1. L8. Reviews Rocky K. C. Chang, May 2011

2. Foci of this course 2 Rocky K. C. Chang  Understand the 3 fundamental cryptographic functions and how they are used in network security.  Understand the main elements in securing today ’ s Internet infrastructure.  Exposed to some current Internet security problems.

3. Types of attacks 3 Rocky K. C. Chang  Passive attacks (eavesdropping), e.g.,  ciphertext-only attacks (recognizable plaintext attacks)  Fred has seen some ciphertext.  known-plaintext attacks  Fred has obtained some pairs.  chosen-plaintext attacks  Fred can choose any plaintext he wants.  Active attacks, e.g.,  pretend to be someone else (impersonation)  introduce new messages in the protocol  delete existing messages  substituting one message for another  replay old messages

4. 4 Rocky K. C. Chang Three cryptographic functions  Hash functions: require 0 key  Secret key functions: require 1 key  Public key functions: require 2 keys

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6. Symmetric cryptography 6 Rocky K. C. Chang  Secret key functions  Stream cipher vs block cipher  Symmetric cryptography based on substitution (confusion) and diffusion  64-bit DES and 128/192/256-bit AES  Secrecy service  Encrypting data of any size: cipher block chaining (CBC)  Security problems with CBC, e.g., identical and nonidentical ciphertext blocks.

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8. Cryptographic hash functions and MAC 8 Rocky K. C. Chang  Hash functions  3 properties: pre-image resistance, collision resistance, and mixing transformation  The birthday problem and attack  k  1.774  q, where q is the number of distinct hash outputs  The length of a secure hash output ≥ 256 bits  Hash function standards (MDx, SHA-x)  2 problems: length extension and partial message collision  Message authentication codes  A successful attack on MAC  CBC-MAC and HMAC

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10. The public-key cryptography 10 Rocky K. C. Chang  Prime numbers, modulo a prime  A group for the set of numbers modulo a prime p without 0 under multiplication  Compute the multiplicative inverse using the extended Euclid algorithm.  Generate a large prime number.  The Rabin-Miller test determines whether an odd integer is prime.  Each party involved in a public-key cryptographic system is one secret and one public “ key ”.

11. The Diffie-Hellman (DH) protocol 11 Rocky K. C. Chang  The DH protocol uses the multiplicative group modulo p, where p is a very large prime.  A generator g generates a set of numbers 1, g, g 2, …, g t-1 (g t = 1 again).  Subgroups (t < p-1) and group (t = p-1)  The basic Diffie-Hellman (DH) protocol  (g, p) and a random number in (1, 2, …, p-1)  The discrete logarithm problem  Security problems  Using a smaller subgroup ({1}, {1, p-1}) and a safe prime  Squares and nonsquares  Man in the middle attack

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13. The RSA algorithm 13 Rocky K. C. Chang  In RSA, we perform modulo a composite number n = p  q, where p and q are large primes.  Use 2 different exponents e (public) and d (private), such that e  d = 1 mod t, where t = lcm(p – 1, q – 1).  To encrypt m, compute c = m e mod n; to decrypt c, compute c d mod n = m.  To sign m, compute s = m 1/e mod n; to verify the signature, compute s e = m mod n.  Choices of e, p, and q  Pitfalls of using RSA, e.g., encrypting a small message, message signing.

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15. Authentication 15 Rocky K. C. Chang  Network-based, password-based  Cryptographic authentication  Symmetric and asymmetric  Challenge and response  Mutual authentication  2 x one-way authentication.  Reflection attack and man in the middle attack  Principles:  One-way: Have the responder influence on what she encrypts or hashes.  Have both parties have some influence over the quantity signed.

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17. Authenticated key exchange 17 Rocky K. C. Chang  Authenticated Diffie-Hellman exchange  Perfect forward secrecy  Allow both sides to agree on the crypto. algorithms and the DH parameters.  A partial solution to denial-of service attacks using cookies  It is prudent to couple the key exchange with authentication.

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19. Secure network protocols in practice

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21. PKI 21 Rocky K. C. Chang  Alice generates her public/private key pair.  Keep the private key.  Take the public key to the CA, say k  The CA has to verify that Alice is who she says she is.  The CA then issues a digital statement stating that k belongs to Alice.  There will never be a single CA for all or most of all.  There are going to be a large number of PKIs.  Use different key pairs in different PKIs.  Choose between a key server approach and a PKI approach.

22. IPSec 22 Rocky K. C. Chang  Unicast, unidirectional security association at the IP layer  Authentication Header and Encapsulation Security Payload  Partial solution to the replay attack  Tunnel mode and transport mode  Encryption without authentication is useless.  Outbound and inbound packet processing

23. IKEv.1 23 Rocky K. C. Chang  IKE phase 1 (ISAKMP association) and phase 2  The main mode consists of 3 message pairs.  1st pair: ISAKMP SA negotiation  2nd pair: a D-H exchange and an exchange of nonces  3rd pair: Peer authentication  The phase 1 is protected with encryption and authentication.  Establish IPSec associations and the necessary keys.  A new issue here is hiding the identities of the end points

24. TLS 1.0/ SSL 3.0 24 Rocky K. C. Chang  Pros and cons of providing security services at the transport layer instead of the IP layer.  The TLS Handshake and Record layers.  Session states and connection states  The session states can be reused to establish a new connection.  Server and client authentication

25. Network security is more than the above 25 Rocky K. C. Chang  Wireless security: IEEE 802.11i, RFID, Bluetooth, IP telephony, etc  Worms and buffer overflow attacks  Denial-of-service and degradation-of-service attacks  Data security  Covert channel, privacy protection

26. Network security is more than the above 26 Rocky K. C. Chang  Security policies  Operational issues  Human issues  Vulnerability analysis  Auditing  Intrusion detection  System security  Program security  etc

27. 27 Rocky K. C. Chang “Security is a chain; it’s only as secure as the weakest link.” “Security is not a product; it itself is a process.” Bruce Schneier