Expose method to find key for a single-key destination

src/keystore.cpp

@@ -136,3 +136,25 @@ bool CBasicKeyStore::HaveWatchOnly() const
     LOCK(cs_KeyStore);
     return (!setWatchOnly.empty());
 }
+
+CKeyID GetKeyForDestination(const CKeyStore& store, const CTxDestination& dest)
+{
+    // Only supports destinations which map to single public keys, i.e. P2PKH,
+    // P2WPKH, and P2SH-P2WPKH.
+    if (auto id = boost::get<CKeyID>(&dest)) {
+        return *id;
+    }
+    if (auto witness_id = boost::get<WitnessV0KeyHash>(&dest)) {
+        return CKeyID(*witness_id);
+    }
+    if (auto script_id = boost::get<CScriptID>(&dest)) {
+        CScript script;
+        CTxDestination inner_dest;
+        if (store.GetCScript(*script_id, script) && ExtractDestination(script, inner_dest)) {
+            if (auto inner_witness_id = boost::get<WitnessV0KeyHash>(&inner_dest)) {
+                return CKeyID(*inner_witness_id);
+            }
+        }
+    }
+    return CKeyID();
+}

src/keystore.h

@@ -78,4 +78,7 @@ public:
 typedef std::vector<unsigned char, secure_allocator<unsigned char> > CKeyingMaterial;
 typedef std::map<CKeyID, std::pair<CPubKey, std::vector<unsigned char> > > CryptedKeyMap;
 
+/** Return the CKeyID of the key involved in a script (if there is a unique one). */
+CKeyID GetKeyForDestination(const CKeyStore& store, const CTxDestination& dest);
+
 #endif // BITCOIN_KEYSTORE_H