byte[] bytes = Convert.FromBase64String(cipherText);

public static class Base64Helper
{
    public static byte[] TryParse(string s)
    {
        if (s == null) throw new ArgumentNullException("s");

        if ((s.Length % 4 == 0) && _rx.IsMatch(s))
        {
            try
            {
                return Convert.FromBase64String(s);
            }
            catch (FormatException)
            {
                // ignore
            }
        }
        return null;
    }

    private static readonly Regex _rx = new Regex(
        @"^(?:[A-Za-z0-9+/]{4})*(?:[A-Za-z0-9+/]{2}[AEIMQUYcgkosw048]=|[A-Za-z0-9+/][AQgw]==)?$",
        RegexOptions.Compiled);
}