script: Add a separate ScriptError for empty pubkeys encountered in Tapscript #33961

1. 
   billymcbip commented at 3:30 pm on November 27, 2025: contributor

   We currently have two callsites for SCRIPT_ERR_PUBKEYTYPE:

   • A pre-tapscript policy error behind the SCRIPT_VERIFY_STRICTENC flag: https://github.com/bitcoin/bitcoin/blob/4de26b111f4d9b116d0d4930f8bbd69889b3cf75/src/script/interpreter.cpp#L220
   • A consensus error in Tapscript: https://github.com/bitcoin/bitcoin/blob/4de26b111f4d9b116d0d4930f8bbd69889b3cf75/src/script/interpreter.cpp#L368

   It would be good for readability and testability to have separate errors for both cases, as they are quite distinct (policy vs. consensus, format vs. emptiness).

   This PR adds SCRIPT_ERR_TAPSCRIPT_EMPTY_PUBKEY for the consensus error path.

   This change would make our error handling more consistent. We have more granular errors for other pubkey error paths already: SCRIPT_ERR_WITNESS_PUBKEYTYPE, SCRIPT_ERR_DISCOURAGE_UPGRADABLE_PUBKEYTYPE. We also have separate errors for MINIMAL_IF: SCRIPT_ERR_MINIMALIF for the policy error pre-tapscript, and SCRIPT_ERR_TAPSCRIPT_MINIMALIF for the consensus error post-tapscript.

   Tests:

   Added a test case to script_tests and ran build/bin/test_bitcoin --run_test=script_tests --log_level=success.

   0test/script_tests.cpp:144: info: check '[["aa","#SCRIPT# 0 CHECKSIG","#CONTROLBLOCK#",0.00000001],"","0x51 0x20 #TAPROOTOUTPUT#","P2SH,WITNESS,TAPROOT","TAPSCRIPT_EMPTY_PUBKEY","TAPSCRIPT: OP_CHECKSIG with empty pubkey must fail"] (with flags 165d5d)' has passed
   1...
   

   Ran DIR_UNIT_TEST_DATA="$(pwd)/../qa-assets/unit_test_data" build/bin/test_bitcoin --run_test=script_assets_tests --log_level=success.

   Updated feature_taproot.py and ran build/test/functional/feature_taproot.py.

   Looking forward to your feedback.

2. 
   DrahtBot commented at 3:30 pm on November 27, 2025: contributor

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

   Code Coverage & Benchmarks

   For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/33961.

   Reviews

   See the guideline for information on the review process.

   Type	Reviewers
   ACK	sedited, darosior, sipa

   If your review is incorrectly listed, please copy-paste <!–meta-tag:bot-skip–> into the comment that the bot should ignore.

3. script: add SCRIPT_ERR_TAPSCRIPT_EMPTY_PUBKEY

   Empty public keys in tapscript are rejected by consensus rules, independent of SCRIPT_VERIFY_STRICTENC. Add SCRIPT_ERR_TAPSCRIPT_EMPTY_PUBKEY to distinguish this from STRICTENC policy failures currently reported as SCRIPT_ERR_PUBKEYTYPE.

   9d5021a05b
4. billymcbip force-pushed on Nov 27, 2025
5. 
   billymcbip commented at 4:29 pm on November 27, 2025: contributor

   Also updatedfeature_taproot.py and ran build/test/functional/feature_taproot.py. Can we re-run CI?
6. DrahtBot added the label CI failed on Nov 27, 2025
7. DrahtBot removed the label CI failed on Nov 28, 2025
8. billymcbip renamed this:
   Add a separate ScriptError for empty pubkeys encountered in Tapscript
   script: Add a separate ScriptError for empty pubkeys encountered in Tapscript
   on Dec 1, 2025
9. DrahtBot added the label Consensus on Dec 1, 2025
10. 
    darosior commented at 8:00 pm on December 4, 2025: member

    Concept/Approach ACK.
11. sedited approved
12. 
    sedited commented at 10:32 pm on December 4, 2025: contributor

    ACK 9d5021a05bd33c73276909eec961777867ddb412
13. DrahtBot requested review from darosior on Dec 4, 2025
14. darosior approved
15. 
    darosior commented at 2:49 pm on December 5, 2025: member

    utACK 9d5021a05bd33c73276909eec961777867ddb412
16. 
    sipa commented at 2:53 pm on December 5, 2025: member

    ACK 9d5021a05bd33c73276909eec961777867ddb412
17. billymcbip closed this on Dec 7, 2025

    ---

18. billymcbip deleted the branch on Dec 7, 2025
19. 
    darosior commented at 3:40 pm on December 7, 2025: member

    Force-push gone wrong @billymcbip? Or is this intentional?
20. billymcbip restored the branch on Dec 7, 2025
21. 
    billymcbip commented at 3:51 pm on December 7, 2025: contributor

    @darosior that was unintentional, sorry! I still intend to ship 9d5021a.
22. billymcbip reopened this on Dec 7, 2025

    ---

23. glozow merged this on Dec 9, 2025
24. glozow closed this on Dec 9, 2025

    ---

25. billymcbip deleted the branch on Dec 10, 2025

Contributors
billymcbip DrahtBot darosior sedited sipa

Labels
Consensus