Detecting word substitutions in text

SeWong Fong, Dmitri Roussinov, David Skillicorn

Research output: Contribution to journalArticlepeer-review

18 Citations (Scopus)
266 Downloads (Pure)


Searching for words on a watchlist is one way in which large-scale surveillance of communication can be done, for example in intelligence and counterterrorism settings. One obvious defense is to replace words that might attract attention to a message with other, more innocuous, words. For example, the sentence the attack will be tomorrow" might be altered to the complex will be tomorrow", since 'complex' is a word whose frequency is close to that of 'attack'. Such substitutions are readily detectable by humans since they do not make sense. We address the problem of detecting such substitutions automatically, by looking for discrepancies between words and their contexts, and using only syntactic information. We define a set of measures, each of which is quite weak, but which together produce per-sentence detection rates around 90% with false positive rates around 10%. Rules for combining persentence detection into per-message detection can reduce the false positive and false negative rates for messages to practical levels. We test the approach using sentences from the Enron email and Brown corpora, representing informal and formal text respectively.
Original languageEnglish
Pages (from-to)1067-1076
Number of pages10
JournalIEEE Transactions on Knowledge and Data Engineering
Issue number8
Early online date26 Jun 2008
Publication statusPublished - 1 Aug 2008


  • pointwise mutual information
  • textual analysis
  • counterterrorism
  • word frequencies
  • data mining
  • co-occurrence


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