Post Quantum Cryptography

Stumbled upon this today: To stop quantum hackers, the US just chose these four quantum-resistant encryption algorithms

so thought to create a general topic for future reference. I’ll edit in some more links later.

While writing the P2SH bulletin post-quantum section which focused on hash functions I dug up some literature:

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While looking at the section, I think you should include the P2SH48 info in the table.

You could have an extra horizontal line to show that it isn’t one of the 3 current ones.

It would allow an easy comparison pf the proposed P2SH48.

P2SH32 (classical): 256-bit/128-bit (preimage/collision)
P2SH48 (quantum): 192-bit/128-bit (preimage/collision)

Is there a specific proposal for P2SH48? Would it use the SHA3-384 algorithm?

Old addresses seem hard to handle.

Some of Satoshi’s original coinbase outputs use raw pay to public key.

Has there been an analysis of how much money is held by the various output types?

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No, there’s no point now when our other primitives (block and TX hashes) would still be (theoretically) vulnerable to quantum collision attack. So, when time comes then we’ll have to upgrade it all to 384bits, and P2SH is easiest since it’d be a non-breaking upgrade while TX and block hashes would break a lot of node-dependent software. Re. choice of hash function, there are other candidates, I think Blake is faster.

Fair enough, hopefully the choice of hash doesn’t derail any future upgrade.

Speed is not the primary target, but not completely unimportant.

I was listening to Bankless & noticed some very interesting comments made here about Quantum stuff.

Specifically, this idea of “one shot signatures”. Apparently this can unlock a whole array of different stuff, much of which sounds like ETH problems rather than BCH problems, but the one which caught my attention was this:

@ 47:17

It also gives us this notion of Quantum money, which is a way to do the Lightning Network without routing. One of the big complications of the Lightning Network is that you need to find this route of channels between the sender and the recipient, and more often than not there just isn’t going to be enough liquidity, whereas with Quantum Money you just need one hop and you don’t need these channels with liquidity.

This is all on the horizon, and I don’t see it being a last minute saviour for the technically regressive BTC network, but is there something here? Is a Lightning Network without routing possible through quantum tech?

The other uses he mentions before and after this segment are also worth listening to.

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I highly doubt that. All of this QC action-at-a-distance stuff is in domain of sci-fi (let’s just wish QC computers the size of a phone into existence) and even if it could be done in a lab it’s too unwieldy for practical applications.

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Good (maxi) video on quantum computing and Bitcoin.

He says it would be up to a decade until quantum is a threat. Is that right? Shouldn’t we prepare earlier?

Besides the dev time, is there any downside to moving to a quantum resistant signing scheme earlier? If not, shouldn’t we get it on the CHIP process and get it done? If we have the upgraded signing as a default earlier than later it will minimise the switchover cost once it is a big deal because Lindy will have put most active coins into quantum safe addresses anyway.

Also, thoughts on freezing out coins that are not quantum safe at some point? Matthew says that would be stealing (from Satoshi etc.) as there’s no way to target effectively, but I think the BCH community might be more pragmatic and after a long enough window to move coins into quantum safe addresses just freeze out the rest to never be spent. Otherwise it’s essentially “”“stealing”"" from the rest of the users by diluting them with the first quantum attacker grabbing up all of Satoshi’s coins & millions of lost/burnt coins.

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I have created an FAQ regarding quantum computing at: What about quantum computing? Are my BCH safe? | The Bitcoin Cash Podcast

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I’ll give you some more things to consider.

How to add support for some post-quantum signature scheme?

That’s relatively easy, it’s a drop-in, we can extend OP_CHECKSIG[VERIFY], OP_CHECKMULTISIG[VERIFY], and OP_CHECKSIG[VERIFY] just like we did when we added support for Schnorr signatures and Schnorr multisig: the opcode reads the length of pushed signature and from that determines how to interpret it and which method to use to validate.
With post-quantum crypto, keys & signatures would be bigger so same method is usable there: opcode can internally switch what scheme it uses based on the length of arguments.

Addresses would stay the same, UTXO size would stay the same, however input sizes would grow since they’d have to reveal bigger pubkeys and signatures, lowering the network’s TPS per megabyte of block size. See here for more info.

How to migrate from safe-at-rest elliptic curve P2PKH to post-quantum P2PKH

If quantum computers would become fast enough, adversary could intercept your transaction, discover your public key, and attempt to generate an alternative signature and replace your TX with his.
If that was a real threat, a new consensus rule could require and intermediate step for transferring funds from such UTXOs, a commit-delay-reveal scheme:

While your pubkey is still secret, you’d create a TX, hash it, then publish the hash on the chain. After the hash has been embedded to a safe depth, you’d publish the TX and reference the hash.
The adversary could generate an alternative TX, but he could not have an aged hash ready, and by the time he makes one, your TX will have been mined.

The paper linked above describes the method for migration in detail:

Can I store BCH in a quantum-safe address today?

Nice thing about Bitcoin Cash having powerful scripting capabilities is that it’s already possible to lock up funds in a quantum-safe contract (P2SH32), one that requires an aged pre-commitment + signature.
I outlined it in the P2SH bulletin at the end of section 5.

Are 160-bit P2PKH addresses that DID NOT reveal their pubkey vulnerable?

Theoretically yes, practically no. Grover’s algorithm could find an alternative spending key in a finite amount of time BUT:

Assuming a clock speed on scale of GHz, this would take about 10 million years. Important to note is that splitting the work and doing it in parallel is not as beneficial as with classic computers because it would offer only a quadratic speedup (Fluhrer, S., Reassessing Grover’s Algorithm). In other words, doing the work in 1 year would require building 100 trillion quantum computers because sqrt(100T) == 10M . Therefore, we can say that breaking a 160-bit hash preimage is physically possible because 10M years is a finite amount of time and less than age of the universe. However, it is still infeasible.

What about abandoned coins with pubkeys revealed?

They’d be free for the taking because original owners wouldn’t act during the window of opportunity to move the coins to quantum-resistant storage and so they’re as good as gone as soon as Shor’s algorithm becomes feasible. I guess it can be considered a bounty for the first to implement Shor’s :slight_smile:

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Without necessarily responding to prior comments, a big question would be how would BCH transition from one algorithm to another, with the dependence on ASICs at some point in the future?

Could a transition algorithm be used… with inspiration taken from the Storm proposal way back when. Take a similar idea, where current SHA-256 ASICs mine weak blocks, and whatever the new algorithm is, and computers that compute it, mine strong blocks. And over time strong blocks will be the only ones mined.

A transitioning of an algorithm will be a challenge. And I highly doubt there will be any algorithm that current ASICs can run that will be resistant to QC.

Is there another method to this that would work? Such as a PoS approach? I’m not a fan of PoS for a multitude of reasons, so I don’t see this as an option.

Out of curiosity, would Avalanche (BCHABC concept from a while back, of which I think they finally began to implement) provide some protection from QC? Or could an idea like that be modified to protect from QC?

Got one more link to add:

I found something: Delgado Segura, Sergi & Pérez-Solà, Cristina & Navarro-Arribas, Guillermo & Herrera-Joancomartí, Jordi. (2018). Analysis of the Bitcoin UTXO set.

image

They didn’t analyze how many of P2PKH UTXOs had their public key revealed.

We won’t need to. Quantum Computers won’t break our PoW. The only thing that’s at risk is the signature scheme (based on ECC - elliptic curve cryptography), nothing else.

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