Base64Url.cs

    // https://tools.ietf.org/html/rfc4648#section-5
    public class Base64Url
    {
        public static string Encode(byte[] inArray)
        {
            var base64 = Convert.ToBase64String(inArray);
            return base64
                .Replace("+", "-")
                .Replace("/", "_")
                .Replace("=", "");
        }

        public static byte[] Decode(string s)
        {
            var sb = new StringBuilder(s);
            sb.Replace("-", "+");
            sb.Replace("_", "/");

            while (sb.Length * 6 % 8 != 0)
            {
                sb.Append("=");
            }

            return Convert.FromBase64String(sb.ToString());
        }
    }