A recent case resulted from an entry in a compromised web server log. The GET request included the string "Mozilla/4.0 (PSP (PlaySation Portable); 2.00)". Our suspect had used a PSP to do dodgy stuff and the PSP eventually came my way. I looked around for some information but did not find a large amount of information, essentially the most useful items were an Encase Message Board post and Chapter 9 of a book entitled Advances in Digital Forensics V which I read via Google Books.
Sony PlayStation Portable hand held consoles have an inbuilt wi-fi adaptor and can therefore connect to the internet. The device utilises the Netfront browser. There are a number of different versions and firmware versions. The one I looked at had a label indicating that it was a PSP1001. This site details the many different types available. A PSP1001 is known as a PSP Fat (as opposed to a PSP Slim). The one I looked had version 4.05 firmware. These type of PSPs have a small amount of internal NAND flash memory and a Memory Stick ProDuo flash media card.
As far as I can ascertain it is not possible to examine the internal NAND memory of devices beyond 1.5 firmware because you would require hacked firmware and modified hardware to do it. The browser does store its cache in this area but I believe as a default only 512KB is used for this purpose. Some information can be derived from the internal memory via a manual exam. Essentially then, we are left with the Memory Stick ProDuo flash media card. Our Tableau USB write blocker would not recognise the card I had however I was able to image it using our Helix imaging box and Guymager. The card had a FAT16 file system and was examinable with Encase.
Files of interest
On the card I looked at only two files were of interest both in the folder \PSP\SYSTEM\BROWSER.
bookmarks.html contained what you would expect -user created bookmarks
historyv.dat contained internet history
Scott Conrad, Carlos Rodriguez, Chris Marberry and Philip Craiger's paper within Advances in Digital Forensics V refer to two further files of interest historyi.dat and historys.dat. I got my hands on a test PSP1001 with the same firmware as my suspects (4.05) and in testing I was not able to populate these files with any data. The files existed but I was not able to cause details of either Google Searches or user typed URLs to be stored in these files. My suspects card had an unpopulated historyi.dat file and no historys.dat file. As noted by Conrad et al I found in testing that I could only cause writes to historyv.dat by shutting the browser down gracefully. Simply turning off the PSP without shutting down the browser did not commit that sessions history to historyv.dat.
Structure of historyv.dat
The structure of historyv.dat is discussed by Conrad et al however they suggest that elements of the file were best decoded by introducing the data into a test PSP for decoding. For example the date of each history entry could be ascertained this way. I would prefer to carry out a completely static examination if possible, not least because on my suspects card I had recovered a number history records in slack space and a manual examination can be a little laborious. I have therefore decoded the records a little bit further as shown below at Figure 2 and Figure 3. Each historyv.dat file is headed with 66 bytes of data starting with the string Ver.01. Within this 66 bytes are two further bits of plain text - NFPKDDAT and BrowserVisit. Immediately following BrowserVisit is the first history record. The most recent record is listed first, the oldest last. Each record can be located using a GREP expression to search for the header - in Encase \x03\x00\x01\x00 -see Figure 1 below. Records can be found in slack space and unallocated clusters.
A significant addition to the research of Conrad et al is the decoding of the date for each record. The date is recorded in the two bytes following the URL and is stored Little Endian. In Encase sweep these two bytes and right click, select Go To and check Little-endian. The value is the number of days since the Unix epoch (1st January 1970). This web site provides a good date calculator.
IMPORTANT NOTE re dates: the dates stored are in accordance with the PSPs internal clock. The clock resets when the battery is exhausted. With the firmware I looked at the reset date was 1st January 2008. This date is 13879 days from the Unix epoch. I speculate that the average user is unlikely to reset the date each time the battery exhausts, therefore I would expect to see a lot of dates in January 2008.
References and thanks
Forensic Analysis of the Sony Playstation Portable - Scott Conrad, Carlos Rodriguez, Chris Marberry and Philip Craiger
Thanks to Pete Lewis-Jones and Simon Maher for their help brain storming the date problem