How Did Nature Inspire Me To Solve The Ciphertext-only Attack

In this presentation I'll demonstrate a genetic algorithm that I develop to attack a given ciphertext and try to find the key (aka ciphertext-only attack).

This presentation was an assignment in security course, Ben Gurion University of Beer Sheva. May 2015.

Algorithm source code is available at http://github.com/lidanh/cbc-attacks

Transcript

1. HOW DID Lidan Hifi Department of So!ware Engineering Ben Gurion University NATURE INSPIRE ME TO SOLVE THE CIPHER-TEXT ONLY ATTACK
2. Quick Review
3. How can we get from here
4. TO THERE?
5. Brute Force
7. Brute Force is OBVIOUS
8. The gradual process by which heritable biological traits become either more or less common in a population as a function of the effect of inherited traits on the differential reproductive success of organisms interacting with their environment. It is a key mechanism of evolution. —wikipedia Natural Selection
9. Srsly it’s not as complicated as it sounds
10. Start with initial population of keys, randomly 1
11. abdcghfe cbaedgfh efcahgbd bdcgfeha fbahedgc hbaecdgf cefgbhad fheadgbc
12. Evaluate the fitness of each key in that population 2
13. Evaluate the fitness Dictionary Letter-frequency Distinct words / Total words = fitness
14. abdcghfe cbaedgfh efcahgbd bdcgfeha fbahedgc hbaecdgf cefgbhad fheadgbc 0.63 0.37 0.43 0.66 0.12 0.71 0.52 0.41
15. cbaedgfh hbaecdgf cefgbhad efcahgbd abdcghfe fheadgbc bdcgfeha fbahedgc 0.71 0.66 0.63 0.52 0.43 0.41 0.37 0.12
16. Breed new keys through crossover and mutation 3
17. Crossover Alpha: abcdefghijklmnopqrstuvwxyz Parent 1: suqhcladfgjknoptvwxzbemriy Parent 2: azbyxcwdevufgtshriqjpklomn
18. Crossover Alpha: abcdefghijklmnopqrstuvwxyz Parent 1: suqhcladfgjknoptvwxzbemriy Parent 2: azbyxcwdevufgtshriqjpklomn Suppose that parent 1’s key is able to find only the word AND
19. Crossover Alpha: and Parent 1: soh Parent 2: aty
20. Crossover Alpha: abcdefghijklmnopqrstuvwxyz Parent 1: suqhcladfgjknoptvwxzbemriy Parent 2: azbyxcwdevufgtshriqjpklomn Suppose that parent 2’s key is able to find only the word BIG
21. Crossover Alpha: big Parent 1: ufa Parent 2: zew
22. Crossover Alpha: abcdefghijklmnopqrstuvwxyz Parent 1: suqhcladfgjknoptvwxzbemriy Child 2: azbyxcwdevufgtshriqjpklomn swap(s,'a) swap(h,'y) swap(o,'t)
23. Crossover Alpha: abcdefghijklmnopqrstuvwxyz Parent 1: suqhcladfgjknoptvwxzbemriy Child 2: szbhxcwdevufgoayriqjpkltmn
24. Crossover Alpha: abcdefghijklmnopqrstuvwxyz Child 1: suqhcladfgjknoptvwxzbemriy Parent 2: azbyxcwdevufgtshriqjpklomn swap(z,'u) swap(w,'a) swap(e,'f)
25. Crossover Alpha: abcdefghijklmnopqrstuvwxyz Child 1: szqhclwdegjknoptvaxubfmriy Parent 2: azbyxcwdevufgtshriqjpklomn
26. A new generation was born Alpha: abcdefghijklmnopqrstuvwxyz Child 1: szqhclwdegjknoptvaxubfmriy Child 2: szbhxcwdevufgoayriqjpkltmn Both children can now find AND and BIG!
27. Mutation suqhcladfgjknoptvwxzbemriy Swap two letters randomly, to keep population variety high
28. Mutation suqhcladfgjknoptvwxzbemriy suqhpladfgjknoctvwxzbemriy
29. Evaluate the fitness of the new offsprings and place them in a new population 4
30. cgeadbfh hbecadgf cfeabhgd efcahgdb dbacghfe fhegadbc cdbgfeha fbaedhgc 0.85 0.79 0.70 0.68 0.57 0.21 0.20 0.11
31. Evolve the population, until the end condition is satisfied 5
32. Return the fittest key in the final population 6
33. Brute Force vs. GA
34. Brute Force vs. GA 20,000 bytes file, same fitness function Key Size Brute Force GA
35. Brute Force vs. GA 20,000 bytes file, same fitness function Key Size Brute Force GA 8 (a-h) 40.9 sec 2.087 sec
36. Brute Force vs. GA 20,000 bytes file, same fitness function Key Size Brute Force GA 8 (a-h) 40.9 sec 2.087 sec 12 (a-l) ~ 9 days * 3.525 sec * 1.3ms per key permutation, 479,001,600 permutations
37. CyActive Challenge
38. CyActive Solution
39. References 1. F.T. Lin, C.Y. Kao. A Genetic Algorithm for Ciphertext-Only Attack in Cryptanalysis. 1995. 2. R. Morelli, R. Walde. A Word-Based Genetic Algorithm for Cryptanalysis of Short Cryptograms. 2003.
40. Thank you. Source-code is available at: https://github.com/lidanh/cbc-attacks @lidanh