Added Base64UrlTextEncoder utility from Security repo

Author: kichalla

src/Microsoft.AspNetCore.WebUtilities/Base64UrlTextEncoder.cs

@@ -0,0 +1,42 @@
+// 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;
+
+namespace Microsoft.AspNetCore.WebUtilities
+{
+    public static class Base64UrlTextEncoder
+    {
+        /// <summary>
+        /// Encodes supplied data into Base64 and replaces any URL encodable characters into non-URL encodable
+        /// characters.
+        /// </summary>
+        /// <param name="data">Data to be encoded.</param>
+        /// <returns>Base64 encoded string modified with non-URL encodable characters</returns>
+        public static string Encode(byte[] data)
+        {
+            return Convert.ToBase64String(data).TrimEnd('=').Replace('+', '-').Replace('/', '_');
+        }
+
+        /// <summary>
+        /// Decodes supplied string by replacing the non-URL encodable characters with URL encodable characters and
+        /// then decodes the Base64 string.
+        /// </summary>
+        /// <param name="text">The string to be decoded.</param>
+        /// <returns>The decoded data.</returns>
+        public static byte[] Decode(string text)
+        {
+            return Convert.FromBase64String(Pad(text.Replace('-', '+').Replace('_', '/')));
+        }
+
+        private static string Pad(string text)
+        {
+            var padding = 3 - ((text.Length + 3) % 4);
+            if (padding == 0)
+            {
+                return text;
+            }
+            return text + new string('=', padding);
+        }
+    }
+}

test/Microsoft.AspNetCore.WebUtilities.Tests/Base64UrlTextEncoderTests.cs

@@ -0,0 +1,30 @@
+// 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 Xunit;
+
+namespace Microsoft.AspNetCore.WebUtilities
+{
+    public class Base64UrlTextEncoderTests
+    {
+        [Fact]
+        public void DataOfVariousLengthRoundTripCorrectly()
+        {
+            for (int length = 0; length != 256; ++length)
+            {
+                var data = new byte[length];
+                for (int index = 0; index != length; ++index)
+                {
+                    data[index] = (byte)(5 + length + (index * 23));
+                }
+                string text = Base64UrlTextEncoder.Encode(data);
+                byte[] result = Base64UrlTextEncoder.Decode(text);
+
+                for (int index = 0; index != length; ++index)
+                {
+                    Assert.Equal(data[index], result[index]);
+                }
+            }
+        }
+    }
+}