Re: [bitcoin-dev] PoW fraud proofs without a soft fork
On Sun, Sep 08, 2019 at 05:39:28AM +0200, Ruben Somsen via bitcoin-dev wrote: > After looking more deeply into Tadge Dryja’s utreexo work [0], it has > become clear to me that this opens up a way to implement PoW fraud > proofs [1] without a soft fork. This is a nifty idea. > [...] you’d need to download: > > [...] > > 3. the utreexo merkle proofs which prove that all inputs of N+1 are > part of the UTXO set (~1MB) I think "~1 MB" is probably a reasonable estimate for the average case but not for the worst case. To allow verification of the spends in block N+1, each UTXO entry must contain its entire scriptPubKey. I believe the current consensus rules allow scriptPubKeys to be up to 10,000 bytes in size. A specially-constructed block can contain a bit more than 20,000 inputs, making the worst case size of just the UTXO entries that needs to be communicated over 200 MB. > If it turns out that one of your peers disagrees on what the correct > hash is, you find the last utreexo hash where that peer still agreed, > let’s say block M, and you simply execute the same three steps to find > out which peer is wrong I think this also expands to a worst-case of over 200 MB. A lying peer will only be able to get you on one of these checks, so it's 200 MB per lying peer. For an honest peer communicating valid blocks, the worst case is that they'll need to communicate both of these state transactions, so over 400 MB. That could be a bandwidth-wasting DoS attack on honest listening nodes if there were a large number of SPV clients using this type of fraud proofs. Additionally, each node capable of providing fraud proofs will need to persistently store the state transition proof for each new block. I assume this is equal to the block undo data currently stored by archival full nodes plus the utreexo partial merkle branches. This data would probably not be stored by pruned nodes, at least not beyond their prune depth, even for pruned nodes that use utreexo. That would mean this system will only work with archival full nodes with an extra "index" containing the utreexo partial merkle branches, or it will require querying utreexo bridge nodes. Given that both of those would require significant additional system resources beyond the minimum required to operate a full node, such nodes might be rare and so make it relatively easy to eclipse attack an SPV client depending on these proofs. Finally, this system depends on SPV clients implementing all the same consensus checks that full nodes can currently perform. Given that most SPV clients I'm aware of today don't even perform the full range of checks it's possible to run on block headers, I have serious doubts that many (or any) SPV clients will actually implement full verification. On top of that, each client must implement those checks perfectly or they could be tricked into a chainsplit the same as a full node that follows different rules than the economic consensus. > [1] Improving SPV security with PoW fraud proofs: > https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2019-April/016873.html One thing I didn't like in your original proposal---which I appologize for keeping to myself---is that the SPV client will accept confirmations on the bad chain until a fork is produced. Even a miner with a minority of the hash rate will sometimes be able to produce a 6-block chain before the remaining miners produce a single block. In that case, SPV clients with a single dishonest peer in collusion with the miner will accept any transctions in the first block of that chain as having six confirmations. That's the same as it is today, but today SPV users don't think fraud proofs help keep them secure. I think that, if we wanted to widely deploy fraud proofs depending on forks as a signal, we'd have to also retrain SPV users to wait for much higher confirmation counts before accepting transactions as reasonably secure. -Dave ___ bitcoin-dev mailing list bitcoin-dev@lists.linuxfoundation.org https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
Re: [bitcoin-dev] PoW fraud proofs without a soft fork
Hi ZmnSCPxj, >I suppose the critical difference is that invalid inflation can fool the SPV >node, the fullnode will not be so fooled. That is correct. If you sybil the SPV node, you can break any consensus rule you like. I believe this is inherent to fraud proofs in general, because you skip consensus checks unless you're able to receive a fraud proof. But note that my goal in the comparison was to assert that there is no security difference between committing or not committing the utreexo hash into a block. The attack your describe works in either situation, so my conclusion remains that committing the hash adds no security. Other weaknesses compared to full nodes are: - the SPV nodes rely on the existence of a healthy network of utreexo supporting full nodes - at least one honest block needs to be mined - consensus slows down, because you need to allow time for an honest minority to produce a block Cheers, Ruben On Mon, Sep 9, 2019 at 8:58 AM ZmnSCPxj wrote: > > Good morning Ruben, > > Yes, I suppose that is correct. > > I suppose the critical difference is that invalid inflation can fool the SPV > node, the fullnode will not be so fooled. > > A somewhat larger-scale attack is to force a miner-supported > miner-subsidy-increase / blocksize-increase hard fork. > If enough such SPV nodes can be sybilled, they can be forced to use the hard > fork, which might incentivize them to support the hard fork rather than > back-compatible consensus chain. > > Regards, > ZmnSCPxj > > > Hi ZmnSCPxj, > > > > Thank you for your comments. You raise an important point that I should > > clarify. > > > > > 1. In event of a sybil attack, a fullnode will stall and think the > > > blockchain has no more miners. > > > > You can still attack the full node by feeding it a minority PoW chain, > > then it won't stall. > > > > > 2. In event of a sybil attack, an SPV, even using this style, will > > > follow the false blockchain. > > > > Correct, but this false blockchain does need to have valid PoW. > > > > So in both cases valid PoW is required to fool nodes. The one > > difference is that for a full node, the blocks themselves also need to > > be valid (except for the fact that they are in a minority chain), but > > the end result is still that a victim can be successfully double spent > > and lose money. > > > > I hope this clarifies why I consider the security for these two > > situations to be roughly equivalent. In either situation, victims can > > be fooled into accepting invalid payments. > > > > Cheers, > > Ruben > > > > On Mon, Sep 9, 2019 at 6:14 AM ZmnSCPxj zmnsc...@protonmail.com wrote: > > > > > Good morning Ruben, > > > > > > > One might intuitively feel that the lack of a commitment is unsafe, > > > > but there seems to be no impact on security (only bandwidth). The > > > > only > > > > way you can be fooled is if all peers lie to you (Sybil), causing > > > > you > > > > to follow a malicious minority chain. But even full nodes (or the > > > > committed version of PoW fraud proofs) can be fooled in this way if > > > > they are denied access to the valid most PoW chain. If there are > > > > additional security concerns I overlooked, I’d love to hear them. > > > > > > > > > > I think it would be better to more precisely say that: > > > > > > 1. In event of a sybil attack, a fullnode will stall and think the > > > blockchain has no more miners. > > > 2. In event of a sybil attack, an SPV, even using this style, will > > > follow the false blockchain. > > > > > > This has some differences when considering automated systems. > > > Onchain automated payment processing systems, which use a fullnode, will > > > refuse to acknowledge any incoming payments. > > > This will lead to noisy complaints from clients of the automated payment > > > processor, but this is a good thing since it warns the automated payment > > > processor of the possibility of this attack occurring on them. > > > The use of a timeout wherein if the fullnode is unable to see a new block > > > for, say, 6 hours, could be done, to warn higher-layer management systems > > > to pay attention. > > > While it is sometimes the case that the real network will be unable to > > > find a new block for hours at a time, this warning can be used to confirm > > > if such an event is occurring, rather than a sybil attack targeting that > > > fullnode. > > > On the other hand, such a payment processing system, which uses an SPV > > > with PoW fraud proofs, will be able to at least see incoming payments, > > > and continue to release product in exchange for payment. > > > Yet this is precisely a point of attack, where the automated payment > > > processing system is sybilled and then false payments are given to the > > > payment processor on the attack chain, which are double-spent on the > > > global consensus chain. > > > And the automated system may very well not be able to notice this. > > > Regards, > >
Re: [bitcoin-dev] PoW fraud proofs without a soft fork
Good morning Ruben, Yes, I suppose that is correct. I suppose the critical difference is that invalid inflation can fool the SPV node, the fullnode will not be so fooled. A somewhat larger-scale attack is to force a miner-supported miner-subsidy-increase / blocksize-increase hard fork. If enough such SPV nodes can be sybilled, they can be forced to use the hard fork, which might incentivize them to support the hard fork rather than back-compatible consensus chain. Regards, ZmnSCPxj > Hi ZmnSCPxj, > > Thank you for your comments. You raise an important point that I should > clarify. > > > 1. In event of a sybil attack, a fullnode will stall and think the > > blockchain has no more miners. > > You can still attack the full node by feeding it a minority PoW chain, > then it won't stall. > > > 2. In event of a sybil attack, an SPV, even using this style, will follow > > the false blockchain. > > Correct, but this false blockchain does need to have valid PoW. > > So in both cases valid PoW is required to fool nodes. The one > difference is that for a full node, the blocks themselves also need to > be valid (except for the fact that they are in a minority chain), but > the end result is still that a victim can be successfully double spent > and lose money. > > I hope this clarifies why I consider the security for these two > situations to be roughly equivalent. In either situation, victims can > be fooled into accepting invalid payments. > > Cheers, > Ruben > > On Mon, Sep 9, 2019 at 6:14 AM ZmnSCPxj zmnsc...@protonmail.com wrote: > > > Good morning Ruben, > > > > > One might intuitively feel that the lack of a commitment is unsafe, > > > but there seems to be no impact on security (only bandwidth). The only > > > way you can be fooled is if all peers lie to you (Sybil), causing you > > > to follow a malicious minority chain. But even full nodes (or the > > > committed version of PoW fraud proofs) can be fooled in this way if > > > they are denied access to the valid most PoW chain. If there are > > > additional security concerns I overlooked, I’d love to hear them. > > > > > > > I think it would be better to more precisely say that: > > > > 1. In event of a sybil attack, a fullnode will stall and think the > > blockchain has no more miners. > > 2. In event of a sybil attack, an SPV, even using this style, will follow > > the false blockchain. > > > > This has some differences when considering automated systems. > > Onchain automated payment processing systems, which use a fullnode, will > > refuse to acknowledge any incoming payments. > > This will lead to noisy complaints from clients of the automated payment > > processor, but this is a good thing since it warns the automated payment > > processor of the possibility of this attack occurring on them. > > The use of a timeout wherein if the fullnode is unable to see a new block > > for, say, 6 hours, could be done, to warn higher-layer management systems > > to pay attention. > > While it is sometimes the case that the real network will be unable to find > > a new block for hours at a time, this warning can be used to confirm if > > such an event is occurring, rather than a sybil attack targeting that > > fullnode. > > On the other hand, such a payment processing system, which uses an SPV with > > PoW fraud proofs, will be able to at least see incoming payments, and > > continue to release product in exchange for payment. > > Yet this is precisely a point of attack, where the automated payment > > processing system is sybilled and then false payments are given to the > > payment processor on the attack chain, which are double-spent on the global > > consensus chain. > > And the automated system may very well not be able to notice this. > > Regards, > > ZmnSCPxj ___ bitcoin-dev mailing list bitcoin-dev@lists.linuxfoundation.org https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
Re: [bitcoin-dev] PoW fraud proofs without a soft fork
Good morning Ruben, > One might intuitively feel that the lack of a commitment is unsafe, > but there seems to be no impact on security (only bandwidth). The only > way you can be fooled is if all peers lie to you (Sybil), causing you > to follow a malicious minority chain. But even full nodes (or the > committed version of PoW fraud proofs) can be fooled in this way if > they are denied access to the valid most PoW chain. If there are > additional security concerns I overlooked, I’d love to hear them. I think it would be better to more precisely say that: 1. In event of a sybil attack, a fullnode will stall and think the blockchain has no more miners. 2. In event of a sybil attack, an SPV, even using this style, will follow the false blockchain. This has some differences when considering automated systems. Onchain automated payment processing systems, which use a fullnode, will refuse to acknowledge any incoming payments. This will lead to noisy complaints from clients of the automated payment processor, but this is a good thing since it warns the automated payment processor of the possibility of this attack occurring on them. The use of a timeout wherein if the fullnode is unable to see a new block for, say, 6 hours, could be done, to warn higher-layer management systems to pay attention. While it is sometimes the case that the real network will be unable to find a new block for hours at a time, this warning can be used to confirm if such an event is occurring, rather than a sybil attack targeting that fullnode. On the other hand, such a payment processing system, which uses an SPV with PoW fraud proofs, will be able to at least see incoming payments, and continue to release product in exchange for payment. Yet this is precisely a point of attack, where the automated payment processing system is sybilled and then false payments are given to the payment processor on the attack chain, which are double-spent on the global consensus chain. And the automated system may very well not be able to notice this. Regards, ZmnSCPxj ___ bitcoin-dev mailing list bitcoin-dev@lists.linuxfoundation.org https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
[bitcoin-dev] PoW fraud proofs without a soft fork
After looking more deeply into Tadge Dryja’s utreexo work [0], it has become clear to me that this opens up a way to implement PoW fraud proofs [1] without a soft fork. With utreexo, we can efficiently verify state transitions between blocks. Verifying a block from a valid utreexo hash requires only about a megabyte worth of merkle proofs. PoW fraud proofs assume that block N is valid if no miner has tried to fork it (read my original post for details [1]). We can extend that assumption to the utreexo hash of block N, and use that to verify fork block N+1, and reject it if the block is invalid, with just 2-3MB of data. For simplicity, I’ll first start by explaining a version with commitments (which would require a soft fork). When a fork (i.e. a PoW fraud proof) occurs at height N+1, indicating that the block might be invalid, you’d need to download: 1. block N+1 from the most PoW chain (~1-2MB) 2. the utreexo hash commitment inside of block N (e.g. a merkle path to the coinbase) 3. the utreexo merkle proofs which prove that all inputs of N+1 are part of the UTXO set (~1MB) Of course step 2 requires a soft fork, but we can also do a non-committed version by relying on the assumption that at least one of your peers is honest and then evaluate disagreements. We simply replace step 2 above with the following: 2. [Download] the utreexo hash of block N from all your peers If it turns out that one of your peers disagrees on what the correct hash is, you find the last utreexo hash where that peer still agreed, let’s say block M, and you simply execute the same three steps to find out which peer is wrong: download block M+1, then get the merkle proofs to verify whether the peer correctly transitioned their utreexo hash from M to M+1. One might intuitively feel that the lack of a commitment is unsafe, but there seems to be no impact on security (only bandwidth). The only way you can be fooled is if all peers lie to you (Sybil), causing you to follow a malicious minority chain. But even full nodes (or the committed version of PoW fraud proofs) can be fooled in this way if they are denied access to the valid most PoW chain. If there are additional security concerns I overlooked, I’d love to hear them. In short, utreexo can enable PoW fraud proofs without a soft fork. At the cost of downloading a couple of MB per stale block (and per malicious peer), an SPV client gains the ability to (eventually) reject the most PoW chain as long as one honest block gets mined, thereby increasing its security beyond 51% honest miners. Finally, while I think this goes without saying, I’d like to reiterate that this is by no means a replacement for running a full node. You’re depending on other full nodes to do full verification and assuming at least some of the miners are honest. If everyone did this, Bitcoin would not be secure. -- Ruben Somsen [0] Utreexo paper: https://eprint.iacr.org/2019/611.pdf [1] Improving SPV security with PoW fraud proofs: https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2019-April/016873.html ___ bitcoin-dev mailing list bitcoin-dev@lists.linuxfoundation.org https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
