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

1. 
   billymcbip commented at 3:30 pm on November 27, 2025: none

   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.

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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: none

   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

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