全套英文版《计算机网络》PPT电子课件教案-Chapter 7 Network security精选.pptVIP

Chapter 7: Network security Foundations: what is security? cryptography authentication message integrity key distribution and certification Security in practice: application layer: secure e-mail transport layer: Internet commerce, SSL, SET network layer: IP security Friends and enemies: Alice, Bob, Trudy well-known in network security world Bob, Alice (lovers!) want to communicate “securely” Trudy, the “intruder” may intercept, delete, add messages What is network security? Secrecy: only sender, intended receiver should “understand” msg contents sender encrypts msg receiver decrypts msg Authentication: sender, receiver want to confirm identity of each other Message Integrity: sender, receiver want to ensure message not altered (in transit, or afterwards) without detection Internet security threats Packet sniffing: broadcast media promiscuous NIC reads all packets passing by can read all unencrypted data (e.g. passwords) e.g.: C sniffs B’s packets Internet security threats IP Spoofing: can generate “raw” IP packets directly from application, putting any value into IP source address field receiver can’t tell if source is spoofed e.g.: C pretends to be B 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 The language of cryptography symmetric key crypto: sender, receiver keys identical public-key crypto: encrypt key public, decrypt key secret Symmetric key cryptography substitution cipher: substituting one thing for another monoalphabetic cipher: substitute one letter for another 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 ma