Cryptanalysis of the Playfair Cipher Using an Evolutionary Algorithm By: Benjamin Rhew

Where We are Going... What is the problem? Why use an Evolutionary Algorithm? The basics of cryptanalysis The workings of the Playfair cipher Implementing the concept Future goals Conclusion

The Problem Cryptography is necessary Data Protection Message Encryption Cryptanalysis is the “other side of the coin” What is it? Why is it needed? Use of the Playfair Cipher Admittedly simple, used for a proof-of-concept

Why use an Evolutionary Algorithm? EAs are able to solve difficult problems with less information about the problems than other methods Automation of the cryptanalysis process will help in making stronger cryptographic systems, and it is hoped that the EA will be better than current methods

The Basics of Cryptanalysis Several different types of attacks Ciphertext only Known plaintext Chosen plaintext Less common methods Chosen ciphertext Chosen text Brute force can be used, but domain knowledge is usually applied to make cryptanalysis easier.

The Workings of the Playfair Cipher Write a message, e.g. This is a message. Create a 5 x 5 matrix using some keyword with no repeating letters, e.g. MONARCHIST Write out the plaintext in digram form, e.g. TH IS IS AM ES SA G E Break up double letters with X or Z, if needed. MORNA CHTI/JS BDGEF KLUPQ VWZXY

The Workings of the Playfair Cipher Encrypt the message using the following: If the digram pair is in the same row, replace by the digram to the right, with wraparound right to left. e.g. ON becomes NA. If the digram pair is in the same column, replace with digram one space below with wraparound. e.g. OH becomes HD. MORNA CHTI/JS BDGEF KLUPQ VWZXY

The Workings of the Playfair Cipher For all other combinations, start with the first letter and move across until lined up with the second letter. Repeat with the second letter. e.g. ET becomes GJ and HE becomes JD MORNA CHTI/JS BDGEF KLUPQ VWZXY

Implementing the Concept Individuals will be based on possible keys for the algorithm – MONARCHJSTBDEFGKLP QUVWXYZ, for example Crossover will take random characters from one parent, keep them in the same position, and use the other parent to fill in the rest. Mutation will swap characters in the possible key. MORNA CHTI/JS BDGEF KLUPQ VWZXY

Implementing the Concept Fitness function is the most difficult to implement Take the possible key, decrypt the ciphertext Use a dictionary look-up on the plaintext generated Fitness increases as the number of real words increases MORNA CHTI/JS BDGEF KLUPQ VWZXY

Future Goals Obtain results Compare method to existing methods Expand the Playfair Cipher concept to larger matrices and compare the EA method to existing methods Use the concepts learned to try to solve more complicated cryptographic systems

Conclusion Cryptanalysis is just as necessary as cryptography The basic idea: cryptanalyze the Playfair Cipher using an Evolutionary Algorithm as a stepping-stone to more difficult problems The most difficult portion to implement is the fitness function Compare results obtained from the EA method to other methods in existence

Questions?