Abstract out IsSolvable from Witnessifier

2 files changed, 25 insertions, 0 deletions

diff --git a/src/script/sign.cpp b/src/script/sign.cpp
index 117a4d8a52..49099bf7b0 100644
--- a/src/script/sign.cpp
+++ b/src/script/sign.cpp
@@ -422,3 +422,22 @@ bool DummySignatureCreator::CreateSig(std::vector<unsigned char>& vchSig, const
     vchSig[6 + 33 + 32] = SIGHASH_ALL;
     return true;
 }
+
+bool IsSolvable(const CKeyStore& store, const CScript& script)
+{
+    // This check is to make sure that the script we created can actually be solved for and signed by us
+    // if we were to have the private keys. This is just to make sure that the script is valid and that,
+    // if found in a transaction, we would still accept and relay that transaction. In particular,
+    // it will reject witness outputs that require signing with an uncompressed public key.
+    DummySignatureCreator creator(&store);
+    SignatureData sigs;
+    // Make sure that STANDARD_SCRIPT_VERIFY_FLAGS includes SCRIPT_VERIFY_WITNESS_PUBKEYTYPE, the most
+    // important property this function is designed to test for.
+    static_assert(STANDARD_SCRIPT_VERIFY_FLAGS & SCRIPT_VERIFY_WITNESS_PUBKEYTYPE, "IsSolvable requires standard script flags to include WITNESS_PUBKEYTYPE");
+    if (ProduceSignature(creator, script, sigs)) {
+        // VerifyScript check is just defensive, and should never fail.
+        assert(VerifyScript(sigs.scriptSig, script, &sigs.scriptWitness, STANDARD_SCRIPT_VERIFY_FLAGS, creator.Checker()));
+        return true;
+    }
+    return false;
+}
diff --git a/src/script/sign.h b/src/script/sign.h
index 400c0c0865..ef6aa7f311 100644
--- a/src/script/sign.h
+++ b/src/script/sign.h
@@ -81,4 +81,10 @@ SignatureData CombineSignatures(const CScript& scriptPubKey, const BaseSignature
 SignatureData DataFromTransaction(const CMutableTransaction& tx, unsigned int nIn);
 void UpdateTransaction(CMutableTransaction& tx, unsigned int nIn, const SignatureData& data);
 
+/* Check whether we know how to sign for an output like this, assuming we
+ * have all private keys. While this function does not need private keys, the passed
+ * keystore is used to look up public keys and redeemscripts by hash.
+ * Solvability is unrelated to whether we consider this output to be ours. */
+bool IsSolvable(const CKeyStore& store, const CScript& script);
+
 #endif // BITCOIN_SCRIPT_SIGN_H