(Requirements in communication) (efficiently) (Information Theory) (certainly) (Coding Theory) (safely) (Cryptography) I 1

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1 (Requirements in communication) (efficiently) (Information Theory) (certainly) (oding Theory) (safely) (ryptography) I 1

2 (Requirements in communication) (efficiently) (Information Theory) (certainly) (oding Theory) (safely) (ryptography) I 1

3 (obstructions for safe communication) (obstruction) (DoS ) (tapping) (tampering) (disguise) etc. I 2

4 DoS (Denial-of-service attack) A B E B E I 3

5 DoS (Denial-of-service attack) A B E B E I 3

6 DoS (Denial-of-service attack) A B E distributed denial-of-service attack (DDos) I 4

7 (tapping) A P P B E P I 5

8 (tapping) A P P B E P I 5

9 (secret communication) A B P E? P P: (plain text), : (ciphertext) P : (encryption) P : (decryption) (cryptanalysis) I 6

10 (tampering) A B P E P A (authentication), (digital signature) I 7

11 (disguise) A B E P P A (authentication), (digital signature) I 8

12 (disguise) A B E P P A (authentication), (digital signature) I 8

13 (cryptography) A B P E? P A P B P E P B I 9

14 (cryptography) A B P E? P A P B P E P B I 9

15 (cryptography) Assumption: open channels (being tapped) ( ) open cryptographic system (symmetric-key cryptography) ( ) (public-key cryptography) I 10

16 (cryptography) Assumption: open channels (being tapped) ( ) open cryptographic system (symmetric-key cryptography) ( ) (public-key cryptography) I 10

17 (cryptography) ( ) ( ) ( ) I 11

18 (symmetric-key cryptography) substitution ciphers ( ) aesar cipher linear block ciphers ( ) Vernam ciphers (one-time pad) DES (Data Encryption Standard) AES (Advances Encryption Standard) I 12

19 Ex. aesar cipher (aesar ) Key ( ) : n Z/26Z Encryption ( ) : n-shift backward Decryption ( ) : n-shift forward XYZABDEFGHIJKLMN OPQRSTUVWXYZAB : n =? :????? KHOOR I 13

20 Ex. aesar cipher (aesar ) Key ( ) : n Z/26Z Encryption ( ) : n-shift backward Decryption ( ) : n-shift forward XYZABDEFGHIJKLMN OPQRSTUVWXYZAB : n = 3 : HELLO KHOOR I 13

21 aesar (Weakness of aesar cipher) DES (Deta Encryption Standard) AES (Advanced Encryption Standard) I 14

22 aesar (Weakness of aesar cipher) DES (Deta Encryption Standard) AES (Advanced Encryption Standard) I 14

23 ( ) (preperties of symmetric-key cryptography) The encryption key and the decryption key are the same. (simple, fast) (need key-sharing) (need a different key for each pair) I 15

24 : ( ) ( ) I 16

25 : ( ) ( ) I 16

26 : ( ) ( ) I 16

27 (Public-key cryptography) ( ) ( ) The encryption key and the decryption key are different. (No need key-sharing in advance) (authentication) (signature) (non-repudiation) I 17

28 (Public-key cryptography) ( ) ( ) The encryption key and the decryption key are different. (No need key-sharing in advance) (authentication) (signature) (non-repudiation) I 17

29 (Public-key cryptography) ( ) ( ) The encryption key and the decryption key are different. (No need key-sharing in advance) (authentication) (signature) (non-repudiation) I 17

30 (Public-key cryptography) (slow) (first share a secret key under public-key cryptosystem) (then communicate with the key under secret-key cryptosystem) I 18

31 A e public: e B d P E? P secret: d I 19

32 A public: e B e d P E? P secret: d A (signature) I 20

33 (signature) A public: e d P E? secret: d B e P I 21

34 (signature) d A public: e B e P E? P secret: d E P I 22

35 (signature) M M (hash value) h(m) A S M B I 23

36 (signature) A public: e A public: e B B S d A h(m) M secret: d A e B d B ea S M h(m) secret: d B I 24

37 (preperties of public-key cryptography) (Everyone can encrypt.) (Decryption requires the secret key.) ( )?!! ( ) I 25

38 (preperties of public-key cryptography) (Everyone can encrypt.) (Decryption requires the secret key.) ( )?!! ( ) I 25

39 (preperties of public-key cryptography) (Everyone can encrypt.) (Decryption requires the secret key.) ( )?!! ( ) I 25

40 (preperties of public-key cryptography) (Everyone can encrypt.) (Decryption requires the secret key.) ( ) (use of problems hard to compute) (prime decomposition) (discrete logarithm) I 26

41 (public-key cryptosystems) RSA cryptosystem (Rivest-Shamir-Adleman) Diffie-Hellman key-exchange ( ) ElGamal encryption I 27

42 (public-key cryptosystems) RSA cryptosystem (Rivest-Shamir-Adleman) Diffie-Hellman key-exchange ( ) ElGamal encryption I 27

43 : RSA Rivest, Shamir, Adleman (1977) p, q n = pq n e d n e d n e d n n = pq ( ) I 28

Block cipher

18 12 9 1 2 1.1............................... 2 1.2.................. 2 1.3................................. 4 1.4 Block cipher............................. 4 1.5 Stream cipher............................