Optimize hashing of generic types

Use a proper hash function instead of f(x)=x to achieve better distribution of hash values

(cherry picked from commit 5eeff1d9c15dfdce6ade9bed455e8fdaab777b31)

src/crypto/generic-ops.h

@@ -32,9 +32,13 @@
 
 #include <cstddef>
 #include <cstring>
+#include <cstdint>
 #include <functional>
 #include <memory>
 #include <sodium/crypto_verify_32.h>
+#include <sodium/crypto_shorthash_siphash24.h>
+
+#include "random.h"
 
 #define CRYPTO_MAKE_COMPARABLE(type) \
 namespace crypto { \
@@ -57,22 +61,29 @@ namespace crypto { \
   } \
 }
 
+namespace crypto {
+  inline std::size_t siphash_to_size_t(const void *data, std::size_t length) {
+    static_assert(sizeof(crypto_siphash_key) == crypto_shorthash_siphash24_KEYBYTES,
+      "crypto_siphash_key size must match the SipHash-2-4 key length");
+    static_assert(sizeof(std::uint64_t) == crypto_shorthash_siphash24_BYTES,
+      "std::uint64_t size must match the SipHash-2-4 digest length");
+    std::uint64_t h;
+    crypto_shorthash_siphash24(reinterpret_cast<unsigned char*>(&h), static_cast<const unsigned char*>(data), length, crypto_siphash_key);
+    return h;
+  }
+}
+
 #define CRYPTO_DEFINE_HASH_FUNCTIONS(type) \
 namespace crypto { \
-  static_assert(sizeof(std::size_t) <= sizeof(type), "Size of " #type " must be at least that of size_t"); \
   inline std::size_t hash_value(const type &_v) { \
-    std::size_t h; \
-    memcpy(&h, std::addressof(_v), sizeof(h)); \
-    return h; \
+    return siphash_to_size_t(std::addressof(_v), sizeof(_v)); \
   } \
 } \
 namespace std { \
   template<> \
   struct hash<crypto::type> { \
     std::size_t operator()(const crypto::type &_v) const { \
-      std::size_t h; \
-      memcpy(&h, std::addressof(_v), sizeof(h)); \
-      return h; \
+      return ::crypto::siphash_to_size_t(std::addressof(_v), sizeof(_v)); \
     } \
   }; \
 }

src/crypto/random.c

@@ -96,6 +96,7 @@ static void generate_system_random_bytes(size_t n, void *result) {
 #endif
 
 static union hash_state state;
+unsigned char crypto_siphash_key[16];
 
 #if !defined(NDEBUG)
 static volatile int curstate; /* To catch thread safety problems. */
@@ -107,10 +108,12 @@ FINALIZER(deinit_random) {
   curstate = 0;
 #endif
   memset(&state, 0, sizeof(union hash_state));
+  memset(crypto_siphash_key, 0, sizeof(crypto_siphash_key));
 }
 
 INITIALIZER(init_random) {
   generate_system_random_bytes(32, &state);
+  generate_system_random_bytes(sizeof(crypto_siphash_key), crypto_siphash_key);
   REGISTER_FINALIZER(deinit_random);
 #if !defined(NDEBUG)
   assert(curstate == 0);

src/crypto/random.h

@@ -32,5 +32,15 @@
 
 #include <stddef.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 void generate_random_bytes_not_thread_safe(size_t n, void *result);
 void add_extra_entropy_not_thread_safe(const void *ptr, size_t bytes);
+
+extern unsigned char crypto_siphash_key[16];
+
+#ifdef __cplusplus
+}
+#endif

src/ringct/rctTypes.h

@@ -789,7 +789,7 @@ inline std::ostream &operator <<(std::ostream &o, const rct::key &v) {
 
 namespace std
 {
-  template<> struct hash<rct::key> { std::size_t operator()(const rct::key &k) const { return reinterpret_cast<const std::size_t&>(k); } };
+  template<> struct hash<rct::key> { std::size_t operator()(const rct::key &k) const { return ::crypto::siphash_to_size_t(&k, sizeof(k)); } };
 }
 
 BLOB_SERIALIZER(rct::key);