Personal Storage Table
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In computing, a Personal Storage Table (.pst) is a file used to store local copies of messages, calendar events, and other items within Microsoft software such as Microsoft Exchange Client, Windows Messaging, and Microsoft Outlook.
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[edit] Overview
In Microsoft Exchange Server, the messages, calendar and other data items are delivered to and stored on the server. In standalone applications (Microsoft Outlook, for example), the messages, calendar and other data items are delivered to and stored locally in a Personal Storage Table (.pst) file that is located on the computer. The size of the file then no longer counts against the size of the mailbox used; by moving files from a server mailbox to .pst files users can free up storage space on their mailserver.[1] To use the .pst files from another location you need to be able to access the files directly over a network from your mail client.
When data files like email messages are added to a .pst file its filesize is automatically adjusted by the mail client (if necessary). When mail is deleted from a .pst file the size of the .pst file will stay the same, creating space to hold future data items. To reduce the size of .pst files you need to compact them.[2] Password protection can be used to protect the content of the .pst files.[3]
But even Microsoft admits http://support.microsoft.com/kb/143241 that the password adds very little protection, due to the existence of commonly available tools which can remove the password.
The .pst file format is fundamentally insecure for multiple reasons. First, the password (actually a crc-32 integer representation of it) is simply stored in the .pst file, and Outlook checks to make sure it matches the user specified password, and refuses to operate if they don't match. But the actual data is still there, and is readable by the libpst project code.
Second, MS offers three values for the encryption setting, none, compressible, and high. No encryption is easy - the .pst file contains your data in plain text, and a simple text editor will show the contents. Compressible encryption is a simple byte substitution cipher with a fixed substitution table. Of course, since the table is fixed it is also widely known, and so must be assumed to be known to any attacker. So they can simply decipher the data and see your plaintext. High (sometimes called "better") encryption moves us up to a WWII German Enigma cipher with three fixed rotors. Again, the rotors are fixed and widely known, and so must be assumed to be known to any attacker. So again, they can simply decipher the data and see your plaintext.
Note that neither of the two encryption modes uses the user specified password as any part of the key for the encryption. Using encryption in this way (with a single global fixed key) is at best obfuscation, and not really encryption.
[edit] Support
The Personal Storage Table format is supported by several Microsoft client applications including Exchange Client, Windows Messaging, Microsoft Outlook, and Microsoft Office Outlook. The PST file format is proprietary and cannot be read directly by third-party applications such as Eudora and Mozilla Thunderbird; many such applications are nonetheless able to import data from PST files via MAPI calls to one of the applications which can read them directly.
The libpst project includes tools to convert .pst files into open formats such as mbox and LDAP Data Interchange Format. libpst is licensed under the GPL and is now included in Fedora 10.
Another problem area for PST files is automatic backup. The Outlook application locks the PST file while it is open. So the end user must close Outlook and then wait a long time to allow the full copy of the PST file since it usually gets very big. There are few PST backup programs that are able to seamlessly backup PST files
[edit] Versions
[edit] Outlook 97
Starting with Microsoft Outlook 97 the Personal Storage Table file is encoded in ANSI format. It has a maximum file size of 2 GB. If the file exceeds this size it will become corrupted. This format is supported by Microsoft Outlook 97, Outlook 98, Outlook 2000, Outlook 2002, and Outlook 2003 Internet Message Access Protocol Version 4rev1 (IMAP4) accounts and HTTP accounts.[4]
[edit] Outlook 2003
Starting with Outlook 2003 the Personal Storage file is encoded in unicode format. It allows a user-definable maximum file size up to 20GB that exceeds the previous limit of 2GB. This format is supported by Microsoft Outlook 2003 and Outlook 2007.[4][5] A file that is created in the personal folders format in Outlook 2003 or Microsoft Office Outlook 2007 is not compatible with earlier versions of Microsoft Outlook and cannot be opened by using those older versions.[1]
[edit] References
- ^ a b "Introduction to Personal Folders files (.pst)". Microsoft.com. http://office.microsoft.com/en-us/outlook/HA012308901033.aspx#1b. Retrieved on 2008-10-04.
- ^ "Reduce the size of a .pst file". Microsoft.com. http://office.microsoft.com/en-us/outlook/HP100807041033.aspx. Retrieved on 2008-10-04.
- ^ "Create a .pst file in Outlook 2003/Office Outlook 2007 format". Microsoft.com. http://office.microsoft.com/en-us/outlook/HA012308911033.aspx#2. Retrieved on 2008-10-04.
- ^ a b "The .pst file has a different format and folder size limit in Outlook 2007 and in Outlook 2003 (KB830336)". Microsoft. 2007-02-05. http://support.microsoft.com/kb/830336. Retrieved on 2007-03-05.
- ^ "How to configure the size limit for both (.pst) and (.ost) files in Outlook 2007 and in Outlook 2003 (KB832925)". Microsoft. 2007-02-13. http://support.microsoft.com/kb/832925/. Retrieved on 2008-04-09.
[edit] External links
- libpst project: format of MS Outlook .pst file
- A great deal of information about the format has been documented by the libpff project, including some of the Personal Folder File format specifications and MAPI definitions.
