Bài giảng Other Block Ciphers - Huỳnh Trọng Thưa

Other Block Ciphers
Huỳnh Trọng Thưa
htthua@ptithcm.edu.vn
What will we learn?
• The most important modes of operation for 
block ciphers in practice
• Security pitfalls when using modes of 
operations
• The principles of key whitening
• Why double encryption is not a good idea, 
and the meet-in-the-middle attack
• Triple encryption
2
Encryption with Block Ciphers: 
Modes of Operation
• Electronic Code Book mode (ECB),
• Cipher Block Chaining mode (CBC),
• Cipher Feedback mode (CFB),
• Output Feedback mode (OFB),
• Counter mode (CTR).
3
Electronic Codebook Mode (ECB)
4
ECB critics
• Advantages
– Block synchronization is not necessary.
• Problem
– identical plaintext blocks result in identical ciphertext 
blocks, as long as the key does not change
– Replay attack
• Usage: 
– not recommended to encrypt more than one block of data
– encryption in database
5
Ex of Substitution attack against electronic bank transfer
• Oscar observes the ciphertexts going through the communication 
network.
• After a while he can recognize the five blocks of his own transfer. 
He now stores blocks 1, 3 and 4 of these transfers.
• The same key is used for several other transfers between bank A 
and B.
• By comparing blocks 1 and 3 of all subsequent messages with the 
ones he has stored, Oscar recognizes all transfers that are made 
from some account at bank A to some account at bank B.
• He now simply replaces block 4 — which contains the receiving 
account number — with the block 4 that he stored before.
6
Encryption of bitmaps in ECB mode
7
Cipher Block Chaining Mode (CBC)
8
CBC critics
9
Decryption of all subsequent blocks yi, i ≥ 2
CBC critics (cont.)
• Good
– Randomized encryption: repeated text gets 
mapped to different encrypted data.
– A ciphertext block depends on all preceding 
plaintext blocks blocks
• reorder affects decryption
• Bad
– Errors in one block propagate to two blocks
– Sequential encryption, cannot use parallel 
hardware
10
Encryption of bitmaps in CBC mode
11
Output Feedback Mode (OFB)
12
Key stream is not generated bitwise but instead in a blockwise fashion.
Cipher Feedback Mode (CFB)
13
Counter Mode (CTR)
14
Exhaustive Key Search Revisited
• A brute-force attack can produce false positive 
results.
• Ex: A cipher with a block width of 64 bit and a 
key size of 80 bit. we find on average 280/264 = 
216 keys that perform the mapping ek(x1)= y1.
15
Multiple keys map between one plaintext and one ciphertext
Increasing the Security of Block Ciphers
• Multiple encryption
– Double Encryption
– Triple Encryption
– Problem: Meet-in-the-Middle Attack
• Key whitening
16
Double Encryption and Meet-in-
the-Middle Attack
• Key length: κ bits
• Brute-force attack: require 2κ ·2κ =22κ
encryptions (or decryptions)
• Meet-in-the-middle attack:
– The total complexity is 2κ +2κ = 2·2κ = 2κ+1.
17
Triple Encryption and Meet-in-the-
Middle Attack
• Key length: κ bits
• Brute-force attack: require 2κ ·2κ ·2κ =23κ encryptions (or 
decryptions)
• Meet-in-the-middle attack:
– The total complexity is 22κ.
– 3DES;:56 bits key => attacker performs 2112 key tests (not 2168)
18
Key Whitening
19