Add BIP: QES2 – Hybrid PQC-based Digital Signature Algorithm

j1729labs commented at 8:03 am on April 18, 2025: none

Summary: This pull request introduces a new Bitcoin Improvement Proposal (BIP) for QES2, a hybrid digital signature algorithm that combines post-quantum cryptography (PQC) with traditional ECDSA. The proposal aims to address the potential vulnerabilities posed by quantum computing while preserving backward compatibility with existing Bitcoin infrastructure.

Details:

Abstract: QES2 leverages a dual-signature mechanism to incorporate both a post-quantum signature and a classical ECDSA signature into Bitcoin transactions.

Motivation: With the emerging threat of quantum computers, classical cryptographic methods may become vulnerable. QES2 presents a transitional solution that enhances security during the shift towards quantum-safe systems.

Specification: The BIP outlines the structure, key generation, signing, and verification methods for the hybrid scheme.

Rationale: The hybrid approach ensures that if one signature method is compromised, the other still provides protection, offering a balanced trade-off between security and backward compatibility.

Reference Implementation: A reference implementation will be linked later for further review and testing.

Create bip-newproposal.mediawiki 7f9114f662

Update bip-newproposal.mediawiki 5414a5c56a

Update bip-newproposal.mediawiki 3a34446077

Update bip-newproposal.mediawiki 28c2aa8a7f

Update bip-newproposal.mediawiki 11a563aeaf

cryptoquick commented at 3:29 pm on April 18, 2025: none

Would it make sense to just add QES2 support to BIP-360?

in bip-newproposal.mediawiki:330 in 11a563aeaf

325+Taproot Compatibility
326+--------------------
327+
328+QES2 can be integrated with BIP-340 (Taproot) by:
329+
330+1. Using the QES2-based signature in place of the Schnorr signature

cryptoquick commented at 3:49 pm on April 18, 2025:

You specify QES2 as ECDSA, but ECDSA doesn’t support all that Schnorr does. This seems like a step backwards that could break Taproot compatibility. Would it not make sense to implement QES2 with Schnorr and remove mention of ECDSA?

murchandamus commented at 1:08 pm on April 23, 2025:

Like @cryptoquick, I’m confused that this proposal focuses only on ECDSA signatures, when about 1/3 of all existing UTXOs use the P2TR output type that employs BIP340 signatures. Could you please provide rationale for this approach and further address the implications for existing P2TR outputs intended to be spent via the scriptpath?

j1729labs commented at 2:38 am on May 12, 2025:

You specify QES2 as ECDSA, but ECDSA doesn’t support all that Schnorr does. This seems like a step backwards that could break Taproot compatibility. Would it not make sense to implement QES2 with Schnorr and remove mention of ECDSA?

I am considering the overall direction of my research as follows:

Initially, I aim to provide a hybrid signature scheme.

Ultimately, I plan to propose a PQC-based signature scheme optimized for Bitcoin.

The QES2 scheme I’m presenting is a hybrid dual-signature model that combines ECDSA and Delicium. The role of ECDSA is to offer a structure that is well-suited for existing Bitcoin systems. As you mentioned, one might question whether this approach is vulnerable to attacks exploiting the Schnorr algorithm. However, even if the ECDSA signature is verified, the Delicium-related verification is not completed, so such concerns do not materialize.

Furthermore, we are in an advanced stage of R&D on a model that mitigates protocol-level vulnerabilities of the Schnorr algorithm in Isogeny-based PQC, one of the NIST Round 3 finalists. We are considering applying this improved model as the final design. (Isogeny-based PQC has the advantage of a small key size, and if the Schnorr-related vulnerabilities are addressed, it becomes a very strong candidate.)

j1729labs commented at 3:43 am on May 12, 2025:

Thank you for the great question. Let me clarify the intention behind QES2 in more detail.

First, QES2 is a general-purpose dual-signature scheme initially designed by combining ECDSA and our Dilithium-based post-quantum signature component. The reason we included ECDSA was to ensure compatibility with the existing Bitcoin infrastructure—especially since many legacy UTXOs and current wallet software still rely on ECDSA. Moreover, in the early stages of our research, we chose to develop on top of ECDSA to maintain control over Dilithium signatures and to align with digital signature types commonly used in the financial sector. We are also exploring the expansion to Schnorr signatures and actively working on methods to control and implement signatures more efficiently. Therefore, this proposal is part of a research roadmap and will be developed step-by-step.

That said, as you rightly pointed out, about one-third of UTXOs utilize the P2TR output type that uses BIP340 (Schnorr) signatures. From a BIP-level integration perspective, it indeed makes more sense to consider Schnorr-based design, and we are currently evaluating this actively. We plan to provide a Schnorr-variant of QES2 in the near future.

Our research roadmap is as follows:

In the short term, we aim to provide an ECDSA + Dilithium hybrid signature model that can be practically deployed and is compatible with existing infrastructure.
In the long term, our goal is to propose a post-quantum signature scheme optimized for Bitcoin, likely based on isogeny-based cryptography.

We are currently developing an isogeny-based signature system that addresses known protocol vulnerabilities (as seen in NIST’s Round 3 competition). We believe this approach offers strong long-term security with small key sizes and resistance to quantum attacks.

To summarize:

We initially adopted ECDSA for general applicability and control over Dilithium.
We are actively researching integration with Schnorr-based signatures.
Our ultimate goal is not to finalize a hybrid scheme, but to fully transition to a PQC-based signature model.

Thank you again for your thoughtful feedback—we see this as a valuable opportunity to ensure QES2 evolves to meet both current and future Bitcoin use cases.

in bip-newproposal.mediawiki:581 in 11a563aeaf

576+
577+1. **Dilithium Security**: The Dilithium signature is secure against quantum adversaries under the hardness assumptions of Module-LWE and Module-SIS problems.
578+
579+2. **ECDSA Security**: While vulnerable to quantum attacks, ECDSA remains secure against classical adversaries.
580+
581+3. **Binding Property**: The ECDSA signature validates the Dilithium signature, creating a binding that requires breaking both schemes or finding hash collisions to forge.

cryptoquick commented at 3:53 pm on April 18, 2025:

Why is it necessary to sign the PQ signature? Can’t it just be included separately and still benefit from the same guarantees if committed to in the same address as BIP-360 does?

j1729labs commented at 3:46 am on May 12, 2025:

My primary concern — and the motivation for this work — is that quantum computing poses a real and growing threat to the cryptographic foundations of Bitcoin and other blockchains. In response, we are actively exploring ways to strengthen these systems with post-quantum cryptography.

In the case of QES2, simply appending a PQ signature is not sufficient, as it does not provide a verifiable commitment to the signature within Bitcoin’s existing validation logic. By signing the PQ signature with a currently supported scheme such as ECDSA or Schnorr, we ensure that the signature is cryptographically bound and verifiable in the current infrastructure, without requiring consensus changes or new opcodes.

This hybrid dual-signature design provides a practical path for backward compatibility while introducing post-quantum security guarantees. Ultimately, our goal is to transition toward a fully post-quantum signature scheme, but this intermediate approach offers a secure and deployable solution today.

cryptoquick commented at 3:54 pm on April 18, 2025: none

Just some questions

jonatack commented at 5:54 pm on April 18, 2025: member

Hi @j1729labs, have you posted about this to the bitcoin-dev mailing list at https://groups.google.com/g/bitcoindev? Please refer to https://github.com/bitcoin/bips/blob/master/bip-0002.mediawiki#user-content-BIP_workflow for details. Thanks!

jonatack added the label New BIP on Apr 18, 2025

murchandamus commented at 11:42 pm on April 18, 2025: contributor

Please take another look at the formatting. The document’s syntax doesn’t seem to be MediaWiki, and especially the preamble does currently not conform to the required formatting.

murchandamus added the label PR Author action required on Apr 18, 2025

in bip-newproposal.mediawiki:129 in 11a563aeaf

124+- ``OP_QES2_CHECKSIG`` is a new opcode that verifies the entire chained signature scheme
125+
126+New Opcode
127+~~~~~~~~~~
128+
129+We introduce a new opcode, tentatively assigned as ``OP_QES2_CHECKSIG (0xba)``, that performs verification of the hybrid QES2 signature by checking both the ECDSA signature (which validates the PQC signature) and the Dilithium signature itself.

murchandamus commented at 1:02 pm on April 23, 2025:

Opcode 186 was designated OP_CHECKSIGADD by BIP 342.

in bip-newproposal.mediawiki:421 in 11a563aeaf

416+======================
417+
418+This BIP maintains backward compatibility through several mechanisms:
419+
420+1. **Opt-in Deployment**: QES2 addresses are distinct from traditional addresses
421+2. **Traditional Scripts**: Existing P2PKH, P2SH, P2WPKH, and P2WSH scripts continue to function normally

murchandamus commented at 1:10 pm on April 23, 2025:

As mentioned, this does not address P2TR outputs which make up about 1/3 of all existing UTXOs.

in bip-newproposal.mediawiki:594 in 11a563aeaf

589+   Where Adv_X(A) represents the advantage of an adversary A against scheme X.
590+
591+Acknowledgments
592+==============
593+
594+This proposal builds on the work of several other BIPs, including BIP-340, BIP-341, and BIP-342 (Taproot), and incorporates concepts from ongoing research in post-quantum cryptography for blockchains.

murchandamus commented at 1:12 pm on April 23, 2025:

I’m confused that this proposal mentions BIP 340 several times, but insufficiently addresses BIP 340 signatures.

in bip-newproposal.mediawiki:117 in 11a563aeaf

112+
113+This BIP introduces a new script template:
114+
115+.. code-block::
116+
117+   <pq_signature_push> <ecdsa_signature_push> <pubkey_push> OP_QES2_CHECKSIG

murchandamus commented at 1:14 pm on April 23, 2025:

Please provide more detail how this new script template would be used in transactions. How is it split between output script, input script, witness section, or a newly introduced transaction section? How would this transaction be serialized? If it is intended to be a soft fork, what mechanism is used to allow unupgraded nodes to accept transactions using this signature scheme?

in bip-newproposal.mediawiki:11 in 11a563aeaf

 6+:Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-XXXX
 7+:Status: Draft
 8+:Type: Standards Track
 9+:Created: 2025-04-18
10+:License: BSD-2-Clause
11+:Requires: 340, 341, 342

murchandamus commented at 1:30 pm on April 23, 2025:

The document must start with the preamble adhering to the required format. Beyond the formatting needing to be amended, the title is too long and the authors need to be on separate lines with the format Name <addr@dom.ain>.

murchandamus changes_requested

murchandamus commented at 1:30 pm on April 23, 2025: contributor

Thanks for your submission. The content of this document shows potential, however it is still lacking some important details (see other review comments). It also currently does not meet the formatting requirements for the BIPs process. Please fix the formatting to conform to the MediaWiki syntax and amend the Preamble to use preformatted text with the required formatting.

As this document is currently not ready to be merged, I’m turning it into a Draft pull request at this time.

murchandamus marked this as a draft on Apr 23, 2025

j1729labs commented at 3:50 am on May 12, 2025: none

Hi @j1729labs, have you posted about this to the bitcoin-dev mailing list at https://groups.google.com/g/bitcoindev? Please refer to https://github.com/bitcoin/bips/blob/master/bip-0002.mediawiki#user-content-BIP_workflow for details. Thanks!

Thank you for the guidance. I will make sure to post this to the mailing list.

Add BIP: QES2 – Hybrid PQC-based Digital Signature Algorithm #1830