More RNG.Fill() and UnixEpoch usage (#18132)

Apply more changes omitted from #18126 and #18128, and use RandomNumberGenerator.Fill() and DateTimeOffset.UnixEpoch for supported TFMs.

Author: martincostello

src/DataProtection/DataProtection/src/Managed/ManagedGenRandomImpl.cs

@@ -8,7 +8,9 @@ namespace Microsoft.AspNetCore.DataProtection.Managed
 {
     internal unsafe sealed class ManagedGenRandomImpl : IManagedGenRandom
     {
+#if NETSTANDARD2_0
         private static readonly RandomNumberGenerator _rng = RandomNumberGenerator.Create();
+#endif
         public static readonly ManagedGenRandomImpl Instance = new ManagedGenRandomImpl();
 
         private ManagedGenRandomImpl()
@@ -18,7 +20,11 @@ private ManagedGenRandomImpl()
         public byte[] GenRandom(int numBytes)
         {
             var bytes = new byte[numBytes];
+#if NETSTANDARD2_0
             _rng.GetBytes(bytes);
+#else
+            RandomNumberGenerator.Fill(bytes);
+#endif
             return bytes;
         }
     }

src/Identity/Extensions.Core/src/Rfc6238AuthenticationService.cs

@@ -1,27 +1,32 @@
 // Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
+using System;
 using System.Diagnostics;
 using System.Net;
 using System.Security.Cryptography;
+using System.Text;
 
 namespace Microsoft.AspNetCore.Identity
 {
-    using System;
-    using System.Text;
-
     internal static class Rfc6238AuthenticationService
     {
-        private static readonly DateTime _unixEpoch = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
         private static readonly TimeSpan _timestep = TimeSpan.FromMinutes(3);
         private static readonly Encoding _encoding = new UTF8Encoding(false, true);
+#if NETSTANDARD2_0
+        private static readonly DateTime _unixEpoch = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
         private static readonly RandomNumberGenerator _rng = RandomNumberGenerator.Create();
+#endif
 
         // Generates a new 80-bit security token
         public static byte[] GenerateRandomKey()
         {
             byte[] bytes = new byte[20];
+#if NETSTANDARD2_0
             _rng.GetBytes(bytes);
+#else
+            RandomNumberGenerator.Fill(bytes);
+#endif
             return bytes;
         }
 
@@ -63,7 +68,11 @@ private static byte[] ApplyModifier(byte[] input, string modifier)
         // More info: https://tools.ietf.org/html/rfc6238#section-4
         private static ulong GetCurrentTimeStepNumber()
         {
+#if NETSTANDARD2_0
             var delta = DateTime.UtcNow - _unixEpoch;
+#else
+            var delta = DateTimeOffset.UtcNow - DateTimeOffset.UnixEpoch;
+#endif
             return (ulong)(delta.Ticks / _timestep.Ticks);
         }
 

src/Identity/Extensions.Core/src/UserManager.cs

@@ -43,7 +43,9 @@ public class UserManager<TUser> : IDisposable where TUser : class
 
         private TimeSpan _defaultLockout = TimeSpan.Zero;
         private bool _disposed;
+#if NETSTANDARD2_0
         private static readonly RandomNumberGenerator _rng = RandomNumberGenerator.Create();
+#endif
         private IServiceProvider _services;
 
         /// <summary>
@@ -2428,7 +2430,11 @@ private IUserRoleStore<TUser> GetUserRoleStore()
         private static string NewSecurityStamp()
         {
             byte[] bytes = new byte[20];
+#if NETSTANDARD2_0
             _rng.GetBytes(bytes);
+#else
+            RandomNumberGenerator.Fill(bytes);
+#endif
             return Base32.ToBase32(bytes);
         }