Tuesday, May 31, 2011

Salvaging Digital Video Fragments

Salvaging Digital Video Fragments: "

Digital video is becoming a more common form of digital evidence with the increasing prevalence of video in computers, mobile devices and cameras. Digital cameras can create high quality videos, most smart phones can create videos, and the iPad2 has two cameras that can create videos. The videos created by such digital devices can be stored on removable storage media and on the devices themselves. Frequent creation and deletion of videos on these kinds of devices can result in fragments of deleted video clips that most file carving tools cannot salvage. In addition, when dealing with Flash memory dumps acquired from mobile devices, data at the physical level is often fragmented. Specialized methods and tools are needed to salvage deleted video fragments as demonstrated in this article using the contents of Flash memory acquired from a Motorola V3 (RAZR) mobile device.

File Carving Limitations

Most file carving tools require a known file header in order to salvage deleted data. For instance, to recover a deleted 3gp file, most carving tools look for the file headers such as the following.

Hex view of 3gp header in the Motorola V3 Flash memory dump

If the file is fragmented or the header is missing, the file carving approach will not salvage the deleted video successfully. In this example, a file carving tool that searched the Motorola V3 memory dump for several 3gp header signatures found two files in as shown in the audit log:

    05/24/2011, 11:26:35
    QuickTime 3GP (3gp), header: ftypisom
    QuickTime 3GP (3gp), header: ftyp3gp
    QuickTime 3GP (3gp), header: ftypmmp4
    Default file size: 1024 KB
    Maximum file size: 100 times (individual file type definition defaults sizes respected)

    E:\Physical GSM Motorola V3 RAZR\Flex Partition 1140000-1fe0000.bin
    Scope: 000000 - E9FFFF
    Extensive byte-level search

    9D0E80 - AD0E7F: 00001.3gp
    B888F0 - C888EF: 00002.3gp

    05/24/2011, 11:26:35
    2 file headers were found. 2 files were retrieved.

However, the salvaged files were invalid because the original files were fragmented. Furthermore, the names and directory paths of these files were not obtained using this method, demonstrating a further limitation of file carving.

Salvaging Video Fragments

When video files are fragmented, it is necessary to consider the video file format in more detail. Fortunately, many digital video formats have a structure that can be used to find and salvage individual frames. A frame is a discrete section of the video that can have a timecode or sequence number and other characteristics that can be useful for salvaging digital video clips.

The defraser tool can be used to identify frames for several video formats in a forensic duplicate of any piece of storage media, including a removable storage card, computer hard drive and Flash dump from a mobile device. The following screenshot shows defraser used to detect video related data in the Motorola V3 memory dump.

Defraser showing video related data in the Motorola V3 memory dump

Although the defraser tool does not automatically piece together the frames into a video that can be played, it does make the frames available for manual reconstruction. With some effort, defraser may be used to combine fragmented frames into a valid video file that can be played.

As with file carving methods that rely on header signatures, the carving methods employed by defraser do not provide the filenames and directory path of salvaged video data in the context of the original file system.

File System Reconstruction

Ultimately, the most effective approach to extracting digital video files from acquired digital evidence such as a Flash memory dump from mobile device is to reconstruct the logical arrangement of data. On mobile devices, this logical structure involves the flash abstraction layer and file system. Using mobile device forensic tools such as Cellebrite Physical and XRY, it is possible to reconstruct and review logical file structure of a Flash memory dump as shown below with a 3gp video stored in an MMS related file in the Motorola V3 memory dump. Note that different tools may interpret the logical structure differently and show more files and folders, clearly demonstrating the importance of validating the results of forensic examination tools.

XRY/XACT showing the logical file system in the Motorola V3 memory dump

Cellebrite Physical showing the logical file system in the Motorola V3 memory dump

Extracting the MMS file using such a mobile device forensic tool and extracting the video content as discussed in the “Delving into Mobile Device File Systems” blog post results in a 3gp file that can be played using VLC media player.

Playing salvaged digital video using VLC Player

Examination of Salvaged Video

After salvaging digital video files it is important to review the resulting data closely for potential anomalies. For instance, using MediaInfo to extract metadata from video files shows details related to its creation and format. The following screenshot shows metadata from a 3gp video extracted from the Motorola V3 memory dump, revealing that the embedded date-time stamp was set to an incorrect date.

Metadata within a 3gp video displayed using MediaInfo

In addition, reviewing individual frames within a salvaged video file can reveal anomalies such as portions of two unrelated videos being combined into one salvage file. The following screenshot shows frames extracted from a 3gp file using DCCI Video Validator revealing footage from two unrelated video files.

Frames extracted from digital video using DCCI Video Validator


When a video file is fragmented or the header of a video file is overwritten, carving methods that rely on header signatures and contiguous files will not salvage video files successfully and may even incorrectly combine unrelated video fragments into a single file or fail to detect the presence of video content altogether. However, using specialized tools such as defraser, a digital investigator may be able to salvage fragments of video files and piece them together into a valid video file. This process of reconstructing video fragments is time consuming and error prone, particularly when dealing with numerous video files on a single piece of storage media or mobile device. Therefore, whenever feasible, it is preferable to reconstruct the logical arrangement of data to extract the complete content of video files. Whichever method is most effective for salvaging digital video, it is important to examine the results closely to ensure the accuracy and completeness of the resulting videos. Such a review includes inspecting embedded metadata for anomalies and reviewing keyframes for possible fragments of unrelated video footage.