操作系统课件PPT第九章.ppt

Security Chapter 9 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 Secret-Key Cryptography Monoalphabetic substitution each letter replaced by different letter Given the encryption key, easy to find decryption key Secret-key crypto called symmetric-key crypto 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 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 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