Remove an array allocation in Rfc2898DeriveBytes::Func.

Changed Helpers::Int to write the value inline, and changed its name to WriteInt.

Author: bartonjs

src/System.Security.Cryptography.DeriveBytes/src/Internal/Cryptography/Helpers.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft. All rights reserved.
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
+using System.Diagnostics;
 using System.Security.Cryptography;
 
 namespace Internal.Cryptography
@@ -25,9 +26,18 @@ public static byte[] GenerateRandom(int count)
         }
 
         // encodes the integer i into a 4-byte array, in big endian.
-        public static byte[] Int(uint i)
+        public static void WriteInt(uint i, byte[] arr, int offset)
         {
-            return unchecked(new byte[] { (byte)(i >> 24), (byte)(i >> 16), (byte)(i >> 8), (byte)i });
+            unchecked
+            {
+                Debug.Assert(arr != null);
+                Debug.Assert(arr.Length >= offset + sizeof(uint));
+
+                arr[offset] = (byte)(i >> 24);
+                arr[offset + 1] = (byte)(i >> 16);
+                arr[offset + 2] = (byte)(i >> 8);
+                arr[offset + 3] = (byte)i;
+            }
         }
     }
 }

src/System.Security.Cryptography.DeriveBytes/src/System/Security/Cryptography/Rfc2898DeriveBytes.cs

@@ -178,11 +178,9 @@ private void Initialize()
         // where i is the block number.
         private byte[] Func()
         {
-            byte[] INT_block = Helpers.Int(_block);
-
-            byte[] temp = new byte[_salt.Length + INT_block.Length];
+            byte[] temp = new byte[_salt.Length + sizeof(uint)];
             Buffer.BlockCopy(_salt, 0, temp, 0, _salt.Length);
-            Buffer.BlockCopy(INT_block, 0, temp, _salt.Length, INT_block.Length);
+            Helpers.WriteInt(_block, temp, _salt.Length);
 
             temp = _hmacSha1.ComputeHash(temp);