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1 CIS 5371 Cryptography 4. Message Authentication Codes B ased on: Jonathan Katz and Yehuda Lindell Introduction to Modern Cryptography.

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Presentation on theme: "1 CIS 5371 Cryptography 4. Message Authentication Codes B ased on: Jonathan Katz and Yehuda Lindell Introduction to Modern Cryptography."— Presentation transcript:

1 1 CIS 5371 Cryptography 4. Message Authentication Codes B ased on: Jonathan Katz and Yehuda Lindell Introduction to Modern Cryptography

2 2 Message Authentication Codes

3 3 Definition 4.1 Message Authentication Code

4 4

5 5 Definition 4.2 -- Secure MAC

6 6 Construction 4.3 A fixed length MAC from any PRF

7 7 Theorem 4.4

8 8 A secure fixed length MAC Proof

9 9 Distinguisher D

10 10 Distinguisher D

11 11 Distinguisher D

12 12 Replay atta cks

13 13 Construction 4.5 A variable length MAC

14 14 Theorem 4.6

15 15 Construction 4.9 CBC- MAC

16 16 Theorem 4.10

17 17 CBC-MAC vs CBC-mode encryption 1.CBC-mode encryption uses a random IV. If we use a random IV for CBS-MAC then we lose security. 2.In CBC-mode encryption all encrypted blocks are output as part of the ciphertext. This is not the case with CBC-MAC. If we do so we loose security.

18 18 Secure CBC-MAC for variable length messages – three options

19 19 Variable length CBC-MAC 

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