Taproot semantics unenforced for signet challenges #36162

Probably should be activated for custom signets

diff --git a/src/signet.cpp b/src/signet.cpp
index 87e9f79c27..d9d263ebad 100644
--- a/src/signet.cpp
+++ b/src/signet.cpp
@@ -27,7 +27,7 @@
 
 static constexpr uint8_t SIGNET_HEADER[4] = {0xec, 0xc7, 0xda, 0xa2};
 
-static constexpr script_verify_flags BLOCK_SCRIPT_VERIFY_FLAGS = SCRIPT_VERIFY_P2SH | SCRIPT_VERIFY_WITNESS | SCRIPT_VERIFY_DERSIG | SCRIPT_VERIFY_NULLDUMMY;
+static constexpr script_verify_flags BLOCK_SCRIPT_VERIFY_FLAGS = SCRIPT_VERIFY_P2SH | SCRIPT_VERIFY_WITNESS | SCRIPT_VERIFY_TAPROOT | SCRIPT_VERIFY_DERSIG | SCRIPT_VERIFY_NULLDUMMY;
 
 static bool FetchAndClearCommitmentSection(const std::span<const uint8_t> header, CScript& witness_commitment, std::vector<uint8_t>& result)
 {

diff --git a/src/test/validation_tests.cpp b/src/test/validation_tests.cpp
--- a/src/test/validation_tests.cpp
+++ b/src/test/validation_tests.cpp
@@ -96,6 +96,14 @@ BOOST_AUTO_TEST_CASE(signet_parse_tests)
     block.vtx.at(0) = MakeTransactionRef(cb);
     BOOST_CHECK(SignetTxs::Create(block, challenge));
     BOOST_CHECK(CheckSignetBlockSolution(block, signet_params->GetConsensus()));
+
+    // A Taproot challenge must not be treated as an unknown, anyone-can-spend
+    // witness version when no solution is provided.
+    CScript taproot_challenge{OP_1};
+    taproot_challenge << std::vector<uint8_t>(32, 0x42);
+    auto taproot_params = signet_params->GetConsensus();
+    taproot_params.signet_challenge.assign(taproot_challenge.begin(), taproot_challenge.end());
+    BOOST_CHECK(!CheckSignetBlockSolution(block, taproot_params));
 
     // no data after header, valid
     std::vector<uint8_t> witness_commitment_section_325{0xec, 0xc7, 0xda, 0xa2};

cc @ajtowns

This is a soft-fork for signet, so should probably be accompanied by an update to BIP 325, even if the current spec isn't very detailed on the subject. I think the only benefit to having a taproot challenge today is it allows use of BIP340 signatures and the potential for musig/frost.

seems hard to "get wrong" except a custom signet accidentally being no-op challenge. Could also just guard against such a misconfig 🤷