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Post-Quantum commit / reveal Fawkescoin variant as a soft fork
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Tadge Dryja
May 28, 2025, 1:28:25 PMMay 28
to Bitcoin Development Mailing List
One of the tricky things about securing Bitcoin against quantum computers is: do you even need to? Maybe quantum computers that can break secp256k1 keys will never exist, in which case we shouldn't waste our time. Or maybe they will exist, in not too many years, and we should spend the effort to secure the system against QCs.
Since people disagree on how likely QCs are to arrive, and what the timing would be if they do, it's hard to get consensus on changes to bitcoin that disrupt the properties we use today. For example, a soft fork introducing a post-quantum (PQ) signature scheme and at the same time disallowing new secp256k1 based outputs would be great for strengthening Bitcoin against an oncoming QC. But it would be awful if a QC never appears, or takes decades to do so, since secp256k1 is really nice.
So it would be nice to have a way to not deal with this issue until *after* the QC shows up. With commit / reveal schemes Bitcoin can keep working after a QC shows up, even if we haven't defined a PQ signature scheme and everyone's still got P2WPKH outputs.
Most of this is similar to Tim Ruffing's proposal from a few years ago here:
https://gnusha.org/pi/bitcoindev/1518710367.3...@mmci.uni-saarland.de/
The main difference is that this scheme doesn't use encryption, but a smaller hash-based commitment, and describes activation as a soft fork. I'll define the two types of attacks, a commitment scheme, and then say how it can be implemented in bitcoin nodes as a soft fork.
This scheme only works for keys that are pubkey hashes (or script hashes) with pubkeys that are unknown to the network. It works with taproot as well, but there must be some script-path in the taproot key, as keypath spends would no longer be secure.
What to do with all the keys that are known is another issue and independent of the scheme in this post (it's compatible with both burning them and leaving them to be stolen)
For these schemes, we assume there is an attacker with a QC that can compute a quickly compute a private key from any secp256k1 public key. We also assume the attacker has some mining power or influence over miners for their attacks; maybe not reliably, but they can sometimes get a few blocks in a row with the transactions they want.
"Pubkey" can also be substituted with "script" for P2SH and P2WSH output types and should work about the same way (with caveats about multisig). The equivalent for taproot outputs would be an inner key proving a script path.
## A simple scheme to show an attack
The simplest commit/reveal scheme would be one where after activation, for any transaction with an EC signature in it, that transaction's txid must appear in a earlier transaction's OP_RETURN output.
When a user wants to spend their coins, they first sign a transaction as they would normally, compute the txid, get that txid into an OP_RETURN output somehow (paying a miner out of band, etc), then after waiting a while, broadcast the transaction. Nodes would check that the txid matches a previously seen commitment, and allow the transaction.
One problem with this scheme is that upon seeing the full transaction, the attacker can compute the user's private key, and create a new commitment with a different txid for a transaction where the attacker gets all the coins. If the attacker can get their commitment and spending transaction in before the user's transaction, they can steal the coins.
In order to mitigate this problem, a minimum delay can be enforced by consensus. A minimum delay of 100 blocks would mean that the attacker would have to prevent the user's transaction from being confirmed for 100 blocks after it showed up in the attacker's mempool. The tradeoff is that longer periods give better safety at the cost of more delay in spending.
This scheme, while problematic, is better than nothing! But it's possible to remove this timing tradeoff.
## A slightly more complex scheme with (worse) problems
If instead of just the txid, the commitment were both the outpoint being spent, and the txid that was going to spend it, we could add a "first seen" consensus rule. Only the first commitment pointing to an outpoint works.
So if nodes see two OP_RETURN commitments in their sequence of confirmed transactions:
C1 = outpoint1, txid1
C2 = outpoint1, txid2
They can ignore C2; C1 has already laid claim to outpoint1, and the transaction identified by txid1 is the only transaction that can spend outpoint1.
If the user manages to get C1 confirmed first, this is great, and eliminates the timing problem in the txid only scheme. But this introduces a different problem, where an attacker -- in this case any attacker, even one without a QC -- who can observe C1 before it is confirmed can flip some bits in the txid field, freezing the outpoint forever.
We want to retain the "first seen" rule, but we want to also be able to discard invalid commitments. In a bit flipping attack, we could say an invalid commitment is one where there is no transaction described by the txid. A more general way to classify a commitment as invalid is a commitment made without knowledge of the (secret) pubkey. Knowledge of the pubkey is what security of coins is now hinging on.
The actual commitment scheme
We define some hash function h(). We'll use SHA256 for the hashing, but it needs to be keyed with some tag, for example "Alas poor Koblitz curve, we knew it well".
Thus h(pubkey) is not equal to the pubkey hash already used in the bitcoin output script, which instead is RIPEMD160(SHA256(pubkey)), or in bitcoin terms, HASH160(pubkey). Due to the hash functions being different, A = HASH160(pubkey) and B = h(pubkey) will be completely different, and nobody should be able to determine if A and B are hashes of the same pubkey without knowing pubkey itself.
An efficient commitment is:
C = h(pubkey), h(pubkey, txid), txid
This commitment includes 3 elements: a different hash of the pubkey which will be signed for, a proof of knowledge of the pubkey which commits to a transaction, and an the txid of the spending transaction. We'll call these "address ID" (AID), sequence dependent proof (SDP), and the commitment txid (CTXID).
For those familiar with the proposal by Ruffing, the SDP has a similar function to the authenticated encryption part of the encrypted commitment. Instead of using authenticated encryption, we can instead just use an HMAC-style authentication alone, since the other data, the CTXID, is provided.
When the user's wallet creates a transaction, they can feed that transaction into a commitment generator function which takes in a transaction, extracts the pubkey from the tx, computes the 3 hashes, and returns the 3-hash commitment. Once this commitment is confirmed, the user broadcasts the transaction.
Nodes verify the commitment by using the same commitment generator function and checking if it matches the first valid commitment for that AID, in which case the tx is confirmed.
If a node sees multiple commitments all claiming the same AID, it must store all of them. Once the AID's pubkey is known, the node can distinguish which commitments are valid, which are invalid, and which is the first seen valid commitment. Given the pubkey, nodes can determine commitments to be invalid by checking if SDP = h(pubkey, CTXID).
As an example, consider a sequence of 3 commitments:
C1 = h(pubkey), h(pubkey', txid1), txid1
C2 = h(pubkey), h(pubkey, txid2), txid2
C3 = h(pubkey), h(pubkey, txid3), txid3
The user first creates tx2 and tries to commit C2. But an attacker creates C1, committing to a different txid where they control the outputs, and confirms it first. This attacker may know the outpoint being spent, and may be able to create a transaction and txid that could work. But they don't know the pubkey, so while they can copy the AID hash, they have to make something up for the SDP.
The user gets C2 confirmed after C1. They then reveal tx2 in the mempool, but before it can be confirmed, the attacker gets C3 confirmed. C3 is a valid commitment made with knowledge of the pubkey.
Nodes can reject transactions tx1 and tx3. For tx1, they will see that the SDP doesn't match the data in the transaction, so it's an invalid commitment. For tx3, they will see that it is valid, but by seeing tx3 they will also be able to determine that C2 is a valid commitment (since pubkey is revealed in tx3) which came prior to C3, making C2 the only valid commitment for that AID.
## Implementation
Nodes would keep a new key/value store, similar to the existing UTXO set. The indexing key would be the AID, and the value would be the set of all (SDP, CTXID) pairs seen alongside that AID. Every time an commitment is seen in an OP_RETURN, nodes store the commitment.
When a transaction is seen, nodes observe the pubkey used in the transaction, and look up if it matches an AID they have stored. If not, the transaction is dropped. If the AID does match, the node can now "clean out" an AID entry, eliminating all but the first valid commitment, and marking that AID as final. If the txid seen matches the remaining commitment, the transaction is valid; if not, the transaction is dropped.
After the transaction is confirmed the AID entry can be deleted. Deleting the entries frees up space, and would allow another round to happen with the same pubkey, which would lead to theft. Retaining the entries takes up more space on nodes that can't be pruned, and causes pubkey reuse to destroy coins rather than allow them to be stolen. That's a tradeoff, and I personally guess it's probably not worth retaining that data but don't have a strong opinion either way.
Short commitments:
Since we're not trying to defend against collision attacks, I think all 3 hashes can be truncated to 16 bytes. The whole commitment could be 48 bytes long. Without truncation the commitments would be 96 bytes.
## Activation
The activation for the commit/reveal requirement can be triggered by a proof of quantum computer (PoQC).
A transaction which successfully spends an output using tapscript:
OP_SHA256 OP_CHECKSIG
is a PoQC in the form of a valid bitcoin transaction. In order to satisfy this script, the spending transaction needs to provide 2 data elements: a signature, and some data that when hashed results in a pubkey for which that signature is valid. If such a pair of data elements exists, it means that either SHA256 preimage resistance is broken (which we're assuming isn't the case) or someone can create valid signatures for arbitrary elliptic curve points, ie a cryptographically relevant quantum computer (or any other process which breaks the security of secp256k1 signatures)
Once such a PoQC has been observed in a confirmed transaction, the requirements for the 3-hash commitment scheme can be enforced. This is a soft fork since the transactions themselves look the same, the only requirement is that some OP_RETURN outputs show up earlier. Nodes which are not aware of the commitment requirement will still accept all transactions with the new rules.
Wallets not aware of the new rules, however, are very dangerous, as they may try to broadcast signed transactions without any commitment. Nodes that see such a transaction should drop the tx, and if possible tell the wallet that they are doing something which is now very dangerous! On the open p2p network this is not really enforceable, but people submitting transactions to their own node (eg via RPC) can at least get a scary error message.
## Issues
My hope is that this scheme would give some peace of mind to people holding bitcoin, that in the face of a sudden QC, even with minimal preparation their coins can be safe at rest and safely moved. It also suggests some best practices for users and wallets to adopt, before any software changes: Don't reuse addresses, and if you have taproot outputs, include some kind of script path in the outer key.
There are still a number of problems, though!
- Reorgs can steal coins. An attacker that observes a pubkey and can reorg back to before the commitment can compute the private key, sign a new transaction and get their commitment in first on the new chain. This seems unavoidable with commit/reveal schemes, and it's up to the user how long they wait between confirming the commitment and revealing the transaction.
- How to get op_returns in
If there are no PQ signature schemes activated in bitcoin when this activates, there's only one type of transaction that can reliably get the OP_RETURN outputs confirmed: coinbase transactions. Getting commitments to the miners and paying them out of band is not great, but is possible and we see this kind of activity today. Users wouldn't need to directly contact miners: anyone could aggregate commitments, create a large transaction with many OP_RETURN outputs, and then get a miner to commit to that parent transaction. Users don't need to worry about committing twice as identical commitments would be a no op.
- Multiple inputs
If users have received more than one UTXO to the same address, they will need to spend all the UTXOs at once. The commitment scheme can deal with only the first pubkey seen in the serialized transaction.
- Multisig and Lightning Network
If your multisig counterparties have a QC, multisig outputs become 1 of N. Possibly a more complex commit / reveal scheme could deal with multiple keys, but the keys would all have to be hashed with counterparties not knowing each others' unhashed pubkeys. This isn't how existing multisig outputs work, and in fact the current trend is the opposite with things like Musig2, FROST and ROAST. If we're going to need to make new signing software and new output types it might make more sense to go for a PQ signature scheme.
- Making more p2wpkhs
You don't have to send to a PQ address type with these transactions -- you can send to p2wpkh and do the whole commit/reveal process again when you want to spend. This could be helpful if PQ signature schemes are still being worked on, or if the PQ schemes are more costly to verify and have high fees in comparison to the old p2wpkh output types. It's possible that in such a scenario a few high-cost PQ transactions commit to many smaller EC transactions. If this actually gets adoption though, we might as well drop the EC signatures and just make output scripts into raw hash / preimage pairs. It could make sense to cover some non-EC script types with the same 3-hash commitment requirement to enable this.
## Conclusion
This PQ commit / reveal scheme has similar properties to Tim Ruffing's, with a smaller commitment that can be done as a soft fork. I hope something like this could be soft forked with a PoQC activation trigger, so that if a QC never shows up, none of this code gets executed. And people who take a couple easy steps like not reusing addresses (which they should anyway for privacy reasons) don't have to worry about their coins.
Some of these ideas may have been posted before; I know of the Fawkscoin paper (https://jbonneau.com/doc/BM14-SPW-fawkescoin.pdf) and the recent discussion which linked to Ruffing's proposal. Here I've tried to show how it could be done in a soft fork which doesn't look too bad to implement.
I've also heard of some more complex schemes involving zero knowledge proofs, proving things like BIP32 derivations, but I think this gives some pretty good properties without needing anything other than good old SHA256.
Hope this is useful & wonder if people think something like this would be a good idea.
-Tadge
Since people disagree on how likely QCs are to arrive, and what the timing would be if they do, it's hard to get consensus on changes to bitcoin that disrupt the properties we use today. For example, a soft fork introducing a post-quantum (PQ) signature scheme and at the same time disallowing new secp256k1 based outputs would be great for strengthening Bitcoin against an oncoming QC. But it would be awful if a QC never appears, or takes decades to do so, since secp256k1 is really nice.
So it would be nice to have a way to not deal with this issue until *after* the QC shows up. With commit / reveal schemes Bitcoin can keep working after a QC shows up, even if we haven't defined a PQ signature scheme and everyone's still got P2WPKH outputs.
Most of this is similar to Tim Ruffing's proposal from a few years ago here:
https://gnusha.org/pi/bitcoindev/1518710367.3...@mmci.uni-saarland.de/
The main difference is that this scheme doesn't use encryption, but a smaller hash-based commitment, and describes activation as a soft fork. I'll define the two types of attacks, a commitment scheme, and then say how it can be implemented in bitcoin nodes as a soft fork.
This scheme only works for keys that are pubkey hashes (or script hashes) with pubkeys that are unknown to the network. It works with taproot as well, but there must be some script-path in the taproot key, as keypath spends would no longer be secure.
What to do with all the keys that are known is another issue and independent of the scheme in this post (it's compatible with both burning them and leaving them to be stolen)
For these schemes, we assume there is an attacker with a QC that can compute a quickly compute a private key from any secp256k1 public key. We also assume the attacker has some mining power or influence over miners for their attacks; maybe not reliably, but they can sometimes get a few blocks in a row with the transactions they want.
"Pubkey" can also be substituted with "script" for P2SH and P2WSH output types and should work about the same way (with caveats about multisig). The equivalent for taproot outputs would be an inner key proving a script path.
## A simple scheme to show an attack
The simplest commit/reveal scheme would be one where after activation, for any transaction with an EC signature in it, that transaction's txid must appear in a earlier transaction's OP_RETURN output.
When a user wants to spend their coins, they first sign a transaction as they would normally, compute the txid, get that txid into an OP_RETURN output somehow (paying a miner out of band, etc), then after waiting a while, broadcast the transaction. Nodes would check that the txid matches a previously seen commitment, and allow the transaction.
One problem with this scheme is that upon seeing the full transaction, the attacker can compute the user's private key, and create a new commitment with a different txid for a transaction where the attacker gets all the coins. If the attacker can get their commitment and spending transaction in before the user's transaction, they can steal the coins.
In order to mitigate this problem, a minimum delay can be enforced by consensus. A minimum delay of 100 blocks would mean that the attacker would have to prevent the user's transaction from being confirmed for 100 blocks after it showed up in the attacker's mempool. The tradeoff is that longer periods give better safety at the cost of more delay in spending.
This scheme, while problematic, is better than nothing! But it's possible to remove this timing tradeoff.
## A slightly more complex scheme with (worse) problems
If instead of just the txid, the commitment were both the outpoint being spent, and the txid that was going to spend it, we could add a "first seen" consensus rule. Only the first commitment pointing to an outpoint works.
So if nodes see two OP_RETURN commitments in their sequence of confirmed transactions:
C1 = outpoint1, txid1
C2 = outpoint1, txid2
They can ignore C2; C1 has already laid claim to outpoint1, and the transaction identified by txid1 is the only transaction that can spend outpoint1.
If the user manages to get C1 confirmed first, this is great, and eliminates the timing problem in the txid only scheme. But this introduces a different problem, where an attacker -- in this case any attacker, even one without a QC -- who can observe C1 before it is confirmed can flip some bits in the txid field, freezing the outpoint forever.
We want to retain the "first seen" rule, but we want to also be able to discard invalid commitments. In a bit flipping attack, we could say an invalid commitment is one where there is no transaction described by the txid. A more general way to classify a commitment as invalid is a commitment made without knowledge of the (secret) pubkey. Knowledge of the pubkey is what security of coins is now hinging on.
The actual commitment scheme
We define some hash function h(). We'll use SHA256 for the hashing, but it needs to be keyed with some tag, for example "Alas poor Koblitz curve, we knew it well".
Thus h(pubkey) is not equal to the pubkey hash already used in the bitcoin output script, which instead is RIPEMD160(SHA256(pubkey)), or in bitcoin terms, HASH160(pubkey). Due to the hash functions being different, A = HASH160(pubkey) and B = h(pubkey) will be completely different, and nobody should be able to determine if A and B are hashes of the same pubkey without knowing pubkey itself.
An efficient commitment is:
C = h(pubkey), h(pubkey, txid), txid
(to label things: C = AID, SDP, CTXID)
For those familiar with the proposal by Ruffing, the SDP has a similar function to the authenticated encryption part of the encrypted commitment. Instead of using authenticated encryption, we can instead just use an HMAC-style authentication alone, since the other data, the CTXID, is provided.
When the user's wallet creates a transaction, they can feed that transaction into a commitment generator function which takes in a transaction, extracts the pubkey from the tx, computes the 3 hashes, and returns the 3-hash commitment. Once this commitment is confirmed, the user broadcasts the transaction.
Nodes verify the commitment by using the same commitment generator function and checking if it matches the first valid commitment for that AID, in which case the tx is confirmed.
If a node sees multiple commitments all claiming the same AID, it must store all of them. Once the AID's pubkey is known, the node can distinguish which commitments are valid, which are invalid, and which is the first seen valid commitment. Given the pubkey, nodes can determine commitments to be invalid by checking if SDP = h(pubkey, CTXID).
As an example, consider a sequence of 3 commitments:
C1 = h(pubkey), h(pubkey', txid1), txid1
C2 = h(pubkey), h(pubkey, txid2), txid2
C3 = h(pubkey), h(pubkey, txid3), txid3
The user first creates tx2 and tries to commit C2. But an attacker creates C1, committing to a different txid where they control the outputs, and confirms it first. This attacker may know the outpoint being spent, and may be able to create a transaction and txid that could work. But they don't know the pubkey, so while they can copy the AID hash, they have to make something up for the SDP.
The user gets C2 confirmed after C1. They then reveal tx2 in the mempool, but before it can be confirmed, the attacker gets C3 confirmed. C3 is a valid commitment made with knowledge of the pubkey.
Nodes can reject transactions tx1 and tx3. For tx1, they will see that the SDP doesn't match the data in the transaction, so it's an invalid commitment. For tx3, they will see that it is valid, but by seeing tx3 they will also be able to determine that C2 is a valid commitment (since pubkey is revealed in tx3) which came prior to C3, making C2 the only valid commitment for that AID.
## Implementation
Nodes would keep a new key/value store, similar to the existing UTXO set. The indexing key would be the AID, and the value would be the set of all (SDP, CTXID) pairs seen alongside that AID. Every time an commitment is seen in an OP_RETURN, nodes store the commitment.
When a transaction is seen, nodes observe the pubkey used in the transaction, and look up if it matches an AID they have stored. If not, the transaction is dropped. If the AID does match, the node can now "clean out" an AID entry, eliminating all but the first valid commitment, and marking that AID as final. If the txid seen matches the remaining commitment, the transaction is valid; if not, the transaction is dropped.
After the transaction is confirmed the AID entry can be deleted. Deleting the entries frees up space, and would allow another round to happen with the same pubkey, which would lead to theft. Retaining the entries takes up more space on nodes that can't be pruned, and causes pubkey reuse to destroy coins rather than allow them to be stolen. That's a tradeoff, and I personally guess it's probably not worth retaining that data but don't have a strong opinion either way.
Short commitments:
Since we're not trying to defend against collision attacks, I think all 3 hashes can be truncated to 16 bytes. The whole commitment could be 48 bytes long. Without truncation the commitments would be 96 bytes.
## Activation
The activation for the commit/reveal requirement can be triggered by a proof of quantum computer (PoQC).
A transaction which successfully spends an output using tapscript:
OP_SHA256 OP_CHECKSIG
is a PoQC in the form of a valid bitcoin transaction. In order to satisfy this script, the spending transaction needs to provide 2 data elements: a signature, and some data that when hashed results in a pubkey for which that signature is valid. If such a pair of data elements exists, it means that either SHA256 preimage resistance is broken (which we're assuming isn't the case) or someone can create valid signatures for arbitrary elliptic curve points, ie a cryptographically relevant quantum computer (or any other process which breaks the security of secp256k1 signatures)
Once such a PoQC has been observed in a confirmed transaction, the requirements for the 3-hash commitment scheme can be enforced. This is a soft fork since the transactions themselves look the same, the only requirement is that some OP_RETURN outputs show up earlier. Nodes which are not aware of the commitment requirement will still accept all transactions with the new rules.
Wallets not aware of the new rules, however, are very dangerous, as they may try to broadcast signed transactions without any commitment. Nodes that see such a transaction should drop the tx, and if possible tell the wallet that they are doing something which is now very dangerous! On the open p2p network this is not really enforceable, but people submitting transactions to their own node (eg via RPC) can at least get a scary error message.
## Issues
My hope is that this scheme would give some peace of mind to people holding bitcoin, that in the face of a sudden QC, even with minimal preparation their coins can be safe at rest and safely moved. It also suggests some best practices for users and wallets to adopt, before any software changes: Don't reuse addresses, and if you have taproot outputs, include some kind of script path in the outer key.
There are still a number of problems, though!
- Reorgs can steal coins. An attacker that observes a pubkey and can reorg back to before the commitment can compute the private key, sign a new transaction and get their commitment in first on the new chain. This seems unavoidable with commit/reveal schemes, and it's up to the user how long they wait between confirming the commitment and revealing the transaction.
- How to get op_returns in
If there are no PQ signature schemes activated in bitcoin when this activates, there's only one type of transaction that can reliably get the OP_RETURN outputs confirmed: coinbase transactions. Getting commitments to the miners and paying them out of band is not great, but is possible and we see this kind of activity today. Users wouldn't need to directly contact miners: anyone could aggregate commitments, create a large transaction with many OP_RETURN outputs, and then get a miner to commit to that parent transaction. Users don't need to worry about committing twice as identical commitments would be a no op.
- Spam
Anyone can make lots of OP_RETURN commitments which are just random numbers, forcing nodes to store these commitments in a database. That's not great, but isn't much different from how bitcoin works today. If it's really a problem, nodes could requiring the commitment outputs to have a non-0 amount of bitcoin, imposing a higher cost for the commitments than other OP_RETURN outputs.
If users have received more than one UTXO to the same address, they will need to spend all the UTXOs at once. The commitment scheme can deal with only the first pubkey seen in the serialized transaction.
- Multisig and Lightning Network
If your multisig counterparties have a QC, multisig outputs become 1 of N. Possibly a more complex commit / reveal scheme could deal with multiple keys, but the keys would all have to be hashed with counterparties not knowing each others' unhashed pubkeys. This isn't how existing multisig outputs work, and in fact the current trend is the opposite with things like Musig2, FROST and ROAST. If we're going to need to make new signing software and new output types it might make more sense to go for a PQ signature scheme.
- Making more p2wpkhs
You don't have to send to a PQ address type with these transactions -- you can send to p2wpkh and do the whole commit/reveal process again when you want to spend. This could be helpful if PQ signature schemes are still being worked on, or if the PQ schemes are more costly to verify and have high fees in comparison to the old p2wpkh output types. It's possible that in such a scenario a few high-cost PQ transactions commit to many smaller EC transactions. If this actually gets adoption though, we might as well drop the EC signatures and just make output scripts into raw hash / preimage pairs. It could make sense to cover some non-EC script types with the same 3-hash commitment requirement to enable this.
## Conclusion
This PQ commit / reveal scheme has similar properties to Tim Ruffing's, with a smaller commitment that can be done as a soft fork. I hope something like this could be soft forked with a PoQC activation trigger, so that if a QC never shows up, none of this code gets executed. And people who take a couple easy steps like not reusing addresses (which they should anyway for privacy reasons) don't have to worry about their coins.
Some of these ideas may have been posted before; I know of the Fawkscoin paper (https://jbonneau.com/doc/BM14-SPW-fawkescoin.pdf) and the recent discussion which linked to Ruffing's proposal. Here I've tried to show how it could be done in a soft fork which doesn't look too bad to implement.
I've also heard of some more complex schemes involving zero knowledge proofs, proving things like BIP32 derivations, but I think this gives some pretty good properties without needing anything other than good old SHA256.
Hope this is useful & wonder if people think something like this would be a good idea.
-Tadge
Sergio Demian Lerner
May 28, 2025, 2:31:18 PMMay 28
to Tadge Dryja, Bitcoin Development Mailing List
Without in-depth reading of your e-mail, but related to Fawkescoin, you can read Mave paper from 2012 , which addressed DoS problems that exist in Fawkescoin.
regards
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Nagaev Boris
May 28, 2025, 5:54:21 PMMay 28
to Tadge Dryja, Bitcoin Development Mailing List
Hi Tadge,
Thanks for writing this up! The proposal is very thoughtful, and it's
great to see concrete work on post-quantum commit/reveal schemes.
I've been exploring a related approach based on a similar
commit/reveal idea. In my scheme, a user creates a QR output that
commits to a hash of a pubkey inside a Taproot leaf. This commitment
is hidden until revealed at spend time. Later, when the user wants to
spend a legacy EC output, they must spend this QR output in the same
transaction, and it must be at least X blocks old.
https://groups.google.com/g/bitcoindev/c/jr1QO95k6Uc/m/lsRHgIq_AAAJ
This approach has a few potential advantages:
1. No need for nodes to track a new commitment store
Because the commitment remains hidden in a Tapleaf until the spend,
observers (including attackers) don't see it, and nodes don't need to
store or validate any external commitment set. The only requirement is
that the QR output must be old enough, and Bitcoin Core already tracks
coin age, which is needed to validate existing consensus rules.
2. Commitment can be made before the transaction is known
Since the commitment doesn't include a txid, the user can precommit to
the pubkey hash far in advance, before knowing the details of the
eventual transaction. This allows greater flexibility: you can delay
choosing outputs, fee rates, etc., until spend time. Only knowledge of
the EC pubkey needs to be proven when creating the QR output.
3. More efficient use of block space
Multiple EC coins can be spent together with a single QR output,
holding EC pubkey commitments in Taproot leaves. If EC coins share the
same EC pubkey (e.g., come from the same address), they can reuse the
same commitment.
Would love to hear your thoughts on this variant. I think this one
might be a simpler, lower-overhead option for protecting EC outputs
post-QC.
Best,
Boris
Best regards,
Boris Nagaev
Thanks for writing this up! The proposal is very thoughtful, and it's
great to see concrete work on post-quantum commit/reveal schemes.
I've been exploring a related approach based on a similar
commit/reveal idea. In my scheme, a user creates a QR output that
commits to a hash of a pubkey inside a Taproot leaf. This commitment
is hidden until revealed at spend time. Later, when the user wants to
spend a legacy EC output, they must spend this QR output in the same
transaction, and it must be at least X blocks old.
https://groups.google.com/g/bitcoindev/c/jr1QO95k6Uc/m/lsRHgIq_AAAJ
This approach has a few potential advantages:
1. No need for nodes to track a new commitment store
Because the commitment remains hidden in a Tapleaf until the spend,
observers (including attackers) don't see it, and nodes don't need to
store or validate any external commitment set. The only requirement is
that the QR output must be old enough, and Bitcoin Core already tracks
coin age, which is needed to validate existing consensus rules.
2. Commitment can be made before the transaction is known
Since the commitment doesn't include a txid, the user can precommit to
the pubkey hash far in advance, before knowing the details of the
eventual transaction. This allows greater flexibility: you can delay
choosing outputs, fee rates, etc., until spend time. Only knowledge of
the EC pubkey needs to be proven when creating the QR output.
3. More efficient use of block space
Multiple EC coins can be spent together with a single QR output,
holding EC pubkey commitments in Taproot leaves. If EC coins share the
same EC pubkey (e.g., come from the same address), they can reuse the
same commitment.
Would love to hear your thoughts on this variant. I think this one
might be a simpler, lower-overhead option for protecting EC outputs
post-QC.
Best,
Boris
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Best regards,
Boris Nagaev
waxwing/ AdamISZ
May 31, 2025, 12:18:28 PMMay 31
to Bitcoin Development Mailing List
Hi Tadge, list,
Appreciate this writeup. I found that the discursive detail avoided some confusions I had with Tim's and Adam's earlier descriptions, though there are clearly a ton of subtle differences in the various designs.
Anyway, one detail: you have h(pubkey), h(pubkey,txid), txid. You note earlier that h should be tagged for domain separation purposes. But if you meant literally the same h for the first two components of the tuple, there's a flaw, depending on h; length extension attacks could allow the creation of h(pubkey,txid') without knowing pubkey. I'm going to ignore the trickier question of whether that matters based on how txid is derived from a transaction (given txid vs wtxid, I guess it actually really does). Presumably just use h2 which differs from h, either a different hash or a different prefix, to avoid this. And/or swapping h(txid,pubkey) etc. Obviously using h() that is not susceptible to LEA avoids the Q being raised, but that may be more or less easy in Bitcoin-land.
Cheers,
AdamISZ/waxwing
AdamISZ/waxwing
Jonathan Voss
Jun 2, 2025, 6:33:13 AMJun 2
to Bitcoin Development Mailing List
As far as I can tell, the main flaw in commit/reveal protocols is in the commit phase: if revealing a commitment with N confirmations is required to spend bitcoins, then, without spending any bitcoins, how do you get the commitment into the blockchain in the first place? Maybe I am just misunderstanding this. If so, then a commit/reveal scheme may be a workable solution.
I really like the PoQC trigger mechanism, but I doubt that a real quantum attacker would voluntarily activate the network's QC defense soft-fork instead of just attacking -- it seems to assume that an honest node will spend the PoQC transaction before a real adversary shows up.
-- Jonathan
Peter Todd
Jun 2, 2025, 9:53:55 AMJun 2
to Jonathan Voss, Bitcoin Development Mailing List
On Fri, May 30, 2025 at 03:00:41PM -0700, Jonathan Voss wrote:
> As far as I can tell, the main flaw in commit/reveal protocols is in the
> commit phase: if revealing a commitment with N confirmations is required to
> spend bitcoins, then, without spending any bitcoins, how do you get the
> commitment into the blockchain in the first place? Maybe I am just
> misunderstanding this. If so, then a commit/reveal scheme may be a workable
> solution.
You can always purchase new BTC to perform the commitment.
> As far as I can tell, the main flaw in commit/reveal protocols is in the
> commit phase: if revealing a commitment with N confirmations is required to
> spend bitcoins, then, without spending any bitcoins, how do you get the
> commitment into the blockchain in the first place? Maybe I am just
> misunderstanding this. If so, then a commit/reveal scheme may be a workable
> solution.
Indeed, this problem is often seen in alt-coins where fees must be paid in a
native asset, while users are trying to send some kind of tokenized asset like
a USD token. You can have funds that you can't move because you don't have the
correct asset. While annoying, this isn't a fatal problem.
--
https://petertodd.org 'peter'[:-1]@petertodd.org
Q C
Jun 2, 2025, 12:02:01 PMJun 2
to Bitcoin Development Mailing List
>>>
You can always purchase new BTC to perform the commitment.
Jonathan's was probably referring to the public-key being exposed when performing the commitment (since this would defeat the purpose).
From first post from Tadge in this thread, the payment needs to be made out of band, so the public key isn't exposed during the commitment phase:
From first post from Tadge in this thread, the payment needs to be made out of band, so the public key isn't exposed during the commitment phase:
>>>
get that txid into an OP_RETURN output somehow (paying a miner out of band, etc)
waxwing/ AdamISZ
Jun 2, 2025, 2:41:39 PMJun 2
to Bitcoin Development Mailing List
Hi Boris, list,
> In my scheme, a user creates a QR output that
commits to a hash of a pubkey inside a Taproot leaf. This commitment
is hidden until revealed at spend time. Later, when the user wants to
spend a legacy EC output, they must spend this QR output in the same
transaction, and it must be at least X blocks old.
commits to a hash of a pubkey inside a Taproot leaf. This commitment
is hidden until revealed at spend time. Later, when the user wants to
spend a legacy EC output, they must spend this QR output in the same
transaction, and it must be at least X blocks old.
There's some nuances here that seem quite interesting. If an output commits (in a quantum-resistant way, i.e. hashing let's say) only to a pubkey, and not a pubkey plus spending-transaction, then, in the general case, that has the weakness that the commitment can be replicated, in another commitment tx, at the time of insertion of commitment into the blockchain; so that if the tiebreaker is "first commitment in the block(chain)" an attacker can mess with you. In your case you refer to "a QR output that commits to the hash of a pubkey inside a taproot leaf" but I'm finding it a tiny bit unclear what you mean there. Taproot itself isn't quantum resistant (QR), so Q = P + H(P,S)G is not QR even if P is NUMS. Whereas you might mean: that same system but with keypath spending invalidated, so it's QR because of the hash function, but also, the tapleaf contains a QR or PQC signing scheme in it (I guess this is what you actually mean). Or, you might mean something that is structurally the same as taproot but not using secp/BIP340, but instead a PQC scheme with a homomorphism so that that same design can be reused; call that "taproot2" .. though I don't think I've heard people talking about that.
Then there's what I think you focus on: the commitment is hidden. To other readers, in case of confusion: I believe the *real* point here is not "nobody knows you committed something" though that may be practically significant, it's instead "nobody knows the exact commitment value (hash) you used".
So, I believe that's a correct/valid point that actually *doesn't* depend on which "version" of taproot as per above. Focusing on current-taproot-but-script-path-only-with-QR-in-tapleaf: we have a protection that the quantum attacker cannot find the S in the (P,S) tuple (indeed, they cannot even know the H in P + H(P,S)G). This commitment is *not* literally perfectly hiding as in a properly formed Pedersen commitment ([1]) but I do think you have the normal preimage resistance against revelation as expected even in post-quantum. So that prevents the "copy the commitment" problem I started out by mentioning. [2]
If "taproot2" instead then we have something for which even the keypath isn't crackable so I guess it's obvious.
> Since the commitment doesn't include a txid, the user can precommit to
the pubkey hash far in advance, before knowing the details of the
eventual transaction.
the pubkey hash far in advance, before knowing the details of the
eventual transaction.
Again, I believe you're right here, but we should try to unpack what's different; because the "reveal" step of commit-reveal is accompanied by the QR signing event of the pre-existing QR output, we have a sane security model, so there's no need to commit to the transaction in the preliminary step, as far as I can tell.
> More efficient use of block space
Makes sense.
I think the only downside I see here is that the initial commitment step requires the PQC scheme to actually exist. That may not seem like a big deal, but I have a suspicion it actually will be. I think a protocol in which we just rely on existing hash primitives and put off the PQC scheme choosing event may be necessary .. though I could be for sure wrong, in more than one way, in saying that.
Apart from that point I think this scheme seems good (as you mention, it has the virtue of not requiring new databases etc which is pretty huge).
Cheers,
AdamISZ/waxwing
[1] In these analyses, I think it's common to overlook a
potentially crucial point: the utxo set is enumerable in practical
time, so we must always remember that we can check our calculations
against existing addresses, even if they are hash-covered keys.
[2] The only caveat is if you're considering the possibility of the attacker knowing the key in advance of even the commitment step; generally, that's "game over", but i know that there is some attempt to analyze that case in some places, too. Not here.
conduition
Jun 2, 2025, 4:03:37 PMJun 2
to waxwing/ AdamISZ, Bitcoin Development Mailing List
Hi list,
One problem I see with Tadge's commit/reveal approach is that verifying
nodes must have an ordering over ALL commitments, including those which
haven't yet been opened. We need every commitment to be visible on-chain
to know if a different commitment for the same pubkey was published
earlier.
This prevents commitment aggregation, and it means we would need one
OP_RETURN output to publish a commitment for every legacy UTXO rescued
by Tadge's protocol.
Let me demonstrate by trying to scale Tadge's proposal with aggregated
commitments. For instance, instead of directly publishing
C = (AID, SDP, CTXID) in an OP_RETURN, we might publish a merkle tree
root which is derived from a list of such commitments. Nodes store all
such merkle roots in a DB. When spending an EC UTXO, we reveal the
pubkey along with a merkle proof for the commitment. The node computes
the merkle root from the proof+pubkey+TXID, and then checks the
database to verify the commitment is sufficiently old.
This won't be secure though. The verifying nodes have no way of
checking if this specific commitment was the FIRST valid commitment
for the relevant pubkey/AID. There might be another commitment hidden
in the same merkle tree, or in an earlier one.
Maybe someone else has a better way to scale this commit/reveal protocol,
but unless that's possible, I think Martin's idea [0] has better potential
because we can rescue multiple EC outputs at once with a single
quantum-resistant UTXO.
As waxwing noted though, we need to better define the commitment protocol
there, including whether to use a post-quantum taproot-like construction,
or an inscription-like envelope with P2SH and a post-quantum checksig
opcode. I'd suggest we move further discussion on that to the relevant
thread [0].
[0]: https://groups.google.com/g/bitcoindev/c/jr1QO95k6Uc/m/qN-pAMS7AgAJ
I do recognize that Tadge's proposal has the advantage of not needing any
quantum-resistant signing algos, which won't be available on BTC for a
long time... but if we don't have a QR signing algo, then where are
you even trying to move your coins to? I agree with waxwing that we
should not pin down our PQ signing algo yet, but we should assume one will
eventually be available. Otherwise why should we bother at all.
regards,
conduition
> To view this discussion visit https://groups.google.com/d/msgid/bitcoindev/402db6ba-2497-4aab-9f84-0d66b4b8efccn%40googlegroups.com.
One problem I see with Tadge's commit/reveal approach is that verifying
nodes must have an ordering over ALL commitments, including those which
haven't yet been opened. We need every commitment to be visible on-chain
to know if a different commitment for the same pubkey was published
earlier.
This prevents commitment aggregation, and it means we would need one
OP_RETURN output to publish a commitment for every legacy UTXO rescued
by Tadge's protocol.
Let me demonstrate by trying to scale Tadge's proposal with aggregated
commitments. For instance, instead of directly publishing
C = (AID, SDP, CTXID) in an OP_RETURN, we might publish a merkle tree
root which is derived from a list of such commitments. Nodes store all
such merkle roots in a DB. When spending an EC UTXO, we reveal the
pubkey along with a merkle proof for the commitment. The node computes
the merkle root from the proof+pubkey+TXID, and then checks the
database to verify the commitment is sufficiently old.
This won't be secure though. The verifying nodes have no way of
checking if this specific commitment was the FIRST valid commitment
for the relevant pubkey/AID. There might be another commitment hidden
in the same merkle tree, or in an earlier one.
Maybe someone else has a better way to scale this commit/reveal protocol,
but unless that's possible, I think Martin's idea [0] has better potential
because we can rescue multiple EC outputs at once with a single
quantum-resistant UTXO.
As waxwing noted though, we need to better define the commitment protocol
there, including whether to use a post-quantum taproot-like construction,
or an inscription-like envelope with P2SH and a post-quantum checksig
opcode. I'd suggest we move further discussion on that to the relevant
thread [0].
[0]: https://groups.google.com/g/bitcoindev/c/jr1QO95k6Uc/m/qN-pAMS7AgAJ
I do recognize that Tadge's proposal has the advantage of not needing any
quantum-resistant signing algos, which won't be available on BTC for a
long time... but if we don't have a QR signing algo, then where are
you even trying to move your coins to? I agree with waxwing that we
should not pin down our PQ signing algo yet, but we should assume one will
eventually be available. Otherwise why should we bother at all.
regards,
conduition
Nagaev Boris
Jun 2, 2025, 7:03:27 PMJun 2
to waxwing/ AdamISZ, Bitcoin Development Mailing List
Hi Adam, hi list,
Thank you for clarifying the nuances of the scheme!
> that same system but with keypath spending invalidated, so it's QR because of the hash function, but also, the tapleaf contains a QR or PQC signing scheme in it
I assume that some future QR signing algorithm might be added in a new
segwit version, resembling the current Taproot structure but with
keypath spending disabled (or otherwise addressed, so it doesn't
present a way to steal funds in a post-quantum world), and a new
opcode added to tapscript to enable QR signing. Also, such an address
type would need to handle the EC opcodes if they remain enabled in
tapscript. I didn't go deep into the details of this hypothetical
address type, as it's a separate complex topic. For the purpose of
this discussion, I assume that some version of it will be deployed
before or alongside the proposed scheme.
> I believe the *real* point here is not "nobody knows you committed something" though that may be practically significant, it's instead "nobody knows the exact commitment value (hash) you used".
I think this is a good observation! We can clarify it further: *nobody
should be able to replicate your commitment*.
I've identified a potential attack that, if it were possible, would
undermine this property. Let's assume, for a second, that a new
address type is just a Merkle root of a Merkle tree. Then an attacker
could build their own Merkle tree on top of the original Merkle root.
The attacker's tree would include the legitimate Merkle tree as a
subtree. If they learn the EC pubkey and the full path in the original
Merkle tree from the pubkey to the root, they could reconstruct the
full path in their tree embedding the original. They could learn this
information when the output is spent, so they would have everything
needed to produce a malicious proof.
Fortunately, with the Taproot we have today, such an extension isn't
possible. There's a step (among others) involving a tagged hash
function (h_tapTweak) that hashes the internal pubkey and Merkle root
together before producing the final public key. This prevents
constructing a Taproot output embedding another tree as a subtree
based solely on the output address. We should ensure that any future
taproot-like address type retains this property to prevent the attack
described above.
The scheme must be carefully analyzed to ensure it satisfies this
crucial property: nobody should be able to replicate your commitment.
I'm curious if anyone sees further edge cases we should consider.
> In these analyses, I think it's common to overlook a potentially crucial point: the UTXO set is enumerable in practical time, so we must always remember that we can check our calculations against existing addresses, even if they are hash-covered keys.
I would propose hardening the requirement further: assume the attacker
already knows exactly which EC address is protected with which QR
output from the beginning. Even with this knowledge, the attacker
should not be able to succeed. If we prove the scheme secure under
these strong assumptions (which are very favorable for the attacker),
then it is also secure under weaker assumptions where the attacker has
less information.
> This commitment is *not* literally perfectly hiding as in a properly formed Pedersen commitment
I agree that the hiding property of a properly formed Pedersen
commitment is stronger than in the proposed scheme. A Pedersen
commitment is perfectly hiding: the committed value is statistically
independent of the commitment, and even an attacker with unbounded
computational power cannot learn anything about the value. In
contrast, the proposed scheme relies on a hash of the pubkey for
hiding, which is only computationally hiding: it depends on the
preimage resistance of the hash function and can be weakened if
quantum attacks on hash functions improve.
However, I believe we cannot use Pedersen commitments here, because
their binding property would be broken by a quantum computer. Pedersen
commitments rely on the hardness of the discrete logarithm problem
(DLP) in an elliptic curve group for binding, and Shor's algorithm can
efficiently solve DLP on a quantum computer. This would allow an
attacker, once the legitimate opening (v,r) is revealed, to compute an
alternative opening (v',r') for the same commitment C. As a result, if
Pedersen commitments were used in the scheme, they would be vulnerable
during the reveal phase under a quantum attack.
Best,
Boris
> To view this discussion visit https://groups.google.com/d/msgid/bitcoindev/402db6ba-2497-4aab-9f84-0d66b4b8efccn%40googlegroups.com.
Thank you for clarifying the nuances of the scheme!
> that same system but with keypath spending invalidated, so it's QR because of the hash function, but also, the tapleaf contains a QR or PQC signing scheme in it
segwit version, resembling the current Taproot structure but with
keypath spending disabled (or otherwise addressed, so it doesn't
present a way to steal funds in a post-quantum world), and a new
opcode added to tapscript to enable QR signing. Also, such an address
type would need to handle the EC opcodes if they remain enabled in
tapscript. I didn't go deep into the details of this hypothetical
address type, as it's a separate complex topic. For the purpose of
this discussion, I assume that some version of it will be deployed
before or alongside the proposed scheme.
> I believe the *real* point here is not "nobody knows you committed something" though that may be practically significant, it's instead "nobody knows the exact commitment value (hash) you used".
should be able to replicate your commitment*.
I've identified a potential attack that, if it were possible, would
undermine this property. Let's assume, for a second, that a new
address type is just a Merkle root of a Merkle tree. Then an attacker
could build their own Merkle tree on top of the original Merkle root.
The attacker's tree would include the legitimate Merkle tree as a
subtree. If they learn the EC pubkey and the full path in the original
Merkle tree from the pubkey to the root, they could reconstruct the
full path in their tree embedding the original. They could learn this
information when the output is spent, so they would have everything
needed to produce a malicious proof.
Fortunately, with the Taproot we have today, such an extension isn't
possible. There's a step (among others) involving a tagged hash
function (h_tapTweak) that hashes the internal pubkey and Merkle root
together before producing the final public key. This prevents
constructing a Taproot output embedding another tree as a subtree
based solely on the output address. We should ensure that any future
taproot-like address type retains this property to prevent the attack
described above.
The scheme must be carefully analyzed to ensure it satisfies this
crucial property: nobody should be able to replicate your commitment.
I'm curious if anyone sees further edge cases we should consider.
> In these analyses, I think it's common to overlook a potentially crucial point: the UTXO set is enumerable in practical time, so we must always remember that we can check our calculations against existing addresses, even if they are hash-covered keys.
I would propose hardening the requirement further: assume the attacker
already knows exactly which EC address is protected with which QR
output from the beginning. Even with this knowledge, the attacker
should not be able to succeed. If we prove the scheme secure under
these strong assumptions (which are very favorable for the attacker),
then it is also secure under weaker assumptions where the attacker has
less information.
> This commitment is *not* literally perfectly hiding as in a properly formed Pedersen commitment
commitment is stronger than in the proposed scheme. A Pedersen
commitment is perfectly hiding: the committed value is statistically
independent of the commitment, and even an attacker with unbounded
computational power cannot learn anything about the value. In
contrast, the proposed scheme relies on a hash of the pubkey for
hiding, which is only computationally hiding: it depends on the
preimage resistance of the hash function and can be weakened if
quantum attacks on hash functions improve.
However, I believe we cannot use Pedersen commitments here, because
their binding property would be broken by a quantum computer. Pedersen
commitments rely on the hardness of the discrete logarithm problem
(DLP) in an elliptic curve group for binding, and Shor's algorithm can
efficiently solve DLP on a quantum computer. This would allow an
attacker, once the legitimate opening (v,r) is revealed, to compute an
alternative opening (v',r') for the same commitment C. As a result, if
Pedersen commitments were used in the scheme, they would be vulnerable
during the reveal phase under a quantum attack.
Best,
Boris
Jonathan Voss
Jun 4, 2025, 3:56:53 AMJun 4
to Bitcoin Development Mailing List
If using a monetary network requires out-of-band payments, then that severely limits the actual utility of the monetary network as a medium of exchange. Imagine if the only way to make a bank transfer was to first go in-person to the bank of the recipient of the transfer to give them something that then allowed your bank to make the transfer -- it would be an unworkable monetary system. Similarly, if future Bitcoin transactions require making out-of-band payments, then it has failed as a monetary network with an endogenous unit of account. The whole system has to work without reliance upon exogenous monetary media or mechanisms. As such, the commit-and-reveal scheme fails to maintain the monetary properties of the network as a whole unless we assert reliance upon altruism to get the commitments into the blockchain, which instead breaks the incentive-based game theoretic design. Maybe it would work as a stop-gap solution in the event of the advent of a relevant quantum computer, but it is certainly not a good long-term plan as currently formulated.
Recall the original premise: "Bitcoin: A Peer-to-Peer Electronic Cash System". If you can't transact with it as cash, i.e. as the ultimate endogenous settlement mechanism, then it is no longer Bitcoin. Requiring an exogenous system fundamentally breaks the model.
-- Jonathan
conduition
Jun 5, 2025, 10:36:07 AMJun 5
to Jonathan Voss, Bitcoin Development Mailing List
Hi Jonathan,
I feel you're making a bigger deal of this than it needs to be.
We're talking about a way for laggards to rescue money locked in
quantum-vulnerable hashed addresses. I don't think anyone wants
a commit/reveal protocol to become the sole primary method of
transferring BTC.
We're more or less designing a system for a wallet which wakes up
after a years-long coma to find quantum computers are a threat.
This wallet can make a single "commitment transaction" followed
by a single "reveal transaction", and this dance moves all of
the funds in the wallet to a new (yet to be defined)
quantum-resistant script. After that, sending Bitcoin should work
mostly as normal.
Even if you're still hard-line opposed to the idea of
requiring a post-quantum UTXO to recover pre-quantum UTXOs,
it's still completely possible to use commit/reveal protocols
without out-of-band purchase of new BTC: You can simply spend
a very small pre-quantum UTXO to create the commitment. A UTXO
worth only a few thousand sats, for instance. This would be
uneconomical for the quantum attacker to double spend, and
so you'd most likely get your commitment mined safely without
buying new BTC. The only situation in which this is not possible
would be for a party with only very high-value pre-quantum UTXOs,
and no small UTXOs. These parties would need to buy a new (small)
UTXO out-of-band.
regards,
conduition
I feel you're making a bigger deal of this than it needs to be.
We're talking about a way for laggards to rescue money locked in
quantum-vulnerable hashed addresses. I don't think anyone wants
a commit/reveal protocol to become the sole primary method of
transferring BTC.
We're more or less designing a system for a wallet which wakes up
after a years-long coma to find quantum computers are a threat.
This wallet can make a single "commitment transaction" followed
by a single "reveal transaction", and this dance moves all of
the funds in the wallet to a new (yet to be defined)
quantum-resistant script. After that, sending Bitcoin should work
mostly as normal.
Even if you're still hard-line opposed to the idea of
requiring a post-quantum UTXO to recover pre-quantum UTXOs,
it's still completely possible to use commit/reveal protocols
without out-of-band purchase of new BTC: You can simply spend
a very small pre-quantum UTXO to create the commitment. A UTXO
worth only a few thousand sats, for instance. This would be
uneconomical for the quantum attacker to double spend, and
so you'd most likely get your commitment mined safely without
buying new BTC. The only situation in which this is not possible
would be for a party with only very high-value pre-quantum UTXOs,
and no small UTXOs. These parties would need to buy a new (small)
UTXO out-of-band.
regards,
conduition
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