@@ -0,0 +1,44 @@
+using System.Text;
+using NUnit.Framework;
+using SpotifyAPI.Web;
+
+namespace SpotifyAPI.Web.Tests
+{
+  [TestFixture]
+  public class Base64UtilTest
+  {
+    [Test]
+    public void Base64UrlDecode_Works()
+    {
+      var encoded = "SGVsbG9Xb3JsZA==";
+
+      Assert.AreEqual("HelloWorld", Encoding.UTF8.GetString(Base64Util.UrlDecode(encoded)));
+    }
+
+    [Test]
+    public void Base64UrlEncode_Works()
+    {
+      var decoded = "HelloWorld";
+
+      Assert.AreEqual("SGVsbG9Xb3JsZA==", Base64Util.UrlEncode(Encoding.UTF8.GetBytes(decoded)));
+    }
+
+    [Test]
+    public void Base64UrlEncode_WorksSpecialChars()
+    {
+      var bytes = new byte[] { 0x04, 0x9f, 0x9c, 0xff, 0x3f, 0x0a };
+
+      // normal base64: BJ+c/z8K
+      Assert.AreEqual("BJ-c_z8K", Base64Util.UrlEncode(bytes));
+    }
+
+    [Test]
+    public void Base64UrlDecode_WorksSpecialChars()
+    {
+      var bytes = new byte[] { 0x04, 0x9f, 0x9c, 0xff, 0x3f, 0x0a };
+
+      // normal base64: BJ+c/z8K
+      Assert.AreEqual(bytes, Base64Util.UrlDecode("BJ-c_z8K"));
+    }
+  }
+}
@@ -0,0 +1,3 @@
+using System.Runtime.CompilerServices;
+
+[assembly: InternalsVisibleTo("SpotifyAPI.Web.Tests")]
@@ -0,0 +1,123 @@
+
+using System;
+using System.Globalization;
+
+namespace SpotifyAPI.Web
+{
+  internal class Base64Util
+  {
+    internal const string WebEncoders_InvalidCountOffsetOrLength = "Invalid {0}, {1} or {2} length.";
+    internal const string WebEncoders_MalformedInput = "Malformed input: {0} is an invalid input length.";
+
+    public static string UrlEncode(byte[] input)
+    {
+      if (input == null)
+      {
+        throw new ArgumentNullException(nameof(input));
+      }
+
+      // Special-case empty input
+      if (input.Length == 0)
+      {
+        return string.Empty;
+      }
+
+      var buffer = new char[GetArraySizeRequiredToEncode(input.Length)];
+      var numBase64Chars = Convert.ToBase64CharArray(input, 0, input.Length, buffer, 0);
+
+      return new string(buffer, startIndex: 0, length: numBase64Chars);
+    }
+
+    public static byte[] UrlDecode(string input)
+    {
+      var buffer = new char[GetArraySizeRequiredToDecode(input.Length)];
+      if (input == null)
+      {
+        throw new ArgumentNullException(nameof(input));
+      }
+
+      if (count == 0)
+      {
+        return Array.Empty<byte>();
+      }
+
+      // Assumption: input is base64url encoded without padding and contains no whitespace.
+
+      var paddingCharsToAdd = GetNumBase64PaddingCharsToAddForDecode(count);
+      var arraySizeRequired = checked(count + paddingCharsToAdd);
+
+      // Copy input into buffer, fixing up '-' -> '+' and '_' -> '/'.
+      var i = 0;
+      for (var j = 0; i < count; i++, j++)
+      {
+        var ch = input[j];
+        if (ch == '-')
+        {
+          buffer[i] = '+';
+        }
+        else if (ch == '_')
+        {
+          buffer[i] = '/';
+        }
+        else
+        {
+          buffer[i] = ch;
+        }
+      }
+
+      // Add the padding characters back.
+      for (; paddingCharsToAdd > 0; i++, paddingCharsToAdd--)
+      {
+        buffer[i] = '=';
+      }
+
+      // Decode.
+      // If the caller provided invalid base64 chars, they'll be caught here.
+      return Convert.FromBase64CharArray(buffer, 0, arraySizeRequired);
+    }
+
+    private static int GetArraySizeRequiredToEncode(int count)
+    {
+      var numWholeOrPartialInputBlocks = checked(count + 2) / 3;
+      return checked(numWholeOrPartialInputBlocks * 4);
+    }
+
+    private static int GetArraySizeRequiredToDecode(int count)
+    {
+      if (count < 0)
+      {
+        throw new ArgumentOutOfRangeException(nameof(count));
+      }
+
+      if (count == 0)
+      {
+        return 0;
+      }
+
+      var numPaddingCharsToAdd = GetNumBase64PaddingCharsToAddForDecode(count);
+
+      return checked(count + numPaddingCharsToAdd);
+    }
+
+    private static int GetNumBase64PaddingCharsToAddForDecode(int inputLength)
+    {
+      switch (inputLength % 4)
+      {
+        case 0:
+          return 0;
+        case 2:
+          return 2;
+        case 3:
+          return 1;
+        default:
+          throw new FormatException(
+              string.Format(
+                  CultureInfo.CurrentCulture,
+                  WebEncoders_MalformedInput,
+                  inputLength));
+      }
+    }
+  }
+
+
+}
Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,3 @@`
	`1`	`+using System.Runtime.CompilerServices;`
	`2`	`+`
	`3`	`+[assembly: InternalsVisibleTo("SpotifyAPI.Web.Tests")]`