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Module 19: Security

The Security Problem（安全问题） Authentication（授权） Program Threats（来自程序的威胁） System Threats（来自系统的威胁） Threat Monitoring（威胁的监控） Encryption（加密）

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安全

保护是个内部问题，应该以一种怎样的方式，提供对存储在计算机系统中的程序和数据的受控访问

安全：除了需要一个适当的保护系统外，还需考虑系统运行的外部环境。

第十八章讨论了操作系统能够提供的机制，它们使用户得以保护属于他们的资源，只有在用户遵守资源的使用和访问规则时，这些机制才能很好地发挥作用。

必须有这样一些机制，它们能够将出现安全缺口的情况控制在一个极小的概率范围内。

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The Security Problem安全问题

Security must consider external environment of the system, and protect it from:（安全必须考虑系统的外环境，以下方面防护：） unauthorized access.（未授权访问） malicious modification or destruction（恶意的修改和破坏） accidental introduction of inconsistency.（意外的引入和不一致）

Easier to protect against accidental than malicious misuse.（防止意外的误用比防止恶意的误用更加容易）

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The Security Problem

恶意破坏的形式： 末经授权的读数据操作 末经授权的修改数据操作 末经授权的数据破坏 阻止系统的正常使用

要保护系统，必须在 4个层次上采取安全机制： 物理 人：减少授权用户授予入侵者访问权限的机会 网络：中途截取数据等 操作系统：防止自身意外

本章将在操作系统层次上讲授安全。

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用户验证

用户持有的物品 用户的信息（用户鉴别、密码） 特征属性（指纹等）

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Authentication

User identity most often established through passwords, can be considered a special case of either keys or capabilities.（用户身份通常使用密码确立，可以认为密码就是某种权限或者钥匙）

Passwords must be kept secret.（密码必须保持秘密的状态） Frequent change of passwords.（经常更换密码） Use of “non-guessable” passwords.（使用难猜的密码） Log all invalid access attempts.（记录所有非法访问企图）

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密码

密码脆弱的一面 猜中 监听、泄露 被一授权用户非法传递给一个未授权用户

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Program Threats程序的威胁

Trojan Horse（特洛伊木马，一个误用自身的代码段） Code segment that misuses its environment.（代码段滥用环境） Exploits mechanisms for allowing programs written by users to be

executed by other users.（利用机制使得某个用户写的程序可以被其他用户运行）

Trap Door（后门） Specific user identifier or password that circumvents normal security

procedures.（特殊的用户标识符或者密码可以绕过通常的安全检查）

Could be included in a compiler.（有可能包含在编译器中）

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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.

Denial of Service Overload the targeted computer preventing it from doing any sueful

work.

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The Morris Internet Worm

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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.

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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

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FireWall

A firewall is placed between trusted and untrusted hosts.

The firewall limits network access between these two security domains.

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Network Security Through Domain Separation Via Firewall

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Intrusion Detection

Detect attempts to intrude into computer systems.

Detection methods: Auditing and logging. Tripwire (UNIX software that checks if certain files and

directories have been altered – I.e. password files)

System call monitoring

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Data Structure Derived From System-Call Sequence

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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 encryption key provided to authorized users via a secure mechanism. Scheme only as secure as the mechanism.

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Encryption (Cont.)

Public-key encryption based on each user having two keys: public key – published key used to encrypt data. private key – key known only to individual user used to

decrypt data. Must be an encryption scheme that can be made public

without making it easy to figure out the decryption scheme. Efficient algorithm for testing whether or not a number is

prime. No efficient algorithm is know for finding the prime

factors of a number.

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Encryption Example - SSL

SSL – Secure Socket Layer

Cryptographic protocol that limits two computers to only exchange messages with each other.

Used between web servers and browsers for secure communication (credit card numbers)

The server is verified with a certificate.

Communication between each computers uses symmetric key cryptography.

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Computer Security Classifications

U.S. Department of Defense outlines four divisions of computer security: A, B, C, and D.

D – Minimal security. C – Provides discretionary protection through auditing.

Divided into C1 and C2. C1 identifies cooperating users with the same level of protection. C2 allows user-level access control.

B – All the properties of C, however each object may have unique sensitivity labels. Divided into B1, B2, and B3.

A – Uses formal design and verification techniques to ensure security.

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Windows NT Example

Configurable security allows policies ranging from D to C2.

Security is based on user accounts where each user has a security ID.

Uses a subject model to ensure access security. A subject tracks and manages permissions for each program that a user runs.

Each object in Windows NT has a security attribute defined by a security descriptor. For example, a file has a security descriptor that indicates the access permissions for all users.