QuBit - P2QRH spending rules

bip-p2qrh.mediawiki

@@ -30,11 +30,11 @@ The primary threat to Bitcoin by CRQCs is [https://en.bitcoin.it/wiki/Quantum_co
 
 The vulnerability of existing bitcoin addresses is investigated in [https://web.archive.org/web/20240715101040/https://www2.deloitte.com/nl/nl/pages/innovatie/artikelen/quantum-computers-and-the-bitcoin-blockchain.html this Deloitte report]. The report estimates that in 2020 approximately 25% of the bitcoin supply is held within addresses vulnerable to quantum attack. As of the time of writing, that number is now closer to 20%. Additionally, cryptographer Pieter Wuille [https://x.com/pwuille/status/1108085284862713856 reasons] even more might be vulnerable.
 
-Ordinarily, when a transaction is signed, the public key can be recovered from the signature. This makes a transaction submitted to the mempool vulnerable to quantum attack until it's mined. One way to mitigate this is to submit the transaction directly to a mining pool, which bypasses the mempool. This process is known as an out-of-band transaction. The mining pool must be trusted not to reveal the transaction public key to attackers.
+Ordinarily, when a transaction is signed, the public key can be recovered from the signature. This makes a transaction submitted to the mempool vulnerable to quantum attack until it's mined. One way to mitigate this is to submit the transaction directly to a mining pool, which bypasses the mempool. This process is known as an out-of-band transaction. The mining pool must be trusted not to reveal the transaction public key to attackers. The problem with this approach is that it requires a trusted third party, which is what this P2QRH proposal aims to avoid.
 
-Not having public keys exposed on-chain is an important step for quantum security. Otherwise funds would need to be spent to new addresses on a regular basis in order to prevent the possibility of a "long-range CRQC attack" recovering the key behind high value addresses. A long-range quantum attack can be considered one performed with chain data, such as that from a used address or one encoded in a spend script. A "short-range quantum attack" would be one performed on keys in the mempool, which is seen as impractical given transaction throughput and block time. As the value being sent increases, so too should the fee in order to commit the transaction to the chain as soon as possible.  This makes useless the public key revealed by spending a UTXO, so long as it is never reused.
+Not having public keys exposed on-chain is an important step for quantum security. Otherwise funds would need to be spent to new addresses on a regular basis in order to prevent the possibility of a "long-range CRQC attack" recovering the key behind high value addresses. A long-range quantum attack can be considered one performed with chain data, such as that from a used address or one encoded in a spend script. This is likely to be more common early on, as early quantum computers must be run for longer in order to overcome errors caused by noise. A "short-range quantum attack" would be one performed on keys in the mempool, which is seen as much more difficult given the block time, and so it requires more sophisticated CRQCs. As the value being sent increases, so too should the fee in order to commit the transaction to the chain as soon as possible. Once the transaction is mined, it makes useless the public key revealed by spending a UTXO, so long as it is never reused.
 
-It is proposed to implement a Pay to Quantum Resistant Hash (P2QRH) address type that relies on a Post-Quantum Cryptographic (PQC) signature algorithm. This new address type protects transactions submitted to the mempool and helps preserve the free market by reducing the need for private, out-of-band mempool transactions.
+It is proposed to implement a Pay to Quantum Resistant Hash (P2QRH) address type that relies on a Post-Quantum Cryptographic (PQC) signature algorithm. This new address type protects transactions submitted to the mempool and helps preserve the free market by preventing the need for private, out-of-band mempool transactions.
 
 The following table is non-exhaustive but intended to inform the average bitcoin user whether their bitcoin is vulnerable to a long-range quantum attack.
 
@@ -65,11 +65,11 @@ The following table summarizes the scenarios in which public keys are revealed w
 |-
 | Early addresses (Satoshi's coins, CPU miners, starts with 04) || Long-range
 |-
-| Reused addresses (any type, except bc1r) || Long-range
+| Reused addresses (any type, except P2QRH) || Long-range
 |-
 | Taproot addresses (starts with bc1p) || Long-range
 |-
-| Any transaction in the mempool (except for bc1r) || Short-range
+| Any transaction in the mempool (except for P2QRH) || Short-range
 |-
 | Unhardened BIP-32 HD wallet keys || Both Long-range or Short-range
 |}
@@ -78,11 +78,11 @@ The only time a short-range attack can occur is when the transaction is in the m
 
 Should quantum advantage manifest, a convention is proposed in spending the full 65-byte P2PK key used by the coinbase output in Block 1 back to itself. It is proposed to call the address in Block 1 the [https://mempool.space/address/0496b538e853519c726a2c91e61ec11600ae1390813a627c66fb8be7947be63c52da7589379515d4e0a604f8141781e62294721166bf621e73a82cbf2342c858ee Canary address] since it can only be spent from by others (assuming Satoshi's continued absence) if secp256k1 is broken. Should the Canary coins move, that will signal that reliance on secp256k1 is presently vulnerable. Without the Canary, or an address like it, there may be some doubt as to whether the coins were moved with keys belonging to the original owner.
 
-As an interesting aside, coinbase outputs to P2PK keys go as far as block 200,000, so it's possible there are between 1-2 million coins that are vulnerable from the first epoch. These coins can be considered "Satoshi's Shield." Any addresses with a balance of less than the original block subsidy of 50 coins can be considered incentive incompatible to capture until all of these are mined.
+As an interesting aside, coinbase outputs to P2PK keys go as far as block 200,000, so there are 1,723,848 coins that are vulnerable from the first epoch at the time of writing in P2PK outputs alone. Since the majority of these have a block reward of 50 coins each, there are roughly 34,000 distinct P2PK addresses that are vulnerable. These coins can be considered "Satoshi's Shield." Any addresses with a balance of less than the original block subsidy of 50 coins can be considered incentive incompatible to capture until all of these are mined, and these addresses serve to provide time to transition Bitcoin to implement post-quantum security.
 
-It's for the above reason that, for those who wish to be prepared for quantum emergency, it is recommended that no more than 50 bitcoin are kept under a single distinct, unused Native SegWit (P2WPKH, "bc1q") address at a time. This is assuming that the attacker is financially-motivated instead of, for example, a nation state looking to break confidence in Bitcoin. Additionally, this assumes that other vulnerable targets such as central banks have upgraded their cryptography already.
+It's for the above reason that, for those who wish to be prepared for quantum emergency, it is recommended that no more than 50 bitcoin are kept under a single, distinct, unused Native SegWit (P2WPKH, "bc1q") address at a time. This is assuming that the attacker is financially-motivated instead of, for example, a nation state looking to break confidence in Bitcoin. Additionally, this assumes that other vulnerable targets such as central banks have upgraded their cryptography by this time.
 
-The Commercial National Security Algorithm Suite (CNSA) 2.0 has a timeline for software and networking equipment to be upgraded by 2030, with browsers and operating systems fully upgraded by 2033.
+The Commercial National Security Algorithm Suite (CNSA) 2.0 has a timeline for software and networking equipment to be upgraded by 2030, with browsers and operating systems fully upgraded by 2033. According to NIST IR 8547, Elliptic Curve Cryptography is planned to be disallowed within the US Federal government after 2035. An exception is made for hybrid cryptography, which is the use of ECC and post-quantum algorithms together.
 
 Lastly, it is worth noting by way of comparison that [https://ethresear.ch/t/how-to-hard-fork-to-save-most-users-funds-in-a-quantum-emergency/18901 Vitalik Buterin's proposed solution] in an Ethereum quantum emergency is quite different from the approach in this BIP. His plan involves a hard fork of the chain, reverting all blocks after a sufficient amount of theft, and using STARKs based on BIP-32 seeds to act as the authoritative secret when signing. These measures are deemed far too heavy-handed for bitcoin.
 
@@ -95,7 +95,7 @@ It is proposed to use SegWit version 3. This results in addresses that start wit
 
 The proposal above also leaves a gap in case it makes sense to use version 2, or bc1z, for implementation of other address formats such as those that employ Cross Input Signature Aggregation (CISA).
 
-P2QRH is meant to be implemented on top of P2TR, combining the security of classical Schnorr signatures along with post-quantum cryptography. This is a form of "hybrid cryptography" such that no regression in security is presented should a vulnerability exist in one of the signature algorithms used. One key distinction between P2QRH and P2TR however is that P2QRH will encode a hash of the public key. This is a significant deviation from how Taproot works by itself, but it is necessary to avoid exposing public keys on-chain where they are vulnerable to attack.
+P2QRH is meant to be implemented on top of P2TR, combining the security of classical Schnorr signatures along with post-quantum cryptography. This is a form of hybrid cryptography such that no regression in security is presented should a vulnerability exist in one of the signature algorithms used. One key distinction between P2QRH and P2TR however is that P2QRH will encode a hash of the public key. This is a significant deviation from how Taproot works by itself, but it is necessary to avoid exposing public keys on-chain where they are vulnerable to attack.
 
 P2QRH uses a 32-byte HASH256 (specifically SHA-256 twice-over, which is similar to that used in [https://github.com/bitcoin/bips/blob/master/bip-0016.mediawiki#specification BIP-16]) of the public key to reduce the size of new outputs and also to increase security by not having the public key available on-chain. This hash serves as a minimal cryptographic commitment to a public key. It goes into the output spend script, which does not receive the witness discount.
 
@@ -260,6 +260,7 @@ TBD
 
 To help implementors understand updates to this BIP, we keep a list of substantial changes.
 
+* 2024-11-20 - Clarifications based on feedback from Murch.
 * 2024-10-21 - Replace XMSS with CRYSTALS-Dilithium due to NIST approval and size constraints.
 * 2024-09-30 - Refactor the ECC vs PoW section. Swap quitness for attestation.
 * 2024-09-29 - Update section on PoW to include partial-preimage.
@@ -269,4 +270,6 @@ To help implementors understand updates to this BIP, we keep a list of substanti
 
 == Acknowledgements ==
 
-Much gratitude to my co-founder, Kyle Crews for proofreading and editing, to David Croisant, who suggested the name "QuBit", and Guy Swann for pointing out the earlier name for the attestation, "quitness", was imperfect. Thank you as well to those who took the time to review and contribute, including Adam Borcany, Antoine Riard, Pierre-Luc Dallaire-Demers, Ethan Heilman, Jon Atack, and Jameson Lopp.
+This document is inspired by [https://github.com/bitcoin/bips/blob/master/bip-0341.mediawiki BIP-341], which introduced the design of the P2TR (Taproot) address type using Schnorr signatures.
+
+Much gratitude to my co-founder, Kyle Crews for proofreading and editing, to David Croisant, who suggested the name "QuBit", and Guy Swann for pointing out the earlier name for the attestation, "quitness", was imperfect. Thank you as well to those who took the time to review and contribute, including Adam Borcany, Antoine Riard, Pierre-Luc Dallaire-Demers, Ethan Heilman, Jon Atack, Jameson Lopp, and Murchandamus.