Use cryptographic RNG to generate the shard connection serial key

src/Rand.cpp

@@ -1,4 +1,5 @@
 #include "Rand.hpp"
+#include "core/Core.hpp"
 
 std::unique_ptr<std::mt19937> Rand::generator;
 
@@ -33,6 +34,58 @@ float Rand::randFloat() {
     return Rand::randFloat(0.0f, 1.0f);
 }
 
+#define RANDBYTES 8
+
+/*
+ * Cryptographically secure RNG. Borrowed from bcrypt_gensalt().
+ */
+uint64_t Rand::cryptoRand() {
+    uint8_t buf[RANDBYTES];
+
+#ifdef _WIN32
+    HCRYPTPROV p;
+
+    // Acquire a crypt context for generating random bytes.
+    if (CryptAcquireContext(&p, NULL, NULL, PROV_RSA_FULL, CRYPT_VERIFYCONTEXT) == FALSE) {
+        goto fail;
+    }
+
+    if (CryptGenRandom(p, RANDBYTES, (BYTE*)buf) == FALSE) {
+        goto fail;
+    }
+
+    if (CryptReleaseContext(p, 0) == FALSE) {
+        goto fail;
+    }
+#else
+    int fd;
+
+    // Get random bytes on Unix/Linux.
+    fd = open("/dev/urandom", O_RDONLY);
+    if (fd < 0) {
+        perror("open");
+        goto fail;
+    }
+
+    if (read(fd, buf, RANDBYTES) < RANDBYTES) {
+        perror("read");
+        close(fd);
+        goto fail;
+    }
+
+    close(fd);
+#endif
+
+    return *(uint64_t*)buf;
+
+fail:
+    std::cout << "[FATAL] Failed to generate cryptographic random number" << std::endl;
+    terminate(0);
+
+    /* not reached */
+    return 0;
+}
+
 void Rand::init(uint64_t seed) {
     Rand::generator = std::make_unique<std::mt19937>(std::mt19937(seed));
 }

src/Rand.hpp

@@ -14,6 +14,8 @@ namespace Rand {
 
     int32_t randWeighted(const std::vector<int32_t>& weights);
 
+    uint64_t cryptoRand();
+
     float randFloat(float startInclusive, float endExclusive);
     float randFloat(float endExclusive);
     float randFloat();

src/servers/CNLoginServer.cpp

@@ -477,7 +477,7 @@ void CNLoginServer::characterSelect(CNSocket* sock, CNPacketData* data) {
     if (lm->plr.iID == 0)
         return invalidCharacter(sock);
 
-    resp.iEnterSerialKey = Rand::rand(); // TODO: cryptographic RNG
+    resp.iEnterSerialKey = Rand::cryptoRand();
 
     // transfer ownership of connection data to CNShared
     CNShared::storeLoginMetadata(resp.iEnterSerialKey, lm);