1. DES Attacks and Triple DES
Cryptography and
Network Security
Reference: Sec 4.1 of Stallings Text

2. DES and Brute Force Attack
Known plaintext attack
Plaintext-ciphertext pairs are available
Brute force attack involves going through all 256 keys, encrypting the plaintext and matching with the ciphertext
SDESCrack1.java and SDESCrack2.java are programs that attack Simplified DES
Brute-force search on all 210 keys
Note the necessity of two plaintext-ciphertext pairs

3. Multiple DES
Brute-force attack on DES will eventually become possible and practical
Multiple applications (multiple keys) of DES increases the key search space for brute-force attacks
e.g., Double DES means two keys and therefore 2112 possible key-pairs to consider

4. Double DES Encryption: C = Ek2[Ek1[P]] Decryption: P = Dk1[Dk2[C]]
Check out SDES2.java for a simplified version
The following fact is essential:
For any two keys k1 and k2, it does not follow that a key k3 exists where Ek2[Ek1[P]] = Ek3[P]
A single-key brute-force attack will not work
Brute-force attack has two go through all key-pairs (see SDES2Crack1.java & SDES2Crack2.java)

5. Meet-in-the-Middle Attack
C = Ek2[Ek1[P]] implies X = Ek1[P] = Dk2[C]
Algorithm: (assume p1,c1 and p2,c2 are known pairs)
Try all 256 possibilities for k1 and tabulate all Ek1[p1]
Try all 256 possibilities for k2, compute for Dk2[c1], and compare against the table
When values match, check if c2 = Ek2[Ek1[p2]]

6. Analysis of Revised Attack on Double DES
Takes only twice the amount of time (approx) as brute-force on Regular DES
Requires a table of size 256
See SDES2MM.java for an attack on Simplified Double DES

7. Triple DES
Meet-in-the-Middle attack does not apply if three DES applications are used
Using three keys possible but aggregate length of keys might be undesirable
Use two keys instead
Reuse a key for first and third application
Make second application a decryption operation (for backward compatibility with Regular DES)
See SDES3.java

8. Programming Homework #2
Write an attack program for simplified triple DES (SDES3.java)
Use a combination of the brute-force method and the meet-in-the-middle method (use a table of size 210) to come up with an algorithm that runs in roughly the same amount of time as DES2Crack1.java