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

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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.

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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

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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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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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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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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 ”.

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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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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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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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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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Secure network protocols in practice

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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.

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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

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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

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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

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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

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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

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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

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