Package nilsimsa is a Go implemenation of Nilsimsa, ported from code.google.com/p/py-nilsimsa but follows the conventions establish by the md5 package in the standard lib

The Java implementaition at https://github.com/weblyzard/nilsimsa/ blob/master/src/main/java/com/weblyzard/lib/string/nilsimsa/Nilsimsa.java was also consulted.

There is a discussion about using hash to score string similarities http://stackoverflow.com/questions/4323977/string-similarity-score-hash

Copyright 2015 Sheng-Te Tsao. All rights reserved. Use of this source code is governed by the same BSD-style license that is used by the Go standard library

From http://en.wikipedia.org/wiki/Nilsimsa_Hash

Nilsimsa is an anti-spam focused locality-sensitive hashing algorithm originally proposed the cmeclax remailer operator in 2001[1] and then reviewed by Damiani et al. in their 2004 paper titled, "An Open Digest-based Technique for Spam Detection".[2]

The goal of Nilsimsa is to generate a hash digest of an email message such that the digests of two similar messages are similar to each other. In comparison with cryptographic hash functions such as SHA-1 or MD5, making a small modification to a document does not substantially change the resulting hash of the document. The paper suggests that the Nilsimsa satisfies three requirements:

1. The digest identifying each message should not vary significantly (sic) for changes that can be produced automatically.
2. The encoding must be robust against intentional attacks.
3. The encoding should support an extremely low risk of false positives.

Subsequent testing[3] on a range of file types identified the Nilsimsa hash as having a significantly higher false positive rate when compared to other similarity digest schemes such as TLSH, Ssdeep and Sdhash.

Nilsimsa similarity matching was taken in consideration by Jesse Kornblum when developing the fuzzy hashing in 2006,[4] that used the algorithms of spamsum by Andrew Tridgell (2002).[5]

References:

[1] http://web.archive.org/web/20050707005338/ http://ixazon.dynip.com/~cmeclax/nilsimsa-0.2.4.tar.gz [2] http://spdp.di.unimi.it/papers/pdcs04.pdf [3] https://www.academia.edu/7833902/TLSH_-A_Locality_Sensitive_Hash [4] http://jessekornblum.livejournal.com/242493.html [5] http://dfrws.org/2006/proceedings/12-Kornblum.pdf

From http://blog.semanticlab.net/tag/nilsimsa/

An Open Digest-based Technique for Spam Detection

Damiani, E. et al., 2004. An Open Digest-based Technique for Spam Detection. In in Proceedings of the 2004 International Workshop on Security in Parallel and Distributed Systems. pp. 15-17.

Summary

This paper discusses the Nilsimsa open digest hash algorithm which is frequently used for Spam detection. The authors describe the computation of the 32-byte code, discuss different attack scenarios and measures to counter them.

Digest computation

1. Slide a five character window through the input text and compute all eight possible tri-grams for each window (e.g. "igram" yields "igr", "gra", "ram", "ira", "iam", "grm", ...)

2. Hash these trigrams using a hash function h() which maps every tri-gram to one of 256 accumulators and increment the corresponding accumulator. Nilsimsa uses the Trans53 hash function for hashing.

3. At the end of the process described below, compute the expected value of the accumulators and set the bits which correspond to each accumulator either to (1) if it exceeds this threshold or (o) otherwise.

Similarity computation

The Nilsimsa similarity is computed based on the bitwise difference between two Nilsimsa hashes. Documents are considered similar if they exceed a pre-defined similarity value.

1. 24 similar bits - conflict probability: 1.35E-4 (suggestions by Nilsimsa's original designer)

2. 54 similar bits - conflict probability: 7.39E-12 (suggested by the article's authors)

Attacks

Spammers can apply multiple techniques to prevent Nilsimsa from detecting duplicates:

1. Random addition: requires >300% of additional text to prevent detection.
2. Thesaurus substitutions: require >20% of replaced text
3. Perceptive substitution (security > s3cur1ty): requires >15% of the text to be altered
4. Aimed attacks (i.e. attacks which specifically target Nilsimsa): ~10% of the text needs to be altered

Aimed attacks manipulate Nilsimsa's accumulators by adding words which introduce new tri-grams that specifically alter the hash value. Although these attacks are relatively effective, they can be easily circumvented by computing the Nilsimsa hash twice with different hash functions.