4 files changed, 48 insertions, 10 deletions
diff --git a/src/script/sign.cpp b/src/script/sign.cpp
index f1ec5e2d13..838e502a0a 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 0fe97a80e8..97c0014cd0 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
diff --git a/src/script/standard.cpp b/src/script/standard.cpp
index 43961d0572..cfb3c58588 100644
--- a/src/script/standard.cpp
+++ b/src/script/standard.cpp
@@ -348,19 +348,14 @@ CScript GetScriptForWitness(const CScript& redeemscript)
     std::vector<std::vector<unsigned char> > vSolutions;
     if (Solver(redeemscript, typ, vSolutions)) {
         if (typ == TX_PUBKEY) {
-            unsigned char h160[20];
-            CHash160().Write(&vSolutions[0][0], vSolutions[0].size()).Finalize(h160);
-            ret << OP_0 << std::vector<unsigned char>(&h160[0], &h160[20]);
-            return ret;
+            return GetScriptForDestination(WitnessV0KeyHash(Hash160(vSolutions[0].begin(), vSolutions[0].end())));
         } else if (typ == TX_PUBKEYHASH) {
-           ret << OP_0 << vSolutions[0];
-           return ret;
+            return GetScriptForDestination(WitnessV0KeyHash(vSolutions[0]));
         }
     }
     uint256 hash;
     CSHA256().Write(&redeemscript[0], redeemscript.size()).Finalize(hash.begin());
-    ret << OP_0 << ToByteVector(hash);
-    return ret;
+    return GetScriptForDestination(WitnessV0ScriptHash(hash));
 }
 
 bool IsValidDestination(const CTxDestination& dest) {
diff --git a/src/script/standard.h b/src/script/standard.h
index 5925f2a512..3b2838a5bb 100644
--- a/src/script/standard.h
+++ b/src/script/standard.h
@@ -73,8 +73,19 @@ public:
     friend bool operator<(const CNoDestination &a, const CNoDestination &b) { return true; }
 };
 
-struct WitnessV0ScriptHash : public uint256 {};
-struct WitnessV0KeyHash : public uint160 {};
+struct WitnessV0ScriptHash : public uint256
+{
+    WitnessV0ScriptHash() : uint256() {}
+    explicit WitnessV0ScriptHash(const uint256& hash) : uint256(hash) {}
+    using uint256::uint256;
+};
+
+struct WitnessV0KeyHash : public uint160
+{
+    WitnessV0KeyHash() : uint160() {}
+    explicit WitnessV0KeyHash(const uint160& hash) : uint160(hash) {}
+    using uint160::uint160;
+};
 
 //! CTxDestination subtype to encode any future Witness version
 struct WitnessUnknown
@@ -144,6 +155,10 @@ bool ExtractDestination(const CScript& scriptPubKey, CTxDestination& addressRet)
  * addressRet is populated with a single value and nRequiredRet is set to 1.
  * Returns true if successful. Currently does not extract address from
  * pay-to-witness scripts.
+ *
+ * Note: this function confuses destinations (a subset of CScripts that are
+ * encodable as an address) with key identifiers (of keys involved in a
+ * CScript), and its use should be phased out.
  */
 bool ExtractDestinations(const CScript& scriptPubKey, txnouttype& typeRet, std::vector<CTxDestination>& addressRet, int& nRequiredRet);
 
@@ -164,6 +179,9 @@ CScript GetScriptForMultisig(int nRequired, const std::vector<CPubKey>& keys);
  * Generate a pay-to-witness script for the given redeem script. If the redeem
  * script is P2PK or P2PKH, this returns a P2WPKH script, otherwise it returns a
  * P2WSH script.
+ *
+ * TODO: replace calls to GetScriptForWitness with GetScriptForDestination using
+ * the various witness-specific CTxDestination subtypes.
  */
 CScript GetScriptForWitness(const CScript& redeemscript);