add Base64UrlHelper (#238)

* add Base64UrlHelper

* Update Base64UrlHelper.cs

Author: VeryEarly

src/Common/Utilities/Base64UrlHelper.cs

@@ -0,0 +1,96 @@
+// ----------------------------------------------------------------------------------
+//
+// Copyright Microsoft Corporation
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+// ----------------------------------------------------------------------------------
+
+using System;
+using System.Globalization;
+using System.Text;
+
+namespace Microsoft.Azure.Commands.Profile.Utilities
+{
+    //The source code is copied from https://github.com/AzureAD/microsoft-authentication-library-for-dotnet/blob/master/src/client/Microsoft.Identity.Client/Utils/Base64UrlHelpers.cs
+    public static class Base64UrlHelper
+    {
+        private const char Base64PadCharacter = '=';
+        private const char Base64Character62 = '+';
+        private const char Base64Character63 = '/';
+        private const char Base64UrlCharacter62 = '-';
+        private const char Base64UrlCharacter63 = '_';
+        private static readonly Encoding TextEncoding = Encoding.UTF8;
+
+        private static readonly string DoubleBase64PadCharacter = string.Format(CultureInfo.InvariantCulture, "{0}{0}",
+            Base64PadCharacter);
+
+        //
+        // The following functions perform base64url encoding which differs from regular base64 encoding as follows
+        // * padding is skipped so the pad character '=' doesn't have to be percent encoded
+        // * the 62nd and 63rd regular base64 encoding characters ('+' and '/') are replace with ('-' and '_')
+        // The changes make the encoding alphabet file and URL safe
+        // See RFC4648, section 5 for more info
+        //
+        public static string Encode(string arg)
+        {
+            if (arg == null)
+            {
+                return null;
+            }
+
+            return Encode(TextEncoding.GetBytes(arg));
+        }
+
+        public static string DecodeToString(string arg)
+        {
+            byte[] decoded = DecodeToBytes(arg);
+            return Encoding.UTF8.GetString(decoded, 0, decoded.Length);
+        }
+
+        public static byte[] DecodeToBytes(string arg)
+        {
+            string s = arg;
+            s = s.Replace(Base64UrlCharacter62, Base64Character62); // 62nd char of encoding
+            s = s.Replace(Base64UrlCharacter63, Base64Character63); // 63rd char of encoding
+
+            switch (s.Length % 4)
+            {
+                // Pad
+                case 0:
+                    break; // No pad chars in this case
+                case 2:
+                    s += DoubleBase64PadCharacter;
+                    break; // Two pad chars
+                case 3:
+                    s += Base64PadCharacter;
+                    break; // One pad char
+                default:
+                    throw new ArgumentException("Illegal base64url string!", nameof(arg));
+            }
+
+            return Convert.FromBase64String(s); // Standard base64 decoder
+        }
+
+        internal static string Encode(byte[] arg)
+        {
+            if (arg == null)
+            {
+                return null;
+            }
+
+            string s = Convert.ToBase64String(arg);
+            s = s.Split(Base64PadCharacter)[0]; // RemoveAccount any trailing padding
+            s = s.Replace(Base64Character62, Base64UrlCharacter62); // 62nd char of encoding
+            s = s.Replace(Base64Character63, Base64UrlCharacter63); // 63rd char of encoding
+
+            return s;
+        }
+    }
+}