Prevent microsoft excel from opening xlsx file.

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Hi, I've got an xlsx file with charts, tables, formulas. Initially, the file was created in Microsoft Excel 2007.

Is there a way for me to prevent the file from being opened by Microsoft Excel? Maybe by me messing with the file header or maybe me writing a custom wrapper or even custom encrypt it?
After encrypting/wrapping it, the xlsx file should be a file that is not recognized by Microsoft Excel, maybe with a custom extension.

The only way to open the xlsx file is to open it through my C# program, which decrypt or unwrap it and then displaying it also in C# by using DSO framer. The decrypting/unwrapping and displaying it is done without any xlsx file saved on the hard disk.

If there is a way, can somebody please point me to the right direction?
The first phase in my multi-phase program is to encrypt/wrap it (not sure if it is the right terminology or not).
I only know how to describe what I plan to do but don't know what it the right term for it so am not sure how to even start searching for it in google.

Please advise. Thanks.

Posted 15-Jul-13 4:47am

JoJo82

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ZurdoDev 15-Jul-13 10:55am

Just rename the file so it ends with something custom, like .jojo That will deter most users. Of course you can also password protect it. What are you really trying to accomplish?

Sergey Alexandrovich Kryukov 15-Jul-13 12:14pm

Exactly. This is the right approach.
—SA

JoJo82 15-Jul-13 11:32am

I have an xlsx file with formulas, graphs, protected information which I do not want my customer to see because those are IP. Lets call it XLSX1.xlsx. Because of that, I want to prevent it from being opened in Microsoft Excel or being opened anywhere. Even if the user were to rename the extension, they still will not be able to open it. That is why I need to encrypt it in such a way that even Excel's password cracker will not be able to crack it.

I have another xlsx file for the customer to enter their information. Lets call it XLSX2.xlsx.
This file, the user can open it normally with Microsoft Excel.

After the user has filled in the information in XLSX2.xlsx, they open my program. My program will decrypt XLSX1.xlsx and load it into filestream, get the necessary formula, do some calculation and then display the result in DSO framer or something. The user will then be able to and save the result in XLSX3.xlsx if they want to.

I hope this paints a much clearer picture of what I plan to do and why.
Thanks.

Sergey Alexandrovich Kryukov 15-Jul-13 12:16pm

Do what ryanb31 advised. Preventing a real Excel file or a file named like an Excel file from opening by Excel is at least silly.
—SA

ZurdoDev 15-Jul-13 12:19pm

Then don't save in Excel format. Write your own proprietary format in binary.

JoJo82 15-Jul-13 12:43pm

Doing as what you suggest would be possible if we're in the initial stage. Currently we already have many XLSX1.xlsx type file which contains the formulas etc and it would take a very long time to convert it to C#. Besides, my colleague is the one that does the excel programming/formula. During the development and design stage, he use Microsoft Excel to design the XLSX1.xlsx. That is why he need to save in Excel format, because everything is done in Microsoft Excel. Now that he already has the formula, he needs it to be protected.

In the future, when he need to change the formula in XLSX1.xlsx, I will decrypt it back to normal Microsoft Excel type file so my colleague can do the necessary changes and then encrypt it back again before ship it to customer.

Hope you understand why I need to do it as what I've written in the above.. Because we're stuck.. And now I'm stuck with the job of figuring out how to program it in C#.

Hope somebody can advice me. Thanks.

ZurdoDev 15-Jul-13 13:10pm

So, what part specifically are you stuck on?

JoJo82 15-Jul-13 20:06pm

I'm now at the very begining. My immediate task is to take XLSX1.xlsx and encrypt it.

Would I be on the right track if I were to follow the article http://support.microsoft.com/kb/307010 and http://www.codeproject.com/Articles/26085/File-Encryption-and-Decryption-in-C ? Which one would you recommend?

After I encrypt the file, would I be able to decrypt the file without saving it to hard disk? This is so that I can display the decrypted XLSX1.xlsx filestream in C# later.

ZurdoDev 15-Jul-13 21:24pm

Absolutely. I use code just like that to encrypt all the time. However, I am normally doing just strings so I am curious how well it works on files as a whole but yes, I would start by following the Microsoft link first.

JoJo82 16-Jul-13 10:55am

Thanks.. at least i know now where to start. I guess my question is answered now.

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ZurdoDev · Accepted Answer · 2013-07-16T05:10:00

Solution 1

As discussed in comments, I would suggest following the article at http://support.microsoft.com/kb/307010[^] to encrypt your file.

Posted 16-Jul-13 5:10am

ZurdoDev