by Martin Schmiedecker and Sebastian Neuner (SBA Research)
Digital rensic investigators currently face numerous challenges, some of which include: the increased digitalisation of our lives, vast case sizes owing to ever increasing storage capacity and the large number of personal devices in use. The goal of our SpeedFor project is to develop new methodologies to reduce the manual work required for digital investigators. Among other things we harvest information from file sharing networks to identify files by extending the forensic process. In an initial proof-of-concept we obtained information from the BitTorrent network to identify up to 2,500 terabytes of data.
Digital forensics has received increasing attention in recent years, as more and more crimes are conducted exclusively by, or with the involvement of, computers. Tools and methods of digital forensics are used by private investigators as well as law enforcement analysts all over the world. One of the challenges in digital forensics is the vast amount of data that needs to be analysed. Commodity hard drives with eight terabytes and more storage capacity are standard nowadays, and are readily obtained. Current forensic processes, however, do not scale well to multi-terabyte workloads. Our project SpeedFor aims to fundamentally increase the performance of current state-of-the-art forensic methods and decrease the manual work necessary for a forensic analyst by developing new methods to increase the use of parallelised data processing within the specific environment of digital forensics, and identifying the best methods to exclude a possibly vast number of files and file system artefacts that are not specific to a case.
We aim to achieve these objectives by leveraging the information specific to large file-sharing networks, in particular we use the BitTorrent protocol as source of information. Prior to sharing a set of files in the network, the data is split in to ‘chunks’ to facilitate parallel data transfers. These chunks are then hashed, and stored in the meta-information of the Torrent swarm . By crawling popular torrent swarms, as well as the few remaining BitTorrent websites, the computational power of the initial seeders can be used for good i.e., a methodology to identify files and file fragments based on data from publicly available file-sharing networks.
Our prototype, dubbed peekaTorrent, will be published at the upcoming DFRWS conference. peekaTorrent is based on the open-source forensic tools bulk extractor  and hashdb , and can be readily integrated into forensic processes. It improves the current state of the art on sub-file hashing twofold: compared to previous approaches we hash sub-file parts larger than pure sector-based hashes. Figure 1 shows a graphical representation. Previously the hard drive sectors were used as input for hash functions like SHA-1, shown as the second layer on the bottom of Figure 1 for hard drives which use 512 bytes respectively 4K sectors. We propose to extract the already available data using larger hash windows which overlap with the BitTorrent specification. Our method is less prone to false-positives for files that share common data segments due to the larger area to be hashed.
Figure 1: Sub-file hashing as used in peekaTorrent.
Secondly, we solve the problem of an a-priori sub-file hash database being required by creating one that can be shared openly. We collected more than 2.5 million torrent files and built their corresponding hashdb databases, which are freely available on the project website. Note that no participation in file-sharing activity is needed as the torrent metadata or ‘metainfo’ already contains all the necessary information including the sub-file hash values. We publish all data sets, tools and our paper openly – you can find the source codes and the hashdb files released under open-source licence on our website https://www.peekatorrent.org. Overall, we expect these methods to help investigators around the globe as this information can be used for file and file fragment identification as well as for effective file whitelisting.
 Bram Cohen. The BitTorrent Protocol Specification, BEP-3, online, http://www.bittorrent.org/beps/bep_0003.html
 Simson L. Garfinkel: “Digital media triage with bulk data analysis and bulk_extractor”, Computers & Security 32: 56-72, 2013
 Simson L. Garfinkel, Michael McCarrin: “Hash-based carving: Searching media for complete files and file fragments with sector hashing and hashdb”, Digital Investigation 14: S95-S105, 2015.
SBA Research, Austria