too much of an issue: any wallet will > have to use new keys for each output and transfer anyway. The statechain > key (used for the ownership proof) and the output key share can be on > different hardened HD paths (following on from a path derived from the > outpoint of the UTXO, similar to the method in BIP175). > > Tom > > > > On Sun, Apr 5, 2020 at 3:17 PM Bob McElrath <b...@mcelrath.org> wrote: > >> Note that this attack requires collaboration with the current UTXO owner. >> Generally if there's some form of address/payment request, the current >> holder is >> trying to transfer the UXTO to some other (non-statechain) entity, and he >> knows >> the target of the transfer, and participates in the protocol to authorize >> it. >> The current holder must obtain the target pubkey for the transfer >> out-of-band >> with respect to the SE, or the SE can MITM that. >> >> It's a stated security assumption that the sender or receiver do not >> collude >> with the SE. If either do, then your attack is generally possible and all >> bets >> are off. So what you've described is simply the SE colluding with the >> receiver. >> The receiver will *already* receive the UTXO, so the receiver here is >> assisting >> the SE in stealing his (the receiver's) funds, or the SE has done a MITM >> on the >> transfer. Various improvements including blind signing, a SE-federation, >> etc >> are valuable to consider to mitigate this. But the SE must be prevented, >> one way >> or another, from "buying the UTXO". The SE cannot be allowed to be both >> operator >> of the SE and a customer of it, as this clearly violates the no-receiver >> collusion principle. >> >> "Adding a new user key" doesn't change the situation. There's already a >> user key >> involved, and the user has already acquiesced to the transfer. >> Acquiescing with >> two keys doesn't change anything. >> >> As far as proving and tracing the fraud, this is where "single use seals" >> come >> in. Each SE transfer can involve an "opening" of a seal, followed by a >> "close" >> when it is transferred, creating a linear history of ownership. If the SE >> obtains the full private key x, one way or another, the spend of that >> UTXO will >> fall outside this seal-based history, and proof of fraud will be evident. >> It >> won't be possible to determine *which* of the old owners collaborated >> with the >> SE, but it gives clear proof that the SE is not to be trusted. A customer >> might >> demand that a seal-based system be in use as an independent entity from >> the SE, >> to audit the honesty of the SE. The seal system does not require any of >> the keys >> required for transfer. See https://mainstay.xyz as a potential >> implementation. >> There are lots of reasons this might required as an AML solution for some >> businesses anyway. >> >> Nadav Kohen via bitcoin-dev [email@example.com] >> wrote: >> > Hey all, >> > >> > So my main concern with the proposal as written is that the Statechain >> Entity >> > (SE) can untraceably scam its users with the following attack: >> > >> > 1) Buy the utxo (have it transferred to a key it knows), this first >> step can be >> > skipped if the utxo was created by the SE. >> > 2) Transfer the UTXO to someone else, let it be for however long >> > 3) When it wishes to steal the UTXO, the SE now knows its own shard s_n >> and it >> > knows the full private key, x, from when it owned the UTXO (and had both >> > shards), and so it can compute x/s_n = the current users shard. It can >> then >> > sign for the current user, and forge a state transition to a key it >> owns before >> > spending the UTXO on chain. >> > >> > The main problem here is that the user who had their funds stolen >> cannot prove >> > to anyone that this has happened since the attack compromises their key. >> > That said, I think this problem is easily fixed by adding a new user >> key to the >> > protocol with which they must sign in order for the transfer to be >> considered >> > valid on the state chain. This way, if the SE wishes to steal the funds >> (which >> > they still can), at least it is traceable/provable that this SE is not >> > trustworthy as there is no evidence of a valid transfer for the funds >> that have >> > been stolen. >> > >> > Best, >> > Nadav >> > >> > On Thu, Apr 2, 2020 at 7:22 PM Tom Trevethan via bitcoin-dev < >> > firstname.lastname@example.org> wrote: >> > >> > Thanks for all of the input and comments - I do now think that the >> > decrementing nSequence relative locktime backup system with kick-off >> > transaction is the way to go, including a fee penalty via CPFP to >> > disincentivise DoS, as suggested. >> > I have started a more detailed document specifying the proposed >> protocol in >> > more detail: https://github.com/commerceblock/mercury/blob/master/ >> > statechains.md which includes improvements to the >> transfer mechanism (and >> > an explanation of how this can be used to transfer/novate positions >> in >> > DLCs). Always happy to get more feedback or PRs. >> > >> > Tom >> > >> > On Tue, Mar 31, 2020 at 12:41 PM Tom Trevethan < >> t...@commerceblock.com> >> > wrote: >> > >> > Hi David, >> > >> > Just for clarity, I left nChain over 2 years ago (having worked >> there >> > since 2016). While there, I (along with other researchers) were >> given >> > free rein to work on any ideas we wanted to. I had been >> interested in >> > the scaling of Bitcoin off-chain, and this was one of several >> things I >> > spent time on (including things like sidechains, pegs and >> threshold >> > signatures). This patent application came out of an idea I had >> to >> > transfer ownership of UTXOs off-chain that has some >> similarities to the >> > statechains proposal, which has shown there is interest and >> demand for >> > this type of system. >> > >> > Although I think the existence of this application is something >> to be >> > mindful of, there are several important things to note: >> > >> > 1. Although there are similarities, the current ideas are >> significantly >> > different to those in the application. >> > 2. The key transfer protocol as described in the application is >> not >> > secure (for several reasons, including as discussed above, by >> Albert >> > and Bob etc.) - and a different mechanism is required. >> > 3. Decrementing timelocks (as suggested in the application) are >> prior >> > art (Decker-Wattenhofer 2015), and in any case any >> implementation will >> > most likely use an 'invalidation tree' relative locktime backup >> > mechanism for open-ended UTXOs. >> > 4. The patent application has not been granted (it was made in >> May >> > 2017) and the international search report rejected it on the >> grounds of >> > prior art. >> > >> > Tom >> > >> > On Tue, Mar 31, 2020 at 11:36 AM David A. Harding < >> d...@dtrt.org> >> > wrote: >> > >> > On Wed, Mar 25, 2020 at 01:52:10PM +0000, Tom Trevethan via >> > bitcoin-dev wrote: >> > > Hi all, >> > > >> > > We are starting to work on an implementation of the >> statechains >> > concept ( >> > > https://medium.com/@RubenSomsen/ >> > >> statechains-non-custodial-off-chain-bitcoin-transfer-1ae4845a4a39), >> > > >> > > [...] >> > > There are two main modifications we are looking at: >> > > [...] >> > > >> > > 2. Replacing the 2-of-2 multisig output (paying to >> statechain >> > entity SE key >> > > and transitory key) with a single P2(W)PKH output where >> the >> > public key >> > > shared between the SE and the current owner. The SE and >> the >> > current owner >> > > can then sign with a 2-of-2 ECDSA MPC. >> > >> > Dr. Trevethan, >> > >> > Would you be able to explain how your proposal to use >> statechains >> > with >> > 2P-ECDSA relates to your patent assigned to nChain Holdings >> for >> > "Secure >> > off-chain blockchain transactions"? >> > >> >  https://patents.google.com/patent/US20200074464A1 >> > >> > Here are some excerpts from the application that caught my >> > attention in >> > the context of statechains in general and your proposal to >> this >> > list in >> > particular: >> > >> > > an exchange platform that is trusted to implement and >> operate the >> > > transaction protocol, without requiring an on-chain >> transaction. >> > The >> > > off-chain transactions enable one computer system to >> generate >> > multiple >> > > transactions that are recordable to a blockchain in >> different >> > > circumstances >> > > >> > > [...] >> > > >> > > at least some of the off-chain transactions are valid for >> > recording on >> > > the blockchain even in the event of a catastrophic >> failure of the >> > > exchange (e.g., exchange going permanently off-line or >> loosing >> > key >> > > shares). >> > > >> > > [...] >> > > >> > > there may be provided a computer readable storage medium >> > including a >> > > two-party elliptic curve digital signature algorithm >> (two-party >> > ECDSA) >> > > script comprising computer executable instructions which, >> when >> > > executed, configure a processor to perform functions of a >> > two-party >> > > elliptic curve digital signature algorithm described >> herein. >> > > >> > > [...] >> > > >> > > In this instance the malicious actor would then also have >> to >> > collude >> > > with a previous owner of the funds to recreate the full >> key. >> > Because >> > > an attack requires either the simultaneous theft of both >> exchange >> > and >> > > depositor keys or collusion with previous legitimate >> owners of >> > funds, >> > > the opportunities for a malicious attacker to compromise >> the >> > exchange >> > > platform are limited. >> > >> > Thank you, >> > >> > -Dave >> > >> > _______________________________________________ >> > bitcoin-dev mailing list >> > email@example.com >> > https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev >> > >> > !DSPAM:5e87670a231323960034969! >> >> > _______________________________________________ >> > bitcoin-dev mailing list >> > firstname.lastname@example.org >> > https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev >> > >> > >> > !DSPAM:5e87670a231323960034969! >> >> -- >> Cheers, Bob McElrath >> >> "For every complex problem, there is a solution that is simple, neat, and >> wrong." >> -- H. L. Mencken >> >>
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