Lec 9: Using Block Ciphers (I)

Mode of operation

• Using a block cipher to encrypt messages of arbitrary length

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• 1976-77 — DES standardized
• 1980 — DES modes of operation standardized
• ECB, CBC, CFB, OFB

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• 2000-01 — AES standardized
• 2001 — AES modes of operation standardized
• Above four + CTR

Block Encryption

Electronic Code Book (ECB)

Cipher Block Chaining (CBC)

Stream Encryption

• Idea: use block cipher (with IV) to generate a “random stream” to be XORed with message

Output Feedback (OFB) mode

Counter (CTR) mode