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Technical Report: DCC-2006-01

Automated Ciphertext-Only Cryptanalysis of the Bifid Cipher

António Machiavelo

Centro de Matemática da Universidade do Porto
R. do Campo Alegre 687, 4169-180 Porto, Portugal
Phone: +351-220 100 707,Fax: +351-220 100 708,
E-mail: ajmachia@fc.up.pt

and

Rogério Reis

DCC-FC & LIACC, Universidade do Porto
R. do Campo Alegre 823, 4150-180 Porto, Portugal
Phone: 351 22 6078830, Fax: 351 22 6003654
E-mail: rvr@ncc.up.pt

March 2006

Abstract

In this paper we describe a fully automated ciphertext-only cryptanalysis attack on the Bifid cipher, for which the original text language is known. We have implemented this attack using Python. We use an easily computable statistical function to find the period of the cipher, and then the key-table is generated in a fairly efficient way. The process is directed in such a way that strongly narrows the search space of possible solutions. This results in a feasible attack to a Bifid cryptogram, provided that its length is enough for accurate statistical analysis.


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