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Lecture 11: Encryption Schemes (IV)

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Recap: CPA-secure encryption scheme

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Modes of Operation

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Variable-length CPA-security

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Electronic Code Book (ECB) mode

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Secure mode of operation 1:�Counter (CTR) mode

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Define “bad event” Repeat

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Secure mode of operation 2:�Output feedback (OFB) mode

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Secure mode of operation 3:�Cipher block chaining (CBC) mode

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CBC mode of operation v. CBC-MAC

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Comparison of CTR, OFB, CBC

	CTR	OFB	CBC
parallel encryption	√	×	×
parallel decryption	√	×	√
pad prepared in advance	√	√	(no pad to speak of)

CTR

OFB

CBC

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PRFs in Practice