Slovak University of Technology in Bratislava
Faculty of electrical engineering and information technology
Degree Course: INFORMATICS
Author: Bc. Juraj Antal
Thesis: The Automated Decryption of Simple Substitution Cipher
Supervisor: Prof. RNDr. Otokar Grošek CSc.
December 1999

This thesis describes a method for unauthorized decrypting of monoalphabetic substitution ciphers. The algorithm relys on statistical properties of English language. It computes the single letters, bigrams and trigrams frequencies of the cipher text and compares them with the plain text frequencies stored in a selected table. The success of the decrypting algorithm, according to the language characteristic, strongly depends on the chosen table. If the algorithm fails, manual user interaction is available.

In the system there is has implemented an intuitive graphic user interface. It also includes a frequency editor which enables to preview, edit and manage the tables of letters frequencies. There is also built-in support for search in large on-line dictionary for words that satisfy constraints on letter position and letter multiplicity. The two search methods are supported. An exhaustive search from a given start letter and the one based on indexes to database. The database index generator is also included.