psbt: accept taproot ALL sigs with sighash unset #36577

SignPSBTInput() assumes SIGHASH_DEFAULT for a taproot input when neither PSBT_IN_SIGHASH_TYPE nor the caller's sighash_type is set. Its check of existing signatures then rejected every 65-byte taproot signature, so a PSBT holding a valid SIGHASH_ALL signature could be neither finalized nor signed further.

When neither the field nor the parameter is set, the default is only assumed, so also accept a 65-byte signature ending in SIGHASH_ALL. NONE, SINGLE and ANYONECANPAY variants are still rejected, as are all mismatches when the field or the parameter is set.

Fixes #36507.

🤖 Generated with Claude Code

<!--e57a25ab6845829454e8d69fc972939a-->

The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

<!--006a51241073e994b41acfe9ec718e94-->

External sites

• Code coverage & benchmarks: https://corecheck.dev/bitcoin/bitcoin/pulls/36577
• Ralph review <img src=https://review.fish.foo/assets/bitcoin/bitcoin/36577.png alt="Ralph review status"> https://git.fish.foo/bitcoin/bitcoin/pulls/36577#author-ralph

<!--021abf342d371248e50ceaed478a90ca-->

Reviews

See the guideline and AI policy for information on the review process.

If your review is incorrectly listed, please copy-paste <code>&lt;!--meta-tag:bot-skip--&gt;</code> into the comment that the bot should ignore.

<!--174a7506f384e20aa4161008e828411d-->

Conflicts

Reviewers, this pull request conflicts with the following ones:

• #36382 _{<img src="https://drahtbot.space/ack_count/bitcoin/bitcoin/36382.svg">} (rpc: flag invalid taproot leaf control blocks in decodepsbt by fametrano)
• #36224 _{<img src="https://drahtbot.space/ack_count/bitcoin/bitcoin/36224.svg">} (test: Add test coverage for PartiallySignedTransaction::Merge() by nebula-21)
• #35747 _{<img src="https://drahtbot.space/ack_count/bitcoin/bitcoin/35747.svg">} (wallet: Fix FillPSBT failing to sign owned inputs when UTXOs disagree by nervana21)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

<!--5faf32d7da4f0f540f40219e4f7537a3-->

LLM Linter (✨ experimental)

Possible places where named args for integral literals may be used (e.g. func(x, /*named_arg=*/0) in C++, and func(x, named_arg=0) in Python):

• src/test/psbt_tests.cpp: SignPSBTInput(DUMMY_SIGNING_PROVIDER, psbt, 0, &txdata, {.finalize = true})

^{2026-10-19 20:49:23}

Tested ACK 1291d0e389 on Windows.

Approach ACK / tACK 1291d0e

Good fix for the referenced issue. When sighash_type is unset, accepting 65-byte Taproot signatures ending in SIGHASH_ALL is safe because it commits to the identical BIP-341 digest as SIGHASH_DEFAULT. Preserving rejection for other variants keeps Core's safety checks intact.

Tested on Ubuntu 24.04:

• psbt_tests passed (including both new absent-field tests).
• rpc_psbt.py passed cleanly too.