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

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

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

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

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

(tapping) A P P B E P I 5

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

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

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

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

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

(cryptography) ( ) ( ) ( ) I 11

(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

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

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

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

( ) (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

: ( ) (1976 77) I 16

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

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

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

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

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

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

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

(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

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

(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

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

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