Block Ciphers & Operation Modes

Question 1

Block Ciphers

Answer 1

Process messages in blocks (64 bits or more), then en/decrypt them; is reversible if and only if 1-to-1 mapping between plain and cipher text exists

Question 2

Stream Ciphers

Answer 2

Process message bit or byte at a time when en/decrypting; Ex: Vigenere cipher

Question 3

Diffusion

Answer 3

Change in statistical structure of plaintext when converted to ciphertext; means small plaintext change=massive ciphertext change

Question 4

Confusion

Answer 4

Make it hard to change ciphertext to plaintext, derive keys, etc.; means larger key size=better confusion

Question 5

DES (Data Encryption Standard)

Answer 5

One symmetric key for en/decrypting; 2^56 distinct keys (not enough; obsolete)

Question 6

Double-DES

Answer 6

DES w/ 2 keys; K1 than K2 to encrypt, K2 than K1 to decrypt; 2^112 distinct key pairs; 2^64 different ciphertext

Question 7

Meet-in-the-middle Attack

Answer 7

For Double-DES; for each possible K1, for each possible K2, if EK1(P1) = DK2(C1) and EK1(P2) = DK2(C2), K1 and K2 are now cracked; O(2^56), NOT THAT LONG

Question 8

Triple-DES

Answer 8

3 keys; no practical attacks; backwards compatible with single-DES even though it only has one key, use it for all encryptions

Question 9

CBC (you know this)

Answer 9

Uses previous ciphertext to create next ciphertext block

Advantages-Excellent Diffusion

Disadvantages-Need unpredictable and integrity-guaranteed IV or plaintext can be easily modified by attackers

Question 10

CFB (you know this)

Answer 10

Like CBC, but some differences (like only using certain bits of ciphertext)

Advantages-Great for streams of data (in bits/bytes)…

Disadvantages-…but not really (does block encryption still)