From ef59ecd9f3b16c8c69bd8aaf14cc2fb231297d68 Mon Sep 17 00:00:00 2001
From: R-Goc <ryszardgoc@gmail.com>
Date: Fri, 11 Apr 2025 22:12:04 +0200
Subject: [PATCH] LibJS/LibCrypto: Cleanup JS Math random() RNG

This commit adds a convenience method to secure random for initializing
single types. It changes the random number generator in JS math random()
to use newer constants by the author as well as initializes it with a
higher quality seed.
---
 Libraries/LibCrypto/SecureRandom.h     |  8 ++++++++
 Libraries/LibJS/Runtime/MathObject.cpp | 17 +++++++++++------
 2 files changed, 19 insertions(+), 6 deletions(-)

diff --git a/Libraries/LibCrypto/SecureRandom.h b/Libraries/LibCrypto/SecureRandom.h
index 9c03d7cafe2..76b37cd3642 100644
--- a/Libraries/LibCrypto/SecureRandom.h
+++ b/Libraries/LibCrypto/SecureRandom.h
@@ -12,4 +12,12 @@ namespace Crypto {
 
 void fill_with_secure_random(Bytes);
 
+template<typename T>
+inline T get_secure_random()
+{
+    T t;
+    fill_with_secure_random({ &t, sizeof(T) });
+    return t;
+}
+
 }
diff --git a/Libraries/LibJS/Runtime/MathObject.cpp b/Libraries/LibJS/Runtime/MathObject.cpp
index 88d846c3888..705561c6b42 100644
--- a/Libraries/LibJS/Runtime/MathObject.cpp
+++ b/Libraries/LibJS/Runtime/MathObject.cpp
@@ -10,6 +10,7 @@
 #include <AK/BuiltinWrappers.h>
 #include <AK/Function.h>
 #include <AK/Random.h>
+#include <LibCrypto/SecureRandom.h>
 #include <LibJS/Runtime/AbstractOperations.h>
 #include <LibJS/Runtime/GlobalObject.h>
 #include <LibJS/Runtime/Iterator.h>
@@ -799,12 +800,15 @@ JS_DEFINE_NATIVE_FUNCTION(MathObject::pow)
     return pow_impl(vm, vm.argument(0), vm.argument(1));
 }
 
-class XorShift128PlusPlusRNG {
+// http://vigna.di.unimi.it/ftp/papers/xorshiftplus.pdf
+class XorShift128PlusRNG {
 public:
-    XorShift128PlusPlusRNG()
+    XorShift128PlusRNG()
     {
-        u64 seed = get_random<u32>();
+        // Splitmix64 is used as xorshift is sensitive to being seeded with all 0s
+        u64 seed = Crypto::get_secure_random<u64>();
         m_low = splitmix64(seed);
+        seed = Crypto::get_secure_random<u64>();
         m_high = splitmix64(seed);
     }
 
@@ -823,6 +827,7 @@ private:
         return z ^ (z >> 31);
     }
 
+    // Apparently this set of constants is better: https://stackoverflow.com/a/34432126
     u64 advance()
     {
         u64 s1 = m_low;
@@ -830,8 +835,8 @@ private:
         u64 const result = s0 + s1;
         m_low = s0;
         s1 ^= s1 << 23;
-        s1 ^= s1 >> 17;
-        s1 ^= s0 ^ (s0 >> 26);
+        s1 ^= s1 >> 18;
+        s1 ^= s0 ^ (s0 >> 5);
         m_high = s1;
         return result + s1;
     }
@@ -845,7 +850,7 @@ Value MathObject::random_impl()
     // This function returns a Number value with positive sign, greater than or equal to +0𝔽 but strictly less than 1𝔽,
     // chosen randomly or pseudo randomly with approximately uniform distribution over that range, using an
     // implementation-defined algorithm or strategy.
-    static XorShift128PlusPlusRNG rng;
+    static XorShift128PlusRNG rng;
     return Value(rng.get());
 }