diff --git a/src/crypto.cpp b/src/crypto.cpp
index 9cf064c..def5e30 100644
--- a/src/crypto.cpp
+++ b/src/crypto.cpp
@@ -159,14 +159,14 @@ class Cache
 public:
 	Cache()
 	{
-		uv_mutex_init_checked(&derivations_lock);
+		uv_rwlock_init_checked(&derivations_lock);
 		uv_rwlock_init_checked(&public_keys_lock);
 		uv_rwlock_init_checked(&tx_keys_lock);
 	}
 
 	~Cache()
 	{
-		uv_mutex_destroy(&derivations_lock);
+		uv_rwlock_destroy(&derivations_lock);
 		uv_rwlock_destroy(&public_keys_lock);
 		uv_rwlock_destroy(&tx_keys_lock);
 	}
@@ -177,33 +177,45 @@ public:
 		memcpy(index.data(), key1.h, HASH_SIZE);
 		memcpy(index.data() + HASH_SIZE, key2.h, HASH_SIZE);
 
+		derivation = {};
 		{
-			MutexLock lock(derivations_lock);
+			ReadLock lock(derivations_lock);
 			auto it = derivations.find(index);
 			if (it != derivations.end()) {
-				derivation = it->second.m_derivation;
-				view_tag = it->second.get_view_tag(output_index);
-				return true;
+				const DerivationEntry& entry = it->second;
+				derivation = entry.m_derivation;
+				if (entry.find_view_tag(output_index, view_tag)) {
+					return true;
+				}
 			}
 		}
 
-		ge_p3 point;
-		ge_p2 point2;
-		ge_p1p1 point3;
+		if (derivation.empty()) {
+			ge_p3 point;
+			ge_p2 point2;
+			ge_p1p1 point3;
 
-		if (ge_frombytes_vartime(&point, key1.h) != 0) {
-			return false;
+			if (ge_frombytes_vartime(&point, key1.h) != 0) {
+				return false;
+			}
+
+			ge_scalarmult(&point2, key2.h, &point);
+			ge_mul8(&point3, &point2);
+			ge_p1p1_to_p2(&point2, &point3);
+			ge_tobytes(reinterpret_cast<uint8_t*>(&derivation), &point2);
 		}
 
-		ge_scalarmult(&point2, key2.h, &point);
-		ge_mul8(&point3, &point2);
-		ge_p1p1_to_p2(&point2, &point3);
-		ge_tobytes(reinterpret_cast<uint8_t*>(&derivation), &point2);
+		derive_view_tag(derivation, output_index, view_tag);
 
 		{
-			MutexLock lock(derivations_lock);
-			auto result = derivations.emplace(index, DerivationEntry{ derivation, {} });
-			view_tag = result.first->second.get_view_tag(output_index);
+			WriteLock lock(derivations_lock);
+
+			DerivationEntry& entry = derivations.emplace(index, DerivationEntry{ derivation, {} }).first->second;
+
+			const uint32_t k = static_cast<uint32_t>(output_index << 8) | view_tag;
+			if (std::find(entry.m_viewTags.begin(), entry.m_viewTags.end(), k) == entry.m_viewTags.end()) {
+				entry.m_viewTags.emplace_back(k);
+			}
 		}
 
 		return true;
@@ -285,7 +297,7 @@ public:
 
 	void clear()
 	{
-		{ MutexLock lock(derivations_lock); derivations.clear(); }
+		{ WriteLock lock(derivations_lock); derivations.clear(); }
 		{ WriteLock lock(public_keys_lock); public_keys.clear(); }
 		{ WriteLock lock(tx_keys_lock); tx_keys.clear(); }
 	}
@@ -296,22 +308,18 @@ private:
 		hash m_derivation;
 		std::vector<uint32_t> m_viewTags;
 
-		uint8_t get_view_tag(size_t output_index) {
+		bool find_view_tag(size_t output_index, uint8_t& view_tag) const {
 			for (uint32_t k : m_viewTags) {
 				if ((k >> 8) == output_index) {
-					return static_cast<uint8_t>(k);
+					view_tag = static_cast<uint8_t>(k);
+					return true;
 				}
 			}
-
-			uint8_t t;
-			derive_view_tag(m_derivation, output_index, t);
-			m_viewTags.emplace_back(static_cast<uint32_t>(output_index << 8) | t);
-
-			return t;
+			return false;
 		}
 	};
 
-	uv_mutex_t derivations_lock;
+	uv_rwlock_t derivations_lock;
 	unordered_map<std::array<uint8_t, HASH_SIZE * 2>, DerivationEntry> derivations;
 
 	uv_rwlock_t public_keys_lock;