|
" Compression-Based Methods for the Automatic Cryptanalysis of Classical Ciphers "
Al-Kazaz, Noor R.
| Document Type
|
:
|
Latin Dissertation
|
| Language of Document
|
:
|
English
|
| Record Number
|
:
|
1057137
|
| Doc. No
|
:
|
TL56254
|
| Main Entry
|
:
|
Al-Kazaz, Noor R.
|
| Title & Author
|
:
|
Compression-Based Methods for the Automatic Cryptanalysis of Classical Ciphers\ Al-Kazaz, Noor R.
|
| College
|
:
|
Bangor University (United Kingdom)
|
| Date
|
:
|
2019
|
| Degree
|
:
|
Ph.D.
|
| student score
|
:
|
2019
|
| Note
|
:
|
220 p.
|
| Abstract
|
:
|
The study documented in this thesis investigates the effectiveness of compression in the field of cryptanalysis, specifically for the automatic cryptanalysis of classical ciphers, initially for the English language. Several new<br/>compression-based cryptanalysis methods are developed against these ciphers.<br/>The new methods use the well-known compression scheme—prediction<br/>by partial matching (PPM)—and have been applied to automatic cryptanalysis<br/>for three main classical ciphers: simple substitution, transposition and<br/>Playfair ciphers. The extensive set of case studies adopted in this research<br/>have validated the new methods, which have proven to be very effective<br/>in the cryptanalysis of these cases with a high success rate—for substitution<br/>ciphers, 92% of the cryptograms were correctly solved with no errors<br/>and 100% with just three errors or less; a 100% decryption success rate was<br/>achieved for transposition ciphers and 87% was achieved for Playfair ciphers.<br/>This study led to the decipherment of more challenging cases, such as very<br/>short ciphertexts with no probable words. The Gzip compression scheme<br/>has also been applied to the automatic decryption of simple substitution<br/>and transposition ciphers, but the results showed that Gzip, in comparison<br/>to PPM, was not as effective. A third compressor, Bzip2, could not be used<br/>as the nature of that scheme made its use unfeasible.<br/>The PPM compression-based cryptanalysis methods offered significant<br/>improvements in decryption accuracy in a diverse range of experiments while<br/>being computationally more efficient compared to previously published techniques. In addition, extensive investigations were conducted to determine<br/>the most appropriate type of PPM scheme to be applied in the cryptanalysis<br/>of these ciphers. These findings have highlighted why better models are<br/>of vital importance in cryptology. In particular, the study has shown how<br/>a good model of the source (i.e. the PPM compression model)–a method<br/>that shows a high level of performance when applied to different language<br/>modelling tasks–can also be effectively used in the automatic decryption of<br/>different classical ciphers.<br/>As spaces have been traditionally omitted from ciphertext, a full cryptanalysis<br/>mechanism which also automatically adds spaces to decrypted texts,<br/>again using a compression-based approach, has also been proposed to achieve<br/>readability.<br/>This work has also investigated whether the newly devised cryptanalysis<br/>methods are applicable to another language (specifically Arabic as it is a<br/>language non-related to English). Arabic is a rich morphological language<br/>with its own characteristics that differentiate it from other languages. The<br/>current study has specifically adapted new compression-based methods for<br/>the automatic cryptanalysis of classical Arabic ciphers (simple substitution,<br/>transposition and Playfair ciphers). Although the experiments conducted<br/>with Arabic ciphers have generally been less effective than those with classical<br/>English ciphers, excellent results have been achieved—for Arabic substitution<br/>ciphers, 72% of the cryptograms were successfully solved without<br/>any errors and over 91% with just three errors or less; a 97% decryption<br/>success rate was achieved for Arabic transposition ciphers, with this result<br/>being 73% for Arabic Playfair ciphers.
|
| Descriptor
|
:
|
Computer science
|
|
|
:
|
Electrical engineering
|
|
|
:
|
Environmental engineering
|
| Added Entry
|
:
|
Bangor University (United Kingdom)
|
| |