Announcements:

1
DTTF/NB479 Dszquphsbqiz Day 15

• Announcements
• Get your ch 1-2 quiz if you havent.
• Grading change
• Homeworks are mixed programming and written,
  makes no sense to separate them.
• Lumping 40 prog 25 written into 65 homework.
• Homework 4 posted, couple details missing
• Today
• Discuss tomorrows quiz
• Attacks on DES
• Questions?

2
Tomorrows quiz

• For each problem, Ill specify the algorithm
• Shift Affine Vigenere Hill LFSR
• and attack
• Ciphertext only known plaintext
• May require you to modify your code some on the
  fly
• Have your algorithms ready to run

3
Breaking DES

• 1975 Whitfield Diffie and Martin Hellmans
  proposition
• 1977 DES made the standard by NBS (NIST)
• 1987 DES 2nd 5-year review. Acknowledged
  weakness. NSA offered to replace it.
• 1992 DES re-certified again!
• 1993 Michael Wieners proposed (special purpose)
  device

4
Distributed approaches to breaking DES

• 1997 DES Challenge issued. 10K prize
• Solved by Rocke Verser in 5 months
• A distributed attack
• Found after searching ___ of keyspace
• 1998 DES Challenge II
• Down to 39 days, 85 of keyspace!
• Also in 1998

5
DES Cracker

• Budget of only 200,000 1998 dollars
• vs 20,000,000 1977 dollars
• Used mixture of software and hardware
• Discuss specialized hardware to prune keys
• Used assumptions about plaintext
• Operated on 2 64-bit blocks of ciphertext
• Result? Cracked in less than 5 days

6
Post-DES

• Let N 256 72,057,594,037,927,936 be the
  number of DES keys
• Brute force attacks that take O(N) DES
  computations are now reasonable.
• Can we just double encrypt to get O(N2)
  computations?
• Use k1, k2
• C Ek2(Ek1(P)), so P Dk1(Dk2(C)) ?

7
Meet-in-the-middle attack

• Assume k completely determines Ek and Dk
• Know P and C Ek2(Ek1(P))

Ek1(P) (for all k1)
Dk2(C) (for all k2)
C
P
Time complexity? O( n ) DES computations, O( n2
) comparisons O(n2 ) memory
8
Triple-DES?
Type DES computations Comparisons Memory Brute force DES
Double CEk2(Ek1(P)) O(N) O(N2) O(N) O(N2)
Triple1 CEk3(Ek2(Ek1(P)))
Triple2 CEk1(Ek2(Ek1(P)))
Triple3 CEk2(Ek1((Ek1(P))
Describe attacks on triple 1-3, fill out chart,
and order by level of security