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Module 20: Security.• The Security Problem.• Authentication.• Program Threats.• System Threats.• Threat Monitoring.• Encryption. 20.1 Silberschatz and Galvin 1999 The Security Problem.• Security must consider external environment of the system, and. protect it from:. – unauthorized access – malicious modification or destruction. – accidental introduction of inconsistency• Easier to protect against accidental than malicious misuse 20.2 Silberschatz and Galvin 1999 Authentication.• User identity most often established through passwords, can be. considered a special case of either keys or capabilities• Passwords must be kept secret – Frequent change of passwords – Use of “non-guessable” passwords – Log all invalid access attempts 20.3 Silberschatz and Galvin 1999 Program Threats.• Trojan Horse. – Code segment that misuses its environment – Exploits mechanisms for allowing programs written by users. to be executed by other users• Trap Door. – Specific user identifier or password that circumvents normal. security procedures – Could be included in a compiler 20.4 Silberschatz and Galvin 1999 System Threats.• Worms – use spawn mechanism; standalone program.• Internet worm. – Exploited UNIX networking features (remote access) and. bugs in finger and sendmail programs – Grappling hook program uploaded main worm program• Viruses – fragment of code embedded in a legitimate program – Mainly effect microcomputer systems – Downloading viral programs from public bulletin boards or. exchanging floppy disks containing an infection – Safe computing 20.5 Silberschatz and Galvin 1999 The Morris Internet Worm. 20.6 Silberschatz and Galvin 1999 Threat Monitoring.• Check for suspicious patterns of activity – i.e., several incorrect. password attempts may signal password guessing• Audit log – records the time, user, and type of all accesses to an. object; useful for recovery from a violation and developing better. security measures• Scan the system periodically for security holes; done when the. computer is relatively unused 20.7 Silberschatz and Galvin 1999 Threat Monitoring (Cont.).• Check for:. – Short or easy-to-guess passwords. – Unauthorized set-uid programs. – Unauthorized programs in system directories. – Unexpected long-running processes. – Improper directory protections. – Improper protections on system data files. – Dangerous entries in the program search path (Trojan. horse). – Changes to system programs: monitor checksum values. 20.8 Silberschatz and Galvin 1999 Network Security Through Domain Separation Via Firewall. 20.9 Silberschatz and Galvin 1999 Encryption.• Encrypt clear text into cipher text• Properties of good encryption technique:. – Relatively simple for authorized users to incrypt and decrypt. data – Encryption scheme depends not on the secrecy of the. algorithm but on a parameter of the algorithm called the. encryption key – Extremely difficult for an intruder to determine the encryption. key• Data Encryption Standard substitutes characters and rearranges. their order on the basis of an enc
