Allow custom sighash flags in tapscript sig verification (align with forfeit tx pattern)

[arkanaai]

Summary

VerifyTapscriptSigs in pkg/ark-lib/txutils/utils.go hardcodes txscript.SigHashDefault when computing the sighash for signature verification. This means any tapscript input signed with a non-default sighash type (e.g. SIGHASH_ALL, SIGHASH_SINGLE, SIGHASH_NONE, or their ANYONECANPAY variants) will fail verification even when the sighash type is correctly encoded in the PSBT.

Current Behavior

In pkg/ark-lib/txutils/utils.go:

sighash, err = txscript.CalcTapscriptSignaturehash(
    txSigHashes,
    txscript.SigHashDefault,   // ← hardcoded
    tx.UnsignedTx,
    inputIndex,
    prevoutFetcher,
    leaf,
)

The sighash type embedded in each TaprootScriptSpendSig entry is ignored.

Expected Behavior

The sighash type should be read from the PSBT sig entry (tapscriptSig.SigHash), exactly as the forfeit tx verification path does in internal/infrastructure/tx-builder/covenantless/builder.go:

preimage, err := txscript.CalcTapscriptSignaturehash(
    txSigHashes,
    tapScriptSig.SigHash,   // ← reads from PSBT sig
    ptx.UnsignedTx,
    index,
    prevoutFetcher,
    txscript.NewBaseTapLeaf(tapLeaf.Script),
)

The signing path in pkg/arkd-wallet/core/application/wallet/service.go already respects input.SighashType from the PSBT when producing signatures, so the signing and verification halves are inconsistent.

Motivation

This is needed to support closures (e.g. smart contract scripts, complex spending conditions) that require non-default sighash semantics — the same flexibility the forfeit tx path already has. Without it, any future closure type that commits to a specific sighash flag cannot be verified through the general VerifyTapscriptSigs utility.

Fix

In pkg/ark-lib/txutils/utils.go, replace the hardcoded txscript.SigHashDefault with tapscriptSig.SigHash:

sighash, err = txscript.CalcTapscriptSignaturehash(
    txSigHashes,
    tapscriptSig.SigHash,
    tx.UnsignedTx,
    inputIndex,
    prevoutFetcher,
    leaf,
)

Relevant Files

• pkg/ark-lib/txutils/utils.go — VerifyTapscriptSigs (needs fix)
• internal/infrastructure/tx-builder/covenantless/builder.go — verifyTapscriptPartialSigs (reference implementation)
• pkg/arkd-wallet/core/application/wallet/service.go — signing already handles custom sighash types
• pkg/ark-lib/script/closure.go — closure types currently document SIGHASH_DEFAULT assumption in comments (should be updated)

[arkanaai]

🤖 Issue Triage — Protocol-Critical Bug

Classification: Bug — Transaction signing inconsistency

Severity: PROTOCOL-CRITICAL — This affects tapscript signature verification, which is core to transaction validity.

Root Cause

VerifyTapscriptSigs in pkg/ark-lib/txutils/utils.go hardcodes txscript.SigHashDefault when computing the sighash for verification, but the signing path in pkg/arkd-wallet/core/application/wallet/service.go already respects the PSBT's input.SighashType. This asymmetry means any tapscript input signed with a non-default sighash type will fail verification even when valid.

Fix Location

• File: pkg/ark-lib/txutils/utils.go — VerifyTapscriptSigs
• Change: Replace txscript.SigHashDefault with tapscriptSig.SigHash (same pattern as internal/infrastructure/tx-builder/covenantless/builder.go:verifyTapscriptPartialSigs)

Impact

• Breaks verification for any closure type with non-default sighash semantics
• Current closures using SIGHASH_DEFAULT are unaffected today, but this will block future closure types
• Referenced in pkg/ark-lib/script/closure.go comments (needs update alongside fix)

⚠️ Flagged for human review — protocol-critical code path (transaction signing). Straightforward fix but needs careful review before merge.

[arkanaai]

Triage analysis (issue-triage agent, 2026-04-08)

Classification: bug (already labeled) — checking current codebase state

Looking at the local clone of pkg/ark-lib/script/verify.go, lines 163–166 already read tapscriptSig.SigHash with a fallback to txscript.SigHashDefault when zero:

sigHashType := tapscriptSig.SigHash
if sigHashType == 0 {
    sigHashType = txscript.SigHashDefault
}

This suggests the fix may already be present in a recent commit (the local HEAD is d372608b). If so, this issue can be closed once the fix is confirmed merged to main. If the fix is in-flight on a branch, it needs a test added for non-default sighash types before merge.

Affected files (for reference):

• pkg/ark-lib/script/verify.go — VerifyTapscriptSigs()
• pkg/ark-lib/script/closure.go — sighash documentation comments may need updating

[arkanaai]

[Triage — issue-triage agent] PROTOCOL-CRITICAL flag

This issue touches tapscript signature verification and forfeit transaction signing, which is protocol-critical territory.

Per Arkana review rules: any code touching forfeit paths, VTXO signing, or tapscript verification requires explicit human review and sign-off, even if the change appears correct.

Please ensure this issue/PR receives human approval before merge.