Bài giảng Other Block Ciphers

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
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 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).
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Electronic Codebook Mode (ECB)
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 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
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 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.
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Encryption of bitmaps in ECB mode
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Cipher Block Chaining Mode (CBC)
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 CBC critics
Decryption of all subsequent blocks yi, i ≥ 2
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 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
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Encryption of bitmaps in CBC mode
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 Output Feedback Mode (OFB)
Key stream is not generated bitwise but instead in a blockwise fashion.
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Cipher Feedback Mode (CFB)
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Counter Mode (CTR)
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 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 = 
 16
 2 keys that perform the mapping ek(x1)= y1.
 Multiple keys map between one plaintext and one ciphertext
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 Increasing the Security of Block Ciphers
• Multiple encryption
 – Double Encryption
 – Triple Encryption
 – Problem: Meet-in-the-Middle Attack
• Key whitening
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 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.
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 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)
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Key Whitening
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