We then pick a random irreducible polynomial p(x) of degree k over GF(2), and we define the fingerprint of m to be the remainder after division of by over GF(2) which can be viewed as a polynomial of degree k-1 or as a k-bit number.
The Low Bandwidth Network Filesystem (LBFS) from MIT uses Rabin fingerprints to implement variable size shift-resistant blocks.  The basic idea is that the filesystem computes the cryptographic hash of each block in a file. To save on transfers between the client and server, they compare their checksums and only transfer blocks whose checksums differ. But one problem with this scheme is that a single insertion at the beginning of the file will cause every checksum to change if fixed-sized (e.g. 4 KB) blocks are used. So the idea is to select blocks not based on a specific offset but rather by some property of the block contents. LBFS does this by sliding a 48 byte window over the file and computing the Rabin fingerprint of each window. When the low 13 bits of the fingerprint are zero LBFS calls those 48 bytes a breakpoint and ends the current block and begins a new one. Since the output of Rabin fingerprints are pseudo-random the probability of any given 48 bytes being a breakpoint is . This has the effect of shift-resistant variable size blocks. Any hash function could be used to divide a long file into blocks (as long as a cryptographic hash function is then used to find the checksum of each block): but the Rabin fingerprint is an efficient rolling hash, since the computation of the Rabin fingerprint of region B can reuse some of the computation of the Rabin fingerprint of region A when regions A and B overlap.
Note that this is a problem similar to that faced by rsync.
- Michael O. Rabin (1981). Fingerprinting by Random Polynomials (PDF). Center for Research in Computing Technology, Harvard University. Tech Report TR-CSE-03-01. Retrieved 2007-03-22.
- Athicha Muthitacharoen, Benjie Chen, and David Mazières "A Low-bandwidth Network File System"
- Andrei Z. Broder (1993). Some applications of Rabin's fingerprinting method. Retrieved 2011-09-12.
- David Andersen (2007). Exploiting Similarity for Multi-Source Downloads using File Handprints. Retrieved 2007-04-12.
- Rabin fingerprint algorithm implemented in C
|This cryptography-related article is a stub. You can help Wikipedia by expanding it.|