Windows 7 Forums
Welcome to Windows 7 Forums. Our forum is dedicated to helping you find support and solutions for any problems regarding your Windows 7 PC be it Dell, HP, Acer, Asus or a custom build. We also provide an extensive Windows 7 tutorial section that covers a wide range of tips and tricks.


Windows 7: File size of zipped password protected files larger than expected

11 Sep 2013   #1
diamondfist

Windows 7 Ultimate 32bit
 
 
File size of zipped password protected files larger than expected

When I compress a password protected Word or Excel file by adding to a zip or archive folder, the file size is much greater than when I add a non protected file. For example, if I compress a 5Mb Excel file, the archive (zip) file is a little less than 5Mb, with an 8% compression ratio. If I compress the same file without the password protection, the archive file is about 1Mb with an 82% compression ratio.

I guess I can somewhat understand this. The act of compressing the file is a manner of changing it. But then why do I get some minimal (8%) compression at all?

I have tried this with combinations of WinXP, Win7, Office 2003, and Office 2010, and it seems the low compression (large file size) with the password protected file is specific to systems running Office 2010. The compression ratio of the file is about 86%-88% with Office 2003, regardless of password protection. Is this part of the "enhanced security" of Office 2010? Is there a workaround for this, or is it an inescapable result of compressing a password protected file when using Office 2010?


My System SpecsSystem Spec
.
11 Sep 2013   #2
UsernameIssues

W7 Pro SP1 64bit
 
 

If you had a simple text file filled with the word TEST over and over - then that would be highly compressible since the compression app just needs to take note of the word (and any spaces) & then record how many times the word (and any spaces) should appear in the decompressed document.

My theory for your findings with spreadsheets is:
An Excel spreadsheet must have lots of repeated patterns that can be highly compressed. When you encrypt that spreadsheet, you scramble some of those repeated patterns - thus less compressibility.

To test this theory, I saved a spreadsheet (Office 2010) and opened it with a HEX editor. Just as I thought, there are lots of repeated patterns in that spreadsheet file. Then I made a copy of that spreadsheet and password protected the entire workbook. The HEX editor showed fewer repeated patterns for the encrypted file... but I cannot put a percentage on it. If I had filled the test spreadsheet with a lot more data/labels/charts/text, then I suspect that the difference in repeated patterns (and thus compressibility) would have been greater. My hope would be that the compression app would not be able to detect a pattern in the part of the encrypted spreadsheet file that represented the secured contents.

I don't feel like installing Office 2003 to try and ferret out why you still saw high compression rates on protected files... but it seems that the protection schemes differ between Office 2003 and Office 2010.

All of the above is just conjecture on my part. I would not put too much stock in my ramblings.
My System SpecsSystem Spec
11 Sep 2013   #3
LMiller7

Windows 7 Pro 64 bit
 
 

UsernameIssues is probably correct. Encryption tends to greatly reduce the compressibility of a file, regardless of it's type or content. Office 2010 improved encryption considerably from Office 2003.

Remember that the purpose of improving encryption is to improve security. The difference in compressibility is rarely even considered.
My System SpecsSystem Spec
.

Reply

 File size of zipped password protected files larger than expected




Thread Tools




Similar help and support threads
Thread Forum
download/upload larger in size files
Hello friends, what is the best way to upload a (20 GB) file and then download the same file ??? thanks in advance
Network & Sharing
Expected Converted File Size ?
I've been converting some WMV files to MP4 files using XMedia Recode. Some of the converted files are much smaller than the original files, but others are not. Does anyone know of any software that can tell the resulting converted file size without having to do the conversion, since the...
Music, Pictures & Video
Password protected files
Is there a way to password protect a file in W7 or do you need 3rd party software?
System Security
list all files, by size, larger than a specified size?
I am running out of space on my system drive (120GB SSD with < 6GB remaining). It happened in only 12 months. I'd like to know where all that space went, which files are using it. So I'm looking for a way to rank files by size regardless of their folder location, largest-to-smallest, ideally...
Performance & Maintenance
Opening a password protected .doc file...
So here's the dealeo guys, I need to see what's inside a Word .doc file that my brother protected several months ago, but the thing is, he doesn't remember the password (shocker right?). Is there a way to find out the password of the .doc file? legally... (sorry if this violates forum rules)....
Software


Our Sites

Site Links

About Us

Find Us

Windows 7 Forums is an independent web site and has not been authorized, sponsored, or otherwise approved by Microsoft Corporation. "Windows 7" and related materials are trademarks of Microsoft Corp.

Designer Media Ltd

All times are GMT -5. The time now is 13:45.
Twitter Facebook Google+