File carving

File carving is the process of reassembling computer files from fragments in the absence of filesystem metadata.

All filesystems contain some metadata that describes the actual file system. At a minimum the following is stored: the hierarchy of folders and files, with names for each. For each file is also stored the physical address on the hard disk where the file is stored. As explained below, a file might be scattered in fragments at different physical addresses.

File carving is the process of trying to recover files without this metadata. This is done by analyzing the raw data and identifying what it is (text, executable, png, mp3, etc.). This can be done in different ways, but the simplest is to look for headers. For instance, every Java class file has as its first four bytes the hexadecimal value CA FE BA BE. Some files contain footers as well, making it just as simple to identify the ending of the file.

Most file systems, such as FAT and UNIX Fast File System, work with the concept of clusters of an equal and fixed size. For example, a FAT32 file system might be broken into clusters of 4 KiB each. Any file smaller than 4 KiB fits into a single cluster, and there is never more than one file in each cluster. Files that take up more than 4 KiB are allocated across many clusters. Sometimes these clusters are all contiguous, while other times they are scattered across two or potentially many more so called fragments, with each fragment containing a number of contiguous clusters storing one part of the file's data. Obviously large files are more likely to be fragmented.

Simson Garfinkel reported fragmentation statistics collected from over 350 disks containing FAT, NTFS and UFS file systems. He showed that while fragmentation in a typical disk is low, the fragmentation rate of forensically important files such as email, JPEG and Word documents is relatively high. The fragmentation rate of JPEG files was found to be 16%, Word documents had 17% fragmentation, AVI had a 22% fragmentation rate and PST files (Microsoft Outlook) had a 58% fragmentation rate (the fraction of files being fragmented into two or more fragments). Pal, Shanmugasundaram, and Memon presented an efficient algorithm based on a greedy heuristic and alpha-beta pruning for reassembling fragmented images. Pal, Sencar, and Memon introduced sequential hypothesis testing as an effective mechanism for detecting fragmentation points. Richard and Roussev presented Scalpel, an open-source file-carving tool.

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