DES Attacks and Triple DES

Cryptography andNetwork Security

Reference: Sec 4.1 of Stallings Text

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

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

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)

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 ifc2 = Ek2[Ek1[p2]]

Analysis of Revised Attackon 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

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

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