Post Quantum Signatures and Scaling Bitcoin

Post Quantum Signatures and Scaling Bitcoin
2025-04-04 17:17 ` Dustin Ray
                   ` (3 more replies)
  
From: Ethan Heilman @ 2025-04-04 16:29 UTC (permalink / raw)
  To: Bitcoin Development Mailing List

I strongly believe Bitcoin will need to move to PQ signatures in the
near future. The rest of this email is premised on this belief.

PQ (Post Quantum) signatures present a problem for Bitcoin:

First, they are large. Of the three proposed in BIP-360 [0], the
smallest is 1.5kb for the public key + signature [1]. Without a
discount this represents a massive reduction in Bitcoin's transaction
volume due to the increase in transaction size of Bitcoin payment
using such signatures.
- Second, even if we discount PQ signatures and public keys so that
the maximum number of transactions that can fit in a block is
unchanged we still have the problem that these blocks and transactions
will be an order of magnitude bigger. If it is the case that we can
handle these extra bytes without degrading performance or
decentralization, then consider the head room we are giving up that
could be used for scalability.

Beyond this there is also the risk that techniques could be developed
to encode JPEGs and other data in these discounted PQ signatures or
public keys. BIP-360 takes steps to make an abuse of this discount
more difficult by requiring that a PQ signature and public key can
only be written to the blockchain if they verify. We do not need PQ
Signatures to be completely “JPEG resistant”, they just need PQ
signatures to not enable significantly cheaper storage than payments.
The degree to which the proposed PQ signature algorithms resist being
repurposed as a storage mechanism is an open question and worth
investigating.

If it turned out PQ signatures could be used to encode data very
cheaply, then Bitcoin faces the dilemma that if you discount PQ
signatures, you make the JPEG problem worse and may price out the
payment use case. If you don't discount PQ, you price out most people
from sending payments in Bitcoin since non-PQ witness data can be used
for storage

I want to draw the community's attention to a solution that could not
only address these problems but also increase Bitcoin’s scalability
(and privacy):

Non-interactive Transaction Compression (NTC) for Transactions
supporting PQ signatures. This is sometimes called Non-Interactive
Witness Aggregation (NIWA) [2].

This would require a new transaction type supporting PQ signatures.
The miner of a block would then pull out the signatures and hash
pointers from transactions to compress transaction data and
non-interactively aggregate all the PQ signatures in all the
transactions in a block, replacing them with one big STARK (STARKS are
a form of SNARK which is PQ). This would make PQ signatures
significantly smaller and cheaper than ECDSA and schnorr signatures.

Consider the following back of the envelope math:

2 bytes per Input = 2 bytes per TXID, 0 bytes per signature
37 bytes per output = 32 bytes pubkey hash + 5 bytes value (max 2.8m
BTC per output)

1-input-2-output transaction would be: 2 + 2*37 = 76 bytes
(4,000,000/76)/(60*10) = ~87 txns/sec

You could shave some bytes off the value, or add some bytes to the
TXID. [3] provides a more detailed estimate, proposing 113.5 weight
units (WU) for a 1-input-2-output transaction with no address reuse.
However it does not consider TXID compression. If desired an
account-based model could push this even further to 12 bytes per
transaction per block [4]. This would enable approximately
4,000,000/(12*60*10) = 555 txns/second.

A secondary benefit of having on-chain PQ payments only be ~76 bytes
in size is that it fundamentally changes the pricing relationship
between payments and on-chain JPEG/complex contracts. The problem with
on-chain JPEGs is not that they are possible, but that they are price
competitive with payments. At ~76 bytes per payment or better yet ~76
bytes per LN channel open/close, JPEGs no longer present the same fee
competition to payments as payments become much cheaper.

Such a system would present scaling issues for the mempool because
prior to aggregation and compression, these transactions would be 2kb
to 100kb in size and there would be a lot more of them. It is likely
parties producing large numbers of transactions would want to
pre-aggregate and compress them in one big many input, many output
transactions. Aggregating prior to the miner may have privacy benefits
but also scalability benefits as it would enable cut-throughs and very
cheap consolidation transactions. ~87/txns a second does not include
these additional scalability benefits.

Consider an exchange that receives and sends a large number of
transactions. For instance between block confirmations customers send
the exchange 10 1-input-2-output transactions in deposits and the
exchange sends out 10 1-input-2-output transactions in withdrawals.
The exchange could consolidate all of the outputs paying the exchange,
including chain outputs, into one output and do the same for inputs.
This would reduce not just size, but also validation costs.

(10 * 2 + 20 * 2 * 37) + (10 * 2 + 20 * 2 * 37)  = 3000 bytes
becomes
(10 * 2 + 11 * 2 * 37) + (2 + 11 * 2 * 37) = 1650 bytes

If constructing these proofs turned out to be as expensive as
performing POW, it would make block generation not progress free.
Essentially you'd have a two step POW: proof generation and then the
actual POW. Such a scenario would be very bad and cause the biggest
miner to always be the one that generates blocks. A critical
assumption I am making is that such proof generation is not
particularly expensive in the scheme of POW. I am optimistic that
proof generation will not be this expensive for two reasons

There are PQ signature schemes which support non-interactive
aggregation such as LaBRADOR [5]. Thus, the STARK wouldn’t need to
perform the block-wide signature aggregation and would only need to
perform transaction compression, cut throughs and consolidation.

We could make use of recursive STARKs [8] to allow miners to
parallelize proof generation to reduce latency or to decentralize
proof generation. Users creating transactions could perform
non-interactive coinjoins with other users or settlement/batching.
This would not only take proof generation pressure off of the miners
and reduce the strain on the mempool but in some circumstances it
would provide privacy if used with payjoin techniques like receiver
side payment batching [7].

The approach we are proposing treats the STARK the miner produces as
free from a blocksize perspective. This is important for bootstrapping
because it means that fees are significantly cheaper for a
transaction, even if it is the only compressed transaction in the
block. This encourages adoption. Adoption helps address the chicken
and egg problem of wallets and exchanges not investing engineering
resources to support a new transaction type if no one is using it and
no one wants to use it because it isn't well supported. By having a
single format, built into the block we both accelerate the switch over
and prevent a fragmented ecosystem that might arise from doing this in
Bitcoin script. Fragmentation reduces the scalability benefits because
validators have to validate multiple STARKs and reduces the privacy
benefits because there are many coinjoins, rather than each being a
coinjoin.

Even if our approach here turns out to be infeasible, we need a way to
reduce the size of PQ signatures in Bitcoin. The ability to move
coins, including the ability to move coins that represent JPEGs, is
the main functionality of Bitcoin. If we make storage/JPEG too price
competitive with the ability to transfer coins, we destroy that
essential functionality and decrease the utility of Bitcoin for
everyone. Currently moving coins securely requires at least one 64
byte signature, which is an unfortunate tax on this most vital of all
use cases. I believe removing that tax with signature aggregation will
be beneficial for all parties.

Consider the world of PQ signatures in Bitcoin without STARKs:
 - The large size of PQ signatures will make it more expensive for
users to use them prior to the invention of a CRQC (Cryptographically
Relevant Quantum Computer). This means that most outputs will not be
protected by PQ signatures. Once a CRQC arises there will be a rush to
move funds under the protection of PQ signatures but due to the large
size of PQ signatures the fees will be too expensive for most outputs.
Users will instead need to move their funds to centralized custodial
wallets that can use a small number of outputs. In such a world it
will be much harder and expensive to self-custody.
- Without a solution here the large sizes of PQ signatures will limit
Bitcoin's functionality to move coins using on-chain payments. This
will also favor centralized custodians and erode the decentralized
nature of Bitcoin.

None of this is an argument against adopting BIP-360 or other PQ
signatures schemes into Bitcoin. On the contrary, having PQ signatures
in Bitcoin would be a useful stepping stone to PQ transaction
compression since it would allow us to gain agreement on which PQ
signature schemes to build on. Most importantly, in the event of a
CRQC being developed it will be far better to have uncompressed PQ
signatures in Bitcoin than none at all.

Acknowledgements:
These ideas arose out of correspondence with Hunter Beast. I want to
thank Neha Narula, John Light, Eli Ben-Sasson for their feedback,
Jonas Nick for his feedback and his idea to use LaBRADOR for signature
aggregation, Tadge Dryja for suggesting the term “JPEG resistance” and
his ideas around its feasibility. I had a number of fruitful
discussions over lunch with members of the MIT DCI and on the Bitcoin
PQ working group. These acknowledgements should not be taken as an
agreement with or endorsement of the ideas in this email.

[0]: Hunter Beast, BIP-360: QuBit - Pay to Quantum Resistant Hash
(2025) https://github.com/bitcoin/bips/pull/1670/files#
[1]: Benchmark Report: Post-Quantum Cryptography vs secp256k1
https://github.com/cryptoquick/libbitcoinpqc/blob/main/benches/REPORT.md
[2]: Ruben Somsen, SNARKs and the future of blockchains (2020)
https://medium.com/@RubenSomsen/snarks-and-the-future-of-blockchains-55b82012452b
[3]: John Light, Validity Rollups on Bitcoin (2022)
https://github.com/john-light/validity-rollups/blob/main/validity_rollups_on_bitcoin.md
[4] Vitalik Buterin, An Incomplete Guide to Rollups (2021)
https://vitalik.eth.limo/general/2021/01/05/rollup.html
[5]: Aardal, Aranha, Boudgoust, Kolby, Takahashi, Aggregating Falcon
Signatures with LaBRADOR (2024) https://eprint.iacr.org/2024/311
[6]: Gidi Kaempfer, Recursive STARKs (2022)
https://www.starknet.io/blog/recursive-starks/
[7]: Dan Gould, Interactive Payment Batching is Better (2023)
https://payjoin.substack.com/p/interactive-payment-batching-is-better
[8] John Tromp, Fee burning and Dynamic Block Size (2018)
https://lists.launchpad.net/mimblewimble/msg00450.html

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 permalink raw reply	[|nested]
2025-04-04 17:17 ` 
                   ` 
  
From: Ethan Heilman @ 2025-04-04 16:29 UTC (permalink / raw)
  To: Bitcoin Development Mailing List

I strongly believe Bitcoin will need to move to PQ signatures in the
near future. The rest of this email is premised on this belief.

PQ (Post Quantum) signatures present a problem for Bitcoin:

First, they are large. Of the three proposed in BIP-360 [0], the
smallest is 1.5kb for the public key + signature [1]. Without a
discount this represents a massive reduction in Bitcoin's transaction
volume due to the increase in transaction size of Bitcoin payment
using such signatures.
- Second, even if we discount PQ signatures and public keys so that
the maximum number of transactions that can fit in a block is
unchanged we still have the problem that these blocks and transactions
will be an order of magnitude bigger. If it is the case that we can
handle these extra bytes without degrading performance or
decentralization, then consider the head room we are giving up that
could be used for scalability.

Beyond this there is also the risk that techniques could be developed
to encode JPEGs and other data in these discounted PQ signatures or
public keys. BIP-360 takes steps to make an abuse of this discount
more difficult by requiring that a PQ signature and public key can
only be written to the blockchain if they verify. We do not need PQ
Signatures to be completely “JPEG resistant”, they just need PQ
signatures to not enable significantly cheaper storage than payments.
The degree to which the proposed PQ signature algorithms resist being
repurposed as a storage mechanism is an open question and worth
investigating.

If it turned out PQ signatures could be used to encode data very
cheaply, then Bitcoin faces the dilemma that if you discount PQ
signatures, you make the JPEG problem worse and may price out the
payment use case. If you don't discount PQ, you price out most people
from sending payments in Bitcoin since non-PQ witness data can be used
for storage

I want to draw the community's attention to a solution that could not
only address these problems but also increase Bitcoin’s scalability
(and privacy):

Non-interactive Transaction Compression (NTC) for Transactions
supporting PQ signatures. This is sometimes called Non-Interactive
Witness Aggregation (NIWA) [2].

This would require a new transaction type supporting PQ signatures.
The miner of a block would then pull out the signatures and hash
pointers from transactions to compress transaction data and
non-interactively aggregate all the PQ signatures in all the
transactions in a block, replacing them with one big STARK (STARKS are
a form of SNARK which is PQ). This would make PQ signatures
significantly smaller and cheaper than ECDSA and schnorr signatures.

Consider the following back of the envelope math:

2 bytes per Input = 2 bytes per TXID, 0 bytes per signature
37 bytes per output = 32 bytes pubkey hash + 5 bytes value (max 2.8m
BTC per output)

1-input-2-output transaction would be: 2 + 2*37 = 76 bytes
(4,000,000/76)/(60*10) = ~87 txns/sec

You could shave some bytes off the value, or add some bytes to the
TXID. [3] provides a more detailed estimate, proposing 113.5 weight
units (WU) for a 1-input-2-output transaction with no address reuse.
However it does not consider TXID compression. If desired an
account-based model could push this even further to 12 bytes per
transaction per block [4]. This would enable approximately
4,000,000/(12*60*10) = 555 txns/second.

A secondary benefit of having on-chain PQ payments only be ~76 bytes
in size is that it fundamentally changes the pricing relationship
between payments and on-chain JPEG/complex contracts. The problem with
on-chain JPEGs is not that they are possible, but that they are price
competitive with payments. At ~76 bytes per payment or better yet ~76
bytes per LN channel open/close, JPEGs no longer present the same fee
competition to payments as payments become much cheaper.

Such a system would present scaling issues for the mempool because
prior to aggregation and compression, these transactions would be 2kb
to 100kb in size and there would be a lot more of them. It is likely
parties producing large numbers of transactions would want to
pre-aggregate and compress them in one big many input, many output
transactions. Aggregating prior to the miner may have privacy benefits
but also scalability benefits as it would enable cut-throughs and very
cheap consolidation transactions. ~87/txns a second does not include
these additional scalability benefits.

Consider an exchange that receives and sends a large number of
transactions. For instance between block confirmations customers send
the exchange 10 1-input-2-output transactions in deposits and the
exchange sends out 10 1-input-2-output transactions in withdrawals.
The exchange could consolidate all of the outputs paying the exchange,
including chain outputs, into one output and do the same for inputs.
This would reduce not just size, but also validation costs.

(10 * 2 + 20 * 2 * 37) + (10 * 2 + 20 * 2 * 37)  = 3000 bytes
becomes
(10 * 2 + 11 * 2 * 37) + (2 + 11 * 2 * 37) = 1650 bytes

If constructing these proofs turned out to be as expensive as
performing POW, it would make block generation not progress free.
Essentially you'd have a two step POW: proof generation and then the
actual POW. Such a scenario would be very bad and cause the biggest
miner to always be the one that generates blocks. A critical
assumption I am making is that such proof generation is not
particularly expensive in the scheme of POW. I am optimistic that
proof generation will not be this expensive for two reasons

There are PQ signature schemes which support non-interactive
aggregation such as LaBRADOR [5]. Thus, the STARK wouldn’t need to
perform the block-wide signature aggregation and would only need to
perform transaction compression, cut throughs and consolidation.

We could make use of recursive STARKs [8] to allow miners to
parallelize proof generation to reduce latency or to decentralize
proof generation. Users creating transactions could perform
non-interactive coinjoins with other users or settlement/batching.
This would not only take proof generation pressure off of the miners
and reduce the strain on the mempool but in some circumstances it
would provide privacy if used with payjoin techniques like receiver
side payment batching [7].

The approach we are proposing treats the STARK the miner produces as
free from a blocksize perspective. This is important for bootstrapping
because it means that fees are significantly cheaper for a
transaction, even if it is the only compressed transaction in the
block. This encourages adoption. Adoption helps address the chicken
and egg problem of wallets and exchanges not investing engineering
resources to support a new transaction type if no one is using it and
no one wants to use it because it isn't well supported. By having a
single format, built into the block we both accelerate the switch over
and prevent a fragmented ecosystem that might arise from doing this in
Bitcoin script. Fragmentation reduces the scalability benefits because
validators have to validate multiple STARKs and reduces the privacy
benefits because there are many coinjoins, rather than each being a
coinjoin.

Even if our approach here turns out to be infeasible, we need a way to
reduce the size of PQ signatures in Bitcoin. The ability to move
coins, including the ability to move coins that represent JPEGs, is
the main functionality of Bitcoin. If we make storage/JPEG too price
competitive with the ability to transfer coins, we destroy that
essential functionality and decrease the utility of Bitcoin for
everyone. Currently moving coins securely requires at least one 64
byte signature, which is an unfortunate tax on this most vital of all
use cases. I believe removing that tax with signature aggregation will
be beneficial for all parties.

Consider the world of PQ signatures in Bitcoin without STARKs:
 - The large size of PQ signatures will make it more expensive for
users to use them prior to the invention of a CRQC (Cryptographically
Relevant Quantum Computer). This means that most outputs will not be
protected by PQ signatures. Once a CRQC arises there will be a rush to
move funds under the protection of PQ signatures but due to the large
size of PQ signatures the fees will be too expensive for most outputs.
Users will instead need to move their funds to centralized custodial
wallets that can use a small number of outputs. In such a world it
will be much harder and expensive to self-custody.
- Without a solution here the large sizes of PQ signatures will limit
Bitcoin's functionality to move coins using on-chain payments. This
will also favor centralized custodians and erode the decentralized
nature of Bitcoin.

None of this is an argument against adopting BIP-360 or other PQ
signatures schemes into Bitcoin. On the contrary, having PQ signatures
in Bitcoin would be a useful stepping stone to PQ transaction
compression since it would allow us to gain agreement on which PQ
signature schemes to build on. Most importantly, in the event of a
CRQC being developed it will be far better to have uncompressed PQ
signatures in Bitcoin than none at all.

Acknowledgements:
These ideas arose out of correspondence with Hunter Beast. I want to
thank Neha Narula, John Light, Eli Ben-Sasson for their feedback,
Jonas Nick for his feedback and his idea to use LaBRADOR for signature
aggregation, Tadge Dryja for suggesting the term “JPEG resistance” and
his ideas around its feasibility. I had a number of fruitful
discussions over lunch with members of the MIT DCI and on the Bitcoin
PQ working group. These acknowledgements should not be taken as an
agreement with or endorsement of the ideas in this email.

[0]: Hunter Beast, BIP-360: QuBit - Pay to Quantum Resistant Hash
(2025) https://github.com/bitcoin/bips/pull/1670/files#
[1]: Benchmark Report: Post-Quantum Cryptography vs secp256k1
https://github.com/cryptoquick/libbitcoinpqc/blob/main/benches/REPORT.md
[2]: Ruben Somsen, SNARKs and the future of blockchains (2020)
https://medium.com/@RubenSomsen/snarks-and-the-future-of-blockchains-55b82012452b
[3]: John Light, Validity Rollups on Bitcoin (2022)
https://github.com/john-light/validity-rollups/blob/main/validity_rollups_on_bitcoin.md
[4] Vitalik Buterin, An Incomplete Guide to Rollups (2021)
https://vitalik.eth.limo/general/2021/01/05/rollup.html
[5]: Aardal, Aranha, Boudgoust, Kolby, Takahashi, Aggregating Falcon
Signatures with LaBRADOR (2024) https://eprint.iacr.org/2024/311
[6]: Gidi Kaempfer, Recursive STARKs (2022)
https://www.starknet.io/blog/recursive-starks/
[7]: Dan Gould, Interactive Payment Batching is Better (2023)
https://payjoin.substack.com/p/interactive-payment-batching-is-better
[8] John Tromp, Fee burning and Dynamic Block Size (2018)
https://lists.launchpad.net/mimblewimble/msg00450.html

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 permalink raw reply	[|nested]
* 
  2025-04-04 16:29 [bitcoindev] Post Quantum Signatures and Scaling Bitcoin Ethan Heilman

  2025-04-05 20:40   ` 'Eli Ben-Sasson' via Bitcoin Development Mailing List
  2025-04-04 18:43 ` 
                   ` 
  3 siblings, 1 reply; 8+ messages in thread
From: Dustin Ray @ 2025-04-04 17:17 UTC (permalink / raw)
  To: Ethan Heilman;  Bitcoin Development Mailing List

[-- Attachment #1: Type: text/plain, Size: 13139 bytes --]

This is a great post, thank you for sharing. I have one small tiny comment
that may or may not be relevant, but there is an existing gap in the
literature for a security proof that STARKs (rather FRI, the underlying
commitment scheme) is secure in a quantum adversary model. We conjecture
that it is, because it relies only on hashes as the primitive in an error
correcting code, but unlike other cryptographic primitives used or proposed
for critical security infrastructure, there is currently no formal security
argument for FRI against a quantum threat model that I am aware of. I'm not
sure how much this matters, but some may argue that stronger security
arguments are warranted for any potential change to the bitcoin signature
model in a PQ landscape. That's just my two cents anyways.

On Fri, Apr 4, 2025 at 9:34 AM Ethan Heilman <eth3rs@gmail•com> wrote:

> I strongly believe Bitcoin will need to move to PQ signatures in the
> near future. The rest of this email is premised on this belief.
>
> PQ (Post Quantum) signatures present a problem for Bitcoin:
>
> First, they are large. Of the three proposed in BIP-360 [0], the
> smallest is 1.5kb for the public key + signature [1]. Without a
> discount this represents a massive reduction in Bitcoin's transaction
> volume due to the increase in transaction size of Bitcoin payment
> using such signatures.
> - Second, even if we discount PQ signatures and public keys so that
> the maximum number of transactions that can fit in a block is
> unchanged we still have the problem that these blocks and transactions
> will be an order of magnitude bigger. If it is the case that we can
> handle these extra bytes without degrading performance or
> decentralization, then consider the head room we are giving up that
> could be used for scalability.
>
> Beyond this there is also the risk that techniques could be developed
> to encode JPEGs and other data in these discounted PQ signatures or
> public keys. BIP-360 takes steps to make an abuse of this discount
> more difficult by requiring that a PQ signature and public key can
> only be written to the blockchain if they verify. We do not need PQ
> Signatures to be completely “JPEG resistant”, they just need PQ
> signatures to not enable significantly cheaper storage than payments.
> The degree to which the proposed PQ signature algorithms resist being
> repurposed as a storage mechanism is an open question and worth
> investigating.
>
> If it turned out PQ signatures could be used to encode data very
> cheaply, then Bitcoin faces the dilemma that if you discount PQ
> signatures, you make the JPEG problem worse and may price out the
> payment use case. If you don't discount PQ, you price out most people
> from sending payments in Bitcoin since non-PQ witness data can be used
> for storage
>
> I want to draw the community's attention to a solution that could not
> only address these problems but also increase Bitcoin’s scalability
> (and privacy):
>
> Non-interactive Transaction Compression (NTC) for Transactions
> supporting PQ signatures. This is sometimes called Non-Interactive
> Witness Aggregation (NIWA) [2].
>
> This would require a new transaction type supporting PQ signatures.
> The miner of a block would then pull out the signatures and hash
> pointers from transactions to compress transaction data and
> non-interactively aggregate all the PQ signatures in all the
> transactions in a block, replacing them with one big STARK (STARKS are
> a form of SNARK which is PQ). This would make PQ signatures
> significantly smaller and cheaper than ECDSA and schnorr signatures.
>
> Consider the following back of the envelope math:
>
> 2 bytes per Input = 2 bytes per TXID, 0 bytes per signature
> 37 bytes per output = 32 bytes pubkey hash + 5 bytes value (max 2.8m
> BTC per output)
>
> 1-input-2-output transaction would be: 2 + 2*37 = 76 bytes
> (4,000,000/76)/(60*10) = ~87 txns/sec
>
> You could shave some bytes off the value, or add some bytes to the
> TXID. [3] provides a more detailed estimate, proposing 113.5 weight
> units (WU) for a 1-input-2-output transaction with no address reuse.
> However it does not consider TXID compression. If desired an
> account-based model could push this even further to 12 bytes per
> transaction per block [4]. This would enable approximately
> 4,000,000/(12*60*10) = 555 txns/second.
>
> A secondary benefit of having on-chain PQ payments only be ~76 bytes
> in size is that it fundamentally changes the pricing relationship
> between payments and on-chain JPEG/complex contracts. The problem with
> on-chain JPEGs is not that they are possible, but that they are price
> competitive with payments. At ~76 bytes per payment or better yet ~76
> bytes per LN channel open/close, JPEGs no longer present the same fee
> competition to payments as payments become much cheaper.
>
> Such a system would present scaling issues for the mempool because
> prior to aggregation and compression, these transactions would be 2kb
> to 100kb in size and there would be a lot more of them. It is likely
> parties producing large numbers of transactions would want to
> pre-aggregate and compress them in one big many input, many output
> transactions. Aggregating prior to the miner may have privacy benefits
> but also scalability benefits as it would enable cut-throughs and very
> cheap consolidation transactions. ~87/txns a second does not include
> these additional scalability benefits.
>
> Consider an exchange that receives and sends a large number of
> transactions. For instance between block confirmations customers send
> the exchange 10 1-input-2-output transactions in deposits and the
> exchange sends out 10 1-input-2-output transactions in withdrawals.
> The exchange could consolidate all of the outputs paying the exchange,
> including chain outputs, into one output and do the same for inputs.
> This would reduce not just size, but also validation costs.
>
> (10 * 2 + 20 * 2 * 37) + (10 * 2 + 20 * 2 * 37)  = 3000 bytes
> becomes
> (10 * 2 + 11 * 2 * 37) + (2 + 11 * 2 * 37) = 1650 bytes
>
> If constructing these proofs turned out to be as expensive as
> performing POW, it would make block generation not progress free.
> Essentially you'd have a two step POW: proof generation and then the
> actual POW. Such a scenario would be very bad and cause the biggest
> miner to always be the one that generates blocks. A critical
> assumption I am making is that such proof generation is not
> particularly expensive in the scheme of POW. I am optimistic that
> proof generation will not be this expensive for two reasons
>
> There are PQ signature schemes which support non-interactive
> aggregation such as LaBRADOR [5]. Thus, the STARK wouldn’t need to
> perform the block-wide signature aggregation and would only need to
> perform transaction compression, cut throughs and consolidation.
>
> We could make use of recursive STARKs [8] to allow miners to
> parallelize proof generation to reduce latency or to decentralize
> proof generation. Users creating transactions could perform
> non-interactive coinjoins with other users or settlement/batching.
> This would not only take proof generation pressure off of the miners
> and reduce the strain on the mempool but in some circumstances it
> would provide privacy if used with payjoin techniques like receiver
> side payment batching [7].
>
> The approach we are proposing treats the STARK the miner produces as
> free from a blocksize perspective. This is important for bootstrapping
> because it means that fees are significantly cheaper for a
> transaction, even if it is the only compressed transaction in the
> block. This encourages adoption. Adoption helps address the chicken
> and egg problem of wallets and exchanges not investing engineering
> resources to support a new transaction type if no one is using it and
> no one wants to use it because it isn't well supported. By having a
> single format, built into the block we both accelerate the switch over
> and prevent a fragmented ecosystem that might arise from doing this in
> Bitcoin script. Fragmentation reduces the scalability benefits because
> validators have to validate multiple STARKs and reduces the privacy
> benefits because there are many coinjoins, rather than each being a
> coinjoin.
>
> Even if our approach here turns out to be infeasible, we need a way to
> reduce the size of PQ signatures in Bitcoin. The ability to move
> coins, including the ability to move coins that represent JPEGs, is
> the main functionality of Bitcoin. If we make storage/JPEG too price
> competitive with the ability to transfer coins, we destroy that
> essential functionality and decrease the utility of Bitcoin for
> everyone. Currently moving coins securely requires at least one 64
> byte signature, which is an unfortunate tax on this most vital of all
> use cases. I believe removing that tax with signature aggregation will
> be beneficial for all parties.
>
> Consider the world of PQ signatures in Bitcoin without STARKs:
>  - The large size of PQ signatures will make it more expensive for
> users to use them prior to the invention of a CRQC (Cryptographically
> Relevant Quantum Computer). This means that most outputs will not be
> protected by PQ signatures. Once a CRQC arises there will be a rush to
> move funds under the protection of PQ signatures but due to the large
> size of PQ signatures the fees will be too expensive for most outputs.
> Users will instead need to move their funds to centralized custodial
> wallets that can use a small number of outputs. In such a world it
> will be much harder and expensive to self-custody.
> - Without a solution here the large sizes of PQ signatures will limit
> Bitcoin's functionality to move coins using on-chain payments. This
> will also favor centralized custodians and erode the decentralized
> nature of Bitcoin.
>
> None of this is an argument against adopting BIP-360 or other PQ
> signatures schemes into Bitcoin. On the contrary, having PQ signatures
> in Bitcoin would be a useful stepping stone to PQ transaction
> compression since it would allow us to gain agreement on which PQ
> signature schemes to build on. Most importantly, in the event of a
> CRQC being developed it will be far better to have uncompressed PQ
> signatures in Bitcoin than none at all.
>
> Acknowledgements:
> These ideas arose out of correspondence with Hunter Beast. I want to
> thank Neha Narula, John Light, Eli Ben-Sasson for their feedback,
> Jonas Nick for his feedback and his idea to use LaBRADOR for signature
> aggregation, Tadge Dryja for suggesting the term “JPEG resistance” and
> his ideas around its feasibility. I had a number of fruitful
> discussions over lunch with members of the MIT DCI and on the Bitcoin
> PQ working group. These acknowledgements should not be taken as an
> agreement with or endorsement of the ideas in this email.
>
> [0]: Hunter Beast, BIP-360: QuBit - Pay to Quantum Resistant Hash
> (2025) https://github.com/bitcoin/bips/pull/1670/files#
> [1]: Benchmark Report: Post-Quantum Cryptography vs secp256k1
> https://github.com/cryptoquick/libbitcoinpqc/blob/main/benches/REPORT.md
> [2]: Ruben Somsen, SNARKs and the future of blockchains (2020)
>
> https://medium.com/@RubenSomsen/snarks-and-the-future-of-blockchains-55b82012452b
> [3]: John Light, Validity Rollups on Bitcoin (2022)
>
> https://github.com/john-light/validity-rollups/blob/main/validity_rollups_on_bitcoin.md
> [4] Vitalik Buterin, An Incomplete Guide to Rollups (2021)
> https://vitalik.eth.limo/general/2021/01/05/rollup.html
> [5]: Aardal, Aranha, Boudgoust, Kolby, Takahashi, Aggregating Falcon
> Signatures with LaBRADOR (2024) https://eprint.iacr.org/2024/311
> [6]: Gidi Kaempfer, Recursive STARKs (2022)
> https://www.starknet.io/blog/recursive-starks/
> [7]: Dan Gould, Interactive Payment Batching is Better (2023)
> https://payjoin.substack.com/p/interactive-payment-batching-is-better
> [8] John Tromp, Fee burning and Dynamic Block Size (2018)
> https://lists.launchpad.net/mimblewimble/msg00450.html
>
> --
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> "Bitcoin Development Mailing List" group.
> To unsubscribe from this group and stop receiving emails from it, send an
> email to bitcoindev+unsubscribe@googlegroups•com.
> To view this discussion visit
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> .
>

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^ permalink raw reply	[|nested] 8+ messages in thread
2025-04-04 18:43 ` Brandon Black

  
From: Ethan Heilman @ 2025-04-04 19:22 UTC (permalink / raw)
  To: Brandon Black;  Bitcoin Development Mailing List

> I'm curious your thoughts on the long term incentive changes for node
runners of such a scheme.
> [...] Such a disjunction between the cost of transaction verification during
relay vs. during block validation also represents a further externality
imposed on node runners which is not compensated (as node runners do not
gain fees for verifying and relaying transactions, and their primary
benefit comes in the form of finality by verifying blocks).

That's an important question to figure out.
I don't frame the problem as the difference in costs between running a
full relay or block only. If we could make blocks-only nodes free to
run without changing the costs for full relay, I'd be in favor of
that. I frame it as running a full relay shouldn't be too expensive,
hopefully no more expensive than it is today.

In theory the transaction aggregation approach could help in two ways:

1. If most transactions are aggregated prior to entering the mempool
then we might be able to reduce verification costs for full relay and
only slightly increase the bandwidth costs.

2. Relay nodes could do the aggregation themselves for users and
collect fees for performing this service and performing relay. The
fact that aggregation is one way, once a relay node performs it,
another relay could not pull the transactions together, allowing each
step in the aggregation pipe to collect fees. I don't have a detailed
design for how this would work. Do you have any thoughts on how such a
design would work?




On Fri, Apr 4, 2025 at 2:43 PM Brandon Black <freedom@reardencode•com> wrote:
>
> Hi Ethan,
>
> Interesting idea for bringing PQ cryptography to bitcoin without
> sacrificing throughput or IBD cost.
>
> On 2025-04-04 (Fri) at 12:29:46 -0400, Ethan Heilman wrote:
> > Such a system would present scaling issues for the mempool because
> > prior to aggregation and compression, these transactions would be 2kb
> > to 100kb in size and there would be a lot more of them. It is likely
> > parties producing large numbers of transactions would want to
> > pre-aggregate and compress them in one big many input, many output
> > transactions. Aggregating prior to the miner may have privacy benefits
> > but also scalability benefits as it would enable cut-throughs and very
> > cheap consolidation transactions. ~87/txns a second does not include
> > these additional scalability benefits.
>
> I'm curious your thoughts on the long term incentive changes for node
> runners of such a scheme.
>
> Currently, running a node in full relay vs. blocks only isn't a
> significant resource difference. Only the smallest of nodes operate in
> blocks only mode afaik. With a scheme like this, the delta would expand
> significantly, potentially weakening the transaction relay network.
>
> Such a disjunction between the cost of transaction verification during
> relay vs. during block validation also represents a further externality
> imposed on node runners which is not compensated (as node runners do not
> gain fees for verifying and relaying transactions, and their primary
> benefit comes in the form of finality by verifying blocks).
>
> All the best,
>
> --
> --Brandon

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 permalink raw reply	[|nested]
2025-04-04 16:29 [bitcoindev] Post Quantum Signatures and Scaling Bitcoin Ethan Heilman
  2025-04-04 17:17 ` Dustin Ray
  2025-04-04 18:43 ` Brandon Black

  2025-04-14 13:47 ` Pieter Wuille
  
From: Matt Corallo @ 2025-04-05 17:39 UTC (permalink / raw)
  To: Ethan Heilman, Bitcoin Development Mailing List



On 4/4/25 12:29 PM, Ethan Heilman wrote:
> I strongly believe Bitcoin will need to move to PQ signatures in the
> near future. The rest of this email is premised on this belief.

Whether this is true or not, none of the non-hash-based PQC signature schemes seem like reasonable 
candidates to include in Bitcoin's consensus today (as far as I'm aware no sensible cryptographer 
anywhere is suggesting migrating to only PQC schemes given the likelihood that they end up broken by 
classical methods at some point in the next decade, which makes putting them in Bitcoin's consensus 
for the long term an incredibly questionable idea). Worse, making STARKs a part of Bitcoin's 
security assumption seems even more far-fetched.

While this is all cool, I don't really see how this is a viable path any time in the next decade, 
sadly. If you believe that we need a PQ signature scheme in the near future, it seems to me the only 
option is something hash-based and we eat the cost (optionally, see eg [1]).

[1] https://groups.google.com/g/bitcoindev/c/oQKezDOc4us/m/F-Pq-Jw2AgAJ

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 permalink raw reply	[|nested]
2025-04-04 17:17 ` Dustin Ray

  
From: 'Eli Ben-Sasson' via Bitcoin Development Mailing List @ 2025-04-05 20:40 UTC (permalink / raw)
  To: Dustin Ray;  Ethan Heilman, Bitcoin Development Mailing List

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I believe there are quite a few works discussing FRI and STARKs in quantum
oracle model, e.g.:
https://doi.org/10.48550/arXiv.2411.05360
https://doi.org/10.1109/FOCS52979.2021.00014




On Fri, Apr 4, 2025 at 8:27 PM Dustin Ray <dustinvonsandwich@gmail•com>
wrote:

> This is a great post, thank you for sharing. I have one small tiny comment
> that may or may not be relevant, but there is an existing gap in the
> literature for a security proof that STARKs (rather FRI, the underlying
> commitment scheme) is secure in a quantum adversary model. We conjecture
> that it is, because it relies only on hashes as the primitive in an error
> correcting code, but unlike other cryptographic primitives used or proposed
> for critical security infrastructure, there is currently no formal security
> argument for FRI against a quantum threat model that I am aware of. I'm not
> sure how much this matters, but some may argue that stronger security
> arguments are warranted for any potential change to the bitcoin signature
> model in a PQ landscape. That's just my two cents anyways.
>
> On Fri, Apr 4, 2025 at 9:34 AM Ethan Heilman <eth3rs@gmail•com> wrote:
>
>> I strongly believe Bitcoin will need to move to PQ signatures in the
>> near future. The rest of this email is premised on this belief.
>>
>> PQ (Post Quantum) signatures present a problem for Bitcoin:
>>
>> First, they are large. Of the three proposed in BIP-360 [0], the
>> smallest is 1.5kb for the public key + signature [1]. Without a
>> discount this represents a massive reduction in Bitcoin's transaction
>> volume due to the increase in transaction size of Bitcoin payment
>> using such signatures.
>> - Second, even if we discount PQ signatures and public keys so that
>> the maximum number of transactions that can fit in a block is
>> unchanged we still have the problem that these blocks and transactions
>> will be an order of magnitude bigger. If it is the case that we can
>> handle these extra bytes without degrading performance or
>> decentralization, then consider the head room we are giving up that
>> could be used for scalability.
>>
>> Beyond this there is also the risk that techniques could be developed
>> to encode JPEGs and other data in these discounted PQ signatures or
>> public keys. BIP-360 takes steps to make an abuse of this discount
>> more difficult by requiring that a PQ signature and public key can
>> only be written to the blockchain if they verify. We do not need PQ
>> Signatures to be completely “JPEG resistant”, they just need PQ
>> signatures to not enable significantly cheaper storage than payments.
>> The degree to which the proposed PQ signature algorithms resist being
>> repurposed as a storage mechanism is an open question and worth
>> investigating.
>>
>> If it turned out PQ signatures could be used to encode data very
>> cheaply, then Bitcoin faces the dilemma that if you discount PQ
>> signatures, you make the JPEG problem worse and may price out the
>> payment use case. If you don't discount PQ, you price out most people
>> from sending payments in Bitcoin since non-PQ witness data can be used
>> for storage
>>
>> I want to draw the community's attention to a solution that could not
>> only address these problems but also increase Bitcoin’s scalability
>> (and privacy):
>>
>> Non-interactive Transaction Compression (NTC) for Transactions
>> supporting PQ signatures. This is sometimes called Non-Interactive
>> Witness Aggregation (NIWA) [2].
>>
>> This would require a new transaction type supporting PQ signatures.
>> The miner of a block would then pull out the signatures and hash
>> pointers from transactions to compress transaction data and
>> non-interactively aggregate all the PQ signatures in all the
>> transactions in a block, replacing them with one big STARK (STARKS are
>> a form of SNARK which is PQ). This would make PQ signatures
>> significantly smaller and cheaper than ECDSA and schnorr signatures.
>>
>> Consider the following back of the envelope math:
>>
>> 2 bytes per Input = 2 bytes per TXID, 0 bytes per signature
>> 37 bytes per output = 32 bytes pubkey hash + 5 bytes value (max 2.8m
>> BTC per output)
>>
>> 1-input-2-output transaction would be: 2 + 2*37 = 76 bytes
>> (4,000,000/76)/(60*10) = ~87 txns/sec
>>
>> You could shave some bytes off the value, or add some bytes to the
>> TXID. [3] provides a more detailed estimate, proposing 113.5 weight
>> units (WU) for a 1-input-2-output transaction with no address reuse.
>> However it does not consider TXID compression. If desired an
>> account-based model could push this even further to 12 bytes per
>> transaction per block [4]. This would enable approximately
>> 4,000,000/(12*60*10) = 555 txns/second.
>>
>> A secondary benefit of having on-chain PQ payments only be ~76 bytes
>> in size is that it fundamentally changes the pricing relationship
>> between payments and on-chain JPEG/complex contracts. The problem with
>> on-chain JPEGs is not that they are possible, but that they are price
>> competitive with payments. At ~76 bytes per payment or better yet ~76
>> bytes per LN channel open/close, JPEGs no longer present the same fee
>> competition to payments as payments become much cheaper.
>>
>> Such a system would present scaling issues for the mempool because
>> prior to aggregation and compression, these transactions would be 2kb
>> to 100kb in size and there would be a lot more of them. It is likely
>> parties producing large numbers of transactions would want to
>> pre-aggregate and compress them in one big many input, many output
>> transactions. Aggregating prior to the miner may have privacy benefits
>> but also scalability benefits as it would enable cut-throughs and very
>> cheap consolidation transactions. ~87/txns a second does not include
>> these additional scalability benefits.
>>
>> Consider an exchange that receives and sends a large number of
>> transactions. For instance between block confirmations customers send
>> the exchange 10 1-input-2-output transactions in deposits and the
>> exchange sends out 10 1-input-2-output transactions in withdrawals.
>> The exchange could consolidate all of the outputs paying the exchange,
>> including chain outputs, into one output and do the same for inputs.
>> This would reduce not just size, but also validation costs.
>>
>> (10 * 2 + 20 * 2 * 37) + (10 * 2 + 20 * 2 * 37)  = 3000 bytes
>> becomes
>> (10 * 2 + 11 * 2 * 37) + (2 + 11 * 2 * 37) = 1650 bytes
>>
>> If constructing these proofs turned out to be as expensive as
>> performing POW, it would make block generation not progress free.
>> Essentially you'd have a two step POW: proof generation and then the
>> actual POW. Such a scenario would be very bad and cause the biggest
>> miner to always be the one that generates blocks. A critical
>> assumption I am making is that such proof generation is not
>> particularly expensive in the scheme of POW. I am optimistic that
>> proof generation will not be this expensive for two reasons
>>
>> There are PQ signature schemes which support non-interactive
>> aggregation such as LaBRADOR [5]. Thus, the STARK wouldn’t need to
>> perform the block-wide signature aggregation and would only need to
>> perform transaction compression, cut throughs and consolidation.
>>
>> We could make use of recursive STARKs [8] to allow miners to
>> parallelize proof generation to reduce latency or to decentralize
>> proof generation. Users creating transactions could perform
>> non-interactive coinjoins with other users or settlement/batching.
>> This would not only take proof generation pressure off of the miners
>> and reduce the strain on the mempool but in some circumstances it
>> would provide privacy if used with payjoin techniques like receiver
>> side payment batching [7].
>>
>> The approach we are proposing treats the STARK the miner produces as
>> free from a blocksize perspective. This is important for bootstrapping
>> because it means that fees are significantly cheaper for a
>> transaction, even if it is the only compressed transaction in the
>> block. This encourages adoption. Adoption helps address the chicken
>> and egg problem of wallets and exchanges not investing engineering
>> resources to support a new transaction type if no one is using it and
>> no one wants to use it because it isn't well supported. By having a
>> single format, built into the block we both accelerate the switch over
>> and prevent a fragmented ecosystem that might arise from doing this in
>> Bitcoin script. Fragmentation reduces the scalability benefits because
>> validators have to validate multiple STARKs and reduces the privacy
>> benefits because there are many coinjoins, rather than each being a
>> coinjoin.
>>
>> Even if our approach here turns out to be infeasible, we need a way to
>> reduce the size of PQ signatures in Bitcoin. The ability to move
>> coins, including the ability to move coins that represent JPEGs, is
>> the main functionality of Bitcoin. If we make storage/JPEG too price
>> competitive with the ability to transfer coins, we destroy that
>> essential functionality and decrease the utility of Bitcoin for
>> everyone. Currently moving coins securely requires at least one 64
>> byte signature, which is an unfortunate tax on this most vital of all
>> use cases. I believe removing that tax with signature aggregation will
>> be beneficial for all parties.
>>
>> Consider the world of PQ signatures in Bitcoin without STARKs:
>>  - The large size of PQ signatures will make it more expensive for
>> users to use them prior to the invention of a CRQC (Cryptographically
>> Relevant Quantum Computer). This means that most outputs will not be
>> protected by PQ signatures. Once a CRQC arises there will be a rush to
>> move funds under the protection of PQ signatures but due to the large
>> size of PQ signatures the fees will be too expensive for most outputs.
>> Users will instead need to move their funds to centralized custodial
>> wallets that can use a small number of outputs. In such a world it
>> will be much harder and expensive to self-custody.
>> - Without a solution here the large sizes of PQ signatures will limit
>> Bitcoin's functionality to move coins using on-chain payments. This
>> will also favor centralized custodians and erode the decentralized
>> nature of Bitcoin.
>>
>> None of this is an argument against adopting BIP-360 or other PQ
>> signatures schemes into Bitcoin. On the contrary, having PQ signatures
>> in Bitcoin would be a useful stepping stone to PQ transaction
>> compression since it would allow us to gain agreement on which PQ
>> signature schemes to build on. Most importantly, in the event of a
>> CRQC being developed it will be far better to have uncompressed PQ
>> signatures in Bitcoin than none at all.
>>
>> Acknowledgements:
>> These ideas arose out of correspondence with Hunter Beast. I want to
>> thank Neha Narula, John Light, Eli Ben-Sasson for their feedback,
>> Jonas Nick for his feedback and his idea to use LaBRADOR for signature
>> aggregation, Tadge Dryja for suggesting the term “JPEG resistance” and
>> his ideas around its feasibility. I had a number of fruitful
>> discussions over lunch with members of the MIT DCI and on the Bitcoin
>> PQ working group. These acknowledgements should not be taken as an
>> agreement with or endorsement of the ideas in this email.
>>
>> [0]: Hunter Beast, BIP-360: QuBit - Pay to Quantum Resistant Hash
>> (2025) https://github.com/bitcoin/bips/pull/1670/files#
>> [1]: Benchmark Report: Post-Quantum Cryptography vs secp256k1
>> https://github.com/cryptoquick/libbitcoinpqc/blob/main/benches/REPORT.md
>> [2]: Ruben Somsen, SNARKs and the future of blockchains (2020)
>>
>> https://medium.com/@RubenSomsen/snarks-and-the-future-of-blockchains-55b82012452b
>> [3]: John Light, Validity Rollups on Bitcoin (2022)
>>
>> https://github.com/john-light/validity-rollups/blob/main/validity_rollups_on_bitcoin.md
>> [4] Vitalik Buterin, An Incomplete Guide to Rollups (2021)
>> https://vitalik.eth.limo/general/2021/01/05/rollup.html
>> [5]: Aardal, Aranha, Boudgoust, Kolby, Takahashi, Aggregating Falcon
>> Signatures with LaBRADOR (2024) https://eprint.iacr.org/2024/311
>> [6]: Gidi Kaempfer, Recursive STARKs (2022)
>> https://www.starknet.io/blog/recursive-starks/
>> [7]: Dan Gould, Interactive Payment Batching is Better (2023)
>> https://payjoin.substack.com/p/interactive-payment-batching-is-better
>> [8] John Tromp, Fee burning and Dynamic Block Size (2018)
>> https://lists.launchpad.net/mimblewimble/msg00450.html
>>
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>>
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 permalink raw reply	[|nested]
2025-04-04 16:29 [bitcoindev] Post Quantum Signatures and Scaling Bitcoin Ethan Heilman
                   ` (2 preceding siblings ...)
  2025-04-05 17:39 ` Matt Corallo

  2025-04-14 19:35   ` Ethan Heilman
  
From: Pieter Wuille @ 2025-04-14 13:47 UTC (permalink / raw)
  To: Ethan Heilman;  Bitcoin Development Mailing List

Hi Ethan,

thank you bringing this up. I'm unconvinced about the practicality, but I'm happy to see thinking and discussion in this area.

Two points addressed below:

On Friday, April 4th, 2025 at 12:29 PM, Ethan Heilman <eth3rs@gmail•com> wrote:

> If it is the case that we can
> handle these extra bytes without degrading performance or
> decentralization, then consider the head room we are giving up that
> could be used for scalability.

I don't disagree with the overall point raised here, but I do think it's worth distinguishing between the "size" (bandwidth/storage) and "computation" (CPU/IO) aspects of scalability.

If it turns out to be the case that PQ schemes need more on-chain size, but have lower per-byte computation cost, a reasonable argument could be made that a higher discount factor for PQ data is acceptable. I don't know what the trade-off here ought to be, and this does not diminish your "JPEG resistance" argument, but I did want to point out that just counting size isn't the only constraint here.

> Such a system would present scaling issues for the mempool because
> prior to aggregation and compression, these transactions would be 2kb
> to 100kb in size and there would be a lot more of them. It is likely
> parties producing large numbers of transactions would want to
> pre-aggregate and compress them in one big many input, many output
> transactions. Aggregating prior to the miner may have privacy benefits
> but also scalability benefits as it would enable cut-throughs and very
> cheap consolidation transactions. ~87/txns a second does not include
> these additional scalability benefits.

I don't think pre-aggregation (beyond a single-transaction-wide one) is realistic, as it effectively breaks in-mempool transaction replacement, turning every pre-aggregated group of transactions that is being relayed together into an atomic package that must be taken or not as a whole. Consider for example the case where transactions P, C1, and C2 are relayed, with C1 and C2 depending on P. One node sees P and C1, but not C2, they may pre-aggregate prior to relay. Another node sees P and C2, but not C1, they may pre-aggregate those prior to relay. These two packages (P+C1, P+C2) cannot be combined, so we've effectively forced the network/miners to choose between one of C1 or C2, unless the individual transactions are still available somewhere.

I fear this is a very fast way to cause mining without direct-to-miner transaction submission from users to become uncompetitive, making entering the mining business permissioned, and effectively removing the point of having a decentralized consensus mechanism in the first place.

-- 
Pieter

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2025-04-14 13:47 ` Pieter Wuille

  
From: Ethan Heilman @ 2025-04-14 19:35 UTC (permalink / raw)
  To: Pieter Wuille;  Bitcoin Development Mailing List

> I'm happy to see thinking and discussion in this area.

Getting this discussion going was exactly my intent. I'm not
presenting so much a solution as we might want to do this at some
point what are problems and can we solve them?

> If it turns out to be the case that PQ schemes need more on-chain size, but have lower per-byte computation cost, a reasonable argument could be made that a higher discount factor for PQ data is acceptable.

I was focused on size because computation is pretty great for most PQ
signature schemes. PQ signatures are far cheaper to validate per byte
and according to BIP-360 Falcon is cheaper than edDSA per signature
verification.

EdDSA Cycles to verify: 130,000
FALCON-512 Cycles to verify: 81,036

This is one of the reasons I am very optimistic that Bitcoin will move
to post-quantum signatures. If research shows that these signature
schemes are sufficiently JPEG resistant, and I think it will, then a
discount is very attractive.

> I don't think pre-aggregation (beyond a single-transaction-wide one) is realistic, as it effectively breaks in-mempool transaction replacement, turning every pre-aggregated group of transactions that is being relayed together into an atomic package that must be taken or not as a whole.

In some circumstances it is possible you could aggregate (P+C1, P+C2)
into (P+C1+C2). If you can prove that P is the same in both
transactions thus the balance and authentication properties are
maintained. However I think what you have described is the shape of
the problem we need to solve.

Consider transactions: T1, T1', T2, T3, T4, T5
where T1 and T1' are double spends, i.e., spend the same output to
different outputs. If half the mempool aggregates TA = (T1, T2, T3)
and the other half aggregates TB = (T1', T4, T5). TA and TB are
mutually exclusive and transactions are needlessly dropped on the
floor. This is a currently existing griefing vector with coinjoins
today and is an issue with mimblewimble aggregation. I don't think we
have seen it abused much, but that doesn't mean we can ignore it.

I believe this is a solvable problem, but it requires careful thought
and I haven't seen a fully baked answer. What follows is my intuition
on how this might be solved.

Approach one: have relay nodes share a map of what UTXOs would be
spent by their mempool prior to performing an aggregation to detect
and resolve doublespends.
Approach two: allow an aggregator to non-interactively aggregate a set
of transactions if they are the sender or receiver of funds in all the
transactions they are aggregating.

My biggest concern here is a conflict between aggregator/relay
incentives and miner incentives that either causes miners to be
aggregators or reduces the profitability of miners. This conflict
arises from the fact that, unless prevented by the protocol, an
aggregator can aggregate high fee transactions with low fee
transactions in such a way as to reduce miner fees and possibility
make fees for themselves.

For the sake of example assume blocksize allows only two transactions per block:
T1 has a 100 sat/vb fee
T2 has a 100 sat/vb fee
T3 has a 50 sat/vb fee
T4 has a 50 sat/vb fee

If the miner was the aggregator they would aggregate (T1 + T2) and
mine it to get the highest fee.
Instead an aggregator who is not a miner could collect a fee from the
creator of T3 and T4 and aggregate (T1 + T3) and (T2 + T4) thereby
raising the average fee of T3 and T4. The miner loses out on fees.

Approach two makes this less of an issue because the creator of T1,
if they are aware T2 exists, is unlikely to consent to having T1
aggregated with T3 since it lowers the total fee.

This relay vs miner conflict isn't an entirely new issue in Bitcoin.
Miners today could run relay nodes and keep the high fee transactions
for themselves. I assume this isn't done very much in 2025 because the
block subsidy still dominates, but is likely to be a bigger issue when
fees dominate.

On Mon, Apr 14, 2025 at 9:47 AM Pieter Wuille <bitcoin-dev@wuille•net> wrote:
>
> Hi Ethan,
>
> thank you bringing this up. I'm unconvinced about the practicality, but I'm happy to see thinking and discussion in this area.
>
> Two points addressed below:
>
> On Friday, April 4th, 2025 at 12:29 PM, Ethan Heilman <eth3rs@gmail•com> wrote:
>
> > If it is the case that we can
> > handle these extra bytes without degrading performance or
> > decentralization, then consider the head room we are giving up that
> > could be used for scalability.
>
> I don't disagree with the overall point raised here, but I do think it's worth distinguishing between the "size" (bandwidth/storage) and "computation" (CPU/IO) aspects of scalability.
>
> If it turns out to be the case that PQ schemes need more on-chain size, but have lower per-byte computation cost, a reasonable argument could be made that a higher discount factor for PQ data is acceptable. I don't know what the trade-off here ought to be, and this does not diminish your "JPEG resistance" argument, but I did want to point out that just counting size isn't the only constraint here.
>
> > Such a system would present scaling issues for the mempool because
> > prior to aggregation and compression, these transactions would be 2kb
> > to 100kb in size and there would be a lot more of them. It is likely
> > parties producing large numbers of transactions would want to
> > pre-aggregate and compress them in one big many input, many output
> > transactions. Aggregating prior to the miner may have privacy benefits
> > but also scalability benefits as it would enable cut-throughs and very
> > cheap consolidation transactions. ~87/txns a second does not include
> > these additional scalability benefits.
>
> I don't think pre-aggregation (beyond a single-transaction-wide one) is realistic, as it effectively breaks in-mempool transaction replacement, turning every pre-aggregated group of transactions that is being relayed together into an atomic package that must be taken or not as a whole. Consider for example the case where transactions P, C1, and C2 are relayed, with C1 and C2 depending on P. One node sees P and C1, but not C2, they may pre-aggregate prior to relay. Another node sees P and C2, but not C1, they may pre-aggregate those prior to relay. These two packages (P+C1, P+C2) cannot be combined, so we've effectively forced the network/miners to choose between one of C1 or C2, unless the individual transactions are still available somewhere.
>
> I fear this is a very fast way to cause mining without direct-to-miner transaction submission from users to become uncompetitive, making entering the mining business permissioned, and effectively removing the point of having a decentralized consensus mechanism in the first place.
>
> --
> Pieter
>

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