Refine RNG usage

- Create instance once, bind it to ShadowsocksController

Signed-off-by: Syrone Wong <wong.syrone@gmail.com>

shadowsocks-csharp/Encryption/IVEncryptor.cs

@@ -1,8 +1,6 @@
 using System;
 using System.Collections.Generic;
 using System.Collections.Concurrent;
-using System.Linq;
-using System.Security.Cryptography;
 using System.Text;
 using System.Net;
 
@@ -237,10 +235,7 @@ namespace Shadowsocks.Encryption
 
         protected static void randBytes(byte[] buf, int length)
         {
-            using (RNGCryptoServiceProvider rng = new RNGCryptoServiceProvider())
-            {
-                rng.GetBytes(buf, 0, length);
-            }
+            RNG.GetBytes(buf, length);
         }
 
         public override void Encrypt(byte[] buf, int length, byte[] outbuf, out int outlength)

shadowsocks-csharp/Encryption/RNG.cs

@@ -0,0 +1,44 @@
+using System;
+using System.Security.Cryptography;
+
+namespace Shadowsocks.Encryption
+{
+    public static class RNG
+    {
+        private static RNGCryptoServiceProvider _rng = null;
+
+        public static void Init()
+        {
+            if (_rng == null)
+                _rng = new RNGCryptoServiceProvider();
+        }
+
+        public static void Close()
+        {
+            if (_rng == null) return;
+            _rng.Dispose();
+            _rng = null;
+        }
+
+        public static void Reload()
+        {
+            Close();
+            Init();
+        }
+
+        public static void GetBytes(byte[] buf, int len)
+        {
+            try
+            {
+                _rng.GetBytes(buf, 0, len);
+            }
+            catch (Exception)
+            {
+                // the backup way
+                byte[] tmp = new byte[len];
+                _rng.GetBytes(tmp);
+                Buffer.BlockCopy(tmp, 0, buf, 0, len);
+            }
+        }
+    }
+}