Introduction to Computer Security

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1 Introduction to Computer Security Instructor: Mahadevan Gomathisankaran CSCE 4550/5550, Fall 2009 Lecture 10 1

2 Announcements Project Group Due today Attendance Mandatory Ave. 85% ( 4 absentees per class ) Control Future assignments will be assigned only for those who are present in the class on the day of announcement CSCE 4550/5550, Fall 2009 Lecture 10 2

3 Firewall Designed to forward some packets and filter (not forward) others. Packet Filter Application Gateway Circuit Gateway Firewall Internet Outgoing Incoming Internal network(s) CSCE 4550/5550, Fall 2009 Lecture 10 3

4 Packet Filter Firewall can forward or block packets based on n/w and transport layer headers: Source/Dest. IP addresses Source/Dest. port addresses Protocol type (TCP or UDP) Decisions made independently on a packetby-packet basis Good for blocking ports ( no incoming HTTP ) or blocking IP addresses/ranges ( blacklists ) Simple and fast included in many routers CSCE 4550/5550, Fall 2009 Lecture 10 4

5 Packet Filter 1 2 Internal n/w(s) Interface Source IP Source Port Destination IP Destination Port * * * 1 * * * 23 1 * * * 2 * 80 * * CSCE 4550/5550, Fall 2009 Lecture 10 5

6 Proxy Firewall Provides filtering based on application layer Doesn t forward packets at all works at application layer (ex: Web proxies) Allows content filtering as well as security Errors All HTTP packets HTTP Proxy Accepted packets Internet HTTP Server Proxy Firewall CSCE 4550/5550, Fall 2009 Lecture 10 6

7 Firewall Types Network-based Specialized devices at network border Filter all traffic to enforce organizational policy Common brands: Cisco PIX, Checkpoint, Host-based (or personal firewalls) Controls traffic into and out of an individual system Software running on general-purpose system Common examples: Linux: iptables Windows: XP-SP2 firewall, Norton personal firewall, ZoneAlarm, BlackIce, and many others CSCE 4550/5550, Fall 2009 Lecture 10 7

8 IP Tables Chains for different packet sources/destinations. Chain FORWARD (policy DROP) target prot opt source destination Policy applies to all packets not explicitly handled by a rule (so this is default deny ). Chain INPUT (policy DROP) target prot opt in out source destination ACCEPT tcp -- * * / /0 state ESTABLISHED ACCEPT udp -- * * / /0 state ESTABLISHED ACCEPT tcp -- * * / /0 tcp dpt:22 ACCEPT tcp -- * * / /0 state RELATED tcp spt:20 ACCEPT all -- lo * / /0 ACCEPT icmp -- * * / /0 ESTABLISHED connections are either initiated locally or accepted through one of the following rules. Accept incoming ssh connections. Special case for ftp Chain OUTPUT (policy ACCEPT) target prot opt source destinationchain All loopback (local) connections and control messages are OK. CSCE 4550/5550, Fall 2009 Lecture 10 8

9 Intrusion Detection Systems Categorization by location: Host-based Intrusion Detection Systems (HIDS) Many just watch system/audit logs for suspicious activity Some with more sophisticated monitoring (ph: monitors system calls) Network-based Intrusion Detection Systems (NIDS) Watches all traffic at a certain point (can use a tap) If just external access point, can miss insider attacks! On switched networks: Use a spanning port Difficulties with encrypted traffic CSCE 4550/5550, Fall 2009 Lecture 10 9

10 Intrusion Detection Systems Categorization by type: Signature-based Monitors traffic for known suspicious patterns Advantages: Fast, few false positives Drawbacks: Can t detect novel attacks, must prioritize warnings Keeping signatures up-to-date leads to subscription services Anomaly-based Tries to learn typical activity and flag anomalies Anything unusual (including novel attacks) can be caught Drawbacks: Slow and atypical behavior doesn t necessarily mean bad behavior (too many false positives) Snort and most commercial IDSs are signature-based (sometimes with simple anomaly-based extensions) CSCE 4550/5550, Fall 2009 Lecture 10 10

11 Traffic Analysis Problem: Often just the fact that two parties are communicating is sensitive Military example: If a forward unit starts receiving a lot of messages from command, then expect an upcoming operation Non-military example: A heavy amount of communication between companies might signal a pending merger or cooperative project How can this information be protected? CSCE 4550/5550, Fall 2009 Lecture 10 11

12 Source to Destination Mapping Example: Sending anonymous The early days: anon.penet.fi Acted as a trusted intermediary Signed up and got a handle like Mail sent to mail forwarder stripped of your real identity, replaced it with a From line, and sent it on to recipient Mail to would be forwarded to you Issues: Had to trust anon.penet.fi Admin was a big privacy advocate considered trustworthy BUT: He was raided by law enforcement as a result of a court order in a lawsuit. Database compromised, so service was shut down. CSCE 4550/5550, Fall 2009 Lecture 10 12

13 Source to Destination Mapping Mixnet idea due to David Chaum Also Onion Routing due to Syverson Modern implementation: Tor Alice E I1 ( To I2, E I2 ( To Bob, E Bob (Message))) Intermediary (I1) E I2 ( To Bob, E Bob (Message)) Intermediary (I2) E Bob (Message) Bob I1 learns Alice is sending through intermediary I2 but doesn t know destination I2 learns Bob is destination but doesn t know who the sender is CSCE 4550/5550, Fall 2009 Lecture 10 13

14 Useful Tools Some of the most important tools nmap: network portscanner see what your machines look like to the network nessus: vulnerability scanner combines scanning with knowledge of vulnerabilities ethereal/wireshark: Nice packet sniffer dsniff: Sniffer with special attacks built in, such as ARP spoofing snort: Combination packet sniffer and IDS Other sources of tools/information Top 100 network security tools at CSCE 4550/5550, Fall 2009 Lecture 10 14

15 Modern Cryptography Goals Confidentiality Integrity Authentication Non-Repudiation Privacy CSCE 4550/5550, Fall 2009 Lecture 10 15

16 Modern Cryptography Building Blocks (Controls) Symmetric Ciphers Block Ciphers Stream Ciphers Asymmetric Ciphers Hash Functions Secure Hash Functions CSCE 4550/5550, Fall 2009 Lecture 10 16

17 Modern Cryptography Attacks cipher-text only attack known plaintext attack chosen plaintext attack adaptive chosen plaintext attack chosen cipher-text attack Order of Complexity CSCE 4550/5550, Fall 2009 Lecture 10 17

18 Symmetric Ciphers Block Cipher Operate on fixed length of group of bits The operation does not vary Stream Cipher Operate on bits The operation varies over time CSCE 4550/5550, Fall 2009 Lecture 10 18

19 Cipher Mode Cipher + Feedback + Additional Operations Security is based on the cipher Operations should not violate the security Objectives Increase the Message size Increase the Key size Improve the Security CSCE 4550/5550, Fall 2009 Lecture 10 19

20 Electronic Code Book (ECB) Electronic Code Book mode: Plaintext Block 1 Block 2 Block 3 Key Key Key Block 1 Block 2 Block 3 Ciphertext CSCE 4550/5550, Fall 2009 Lecture 10 20

21 ECB Security Ciphertext only attack Block replay Known plaintext attack Predefined structures in messages Chosen plaintext attack CSCE 4550/5550, Fall 2009 Lecture 10 21

22 Block Chaining Cipher Block Chaining mode: Plaintext Initialization Vector (IV) Block 1 Block 2 Block 3 Key Key Key Block 1 Block 2 Block 3 Ciphertext CSCE 4550/5550, Fall 2009 Lecture 10 22

23 CBC Initialization Vector Does it have to be secret? Padding Error propagation plaintext error propagates one block of ciphertext error affects two blocks of plaintext self recovering Latency Encryption is not parallelizable Decryption is parallalizable CSCE 4550/5550, Fall 2009 Lecture 10 23

24 Cipher Feedback Mode Source: Wikipedia CSCE 4550/5550, Fall 2009 Lecture 10 24

25 CFB Self synchronizing stream cipher Error propagation One block ciphertext error propagates to n/m plaintext blocks Shift Register (n) Key(k) Encrypt (n) (m) Plaintext block (n) Ciphertext block (n) CSCE 4550/5550, Fall 2009 Lecture 10 25

26 Output Feedback Mode Source: Wikipedia CSCE 4550/5550, Fall 2009 Lecture 10 26

27 Counter Mode Source: Wikipedia CSCE 4550/5550, Fall 2009 Lecture 10 27

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