Insider Brief

  • Bitcoin Cash developers are reviewing a proposal to support post-quantum and hybrid signatures, with potential lock-in on Nov. 15, 2026, and activation targeted for May 15, 2027.
  • The proposed OP_SIGHASH opcode would simplify contracts using alternative signature schemes by exposing transaction data already calculated during signature verification.
  • Hybrid signatures would require both classical and post-quantum checks to succeed, providing a migration path designed to preserve existing address formats and wallet compatibility.
  • Photo by Shubham Dhage on Unsplash

PRESS RELEASE — The Bitcoin Cash (BCH) development community has formalized CHIP-2026-06, a Cash Improvement Proposal introducing the `OP_SIGHASH` opcode. The proposal is under active discussion and implementation review for potential lock-in on November 15, 2026, with mainnet activation targeted for May 15, 2027, following the established CHIP process. Bitcoin Cash contributor mainnet_pat has published CHIP-2026-06: Post-Quantum and Hybrid Signatures, a Cash Improvement Proposal that extends the Bitcoin Cash virtual machine’s check-signature opcodes to support multiple signature schemes, including post-quantum (PQ) algorithms and hybrid combinations.

CHIP-2026-06 adds a single opcode that computes a transaction’s signature hash (sighash) according to the same algorithm, type flags, and `scriptCode` conventions already used by `OP_CHECKSIG`, then pushes the resulting 32-byte digest onto the stack. No new cryptography or sighash semantics are introduced; the opcode simply exposes work the virtual machine already performs internally. The proposal creates a generic signature dispatch mechanism. A public key’s first byte selects the scheme from a registry (secp256k1 for current ECDSA/Schnorr signatures, with post-quantum schemes such as SPHINCS+ and Falcon-512 added over time). Compound (hybrid) keys bind two schemes under AND semantics, so a hybrid spend (for example, secp256k1 + SPHINCS+) is valid only when every component verifies.

Importantly, no new opcodes or output types are required with the hybrid scheme. PQ and hybrid keys spend from ordinary Pay-to-Public-Key-Hash (P2PKH) outputs that commit to `HASH160(public-key element)`. This design preserves compatibility with existing address formats, wallet tooling, and Double-Spend Proof (DSP) coverage.

Key benefits include:

– Dramatically smaller and simpler contracts for alternative signature schemes. Projects implementing Quantumroot post-quantum vaults, Lamport signatures, secp256r1 verification, and similar constructions currently reconstruct the sighash manually in Script—often requiring hundreds of bytes. `OP_SIGHASH` reduces this to two bytes while guaranteeing the exact digest produced by node implementations.
– Improved reliability and auditability. Contract authors no longer need to re-implement complex edge cases around `SIGHASH_SINGLE`, `ANYONECANPAY`, ForkID, UTXOS, and related rules.
– Enhanced support for covenants, oracles, and novel cryptographic patterns that combine the transaction sighash with additional data before verification.

– Providing a reliable, standardized post-quantum target (initially favoring the hash-based, stateless SPHINCS+ / SLH-DSA) that wallets and users can commit to today as a long-term “bunker” against a future cryptographically relevant quantum computer.
– Supporting hybrid signatures for a smoother migration path that maintains classical security while adding quantum resistance.
– Avoiding stateful signature schemes in the core `OP_CHECKSIG` path, because those schemes carry key-reuse risks that conflict with the long-standing expectation that signature verification is safe under key reuse.
– Enabling future extensibility so additional schemes can be registered without further consensus changes to the dispatch mechanism itself.

Community feedback has been constructive. Bitcoin Cash Node developers have already opened a merge request implementing the opcode, and multiple stakeholders have described it as low-hanging fruit that meaningfully lowers the barrier for advanced CashVM applications.

The hybrid proposal is positioned as infrastructure for quantum readiness rather than an immediate replacement for efficient everyday secp256k1 usage. Discussion on Bitcoin Cash Research notes that script-level solutions such as Quantumroot vaults (already live on mainnet following the May 2026 Layla upgrade) already provide practical post-quantum options, while CHIP-2026-06 would add a native, drop-in path at the signature verification layer.

“This completes a useful unbundling of checksig functionality and gives every builder the same battle-tested sighash the nodes already use,” noted discussions on the Bitcoin Cash Research forum. The proposal builds directly on the capabilities delivered by the May 2026 “Layla” upgrade (Loops, Functions, Bitwise operations, and Pay-to-Script).

The full specification is available at the author’s GitLab repository and is under active discussion on the Bitcoin Cash Research forum. Stakeholders, wallet developers, node implementers, and the broader community are invited to review the text, provide feedback, and help refine the choice of initial schemes and parameter sets.

The CHIP remains open for review, refinement, and stakeholder statements ahead of the November 15 lock-in deadline. Full specification, rationale, and discussion are available on Bitcoin Cash Research.

Bitcoin Cash continues its track record of predictable, decentralized protocol upgrades. Six consecutive annual upgrades from 2021 through 2026 activated on schedule with zero chain splits, demonstrating the effectiveness of the open CHIP process.

About the Proposal
CHIP-2026-06 was introduced by mainnet_pat in June 2026. Full text and discussion:  
https://bitcoincashresearch.org/t/chip-2026-06-post-quantum-and-hybrid-signatures/1864  
Specification repository: https://gitlab.com/mainnet-pat/chip-2026-06-generic-sigs

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