File Carving, or sometimes simply Carving, is the practice of searching an input for files or other kinds of objects based on content, rather than on metadata. File carving is a powerful tool for recovering files and fragments of files when directory entries are corrupt or missing, as may be the case with old files that have been deleted or when performing an analysis on damaged media. Memory carving is a useful tool for analyzing physical and virtual memory dumps when the memory structures are unknown or have been overwritten.
Most file carvers operate by looking for file headers and/or footers, and then "carving out" the blocks between these two boundaries. Semantic Carving performs carving based on an analysis of the contents of the proposed files.
File carving should be done on a disk image, rather than on the original disk.
File carving tools are listed on the Tools:Data_Recovery wiki page.
Many carving programs have an option to only look at or near sector boundaries where headers are found. However, searching the entire input can find files that have been embedded into other files, such as JPEGs being embedded into Microsoft Word documents. This may be considered an advantage or a disadvantage, depending on the circumstances.
The majority of file carving programs will only recover files that are contiguous on the media (in other words files that are not fragmented).
Fragmented File Recovery
Simson Garfinkel estimated that upto 58% of outlook, 17% of jpegs and 16% of MS-Word files are fragmented and, therefore, appear corrupted or missing to a user using traditional data carving. The first set of file carving programs that can handle fragmented files automatically have finally arrived.
A. Pal, Nasir Memon, T. Sencar and K. Shanmugasundaram have introduced a technique called SmartCarving that can recover fragmented files.
File Carving Taxonomy
Simson Garfinkel and Joachim Metz have proposed the following file carving taxonomy:
Carving General term for extracting data (files) out of undifferentiated blocks (raw data), like "carving" a sculpture out of soap stone.
Block-Based Carving Any carving method (algorithm) that analyzes the input on block-by-block basis to determine if a block is part of a possible output file. This method assumes that each block can only be part of a single file (or embedded file).
Statistical Carving Any carving method (algorithm) that analyzes the input on characteristic or statistic for example, entropy) to determine if the input is part of a possible output file.
Header/Footer Carving A method for carving files out of raw data using a distinct header (start of file marker) and footer (end of file marker).
Header/Maximum (file) size Carving A method for carving files out of raw data using a distinct header (start of file marker) and a maximum (file) size. This approach works because many file formats (e.g. JPEG, MP3) do not care if additional junk is appended to the end of a valid file.
Header/Embedded Length Carving A method for carving files out of raw data using a distinct header and a file length (size) which is embedded in the file format
File structure based Carving A method for carving files out of raw data using a certain level of knowledge of the internal structure of file types. Garfinkel called this approach "Semantic Carving" in his DFRWS2006 carving challenge submission, while Metz and Mora called the approach "Deep Carving."
Semantic Carving A method for carving files based on a linguistic analysis of the file's content. For example, a semantic carver might conclude that six blocks of french in the middle of a long HTML file written in English is a fragment left from a previous allocated file, and not from the English-language HTML file.
Carving with Validation A method for carving files out of raw data where the carved files are validated using a file type specific validator.
Fragment Recovery Carving A carving method in which two or more fragments are reassembled to form the original file or object. Garfinkel previously called this approach "Split Carving."
Repackaging Carving A carving method that modifies the extracted data by adding new headers, footers, or other information so that it can be viewed with standard utilities. For example, Garfinkel's ZIP Carver looks for individual components of a ZIP file and repackages them with a new Central Directory so that they can be opened with a standard unzip utility.
File Carving challenges and test images
File Carving Challenge - DFRWS 2006
File Carving Challenge - DFRWS 2007
FAT Undelete Test #1 - Digital Forensics Tool Testing Image (dftt #6)
NTFS Undelete (and leap year) Test #1 - Digital Forensics Tool Testing Image (dftt #7)
Basic Data Carving Test - fat32, by Nick Mikus - Digital Forensics Tool Testing Image (dftt #11)
Basic Data Carving Test - ext2, by Nick Mikus - Digital Forensics Tool Testing Image (dftt #12)
- Measuring and Improving the Quality of File Carving Methods, by Bas Kloet, October 2007
- Advanced File Carving Approaches for Multimedia Files (pdf), by Rainer Poisel, Simon Tjoa, and Paul Tavolato, December 2011