Computer and Network Security

1. Computer and Network Security Mini Lecture by Milica Barjaktarovic

2. Why do we need computer security? • Potentially very costly loss of data and/or equipment due to: • Hardware and software failures • Natural disasters • External attacks: • From the Internet • Internal attacks: • From employees

3. Disaster prevention and recovery • Disaster scenarios • Backup/restore procedures • Network fault tolerance • Attack protection: • Network-based intrusion detection • Detect dangers coming into our network from the outside and going from our network to the outside • Host-based intrusion detection • Detect tampering with individual hosts

4. Protecting Data and Networks • Data/file types: • Public, internal, confidential, secret • On UNIX: set file permission with chmod • On PC: file permission window • Network access levels: • Local, remote, public • Solution: LAN behind a firewall

5. Attacks 101 • Types: • Internal attack • Organizational attacks • Accidental security breaches • Ways of attacking: • Social engineering • Denial of Service (DoS) • Automated computer attacks • Probing (precursor to a real attack) • SATAN, ISS tools • Spoofing • Viruses, worms, trojan horses • Spamming • Steganography • Players: • Hackers • Security analysts • Security watchdogs (e.g. CERT) and resources (e.g. SANS)

6. Organizational Attacks and Defense • Organizational attacks: • For (financial) crime • For terrorism/espionage • Organizational defense: • By the military: mandatory access controls, levels of security, Orange Book, professional and numerous security analysts • By corporations: system administrators often doubling as security analysts • Firewalls • Network and host intrusion detection • Tight grip on employees • Security evaluation and certification • Cryptographic services

7. Cryptography 101 • Cryptography allows production and exchange of “secret messages” • Cryptography is used to provide security services: • Privacy • Only the intended recipient can access data • Authentication • The identity of communicating parties can be verified • Message integrity • Nobody tampered with the message • Cryptography utilizes: • cryptographic hash functions: • provide a way to “scramble” data. No possibility of unscrambling. • cryptographic algorithms: • provide a way to “scramble” data using a specific key. The data can be “unscrambled” only with another specific key.

8. Cryptographic Hash Functions • A hash functionH is a mathematical transformation that takes an input message m and returns a fixed-size string, which is called the hash value h • h = H(m) • A cryptographic hash function is a hash function with additional properties: • The input can be of any length. • The output has a fixed length. • H(x) is relatively easy to compute for any given x. • H(x) is one-way. • H(x) is collision-free. • A hash function H is said to be one-way if it is hard to invert, where ``hard to invert'' means that given a hash value h, it is computationally infeasible to find some input x such that H(x) = h. • A hash function H is said to be a weakly collision-free if, given a message x, it is computationally infeasible to find a message y not equal to x such that H(x) = H(y). • A hash function H is said to be strongly collision-free if it is computationally infeasible to find any two messages x and y such that H(x) = H(y).

9. Cryptographic Algorithms • Secret key (e.g. DES) • The same secret key is used to scramble and unscramble data • Pros: only one key • Cons: both parties must share the same key • Public key (e.g. RSA) • The sender scrambles with receiver’s public key, the receiver unscrambles with his private key • Pros: the public keys can be publicly posted • Cons: how do you distribute public keys in a trustworthy manner • PKI (Public Key Infrastructure) and X.509 standard for public key distribution • Chain of trust of Certification Authorities (CAs)

10. Protecting a Message: Levels of Protection Strength • CRC • Message digest (i.e. message hash) • Message digest is the string obtained by applying a cryptographic hash function to message • Cryptographic hash function is an irreversible, collision-free hash function that takes as input data of any length and produces a fixed length string • Sample algorithms: MD2, MD5, SHA. • Encrypted message • Obtained by applying a cryptographic algorithm (public or secret key) to message • Sample algorithms: RSA, DES, Blowfish, IDEA, etc. • Crypto++ library http://www.amasci.com/~weidai/cryptlib.html

11. Cryptographic Applications • Message Integrity Code (MIC): • A fixed-length quantity generated cryptographically and associated with the message. Usually: compute message digest (i.e. message hash) and encrypt it, usually using secret key cryptography. • Digital Signature (Digital Signature Algorithm (DSA)) • the sender encrypts message using his private key, recipient verifies it using sender’s public key. Usually: compute message digest and then encrypt it. • Secure email • PGP assumes that each user decides whom to trust • PEM assumes a rigid hierarchy of CAs • Transmitting over insecure channel (virtual encrypted tunnel) • Tunneling protocols: • Point-to-point • Layer 2 tunneling protocol (L2TP) / IPsec • Secure storage on insecure media • Authentication • 3-way handshake • Third trusted party • Digital signature: the sender signs using his private key, others verify it using the sender’s public key

12. Network Security • Firewalls: • Filter based • Proxy based • Application level security (e.g. HTTPS) • Transport layer security • TSL (Secure Transport Layer) • E-commerce, public key, 3-way handshake • Network Layer Security: • IPsec • SSL (Secure Sockets Layer)