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Operating Systems

This chapter covers key security concepts and measures in operating systems, including cryptography basics, user authentication methods, protection mechanisms, and defense against internal and external attacks.

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Operating Systems

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  1. Operating Systems 软件学院 高海昌 hchgao@xidian.edu.cn

  2. Contents 1. Introduction ** 2. Processes and Threads ******* 3. Deadlocks ** 4. Memory Management ***** 5. Input/Output *** 6. File Systems **** 8. Multiple Processor Systems * 9. Security ** • 2

  3. Chapter 9: Security 9.1 The security environment 9.2 Basics of cryptography 9.3 User authentication 9.4 Attacks from inside the system 9.5 Attacks from outside the system 9.6 Protection mechanisms 9.7 Trusted systems • 3

  4. Threats Security goals and threats 机密性 篡改

  5. Intruders Common Categories • Casual prying (窥探) by nontechnical users • Snooping by insiders • Determined attempt to make money • Commercial or military espionage

  6. Accidental Data Loss Common Causes • Acts of God • fires, floods, wars • Hardware or software errors • CPU malfunction, bad disk, program bugs • Human errors • data entry, wrong tape mounted

  7. Chapter 9: Security 9.1 The security environment 9.2 Basics of cryptography 9.3 User authentication 9.4 Attacks from inside the system 9.5 Attacks from outside the system 9.6 Protection mechanisms 9.7 Trusted systems • 7

  8. Basics of Cryptography Relationship between the plaintext and the ciphertext

  9. Secret-Key Cryptography • Monoalphabetic substitution 单字符替换 • each letter replaced by different letter • Given the encryption key, easy to find decryption key • Secret-key crypto called symmetric-key crypto A B C D E F G…… Q W E R T Y U…… ATTACK QZZQEA

  10. Public-Key Cryptography • All users pick a public key/private key pair • publish the public key • private key not published • Public key is the encryption key, private key is the decryption key • RSA, slow than symmetric cryptography Q1: How much is 314159265358979 x 314159265358979 ? Q2: What is the square root of 3912571506419387090594828508241?

  11. One-Way Functions • Function such that given formula for f(x) • easy to evaluate y = f(x) • But given y， • computationally infeasible to find x

  12. Digital Signatures • (a) Computing a signature block • (b) What the receiver gets • MD5 16-byte, SHA 20-byte • E(D(x))=x (b)

  13. Chapter 9: Security 9.1 The security environment 9.2 Basics of cryptography 9.3 User authentication 9.4 Attacks from inside the system 9.5 Attacks from outside the system 9.6 Protection mechanisms 9.7 Trusted systems • 13

  14. User Authentication Basic Principles. Authentication must identify: • Something the user knows • Something the user has • Something the user is This is done before user can use the system

  15. Authentication Using Passwords (a) A successful login (b) Login rejected after name entered (c) Login rejected after name and password typed

  16. How crackers break in • How a cracker broke into LBL • a U.S. Dept. of Energy research lab • Ping / telnet / dnsquery / passwd / packet sniffer

  17. Salt Password UNIX password security UNIX: the user password is “encrypted” by using it as a key to encrypt a fixed block of data. The use of salt to defeat precomputation of encrypted passwords

  18. One-Time passwords • One way function y=f(x) • given x it is easy to find y, but given y it is computational infeasible to find x. • The input and output should be the same length, i.e. 128 bits • pi-1 = f(pi) p0 = f(p1), p1 = f(p2), p2 = f(p3), p3 = f(p4) p1 = f(f(f(f(s)))), p2 = f(f(f(s))), p3 = f(f(s)) , p4 = f(s) • Challenge-Response authentication • client get r, send f(r,k)

  19. Authentication Using a Physical Object • Magnetic cards • magnetic stripe cards • chip cards: stored value cards, smart cards

  20. Authentication Using Biometrics A device for measuring finger length.

  21. Countermeasures 对策 • Limiting times when someone can log in • Automatic callback at number prespecified • Limited number of login tries • A database of all logins • Simple login name/password as a trap • security personnel notified when attacker bites

  22. Chapter 9: Security 9.1 The security environment 9.2 Basics of cryptography 9.3 User authentication 9.4 Attacks from inside the system 9.5 Attacks from outside the system 9.6 Protection mechanisms 9.7 Trusted systems • 22

  23. Trojan Horses • Free program made available to unsuspecting user • Actually contains code to do harm • Place altered version of utility program on victim's computer • trick user into running that program

  24. Login Spoofing (a) Correct login screen (b) Phony login screen

  25. Logic Bombs • Company programmer writes program • potential to do harm • OK as long as he/she enters password daily • if programmer fired, no password and bomb explodes

  26. Trap Doors (a) Normal code. (b) Code with a trapdoor inserted

  27. Buffer Overflow • (a) Situation when main program is running • (b) After program A called • (c) Buffer overflow shown in gray

  28. Generic Security Attacks Typical attacks • Request memory, disk space, tapes and just read • Try illegal system calls • Start a login and hit DEL, RUBOUT, or BREAK • Try modifying complex OS structures • Try to do specified DO NOTs • Convince a system programmer to add a trap door • Beg admin's sec’y to help a poor user who forgot password

  29. Famous Security Flaws The TENEX – password problem (a) (b) (c)

  30. Design Principles for Security • System design should be public • Default should be no access • Check for current authority • Give each process least privilege possible • Protection mechanism should be • simple • uniform • in lowest layers of system • Scheme should be psychologically acceptable And … keep it simple

  31. Operating Systems Lesson 2

  32. Chapter 9: Security 9.1 The security environment 9.2 Basics of cryptography 9.3 User authentication 9.4 Attacks from inside the system 9.5 Attacks from outside the system 9.6 Protection mechanisms 9.7 Trusted systems • 32

  33. Network Security • External threat • code transmitted to target machine • code executed there, doing damage • Goals of virus writer • quickly spreading virus • difficult to detect • hard to get rid of • Virus = program can reproduce itself • attach its code to another program • additionally, do harm

  34. Virus Damage Scenarios • Blackmail • Denial of service as long as virus runs • Permanently damage hardware • Target a competitor's computer • do harm • espionage 间谍活动 • Intra-corporate dirty tricks • sabotage (阴谋破坏)another corporate officer's files

  35. How Viruses Work • Virus written in assembly language • Inserted into another program • use tool called a “dropper” • Virus dormant (潜伏) until program executed • then infects other programs • eventually executes its “payload”

  36. How Viruses Work (1) Several kinds of Viruses based on what is infected companion executable program memory boot sector device driver macro source code viruses • 36

  37. How Viruses Work (2) Virus could infect them all Recursive procedure that finds executable files on a UNIX system

  38. How Viruses Work (3) • An executable program • With a virus at the front • With the virus at the end • With a virus spread over free space within program Parasitic virus 寄生病毒

  39. How Viruses Work (5) • After virus has captured interrupt, trap vectors • After OS has retaken printer interrupt vector • After virus has noticed loss of printer interrupt vector and recaptured it

  40. How Viruses Spread • Virus placed where likely to be copied • When copied • infects programs on hard drive, floppy • may try to spread over LAN • Attach to innocent (无辜) looking email • when it runs, use mailing list to replicate

  41. Antivirus and Anti-Antivirus Techniques (a) A program (b) Infected program (c) Compressed infected program (d) Encrypted virus (disguise) (e) Compressed virus with encrypted compression code

  42. Antivirus and Anti-Antivirus Techniques Examples of a polymorphic virus 多形态病毒 All of these examples do the same thing

  43. Antivirus and Anti-Antivirus Techniques • Integrity checkers • Behavioral checkers • Virus avoidance • good OS • install only shrink-wrapped software • use antivirus software • do not click on attachments to email • frequent backups • Recovery from virus attack • halt computer, reboot from safe disk, run antivirus

  44. The Internet Worm (Morris) • Consisted of two programs • bootstrap to upload worm • the worm itself • Worm first hid its existence • Next replicated itself on new machines

  45. Mobile Code (1) Sandboxing (a) Memory divided into 1-MB sandboxes (b) One way of checking an instruction for validity

  46. Mobile Code (2) Applets can be interpreted (解释)by a Web browser

  47. Mobile Code (3) How code signing works

  48. Java Security • A type safe language • compiler rejects attempts to misuse variable • Checks include … • Attempts to forge 伪造 pointers • Violation of access restrictions on private class members • Misuse of variables by type • Generation of stack over/underflows • Illegal conversion of variables to another type

  49. Chapter 9: Security 9.1 The security environment 9.2 Basics of cryptography 9.3 User authentication 9.4 Attacks from inside the system 9.5 Attacks from outside the system 9.6 Protection mechanisms 9.7 Trusted systems • 49

  50. Protection Domains (1) Examples of three protection domains

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