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Last Notes

2023-06-07 15:39:01 UTC

- reply

📅 Original date posted:2015-06-19
📝 Original message:It is disappointing that F2Pool would enable full RBF when the safe
alternative, first-seen-safe RBF, is also available, especially since the
fees they would gain by supporting full RBF over FSS RBF would likely be
negligible. Did they consider using FSS RBF instead?
Best,
Stephen
On Fri, Jun 19, 2015 at 6:39 AM, Peter Todd <pete at petertodd.org> wrote:
> Yesterday F2Pool, currently the largest pool with 21% of the hashing
> power, enabled full replace-by-fee (RBF) support after discussions with
> me. This means that transactions that F2Pool has will be replaced if a
> conflicting transaction pays a higher fee. There are no requirements for
> the replacement transaction to pay addresses that were paid by the
> previous transaction.
>
>
> I'm a user. What does this mean for me?
> ---------------------------------------
>
> In the short term, very little. Wallet software aimed at average users
> has no ability to reliably detect conditions where an unconfirmed
> transaction may be double-spent by the sender. For example, Schildbach's
> Bitcoin Wallet for Android doesn't even detect double-spends of
> unconfirmed transactions when connected to a RBF or Bitcoin XT nodes
> that propagate them. The least sophisticated double-spend attack
> possibly - simply broadcasting two conflicting transactions at the same
> time - has about 50% probability of success against these wallets.
>
> Additionally, SPV wallets based on bitcoinj can't even detect invalid
> transactions reliably, instead trusting the full node(s) it is connected
> too over the unauthenticated, unencrypted, P2P protocol to do validation
> for them. For instance due to a unfixed bug¹ Bitcoin XT nodes will relay
> double-spends that spend the output of the conflicting transaction. I've
> personally tested this with Schildbach's Bitcoin Wallet for Android,
> which shows such invalid transactions as standard, unconfirmed,
> transactions.
>
> Users should continue to assume that unconfirmed transactions could be
> trivially reversed by the sender until the first confirmation. In
> general, only the sender can reverse a transaction, so if you do trust
> the sender feel free to assume an unconfirmed transaction will
> eventually confirm. However, if you do not trust the sender and/or have
> no other recourse if they double-spend you, wait until at least the
> first confirmation before assuming the transaction will go through.
>
> In the long term, miner support of full RBF has a number of advantages
> to users, allowing you to more efficiently make transactions, paying
> lower fees. However you'll need a wallet supporting these features; none
> exist yet.
>
>
> I'm a business. What does this mean for me?
> -------------------------------------------
>
> If you use your own node to verify transactions, you probably are in a
> similar situation as average users, so again, this means very little to
> you.
>
> If you use a payment processor/transaction API such as BitPay, Coinbase,
> BlockCypher, etc. you may or may not be accepting unconfirmed
> transactions, and they may or may not be "guaranteed" by your payment
> processor even if double-spent. If like most merchants you're using the
> API such that confirmations are required prior to accepting orders (e.g.
> taking a meaningful loss such as shipping a product if the tx is
> reversed) nothing changes for you. If not I recommend you contact your
> payment processor.
>
>
> I'm a miner. Why should I support replace-by-fee?
> -------------------------------------------------
>
> Whether full or first-seen-safe⁵ RBF support (along with
> child-pays-for-parent) is an important step towards a fully functioning
> transaction fee market that doesn't lead to users' transactions getting
> mysteriously "stuck", particularly during network flooding
> events/attacks. A better functioning fee market will help reduce
> pressure to increase the blocksize, particularly from the users creating
> the most valuable transactions.
>
> Full RBF also helps make use of the limited blockchain space more
> efficiently, with up to 90%+ transaction size savings possible in some
> transaction patterns. (e.g. long payment chains⁶) More users in less
> blockchain space will lead to higher overall fees per block.
>
> Finally as we'll discuss below full RBF prevents a number of serious
> threats to the existing level playing field that miners operate in.
>
>
> Why can't we make accepting unconfirmed txs from untrusted people safe?
> -----------------------------------------------------------------------
>
> For a decentralized wallet, the situation is pretty bleak. These wallets
> only have a handful of connections to the network, with no way of
> knowing if those connections give an accurate view of what transactions
> miners actually know about.
>
> The only serious attempt to fix this problem for decentralized wallets
> that has been actually deployed is Andresen/Harding's double-spend
> relaying, implemented in Bitcoin XT. It relays up to one double-spend
> transaction per double-spent txout, with the intended effect to warn
> recipients. In practice however this functionality makes it easier to
> double-spend rather than harder, by giving an efficient and easy way to
> get double-spends to miners after the fact. Notably my RBF
> implementation even connects to Bitcoin XT nodes, reserving a % of all
> incoming and outgoing connection slots for them.
>
> Additionally Bitcoin XT's double-spend relaying is subject to attacks
> include bandwidth exhaustion, sybil attacks, and Gervais's non-sybil
> interactive attacks⁷ among many others.
>
>
> What about centralised wallets?
> -------------------------------
>
> Here the solutions being deployed, planned, and proposed are harmful,
> and even represent serious threats to Bitcoin's decentralization.
>
>
> Confidence factors
> ------------------
>
> Many services such as BlockCypher² have attempted to predict the
> probability that unconfirmed transactions will be mined, often
> guaranteeing merchants payment³ even in the event of a double-spend. The
> key component of these predictions is to sybil attack the P2P network as
> a whole, connecting to as many nodes as possible to measure transaction
> propagation. Additionally these services connect to pools directly via
> the getblocktemplate protocol, repeatedly downloading via GBT the lists
> of transactions in the to-be-mined blocks to determine what transactions
> miners are attempting to mine.
>
> None of these measures scale, wasting significant network and miner
> resources; in one instance a sybil attack by Chainalysis even completely
> blocked the users of the SPV wallet Breadwallet⁴ from accessing the
> network. These measures also don't work very well, giving double-spend
> attackers incentives to sybil attack miners themselves.
>
>
> Transaction processing contracts with miners
> --------------------------------------------
>
> The next step after measuring propagation fails is to contract with
> miners directly, signing contracts with as much of the hashing power as
> possible to get the transactions they want mined and double-spends
> rejected. The miners/pools would then provide an authenticated API
> endpoint for exclusive use of this service that would allow the service
> to add and remove specific transactions to the mempool on demand.
>
> There's a number of serious problems with this:
>
> 1) Mining contracts can be used to double-spend
>
> ...even when they're being used "honestly".
>
> Suppose Alice is a merchant using CoinPayCypher, who has contracts with
> 75% of the hashing power. Bob, another merchant, meanwhile uses a
> decentralized Bitcoin Core backend for payments to his website.
>
> Mallory wants to double-spend Bob's to buy his expensive products. He
> can do this by creating a transaction, tx1, that pays Alice, followed by
> a second transaction, tx2, that pays Bob. In any circumstance when
> Mallory can convince Bob to accept tx2, but prevent Bob from seeing tx1,
> the chance of Malory's double-spend succeeding becomes ~75% because
> CoinPayCypher's contracts with mining ensure the transaction paying
> Alice will get mined.
>
> Of course, dishonest use and/or compromise makes double-spending
> trivial: Malory can use the API credentials to ask miners to reject
> Bob's payment at any time.
>
>
> 2) They still don't work, without 51% attacking other miners
>
> Even if CoinPayCypher has 75% of the hashing power on contract, that's
> still a potentially 75% chance of being double-spent. The 25% of miners
> who haven't signed contracts have no _decentralized_ way of ensuring
> they don't create blocks with double-spends, let alone at low cost. If
> those miners won't or can't sign contracts with CoinPayCypher the only
> next step available is to reject their blocks entirely.
>
>
> 3) Legal contracts give the advantage to non-anonymous miners in
> Western jurisdictions
>
> Suppose CoinPayCypher is a US company, and you're a miner with 1%
> hashing power located in northern China. The barriers to you succesfully
> negotiating a contract with CoinPayCypher are significant. You don't
> speak the same langauge, you're in a completely different jurisdiction
> so enforcing the legal contract is difficult, and being just 1%,
> CoinPayCypher sees you as insignificant.
>
> Who's going to get the profitable hashing power contracts first, if at
> all? Your English speaking competitors in the west. This is inherently a
> pressure towards centralization of mining.
>
>
> Why isn't this being announced on the bitcoin-security list first?
> ------------------------------------------------------------------
>
> I've had repeated discussions with services vulnerable to double-spends;
> they have been made well aware of the risk they're taking. If they've
> followed my own and others' advice they'll at minimum have constant
> monitoring of the rate of double-spends both on their own services and
> on the P2P network in general.
>
> If you choose to take a risk you should accept the consequences.
>
>
> How do I actually use full RBF?
> -------------------------------
>
> First get the full-RBF patch to v0.10.2:
>
> https://github.com/petertodd/bitcoin/tree/replace-by-fee-v0.10.2
>
> The above implementation of RBF includes additional code to find and
> preferentially connect to other RBF nodes, as well as Bitcoin XT nodes.
> Secondly, try out my replace-by-fee-tools at:
>
> https://github.com/petertodd/replace-by-fee-tools
>
> You can watch double-spends on the network here:
>
> http://respends.thinlink.com/
>
>
> References
> ----------
>
> 1) "Replace-by-fee v0.10.2 - Serious DoS attack fixed! - Also novel
> variants of existing attacks w/ Bitcoin XT and Android Bitcoin Wallet",
> Peter Todd, May 23rd 2015, Bitcoin-development mailing list,
>
> http://www.mail-archive.com/[email protected]/msg07795.html
>
> 2) "From Zero to Hero: Bitcoin Transactions in 8 Seconds",
> June 2nd, 2014, Erik Voorhees,
>
> https://medium.com/blockcypher-blog/from-zero-to-hero-bitcoin-transactions-in-8-seconds-7c9edcb3b734
>
> 3) Coinbase Merchant API, Accessed Jun 19th 2015,
> https://developers.coinbase.com/docs/merchants/callbacks#confirmations
>
> 4) "Chainalysis CEO Denies 'Sybil Attack' on Bitcoin's Network",
> March 14th 2015, Grace Caffyn, Coindesk,
>
> http://www.coindesk.com/chainalysis-ceo-denies-launching-sybil-attack-on-bitcoin-network/
>
> 5) "First-Seen-Safe Replace-by-Fee",
> May 25th 2015, Peter Todd, Bitcoin-development mailing list,
>
> http://www.mail-archive.com/bitcoin-development%40lists.sourceforge.net/msg07829.html
>
> 6) "Cost savings by using replace-by-fee, 30-90%",
> May 25th 2015, Peter Todd, Bitcoin-development mailing list,
>
> http://www.mail-archive.com/[email protected]/msg07813.html
>
> 7) "Tampering with the Delivery of Blocks and Transactions in Bitcoin",
> Arthur Gervais and Hubert Ritzdorf and Ghassan O. Karame and Srdjan
> Capkun,
> Cryptology ePrint Archive: Report 2015/578, Jun 10th 2015,
> http://eprint.iacr.org/2015/578
>
> --
> 'peter'[:-1]@petertodd.org
> 0000000000000000070a2bb3b92c20d5c2c971e6e1a7abe55cdbbe6a2dd9a5ad
>
>
> ------------------------------------------------------------------------------
>
> _______________________________________________
> Bitcoin-development mailing list
> Bitcoin-development at lists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/bitcoin-development
>
>
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2023-06-07 15:36:26 UTC

- reply

📅 Original date posted:2015-06-02
📝 Original message:Hi Mark,
Overall, I like this idea in every way except for one: unless I am missing
something, we may still need an OP_RCLTV even with this being implemented.
In use cases such as micropayment channels where the funds are locked up by
multiple parties, the enforcement of the relative locktime can be done by
the first-signing party. So, while your solution would probably work in
cases like this, where multiple signing parties are involved, there may be
other, seen or unforeseen, use cases that require putting the relative
locktime right into the spending contract (the scriptPubKey itself). When
there is only one signer, there's nothing that enforces using an nSequence
and nVersion=2 that would prevent spending the output until a certain time.
I hope this is received as constructive criticism, I do think this is an
innovative idea. In my view, though, it seems to be less fully-featured
than just repurposing an OP_NOP to create OP_RCLTV. The benefits are
obviously that it saves transaction space by repurposing unused space, and
would likely work for most cases where an OP_RCLTV would be needed.
Best,
Stephen
On Mon, Jun 1, 2015 at 9:49 PM, Mark Friedenbach <mark at friedenbach.org>
wrote:
> I have written a reference implementation and BIP draft for a soft-fork
> change to the consensus-enforced behaviour of sequence numbers for the
> purpose of supporting transaction replacement via per-input relative
> lock-times. This proposal was previously discussed on the mailing list in
> the following thread:
>
> http://sourceforge.net/p/bitcoin/mailman/message/34146752/
>
> In short summary, this proposal seeks to enable safe transaction
> replacement by re-purposing the nSequence field of a transaction input to
> be a consensus-enforced relative lock-time.
>
> The advantages of this approach is that it makes use of the full range of
> the 32-bit sequence number which until now has rarely been used for
> anything other than a boolean control over absolute nLockTime, and it does
> so in a way that is semantically compatible with the originally envisioned
> use of sequence numbers for fast mempool transaction replacement.
>
> The disadvantages are that external constraints often prevent the full
> range of sequence numbers from being used when interpreted as a relative
> lock-time, and re-purposing nSequence as a relative lock-time precludes its
> use in other contexts. The latter point has been partially addressed by
> having the relative lock-time semantics be enforced only if the
> most-significant bit of nSequence is set. This preserves 31 bits for
> alternative use when relative lock-times are not required.
>
> The BIP draft can be found at the following gist:
>
> https://gist.github.com/maaku/be15629fe64618b14f5a
>
> The reference implementation is available at the following git repository:
>
> https://github.com/maaku/bitcoin/tree/sequencenumbers
>
> I request that the BIP editor please assign a BIP number for this work.
>
> Sincerely,
> Mark Friedenbach
>
>
> ------------------------------------------------------------------------------
>
> _______________________________________________
> Bitcoin-development mailing list
> Bitcoin-development at lists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/bitcoin-development
>
>
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2023-06-07 15:36:19 UTC

- reply

📅 Original date posted:2015-06-01
📝 Original message:This exact question came up on the Bitcoin Stack Exchange once. I gave an
answer here:
http://bitcoin.stackexchange.com/questions/37292/whats-the-purpose-of-a-maximum-block-size/37303#37303
On Mon, Jun 1, 2015 at 2:32 PM, Jim Phillips <jim at ergophobia.org> wrote:
> Ok, I understand at least some of the reason that blocks have to be kept
> to a certain size. I get that blocks which are too big will be hard to
> propagate by relays. Miners will have more trouble uploading the large
> blocks to the network once they've found a hash. We need block size
> constraints to create a fee economy for the miners.
>
> But these all sound to me like issues that affect some, but not others. So
> it seems to me like it ought to be a configurable setting. We've already
> witnessed with last week's stress test that most miners aren't even
> creating 1MB blocks but are still using the software defaults of 730k. If
> there are configurable limits, why does there have to be a hard limit?
> Can't miners just use the configurable limit to decide what size blocks
> they can afford to and are thus willing to create? They could just as
> easily use that to create a fee economy. If the miners with the most
> hashpower are not willing to mine blocks larger than 1 or 2 megs, then they
> are able to slow down confirmations of transactions. It may take several
> blocks before a miner willing to include a particular transaction finds a
> block. This would actually force miners to compete with each other and find
> a block size naturally instead of having it forced on them by the protocol.
> Relays would be able to participate in that process by restricting the
> miners ability to propagate large blocks. You know, like what happens in a
> FREE MARKET economy, without burdensome regulation which can be manipulated
> through politics? Isn't that what's really happening right now? Different
> political factions with different agendas are fighting over how best to
> regulate the Bitcoin protocol.
>
> I know the limit was originally put in place to prevent spamming. But that
> was when we were mining with CPUs and just beginning to see the occasional
> GPU which could take control over the network and maliciously spam large
> blocks. But with ASIC mining now catching up to Moore's Law, that's not
> really an issue anymore. No one malicious entity can really just take over
> the network now without spending more money than it's worth -- and that's
> just going to get truer with time as hashpower continues to grow. And it's
> not like the hard limit really does anything anymore to prevent spamming.
> If a spammer wants to create thousands or millions of transactions, a hard
> limit on the block size isn't going to stop him.. He'll just fill up the
> mempool or UTXO database instead of someone's block database.. And block
> storage media is generally the cheapest storage.. I mean they could be
> written to tape and be just as valid as if they're stored in DRAM. Combine
> that with pruning, and block storage costs are almost a non-issue for
> anyone who isn't running an archival node.
>
> And can't relay nodes just configure a limit on the size of blocks they
> will relay? Sure they'd still need to download a big block occasionally,
> but that's not really that big a deal, and they're under no obligation to
> propagate it.. Even if it's a 2GB block, it'll get downloaded eventually.
> It's only if it gets to the point where the average home connection is too
> slow to keep up with the transaction & block flow that there's any real
> issue there, and that would happen regardless of how big the blocks are. I
> personally would much prefer to see hardware limits act as the bottleneck
> than to introduce an artificial bottleneck into the protocol that has to be
> adjusted regularly. The software and protocol are TECHNICALLY capable of
> scaling to handle the world's entire transaction set. The real issue with
> scaling to this size is limitations on hardware, which are regulated by
> Moore's Law. Why do we need arbitrary soft limits? Why can't we allow
> Bitcoin to grow naturally within the ever increasing limits of our
> hardware? Is it because nobody will ever need more than 640k of RAM?
>
> Am I missing something here? Is there some big reason that I'm overlooking
> why there has to be some hard-coded limit on the block size that affects
> the entire network and creates ongoing issues in the future?
>
> --
>
> *James G. Phillips IV*
> <https://plus.google.com/u/0/113107039501292625391/posts>
>
> *"Don't bunt. Aim out of the ball park. Aim for the company of immortals."
> -- David Ogilvy*
>
> *This message was created with 100% recycled electrons. Please think
> twice before printing.*
>
>
> ------------------------------------------------------------------------------
>
> _______________________________________________
> Bitcoin-development mailing list
> Bitcoin-development at lists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/bitcoin-development
>
>
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2023-06-07 15:32:32 UTC

- reply

📅 Original date posted:2015-04-25
📝 Original message:Hi Gregory,
In particular not covering the ID allows for transaction replay which
> can result in monetary losses far more severe than any possible
> mishandling of malleability could result in. Byzantine attackers can
> costlessly replay your old transactions any time anyone reuses an
> address, even accidentally (which cannot be easily prevented since
> they can race).
>
With the SIGHASH_WITHOUT_PREV_VALUE flag, signatures have to explicitly
specify that they are to be signed without the previous UTXO's
value/amount. This means that, at worst, replay attacks can send the money
to the same place it was sent before (which in many cases is likely not be
a loss of funds), and only if the amount sent to the reused address is the
exact same as it was before. I don't think this is worse than an attacker
being able to mutate their transaction and extort a merchant who accepts
zero-conf transactions. Anyway, not signing the input ID wouldn't exactly
be the norm, there would be a defined set of flags for standard use cases.
Not signing the input TXID would only be used in specialized cases, such as
setting up micropayment channels.
> There are no free lunches; the proposal linked to there is itself a
> game of wack-a-mole with assorted masking flags;
I agree that it is also a bit of wac-a-mole, but the defined space of
issues is possibly more limited here. There are only X number of things
that can be signed/not signed in a transaction, and the 'Build your own
nHashType' proposal enables you to fully specify which of those are being
signed. If you don't want to get burned by not fully signing your
transactions, then don't use the non-standard sighash flags.
many of which we have
> no notion of if they're useful for any particular application(s);
A few of the flags, indeed, may not ever be useful. But we can't predict
the future, and I think it's better to build in a more flexible solution
now than to wish we had more flexible nHashTypes later.
To the original point of this thread, hopefully the suggested proposal
won't be necessary as wallets will upgrade to use version 3 transactions
and the rules associated with them over time.
Best,
Stephen
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