TXO commitments do not need a soft-fork to be useful
6 messages, 23 Feb to 16 May 2017. Started by Peter Todd.
Peter Todd (3), Eric Lombrozo, praxeology_guy, Alex Mizrahi
Something I've recently realised is that TXO commitments do not need to be implemented as a consensus protocol change to be useful. All the benefits they provide to full nodes with regard to allowing for old UTXO data to be pruned - and thus solving the UTXO bloat problem - can be implemented even without having miners commit to the TXO commitment itself. This has a significant deployment advantage too: we can try out multiple TXO commitment schemes, in production, without the need for consensus changes.
# Reasoning
1) Like any other merkelized data structure, a TXO commitment allows a data set - the TXO set - to be securely provided by an untrusted third party, allowing the data itself to be discarded. So if you have a valid TXO commitment, you can discard the TXO data itself, and rely on untrusted entities to provide you that data on demand.
2) The TXO set is a super-set of the UTXO set; all data in the UTXO set is also present in the TXO set. Thus a TXO commitment with spent TXO's pruned is equivalent to a UTXO set, doubly so if inner nodes in the commitment tree commit to the sum-unspent of their children.
3) Where a outpoint-indexed UTXO set has a uniform access pattern, an insertion-ordered TXO set has a delibrately *non-uniform* access pattern: not only are new entries to the TXO set always appended to the end - an operation that requires a known, log2(n), sized set of merkle tips - but due to lost coins alone we can guarantee that older entries in the TXO set will be less frequently updated than newer entries.
4) Thus a full node that doesn't have enough local storage to maintain the full UTXO set can instead keep track of a TXO commitment, and prune older UTXO's from it that are unlikely to be spent. In the event those UTXO's are spent, transactions and blocks spending them can trustlessly provide the necessary data to temporarily fill-in the node's local TXO set database, allowing the next commitment to be calculated.
5) By *not* committing the TXO commitment in the block itself, we obsolete my concept of delayed TXO commitments: you don't need to have calculated the TXO commitment digest to validate a block anyway!
# Deployment Plan
1) Implement a TXO commitment scheme with the ability to efficiently store the last n versions of the commitment state for the purpose of reorgs (a reference-counted scheme naturally does this).
2) Add P2P support for advertising to peers what parts of the TXO set you've pruned.
3) Add P2P support to produce, consume, and update TXO unspentness proofs as part of transaction and block relaying.
4) Profit.
# Bootstrapping New Nodes
With a TXO commitment scheme implemented, it's also possible to produce serialized UTXO snapshots for bootstrapping new nodes. Equally, it's obviously possible to distribute those snapshots, and have people you trust attest to the validity of those snapshots.
I argue that a snapshot with an attestation from known individuals that you trust is a *better* security model than having miners attest to validity: the latter is trusting an unknown set of unaccountable, anonymous, miners.
This security model is not unlike the recently implemented -assumevalid scheme(1), in that auditing the validity of the assumed valid TXO commitments is something anyone can do provided they have a full node. Similarly, we could ship Bitcoin nodes with an assumed-valid TXO commitment, and have those nodes fill in the UTXO data from their peers.
However it is a weaker security model, in that a false TXO commitment can more easily be used to trick a node into accepting invalid transactions/blocks; assumed valid blocks requires proof-of-work to pull off this attack. A compromise may be to use assumed valid TXO commitments, extending my partial UTXO set(2) suggestion of having nodes validate the chain backwards, to eventually validate 100% of the chain.
# References
1) https://github.com/bitcoin/bitcoin/pull/9484 2) [Bitcoin-development] SPV bitcoind? (was: Introducing BitcoinKit.framework), Peter Todd, Jul 17th 2013, Bitcoin development mailing list, https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2013-July/002917.html
This kind of thing is long overdue!
I think it’s a great idea to attempt this without soft forking TXO commitments yet so we can see what works best.
- E
On Wed, Feb 22, 2017 at 5:11 PM, Peter Todd via bitcoin-dev < bitcoin-dev@lists•linuxfoundation•org> wrote:
Quoted text (123 lines)
> Something I've recently realised is that TXO commitments do not need to be > implemented as a consensus protocol change to be useful. All the benefits > they > provide to full nodes with regard to allowing for old UTXO data to be > pruned - > and thus solving the UTXO bloat problem - can be implemented even without > having miners commit to the TXO commitment itself. This has a significant > deployment advantage too: we can try out multiple TXO commitment schemes, > in > production, without the need for consensus changes. > > > # Reasoning > > 1) Like any other merkelized data structure, a TXO commitment allows a > data set > - the TXO set - to be securely provided by an untrusted third party, > allowing > the data itself to be discarded. So if you have a valid TXO commitment, > you can > discard the TXO data itself, and rely on untrusted entities to provide you > that > data on demand. > > 2) The TXO set is a super-set of the UTXO set; all data in the UTXO set is > also > present in the TXO set. Thus a TXO commitment with spent TXO's pruned is > equivalent to a UTXO set, doubly so if inner nodes in the commitment tree > commit to the sum-unspent of their children. > > 3) Where a outpoint-indexed UTXO set has a uniform access pattern, an > insertion-ordered TXO set has a delibrately *non-uniform* access pattern: > not > only are new entries to the TXO set always appended to the end - an > operation > that requires a known, log2(n), sized set of merkle tips - but due to lost > coins alone we can guarantee that older entries in the TXO set will be less > frequently updated than newer entries. > > 4) Thus a full node that doesn't have enough local storage to maintain the > full > UTXO set can instead keep track of a TXO commitment, and prune older UTXO's > from it that are unlikely to be spent. In the event those UTXO's are spent, > transactions and blocks spending them can trustlessly provide the necessary > data to temporarily fill-in the node's local TXO set database, allowing the > next commitment to be calculated. > > 5) By *not* committing the TXO commitment in the block itself, we obsolete > my > concept of delayed TXO commitments: you don't need to have calculated the > TXO > commitment digest to validate a block anyway! > > > # Deployment Plan > > 1) Implement a TXO commitment scheme with the ability to efficiently store > the > last n versions of the commitment state for the purpose of reorgs (a > reference-counted scheme naturally does this). > > 2) Add P2P support for advertising to peers what parts of the TXO set > you've > pruned. > > 3) Add P2P support to produce, consume, and update TXO unspentness proofs > as > part of transaction and block relaying. > > 4) Profit. > > > # Bootstrapping New Nodes > > With a TXO commitment scheme implemented, it's also possible to produce > serialized UTXO snapshots for bootstrapping new nodes. Equally, it's > obviously > possible to distribute those snapshots, and have people you trust attest > to the > validity of those snapshots. > > I argue that a snapshot with an attestation from known individuals that you > trust is a *better* security model than having miners attest to validity: > the > latter is trusting an unknown set of unaccountable, anonymous, miners. > > This security model is not unlike the recently implemented -assumevalid > scheme(1), in that auditing the validity of the assumed valid TXO > commitments > is something anyone can do provided they have a full node. Similarly, we > could > ship Bitcoin nodes with an assumed-valid TXO commitment, and have those > nodes > fill in the UTXO data from their peers. > > However it is a weaker security model, in that a false TXO commitment can > more > easily be used to trick a node into accepting invalid transactions/blocks; > assumed valid blocks requires proof-of-work to pull off this attack. A > compromise may be to use assumed valid TXO commitments, extending my > partial > UTXO set(2) suggestion of having nodes validate the chain backwards, to > eventually validate 100% of the chain. > > > # References > > 1) https://github.com/bitcoin/bitcoin/pull/9484 > 2) [Bitcoin-development] SPV bitcoind? (was: Introducing > BitcoinKit.framework), > Peter Todd, Jul 17th 2013, Bitcoin development mailing list, > https://lists.linuxfoundation.org/pipermail/bitcoin-dev/ > 2013-July/002917.html > > -- > https://petertodd.org 'peter'[:-1]@petertodd.org > > _______________________________________________ > bitcoin-dev mailing list > bitcoin-dev@lists•linuxfoundation•org > https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev > >
On Wed, Feb 22, 2017 at 08:11:47PM -0500, Peter Todd via bitcoin-dev wrote:
Quoted text (3 lines)
> 5) By *not* committing the TXO commitment in the block itself, we obsolete my > concept of delayed TXO commitments: you don't need to have calculated the TXO > commitment digest to validate a block anyway!
Thinking about this a bit more, by not being forced to calculate a TXO commitment for every block, we may be able to do significantly better than delayed TXO commitments by lazily hashing.
Suppose we have the following perfect merkle tree, which we're using as a key-value map. We'll represent inner nodes for which we've calculated digests with "0"'s to represent what version of the tree they correspond too:
0
/ \
/ \
/ \
/ \
/ \
0 0
/ \ / \
/ \ / \
0 0 0 0
/ \ / \ / \ / \
a b c d e f g h
If a value is updated, digests above it become out of date and need to be recalculated:
1
/ \
/ \
/ \
/ \
/ \
0 1
/ \ / \
/ \ / \
0 0 0 1
/ \ / \ / \ / \
a b c d e f g H
2
/ \
/ \
/ \
/ \
/ \
0 2
/ \ / \
/ \ / \
0 0 2 1
/ \ / \ / \ / \
A b c d e F g H
3
/ \
/ \
/ \
/ \
/ \
0 3
/ \ / \
/ \ / \
0 0 2 3
/ \ / \ / \ / \
a b c d e F G H
Suppose however that your implementation does lazy hashing; after the 3rd update your state will be:
.
/ \
/ \
/ \
/ \
/ \
0 .
/ \ / \
/ \ / \
0 0 . .
/ \ / \ / \ / \
a b c d e F G H
Basically all the digests on the right side is out of date and need to be recalculated. Now, first of all it's obviously possible for your implementation to keep updating values in the tree given their keys - you've essentially regressed to a bog standard binary tree.
But what happens if you discard part of your dataset? Let's suppose you've discarded the left half:
.
/ \
/ \
/ \
/ \
/ \
0 .
/ \
/ \
. .
/ \ / \
e F G H
Note how you still have sufficient information to calculate the current merkle tip commitment: the left side hasn't changed yet. But what happens when someone gives you an update proof? Specifically, suppose they want to change b -> B. That requires them to provide you with the part of the merkle tree proving that position #1 is b. Now you might think that's this data:
3
/ \
/ \
/ \
/ \
/ \
0 3
/ \
/ \
0 0
/ \
a b
But the inner node digests marked "3" are useless to you: you haven't calculated those digests yet so you can't compare them to anything. What you can compare is the following:
0
/ \
/ \
0 0
/ \
a b
With that extra data your local knowledge is now:
.
/ \
/ \
/ \
/ \
/ \
0 .
/ \ / \
/ \ / \
0 0 . .
/ \ / \ / \
a b e F G H
Allowing you to apply the update:
.
/ \
/ \
/ \
/ \
/ \
. .
/ \ / \
/ \ / \
. 0 . .
/ \ / \ / \
a B e F G H
If you want to again prune that data, simply recalculate the digests so you can verify a copy given to you by a peer in the future:
.
/ \
/ \
/ \
/ \
/ \
4 .
/ \ / \
/ \ / \
4 0 . .
/ \ / \ / \
a B e F G H
And prune, leaving you with:
.
/ \
/ \
/ \
/ \
/ \
4 .
/ \
/ \
. .
/ \ / \
e F G H
So tl;dr: the reason this works is that we can substitute commitments for pointers: our merkle tree can also be viewed as a binary tree. So a reasonable real-world implementation would be to delay computation of digests for anything we have in RAM, and only compute digests as in-RAM data is flushed to disk. Equally, on disk we can use standard time-space tradeoffs to only store a subset of the digests, recalculating the rest on the fly. Given that'd we could effectively combine both a cryptographic data structure and a standard pointer-based data structure in one, I suspect we can get good performance out of this.
The main subtlety of this approach will be how exactly to handle the proofs: the level of verification possible depends on what digests a given node has calculated, and we want to avoid making network splitting attacks possible by attackers deliberately giving nodes proofs with upper digests that are incorrect, something only some nodes can detect. Not sure yet exactly what's the right approach there.
Finally, notice how this entire approach depends on schemes like MMR's where the overall structure of the tree does not change as nodes are added and updated; it would be much harder to implement this idea for something like a merklized red-black tree where the structure changes as the tree is rebalanced.
Peter Todd & Eric Lombrozo,
I also think communicating the UTXO would be increadibly useful. I just made a writeup called "Synchronization Checkpoints" on github. "https://github.com/bitcoin/bitcoin/issues/9885" This idea also doesn't use commitments.
But... Commitments would be a plus, because then we having all of the miners verifying the UTXO. Below I brainstorm on how to make this happen with my "Synchronization Checkpoints" idea.
I think if there were commitments, such would not be feasible without it being a commitment on the UTXO as it was N blocks in the past rather than the highest block's UTXO set... because just one little fork of height 1 would be a big waste of effort for the miners.
- Miners would put a commitment at the current Checkpoint Block that would be a hash of the full state of the UTXO at the previous Checkpoint Block. - I'll point out that blocks are like "incremental diffs" to the UTXO state.
I was thinking that say if a miner and other nodes are OK with storing multiple copies/backups of the UTXO state then to make this work with high performance: 1. Maintain a DB table who's only purpose is to sort UTXO.txid concat UTXO.vout.index. 2. Some Wait for no Forks blocks after a CheckPoint Block is made, begin populating a new UTXO Checkpoint File that is a serialized sorted UTXO set. 3. Merkle tree or bittorrent style hash the UTXO Checkpoint File 4. Party!
Cheers, Praxeology
Quoted text (2 lines)
> Something I've recently realised is that TXO commitments do not need to be > implemented as a consensus protocol change to be useful.
You're slow, Peter. I figured this out back in 2013:
On Tue, May 16, 2017 at 03:15:17PM +0300, Alex Mizrahi via bitcoin-dev wrote:
Quoted text (7 lines)
> > Something I've recently realised is that TXO commitments do not need to be > > implemented as a consensus protocol change to be useful. > > > You're slow, Peter. I figured this out back in 2013: > > https://bitcointalk.org/index.php?topic=153662.10
Lol, good job! And you even figured out that lovely "distributed file system" explanation first.
Though, it does look like I'm still the person who made it 100% *clear* the first time - you're explanation is easy to read the wrong way, particularly when you say:
"Next time I will teach you how to implement a blockchain-based cryptocurrency in such a way that new miners can start mining right away without downloading whole blockchain, stay tuned..."
After all, at the time UTXO commitments had been already discussed. Also, talking about a DHT in relation to this stuff probably made the explanation get missed by some people.
Unfortunately, I think this is a good example of how important coming up with good explanations and analogies is. :/