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Microsoft Outlook allows you to compress and/or encrypt your PST file. We'll look at what the options are, and when you might choose them.

What kind of Compression or Encryption do I want for Outlook's .PST files?

In an earlier article I discussed compacting an existing .PST, and creating additional .PSTs. One thing I didn't talk about was what kind of encryption or compression to select. In most cases the default is fine, but understanding the differences may help you select something more appropriate for your situation.

When you create a new .PST, Outlook offers you three different types of encryption: none, compressible, and high.

No Encryption means just that. Outlook will store your email in such a way that anyone with access to your .PST and notepad will be able to view your mail. It won't be pretty because it'll be intermixed with Outlook's control information and anything in HTML will have all of the HTML tags intermingled. But it will be easily visible.

Compressible Encryption will obfuscate the data in your .PST such that simple tools like notepad will see only garbage. In fact, it'll take some hacker's tool to actually decrypt the data. The term "Compressible" is used because the type of encryption used does not defeat the techniques used by programs such as ZIP, or even the built-in compression that's available on some Windows file systems.

High Encryption uses a stronger form of encryption that is more difficult to hack. It's also potentially more time consuming for Outlook to read and write. And as you might guess, this form of encryption does not compress well if at all. You can still attempt to compress a copy of your .PST for backup if you like, but it's not likely to get much smaller if at all.

So the bottom line is it depends. If you don't care that your .PST might be readable by someone or perhaps you use other forms of security to prevent access, then perhaps No Encryption is right for you. If you're storing sensitive data or have other reasons to be concerned, then perhaps High Encryption is best.

As for me I leave it at the default: Compressible Encryption. I do occasionally want to be able to compress my .PSTs (they do get big), but I'd at least like to keep the casual browser from easily reading my mail.

Article C1882 - January 16, 2004 « »

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Leo Leo A. Notenboom has been playing with computers since he was required to take a programming class in 1976. An 18 year career as a programmer at Microsoft soon followed. After "retiring" in 2001, Leo started Ask Leo! in 2003 as a place for answers to common computer and technical questions. More about Leo.

Not what you needed?

February 9, 2005 9:29 PM

how cam i open a pst file of outlook through vb or some other langauage

February 9, 2005 9:35 PM

By driving Outlook progamaticly, yes. Check Office Developer Resources:

April 7, 2005 11:56 AM

I clicked through the making of an PST file in Outlook 2003 and when i go to import it back in after a system reinstall, i get access is denied.

I did not set a password for the pst.

If you can help it would be great

please reply to my gmail address.

April 7, 2005 3:36 PM

Has nothing to do with passwords. More likely the .pst file is marked "read/only" or it's open by another process.

April 8, 2005 8:20 AM

YOur right Leo, i found out that when you burn it to a CD it marks it "read ONly" i copied it off the CD and checked the properties and it was not checked as read only. But after checking and then unchecking the box, it worked

May 15, 2005 6:15 AM

i choosed Compressible Encryption to export pst, i formatted and now i can't import my pst :(
is there any tool that decrypt the pst? If anybody has any suggestion, please email me.
thanks in advance

May 15, 2005 10:16 PM

You shouldn't have to "import" it. Just open it in Outlook.

May 18, 2005 2:32 AM

i want extract massage from a damage pst.Please
give me suggesion.


May 18, 2005 9:58 PM

I would try the inbox repair tool:

May 26, 2005 11:08 PM

I am writing a software which should automatically read the pst file of outlook and check the calender appoinments. To do so we need a clear knowledge of pst file , it's format and any encryption details. could you help me on that matter

May 27, 2005 7:35 PM

I do not believe that information is public. The appropriate way to do that would be to use the Outlook Object model and programatic API. Which requires outlook be installed and available, of course.

jimmy ul-haq
October 21, 2005 7:19 AM

hi could you plaz explain what does Encryption mean? and what it does?

thank you very muich

i look forward to hear from you

Michael Manzo
July 19, 2006 9:22 AM

If the compressible encryption option is used, do you know what encryption algorythm is used? For exampel AES256, AES128, 3DES....

August 20, 2006 11:20 PM

I want to read a pst file through my own program, but i couldn't find any material describing the structure of the pst file . Can u help me in getting that.

Looking forward to hear from you

Leo A. Notenboom
August 21, 2006 9:05 AM

The file format is proprietary, and to the best of my knowledge, not published. You need outlook.

CB Jackson
April 27, 2007 11:06 AM

I found you through Google, please help. What's the difference b/w encryption "none" and encryption "AES-128"? I'm trying to use/format a brand new blank disk on my PC that has Microsoft Office Suite. Thanks so much!!

January 24, 2008 10:20 PM

i am having a problem in outlook 2007 regarding to export pst file with no encryption,By default Microsoft outlook 2007 export pst file with outlook 2003,after going through all steps of exporting pst file,finally a dialog box appears having encryption settings,but in outlook 2007 that dialog box doesn't have encryption setting options.
so how can i enable those settings?

January 24, 2008 10:26 PM

i am having a problem in outlook 2007 regarding to export pst file with no encryption,By default Microsoft outlook 2007 export pst file with outlook 2003,after going through all steps of exporting pst file,finally a dialog box appears having encryption settings,but in outlook 2007 that dialog box doesn't have encryption setting options.
so how can i enable those settings?

June 3, 2008 12:27 PM

In Outlook 2007 there indeed seems to be no option anymore to set no encryption! Why Microsoft makes a new Outlook version not compatible with the older versions?? Anyone knows a fix for this? In order to use certain programs like Pst2mail the pst-file should be set to no encryption.........

Carl Byington
January 18, 2009 2:58 PM

With respect to security, it does not matter which of the three so-called encryption options you pick, they are all essentially equivalent. Compressible encryption is a simple Caesar cipher (crypt = table[plain]) for each plain text byte with a fixed permutation table that does not depend on the password. Of course, since the permutation table is fixed, it is also known. Therefore, anyone can trivially decrypt these .pst files.

High encryption moves us all the way up to an early WWII German Enigma three rotor cipher, where again, the content of the three rotors are fixed and don't depend on the password. Of course, since the rotors are fixed and known, anyone can trivially decrypt these .pst files as well.

All in all, a complete joke as far as security is concerned.

Carl Byington
January 18, 2009 3:17 PM

My mistake. The compressible encryption is not a Caesar cipher, it is instead a byte substitution cipher with a fixed substitution table.

April 25, 2009 7:22 AM

Im using to encrypt my data. It is userfriendly and really fast.

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