Until recently, if a digital investigator wished to use multiple tools to hasten the time-intensive labor of analyzing data from a PC hard drive, the raw format – an exact byte-for-byte copy of data – was the only format available. But being uncompressed, raw data is unwieldy and also unable to store critical metadata about the data’s source.
Use of proprietary file formats means converting from one format to another to use multiple tools. Converting between formats risks data corruption if the formats are not well understood. Metadata may be lost if all formats do not support the same forms of metadata.
AFF is an open and extensible file format to store disk images and associated metadata. Using AFF, the user is not locked into a proprietary format that may limit how he or she may analyze it. An open standard enables investigators to quickly and efficiently use their preferred tools to solve crimes, gather intelligence, and resolve security incidents.
Use AFF to store any type of metadata such as GPS coordinates, chain of custody information, or any other user-defined data.
AFF supports the definition of arbitrary metadata by storing all data as name and value pairs, called segments. Some segments store the disk data and others store metadata. Because of this general design, any metadata can be defined by simply creating a new name and value pair. Each of the segments can be compressed to reduce the size of drive images, and cryptographic hashes can be calculated for each segment to ensure data integrity.
For flexibility, there are three variations of AFF files – AFF, AFD and AFM – and freely available tools to easily convert between the variations.
The original AFF format is a single file that contains segments with drive data and metadata. Its contents can be compressed, but it can be quite large as the data on modern hard disks often reach 100GB in size.
For ease of transfer, large AFF files can be broken into multiple AFD format files. The smaller AFD files can be readily moved around a FAT32 file system which limits files to 2GB or stored on DVDs, which have similar size restrictions.
The AFM format stores the metadata in an AFF file, and the disk data in a separate raw file. This format allows analysis tools that support the raw format to access the data, but without losing the metadata.
Compression and Encryption
AFF supports two compression algorithms: zlib, which is fast and reasonably efficient, and LZMA, which is slower but dramatically more efficient. zlib is the same compression algorithm used by EnCase. As a result, AFF files compressed with zlib are roughly the same size as the equivalent EnCase file. AFF files can be recompressed using the LZMA algorithm. These files are anywhere from 1/2 to 1/10th the size of the original AFF/EnCase file.
AFF2.0 supports compression of disk images. Unlike the password implemented by EnCase, encrypted images cannot be accessed without the necessary encryption key.