Cryptanalysis of the Playfair Cipher Using an Evolutionary Algorithm

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

2. 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

3. 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

4. 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

5. 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.

6. 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. M O N A R C H I/J S T B D E F G K L P Q U V W X Y Z

7. 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. M O N A R C H I/J S T B D E F G K L P Q U V W X Y Z

8. 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 M O N A R C H I/J S T B D E F G K L P Q U V W X Y Z

9. 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. M O N A R C H I/J S T B D E F G K L P Q U V W X Y Z

10. 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 M O N A R C H I/J S T B D E F G K L P Q U V W X Y Z

11. 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

12. 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

13. Questions?