Bài giảng Hệ điều hành - Chapter 9: Security

Security Chapter 9 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 The Security EnvironmentThreats Security goals and threats Intruders Common Categories Casual prying by nontechnical users Snooping by insiders Determined attempt to make money Commercial or military espionage 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 Basics of Cryptography Relationship between the plaintext and the ciphertext Monoalphabetic substitution each letter replaced by different letter Given the encryption key, easy to find decryption key Secret-key crypto called symmetric-key crypto Secret-Key Cryptography 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 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 Digital Signatures Computing a signature block What the receiver gets (b) 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 Authentication Using Passwords (a) A successful login (b) Login rejected after name entered (c) Login rejected after name and password typed Authentication Using Passwords How a cracker broke into LBL a U.S. Dept. of Energy research lab Authentication Using Passwords The use of salt to defeat precomputation of encrypted passwords Salt Password , , , , Authentication Using a Physical Object Magnetic cards magnetic stripe cards chip cards: stored value cards, smart cards Authentication Using Biometrics A device for measuring finger length. Countermeasures 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 Operating System SecurityTrojan 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 Login Spoofing (a) Correct login screen (b) Phony login screen Logic Bombs Company programmer writes program potential to do harm OK as long as he/she enters password daily ff programmer fired, no password and bomb explodes Trap Doors (a) Normal code. (b) Code with a trapdoor inserted Buffer Overflow (a) Situation when main program is running (b) After program A called (c) Buffer overflow shown in gray 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 Famous Security Flaws The TENEX – password problem (a) (b) (c) Design Principles for Security System design should be public Default should be n 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 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 Virus Damage Scenarios Blackmail Denial of service as long as virus runs Permanently damage hardware Target a competitor's computer do harm espionage Intra-corporate dirty tricks sabotage another corporate officer's files How Viruses Work (1) Virus written in assembly language Inserted into another program use tool called a “dropper” Virus dormant until program executed then infects other programs eventually executes its “payload” How Viruses Work (2) Recursive procedure that finds executable files on a UNIX system Virus could infect them all 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 How Viruses Work (4) 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 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 looking email when it runs, use mailing list to replicate Antivirus and Anti-Antivirus Techniques (a) A program (b) Infected program (c) Compressed infected program (d) Encrypted virus (e) Compressed virus with encrypted compression code Antivirus and Anti-Antivirus Techniques Examples of a polymorphic virus All of these examples do the same thing 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 The Internet Worm Consisted of two programs bootstrap to upload worm the worm itself Worm first hid its existence Next replicated itself on new machines Mobile Code (1) Sandboxing (a) Memory divided into 1-MB sandboxes (b) One way of checking an instruction for validity Mobile Code (2) Applets can be interpreted by a Web browser Mobile Code (3) How code signing works Java Security (1) A type safe language compiler rejects attempts to misuse variable Checks include … Attempts to forge 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 Java Security (2) Examples of specified protection with JDK 1.2 Protection Mechanisms Protection Domains (1) Examples of three protection domains Protection Domains (2) A protection matrix Protection Domains (3) A protection matrix with domains as objects Access Control Lists (1) Use of access control lists of manage file access Access Control Lists (2) Two access control lists Capabilities (1) Each process has a capability list Cryptographically-protected capability Generic Rights Copy capability Copy object Remove capability Destroy object Capabilities (2) Trusted SystemsTrusted Computing Base A reference monitor Formal Models of Secure Systems (a) An authorized state (b) An unauthorized state Multilevel Security (1) The Bell-La Padula multilevel security model Multilevel Security (2) The Biba Model Principles to guarantee integrity of data Simple integrity principle process can write only objects at its security level or lower The integrity * property process can read only objects at its security level or higher Orange Book Security (1) Symbol X means new requirements Symbol -> requirements from next lower category apply here also Orange Book Security (2) Covert Channels (1) Client, server and collaborator processes Encapsulated server can still leak to collaborator via covert channels Covert Channels (2) A covert channel using file locking Covert Channels (3) Pictures appear the same Picture on right has text of 5 Shakespeare plays encrypted, inserted into low order bits of color values Zebras Hamlet, Macbeth, Julius Caesar Merchant of Venice, King Lear