Sounds like a commit-delay-reveal scheme, I first saw it described here: Stewart, I., et. al. (2018). “Committing to quantum resistance: A slow defence for Bitcoin against a fast quantum computing attack”
On BCH, we can implement that as a smart-contract, here’s the proof-of-concept: Quantum-resistant One-time-use Lock
I assume that requires moving all P2SH outputs to new outputs (with the smart contract) manually first to protect them?
In that case, they could just be moved to quantum resistant outputs.
If a quantum computer was suddenly able to break the elliptic curve signature algorithm, then there would need to be a soft fork to force all P2SH outputs to additionally require a commit transaction.
After the soft fork, you have to send the commit first because P2SH outputs would require the normal signature and a reference to a commit transaction.
Edit:
Also, it would be a good idea if it could handle pre-signed transactions. If someone had a valid transaction in a safe, then the client would take that transaction and produce a valid commit transaction for sending.
Really good talk from the BTC guys at OP_NEXT. I’ll need to watch this again, but for me the key points really were that the government is looking to be quantum secure 2030 - 2033, and somewhere in the range of 2026 to 2030 is probably ideal for Bitcoin.
This means if we’ve already locked in 2025 that we do need to get this onto the radar so that we can ship BCH quantum resistance changes 2026 - 2028 sometime.
It’s also a point of community discussion as to the economic effect. For instance, is there a case to hard fork in a “burn” of all Satoshi + other vulnerable coins after an announced window to move to quantum secure? Or do we let a huge amount of value in the UTXO set get grabbed by quantum computers?
The 2025 VM Limits upgrade may also help us out significantly I guess in terms of opening the ability for more cryptographic defenses.
See this graph of how you need to prep & migrate coins (with some margin for error) BEFORE the attack becomes viable obviously.
Government timeline:
The PoC smart contract is only good for someone who’d want to preemtively move his funds to a quantum-resistant address. Once QCs become available, you’d want a fork to enable safely migrating funds from any stranded and vulnerable contracts (including any non-standard variations of P2PKH, P2PKH, P2SH) where a commit-delay-reveal could work to prove original ownership.
Caveat with commit-delay-reveal is that miners could collude to steal, making a 51% potentially more damaging (right now, 51% could only censor transactions, or execute a double-spend fraud, but not out-right steal someone else’s funds) but once signatures are broken there’s no way around it but to use your P2PKH / P2SH address as an aged hashlock. With a long enough aging requirement, I think this would not be a deal-breaker.
I don’t consider P2SH or P2PKH preimage to be breakable, so consideration here is just vulnerability of redeem scripts, where they’d become vulnerable only after exposing a vulnerable redeem script, same how P2PKH becomes vulnerable only after exposing the committed key.
With P2SH, it is more complex, because some contracts may already be quantum-resistant (signatureless covenants etc.), and only some would be vulnerable to QCs. Having a SF to require a commit for all of P2SH could inconvenience non-vulnerable contracts or could even break them (if contract requires a particular number of inputs & outputs and commitment would be provided by an additional input or output).
Therefore, I think a complete solution would be to extend the TX format so that this commit-delay-reveal happens “outside” the legacy TX, so that it doesn’t interfere with functioning of existing contracts and it would only be required for any script that executes OP_CHECKSIG, CHECKDATASIG, or CHECKMULTISIG with elliptic curve keys.
BTC Core has a speculative PR for Quantum resistant addresses as a soft fork: QuBit - P2QRH spending rules by cryptoquick · Pull Request #1670 · bitcoin/bips · GitHub
Of course it’s overengineered, yet another level of segregating the witness.
But anyway, nice doc, here’s the link to view the doc: https://github.com/bitcoin/bips/blob/e186b52cff5344c789bc5996de86697e62244323/bip-p2qrh.mediawiki
smells like FUD, and conveniently there’s a bunch of new coins claiming quantum resistance
Check the community note. This is an industry/insider joke.
Look guys, you need to not just follow what some companies or governments say.
Companies and universaties have had this quantum computing “dream” for more than a century and billions have gone into actual research for 40 years. The results are basically that we see papers and results published at a rate that is really only about getting more funding.
Governments are a really big part of getting funding nowadays, they are afterall experts in spending other people’s money.
And in the meantime we see no real progress. And the next billions burned in research and building something to make it look good.
There is basically zero risk to old funds stored in p2pkh transactions, even if some quantum crypto is available tomorrow with actually a useful amount of bits. So stop worrying and wasting money on protecting against something that literally doesn’t exist.
You have some reading up to do. It is evolving, it is not just for more research dollars. There is and will be a market. Things take time.
Google’s latest QC breakthrough
This is not to say it is close to being a threat, on that I would agree, but it also does not mean we should ignore it or assume:
That is not an assumption. A ripe160 hashed pubkey on chain is safe from quantum computers for quite some time yet.
(also, as ps. Please be more careful about quoting actual statements, not one that looks like one but isn’t)
Interesting, this is your response to my stating it is all about getting more funding. You might make the connection yourself, how google is indeed getting work from nasa based on this, how the Department of Energy is stating it will buy contracts based on this. (that took me 5 minutes of research, follow the money is not that hard!)
So, do your research and again: you need to not just follow what some companies or governments say.
Welcome to BCH Research! Good to see you!
I love how terrified they are of a blocksize increase, author specifically mentions it several times (emphasis mine).
This approach for adding a post-quantum secure output type does not require a hard fork or block size increase.
Also pretty funny they have to split things up even further with a new type of address as they’re trying to avoid even more Inscriptions & SegWit disaster. Tech debt always stacking up for them.
But otherwise yeah a pretty good summary and worth us keeping an eye on.
Good thread to keep an eye on with regards to Quantum Discussion from the BTC guys: https://groups.google.com/g/bitcoindev/c/Aee8xKuIC2s/m/cu6xej1mBQAJ
It would depend on how fast the QCs are.
You could spend P2SH / P2PKH outputs safely if the time to break them was much longer than the block time and there was at least 1 honest miner. By the time an attacker could find the private key, your transaction would be buried in the blockchain.
There are effectively 3 cases
A) QC can’t break ECC: current situation
B) QC can break ECC slowly (it takes days, or more, to find the private key)
C) QC can break ECC quickly (it takes < 1 minute to find the private key)
We would probably move through B for a while before we hit C. A massive breakthough might move things from A to C suddenly though.
If the first seen rule is enforced widely, then that helps even in case C. An attacker would have to break your key and then get a new transaction broadcast before your transaction got to any of the miners.
I take your point that some P2SH outputs might already be quantum safe. It would be sufficient if there was a way to “lock” outputs before releasing the key.
For example, with P2SH, you could release the RIPEMD preimage without releasing the actual script.
You could publish
<SHA256(old_script), SHA256(nonce, old_script, new_script>>
in a commit transaction.
This can be checked by computing RIPEMD160<SHA256<old_script>>. This proves that the owner of the output wants it locked.
An attacker could publish alternative versions of the commit script, but none of them would be valid since they wouldn’t know old_script.
Once it is buried deeply enough, then the owner can publish nonce, old_script, new_script and spend their coins.
There is still a potential timing issue when the actual publication occurs. A dishonest miner (cartel) could try to block publication of the spending transaction while submitting an “updated” lock/commit transaction.
“them” here refers to their pubkey which would be exposed on 1st spend, so the attacker isn’t breaking the P2SH or P2PKH wrapping - he’s breaking the pubkey
the key is vulnerable between unwrapping and next wrapping, e.g. if you avoid address reuse then you limit the window of opportunity for the attacker, as you correctly pointed out
my comment was regarding the wrapping itself - which would require breaking the preimage (Grover’s algo) and is a much harder problem than ECC and in theory possible against 160 bits: address - Post-quantum preimage resistance of HASH160 addresses - Bitcoin Stack Exchange
On BCH, there is a way because we can make such a lock using our new smart contract capabilities, see here: Quantum-resistant One-time-use Lock
Ahh I see.
The commit must be at least N blocks deep.
Thus if a miner creates his own fake commit as soon as he sees the private information, it won’t be buried deep enough.
Thus a miner would have to win the next N blocks to use his fake commit. If a different miner mined any of those blocks, then they could include the legitimate transaction and negate his fake commit.
It does mean that a dishonest miners majority could block any other honest miner. However, a 51% attack is pretty damaging either way.
It does mean that a long duration 51% fork might be able to pay for itself, since it can steal all coins in the fork that is replaced.
This is inherent in any such scheme. If, say, 50 BCH of non-quantum protected outputs were redeemed in each block, then that would pay for the hash power for the alternative fork (assuming it is for sale).
Ideally, less than the block reward would be redeemed on average per block.
Fawkescoin was a proposal for a hash-only commit based currency (no signatures at all).
