Unique file identification in the National Software Reference Library

The National Software Reference Library (NSRL) provides a repository of known software, file profiles, and file signatures for use by law enforcement and other organizations involved with computer forensic investigations. The NSRL is comprised of three major elements: 1. A physical library of commercial software packages. 2. A database of information about each file within each software package. 3. A smaller database of the most widely used information that is updated and released quarterly. This database is called the NSRL Reference Data Set (RDS) and is NIST Special Database #28 [18]. During a forensic investigation, hundreds of thousands of files may be encountered. The NSRL is used to identify known files. This can reduce the amount of time spent examining a computer. Matches for common operating systems and applications do not need to be searched, either manually or electronically, for evidence. Additionally, the NSRL is used to determine which software applications are present on a system. This may suggest how the computer was being used and provide information on how and where to search for evidence. This paper examines whether the techniques used to create file signatures in the NSRL produce unique results—a core characteristic that the NSRL depends on for the majority of its uses. The uniqueness of the file identification is analyzed via two methods: an empirical analysis of the file signatures within the NSRL and research into the recent attacks on the hash algorithms used to generate the file signatures within the NSRL. The research addresses the following questions: 1. Are the file signatures in the NSRL unique? The NSRL was examined for distinct files that generated the same signature (i.e., a collision). 2. How likely is it that collisions will occur in the future? The probability of future collisions depends directly on the randomness of the file signatures. We ran statistical tests to answer the following questions: • Do file signatures appear to be random?