Internet security threats Denial of service(DOS: o flood of maliciously generated packets "swamp receiver o Distributed DOS(DDOS): multiple coordinated Sources swamp receiver oe.g. C and remote host SyN-attack A A ADC SYN SYN SYN SYN SYN SYN SYN 7: Network Security 6
7: Network Security 6 Internet security threats Denial of service (DOS): flood of maliciously generated packets “swamp” receiver Distributed DOS (DDOS): multiple coordinated sources swamp receiver e.g., C and remote host SYN-attack A A B C SYN SYN SYN SYN SYN SYN SYN
The language of cryptography plaintext VA B→ plaintext ciphertext Encryption Decryption algorithm algoritnm channe Alice Tr symmetric key crypto: sender, receiver keys identical public-key cryp to: encry ypt key public, decrypt key secret 7: Network Security 7
7: Network Security 7 The language of cryptography symmetric key crypto: sender, receiver keys identical public-key crypto: encrypt key public, decrypt key secret Figure 7.3 goes here plaintext plaintext ciphertext K A K B
Symmetric key cryptograph substitution cipher: substituting one thing for another o monoalphabetic cipher: substitute one letter for another plaintext: abcdefghijklmnopqrstuvwxyz ciphertext: mnbvcxzasdfghjklpoiuytrewg E.g. Plaintext: bob. i love you. alice ciphertext: nkn. s gktc wky. mgsbc Q: How hard to break this simple cipher? brute force(how hard? ° other? 7: Network Security 8
7: Network Security 8 Symmetric key cryptography substitution cipher: substituting one thing for another monoalphabetic cipher: substitute one letter for another plaintext: abcdefghijklmnopqrstuvwxyz ciphertext: mnbvcxzasdfghjklpoiuytrewq Plaintext: bob. i love you. alice ciphertext: nkn. s gktc wky. mgsbc E.g.: Q: How hard to break this simple cipher?: •brute force (how hard?) •other?
Symmetric key crypto: DES DES: Data Encryption Standard D US encryption standard [NIST 1993 0 56-bit symmetric key, 64 bit plaintext input 门 How secure is Des? o DES Challenge: 56-bit-key-encrypted phrase CStrong cryptography makes the world a safer place")decrypted(brute force)in 4 months o no known"backdoor"decryption approach o making DES more secure o use three keys sequentially (3-DES)on each datum o use cipher-block chaining 7: Network Security 9
7: Network Security 9 Symmetric key crypto: DES DES: Data Encryption Standard US encryption standard [NIST 1993] 56-bit symmetric key, 64 bit plaintext input How secure is DES? DES Challenge: 56-bit-key-encrypted phrase (“Strong cryptography makes the world a safer place”) decrypted (brute force) in 4 months no known “backdoor” decryption approach making DES more secure use three keys sequentially (3-DES) on each datum use cipher-block chaining
64-bit input 56bit key termite Symmetric Key L1 RI crypto: DES 48-bit KI fILL, RL, KID DES operation 12R2 initial permutation 48-bit K2 2R2K2 16 identical"rounds"of function application 13 each using different 48 bits of key final permutation 48-bit K16 t7R17 permu:e 64-bit output /: Network Security 10
7: Network Security 10 Symmetric key crypto: DES initial permutation 16 identical “rounds” of function application, each using different 48 bits of key final permutation DES operation