Base64Encode.cs

public static string Base64Encode(string plainText)
{
    var plainTextBytes = System.Text.Encoding.UTF8.GetBytes(plainText);
    return Convert.ToBase64String(plainTextBytes);
}

public static string Base64Decode(string base64EncodedData)
{
    var base64EncodedBytes = System.Convert.FromBase64String(base64EncodedData);
    return Encoding.UTF8.GetString(base64EncodedBytes);
}