Hashing is a method for reducing large inputs to a smaller fixed size output. When doing forensics, typically cryptographic hashing algorithms like MD5 and SHA-1 are used. These functions have a few properties useful to forensics. Other types of hashing, such as Context Triggered Piecewise Hashing can also be used.
There are literally hundreds of hashing programs out there, but a few related to forensics are:
- md5sum - Part of the GNU coreutils suite, this program is standard on many computers.
- md5deep - Computes hashes, recursively if desired, and can compare the results to known values.
- ssdeep - Computes and matches Context Triggered Piecewise Hashes.
- National Software Reference Library
- The largest hash database.
- National Drug Intelligence Center
- Solaris Fingerprint Database lookup for files distributed by Sun Microsystems
Online NSRL Lookup
- Allows searching of NSRL 2.17 by MD5 or SHA1. Reportedly the dataset contains 43,103,492 files.
- (Infrequently available, and likely only when the site owner (Jason Spashett) needs to use it himself.)
MD5 Reverse Hash Services
There are several online services that allow you to enter a hash code and find out what the preimage might have been. One way to find these services is to google for 'd41d8cd98f00b204e9800998ecf8427e' (the MD5 of the null string).
Here are some services that we have been able to find:
- A nice forward and reverse demonstration system, with an XML and AJAX interface.
- Reverse hash lookup of MD5, SHA1, MySQL, NTLM, and Lanman hashes. Claims 75 million hashes of 13.2 million unique words.
- MD5 reverse lookup with approximately 1 million entries.
- Hash database from InsidePro (MD5, NTLM).
- This site is another simple MD5 reverse lookup. It claims a database with "billions" of entries. Mostly for password cracking. (Who uses straight MD5s for passwords?)
Online Malware Hash Lookups
- Malware Hash Registry by Team Cymru.
- Utilizes a DNS query interface to lookup MD5 or SHA-1 Hashes for malware
- VirusTotal.com Online hash lookup no api/automation yet like Team Cymru but does frequently have hashes for current new malware
Segmented hashing produces not a single hash value for the entire image, but a list of hashes of corresponding LBA ranges of the image. By validating all hashes in a set it is still verify image integrity. Segmented Hashes are saved in a CSV file with the following format: hash, start LBA, end LBA
- When Segmented hashing is useful
- Segmented hashes support multi-pass imaging and handling of bad sectors: Hashes are calculated only for the imaged regions, while all bad sectors are excluded from calculation. This allows to validate a hash even when the source drive is damaged.
- Better resiliency against data corruption: If an acquired image gets damaged later, regular hash is invalid upon verification making the entire image useless. With segmented hashing, only a single hash value becomes invalid, while the rest of the image can still be validated.
Seghash on GitHub is a free open-source tool for both calculating and validating segmented hashes.