Ssdeep -
ssdeep is a program for computing and matching Context Triggered Piecewise Hashing values. It is based on a spam detector called spamsum by Andrews Trigdell.
File Format
The program uses an ASCII text file to record fuzzy hashes. The format changed slightly in version 2.6 in Sep 2010. Hashes created by the version 2.6 or later of the program cannot be used in earlier versions ref. The contains a header followed by one hash per line. The current header is:
ssdeep,1.1--blocksize:hash:hash,filename 
Usage Scenarios
Truncated Files
The program can be used to associate two files where one is a truncated version of the other. In this example, the examiner has a file all-the-kings-men.avi. She computes a fuzzy hash of his file:
$ ls -lsh -rwxr-xr-x 1 jvalenti users 699M Sep 29 2006 all-the-kings-men.avi $ ssdeep -b all-the-kings-men.avi > sig.txt $ cat sig.txt ssdeep,1.0--blocksize:hash:hash,filename 12582912:fgQl/nUjQAbaBQvHf8yLr5CHJu3dyh YJ27TuXyphJs3wHC6 rEfAV wDrw6C/AT:fPl8cdAUyLr5CHJu3dyh8uzwHC6 reAS,"all-the-kings-men.avi" 
The examiner then creates a second file that contains the first 29% of the original. This simulates recovering a partial file in some manner.
$ dd if=all-the-kings-men.avi of=partial.avi bs=1m count=200 200 0 records in 200 0 records out 209715200 bytes transferred in 14.510224 secs (14452926 bytes/sec) $ ls -lsh partial.avi -rw-r--r-- 1 jvalenti users 200M Oct 6 06:40 partial.avi 
The examiner can then use the matching mode of ssdeep, the -m option, to read the known signature generated above and match it against the partial file.
$ ssdeep -bm sig.txt partial.avi partial.avi matches all-the-kings-men.avi (57) 
The files are associated!
Source Code Reuse
The source code for ssdeep was originally obtained from another open source project called md5deep. An examiner with access to both source code directory trees could use ssdeep to find any similarities between the two. In this example we have two folders, ssdeep-1.1 and md5deep-1.12. First we record the fuzzy hashes, with relative filenames (the -l switch) to a file:
C:\> ssdeep -lr md5deep-1.12 > hashes.txt 
Then we compare those saved hashes with the other directory:
C:\> ssdeep -lrm hashes.txt ssdeep-1.1 ssdeep-1.1\cycles.c matches md5deep-1.12\cycles.c (94) ssdeep-1.1\dig.c matches md5deep-1.12\dig.c (35) ssdeep-1.1\helpers.c matches md5deep-1.12\helpers.c (57) 
Those matches indicate source code reuse! A manual examination of the files in question is required to tell exactly what kind of copying occurred, but we've saved the examiner a lot of work.
An advanced examiner can accomplish this matching with just one command line, but it will also include all of the matches internal to each directory.
C:\> ssdeep -lrd md5deep-1.12 ssdeep-1.1 md5deep-1.12\md5.h matches md5deep-1.12\cycles.c (27) md5deep-1.12\sha1.h matches md5deep-1.12\cycles.c (25) md5deep-1.12\sha1.h matches md5deep-1.12\md5.h (58) md5deep-1.12\sha256.h matches md5deep-1.12\cycles.c (25) md5deep-1.12\sha256.h matches md5deep-1.12\md5.h (61) md5deep-1.12\sha256.h matches md5deep-1.12\sha1.h (57) md5deep-1.12\tiger.h matches md5deep-1.12\cycles.c (29) md5deep-1.12\tiger.h matches md5deep-1.12\md5.h (65) md5deep-1.12\tiger.h matches md5deep-1.12\sha1.h (63) md5deep-1.12\tiger.h matches md5deep-1.12\sha256.h (61) ssdeep-1.1\cycles.c matches md5deep-1.12\cycles.c (94) ssdeep-1.1\dig.c matches md5deep-1.12\dig.c (35) ssdeep-1.1\helpers.c matches md5deep-1.12\helpers.c (57) 
If you'd like to see the matches in both directions (i.e. for two files A and B that match, see that A matches B and B matches A), use the -p flag instead of -d.
External Links
Official website