Detecting a password-protected document

Question

Is there any way to know whether a doc/ppt/xls file is password-protected even before opening the file?

Answer 1

Here is a crude version of a password detecter i made. Does not need to open any Office objects.

    public static bool IsPassworded(string file) {
        var bytes = File.ReadAllBytes(file);
        return IsPassworded(bytes);
        return false;
    }
    public static bool IsPassworded(byte[] bytes) {
        var prefix = Encoding.Default.GetString(bytes.Take(2).ToArray());
        if (prefix == "PK") {
            //ZIP and not password protected
            return false;
        }
        if (prefix == "ÐÏ") {
            //Office format.

            //Flagged with password
            if (bytes.Skip(0x20c).Take(1).ToArray()[0] == 0x2f) return true; //XLS 2003
            if (bytes.Skip(0x214).Take(1).ToArray()[0] == 0x2f) return true; //XLS 2005
            if (bytes.Skip(0x20B).Take(1).ToArray()[0] == 0x13) return true; //DOC 2005

            if (bytes.Length < 2000) return false; //Guessing false
            var start = Encoding.Default.GetString(bytes.Take(2000).ToArray()); //DOC/XLS 2007+
            start = start.Replace("\0", " ");
            if (start.Contains("E n c r y p t e d P a c k a g e")) return true;
            return false;
        }

        //Unknown.
        return false;
    }

It might not be 100%. The flags I found by comparing several Excel and Word documents with and without password. To add for PowerPoint just do the same.