بسم الله الرحمن الرحيم NETWORK SECURITY

1. بسم الله الرحمن الرحيمNETWORK SECURITY Done By: Saad Al-ShahraniSaeed Al-Smazarkah May 2006

2. Network Security Goals • Confidentiality or Privacy: Only sender, intended receiver should ”understand” message contents • Sender encrypts message • Receiver decrypts message • Authentication:Sender, receiver want to confirm identity of each other • Integrity:Sender, receiver want to ensure messages are not altered during transmission without detection • Access and Availability: Services must be accessible and available to users

3. People want to communicate securely • Eve (intruder/eavesdropper): may intercept, delete, add messages

4. There are Eavesdroppers out there! Q: what can an eavesdropper do? A: a lot! • Observing and recording information • Unauthorized access to a server • Denial of service: prevent service from being used by others (e.g. by Flooding a server with requests) • Impersonating a legitimate server • Hijacking: “take over” ongoing connection by removing sender or receiver, inserting himself in place

5. Network Security: Agenda • Main weapon: cryptography • Confidentiality (encryption) • Message authentication • Signatures and Certificates • Internet Threats, Attacks and Defenses • Secure request/ response protocols • Secure connection ‘tunnels’ • Denial of Service attacks • Firewalls

6. Encryption Protects Confidentiality • Secret Key crypto:Shared secret key (KE,B=KD,B) • Public key crypto:A user has a public encryption key KE,B and a matching private decryption key KD,B

7. Secret Key Crypto • Two operations (“encrypt”, “decrypt”) which are inverses of each other. Like multiplication/division • One parameter (“the key”) • Even the person who designed the algorithm can’t break it without the key • Ideally, a different key for each pair of users

8. Secret key crypto • encrypt= f(K, plaintext)=ciphertext • decrypt= f(K, ciphertext)=plaintext • authentication:send f(K, challenge) • integrity check:f(K, msg)=X • verify integrity check:f(K, X, msg)

9. Public Key Crypto • Two keys per user, keys are inverses of each other • Public key “BPub” you tell to the world • Private key “BPriv” you keep private • Yes it’s magic. Why can’t you derive “BPriv” from “BPub”? • And if it’s hard, where did (BPub, BPriv) come from?

10. Encryption and Integrity • Encryption hides the message from eavesdropper • Question:can eavesdropper change message? • Or:dose encryption ensure message integrity? • With Public Key Encryption: • Eve can replace EBPub(m) with fake: EBPub (m’) • With Secret (shared) Key Encryption: • This seems harder to do • But given c=mk, attacker can send c mask, to invert any bit in decrypted message (use mask) • Encryption does not ensure integrity!

11. Public Key VS. Secret Key • In terms of capabilities public key is more powerful. • Public key provides digital signature. • Secret key is much faster than public key

12. Public Key Digital Signatures: concept • One of the best features of public key • An integrity check • calculated as f(priv key, data) • verified as f(public key, data, signature) • Verifiers don’t need to know secret key • RSA can be used for digital signature scheme

13. Popular Secret Key Algorithms • DES:old standard, 56-bit key, slow • 3DES:fix key size but 3 times as slow • RC4:variable length key, “stream cipher” (generate stream from key, XOR with data) • AES:replacement for DES, will probably take over

14. Popular Public Key Algorithms • RSA:public key operations can be made very fast, but private key operations will be slow. • ECC (elliptic curve crypto):smaller keys, so faster than RSA (but not for public key ops).

15. Secret Key Problem: How do two entities establish shared secret key over the network? Solution: Trusted key distribution (KDC) acting as intermediary between entities KDC needs shared key with each entity. Public Key Problem: When Saad obtains Saeed’s public key (from web site, e-mail, diskette), how does he know it is Saeed’s public key, not eavesdropper’s? Solution: Trusted certification authority (CA) Key Distribution Problem

16. Key Distribution - Secret Keys • Could configure n2 keys • Makes the systems unfeasible for large-scale use • Instead use Key Distribution Center (KDC) • Every user has a secret key shared with KDC • The KDC knows all the users • The KDC assigns a key to any pair who need to talk

17. Key Distribution - Public Keys • Certification Authority (CA) signs “Certificates” • Certificate =a signed message saying “I, the CA, vouch that 489024729 is Saad’s public key” • If everyone has a certificate, a private key, and the CA’s public key, they can authenticate

18. Key Distribution - Public Keys Saeed Saad [“Saad”, key=342872]CA [“Saeed”, key=8294781]CA Auth, encryption, etc.

19. Denial Of Service Attack • Attacker tries to exhaust resources of host/ server/ router/ user • Resources include: • Computation (CPU time) • Storage (e.g. for state of requests/connections) • Open TCP connections • Limited (10s to several thousand connections – depending on hardware, operating system) • SYN flooding DOS attack: attacker sends ‘SYN’ flow (open connection); server waits

20. SYN flooding DOS attack • Attacker sends many SYN requests (using different spoofed client IP address), no ACK • Uses up server’s capacity for open connections

21. Firewalls • Firewall: A secure machine (or program), isolating organization’s internal net from larger Internet (or another net), allowing some packets to pass, blocking others

22. Packet Filtering • Internal network connected to Internet via Router Firewall (packet filtering) • Router filters packet-by-packet, decision to forward/ drop packet based on: • Source IP address, destination IP address • TCP/UDP source and destination port numbers • TCP SYN and ACK bit (identify client vs. server)

23. References • Our textbook • http://www.aw.com/kurose-ross/ • http://www.iec.org • http://amir.herzberg.name