Bugfix for Microsoft.AspNetCore.Identity.Base32.ToBase32 function (#19621)

* Bugfix for Microsoft.AspNetCore.Identity.Base32.ToBase32 function

* Add Base32 class tests

src/Identity/Extensions.Core/src/Base32.cs

@@ -89,7 +89,7 @@ namespace Microsoft.AspNetCore.Identity
             uint b1, b2, b3, b4, b5;
 
             int retVal;
-            switch (offset - input.Length)
+            switch (input.Length - offset)
             {
                 case 1: retVal = 2; break;
                 case 2: retVal = 4; break;

src/Identity/test/Identity.Test/Base32Test.cs

@@ -0,0 +1,40 @@
+// 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.Security.Cryptography;
+using Xunit;
+
+namespace Microsoft.AspNetCore.Identity.Test
+{
+    public class Base32Test
+    {
+        [Fact]
+        public void ConversionTest()
+        {
+            var data = new byte[] { 1, 2, 3, 4, 5, 6 };
+            Assert.Equal<byte>(data, Base32.FromBase32(Base32.ToBase32(data)));
+
+            int length;
+            do {
+                length = GetRandomByteArray(1)[0];
+            } while (length % 5 == 0);
+            data = GetRandomByteArray(length);
+            Assert.Equal<byte>(data, Base32.FromBase32(Base32.ToBase32(data)));
+
+            length = (int)(GetRandomByteArray(1)[0]) * 5;
+            data = GetRandomByteArray(length);
+            Assert.Equal<byte>(data, Base32.FromBase32(Base32.ToBase32(data)));
+        }
+
+
+        private static readonly RandomNumberGenerator _rng = RandomNumberGenerator.Create();
+
+        private static byte[] GetRandomByteArray(int length) {
+            byte[] bytes = new byte[length];
+            _rng.GetBytes(bytes);
+            return bytes;
+        }
+    }
+
+}