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Kambhatla, Nishant - Decipherment of substitution ciphers with neural language models

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This thesis has been approved for inclusion in the SFU Library.

Term : Summer 2018

Degree : M.Sc.

Degree type : Thesis

Department : School of Computing Science

Faculty : Applied Sciences

Senior supervisor : Anoop Sarkar

Thesis title : Decipherment of substitution ciphers with neural language models

Given names : Nishant

Surname : Kambhatla

Abstract : The decipherment of homophonic substitution ciphers using language models (LMs) is a well-studied task in Natural Language Processing (NLP). Previous work in this topic score short local spans of possible plaintext decipherments using n-gram LMs. The most widely used technique is the use of beam search with n-gram LMs proposed by Nuhn et al. (2013). We propose a new approach on decipherment using a beam search algorithm that scores the entire candidate plaintext at each step with a neural LM. We augment beam search with a novel rest cost estimation that exploits the predictive power of a neural LM. This work, to our knowledge, is the first to use a large pre-trained neural language model for decipherment. Our neural decipherment approach outperforms the state-of-the-art n-gram based methods on many different ciphers. On challenging ciphers such as the Beale cipher our system reports significantly lower error rates with much smaller beam sizes.

Keywords : Natural Language Processing; decipherment; neural decipherment; neural language models; beam search

Total pages : 42