本文是我在上UCSD的 CSE 120: Principles of Operating Systems (Winter 2020) 整理的笔记，这一课主要介绍了操作系统里面的不同环境以及系统之间的威胁，包括病毒等，除此之外还介绍了一些安全机制，包括公钥和私钥的方法。

Basic

1. What is Computer Security?

   • How to protect computer systems

     • System contents: data, software, hardware
     • System operation: performance, reliability
     • System service: what the user sees and expects

   • From various threats

     • Theft
     • Damage
     • Disruption

2. Aspects of Security

   • Confidentiality

     • keeping a secret secret; for authorized eyes only

   • Integrity

     • maintaining accuracy; only authorized changes

   • Authenticity

     • Is it really who/what it claims to be?

   • Availablity

     • access to info/resources you need, when needed

3. Security Threats

   • Interception

     • eavesdropping

   • Interruption

     • destroying, denial of service

   • Modification

     • tampering with data or programs

   • Fabrication

     • new data/programs, replaying message

User Authentication

1. Password

   • Passwords are most common method

     • User and computer know secret
     • User proves knowledge of secret
     • Computer checks

   • Encrypted passwords

     • Computer stores only encrypted passwords
     • User provides password
     • Computer encrypts, checks

   • Problem with passwords

     • Assume 100 possible characters for passwords

       # chars	# passwords	100G/s machine	100T/s machine
       6	$100^6$	10 sec	10 msec
       7	$100^7$	17 min	10 msec
       8	$100^8$	1.2 days	1.7min
       9	$100^9$	116 days	2.8 hr

     • But most characters are uncommon, hard to remember

     • Using dictionary words (~250,000): only 2.5 usec!

2. Challenge/Response Protocol

   • Challenge/response, algorithm passwords

     • User and system know secret algorithm
     • System challenge user’s knowledge, user responds

   • Example: say secret algorithm is $f(x) = x^2$

     • System challenges user: sends system 9
     • User computes $f(x) = 9$, sends system 9
     • System concludes user must know secret algorithm
     • Next time, system can provide different challenge

   • Secret is never sent, only challenge/response

Threats

1. Trojan Horse

   • Greeks invaded Troy in hollow wooden horse

   • Program that contains hidden malicious code

     • User thinks program does something useful
     • In actuality, it (also) does something harmful

   • Program runs as process in user’s domain

     • Can do harm to user’s environment
     • Can do harm under that user’s name

2. Trap Door

   • Secret access point in program

   • Designer develops program for someone else

   • Once loaded in system, designer can access

   • Consider if trap door is added by compiler

     • Compiler adds trap doors to programs
     • Designer of compiler can then access
     • Can’t tell from program source code
     • Even if new compiler written, must be compiled!

3. Virus

   • Code attached to legitimate program

   • When program runs, the virus runs

     • causes damage
     • spreads, attaching itself to other programs

   • Disinfectants

     • Check that programs look normal (modified)
     • Check for known virus patterns in programs

4. Internet Worm

   • Worm: program that copies itself over network by email, finger, rsh attack

5. Denial of Service

   • Preventing others from using system

     • by using lots of resources
     • by bombarding with network requests or traffic

   • Example

     • Repeatly request TCP connection
     • Don’t answer responses; system times them out
     • Eventually, no TCP ports left available

Approach to clear (prevent) threats

Intrusion Detection

1. Intrusion Detection

   • Detecting if there is an intruder, or an attack

   • Signature-based

     • Look for specific patterns of attack behavior
     • Example: repeated login attempts

   • Anomaly-based

     • Look for unusual behavior
     • Example: unusual command/system call patterns

   • Solution: create audit trail (log), then analyze it

Crytography

1. Basics

   • Encoding messages to

     • limit who can view the original message

     • determine who sent a message

2. Secret Key Encryption

   • Secret key (symmetric)

     • Same key K is used to encrypt and decrypt
     • Sender encrypts $E_k(P)$, Receiver decrypts $D_k(E_k(P))$

   • DES: Data Encryption Standard (1997)

     • Weak due to 56-bit keys

   • AES: Advanced Encryption Standard (2001)

     • 128, 192, 256-bit keys

3. Public Key Encryption

   • Public key (asymetric)

     • Different keys to encrpyt and decrypt
     • Each user has two keys: one public, one private

   • If A wants to send to B

     • A encrypts using B’s public key
     • B decrypts using its private key

   • RSA (Rivest, Shamir and Adelman)

4. Public key vs. Secret key

   • Secret key

     • Operates fast
     • Difficult to distribute keys

   • Public key

     • Time-consuming operation (generating random/prime number, see example below)
     • Conveninet for key distribution

   • Example: Alice chats with Bob

   • Bob authenticates Alice

   • Alice authenticates Bob

   • Authentication using public key

     • Alice: sends $K_{B,pub}(A, R_A)$ to Bob (uses Bob’s public key)

     • Bob:

       • Decrypts: $K_{B,priv}(K_{B,pub}(A, R_A))\rightarrow (A, R_A)$
       • Encrypts and sends $K_{A,pub}(R_A, R_B, K)$ to Alice

     • Alice:

       • Decrypts: $K_{A,priv}(K_{A,pub}(R_A, R_B, K))\rightarrow$ “It’s Bob”
       • Encrypts and sends $K(R_B)$ to Bob

     • Bob: Decrypts $K(K(R_B)) = R_B \rightarrow$ “it’s Alice”

     • whole process:

5. Digitial Signatures

   • If Alice wants to digitially sign message to Bob

     • Encrypt M using $K_{A,priv}$ and send $K_{A,priv}(M)$ to Bob

   • When Bob receives, decrypts using $K_{A, pub}$

     • can decrypt only if from Alice

   • To sign and keep private

     • Alice sends $K_{B,pub}(M, K_{A, pirv}(M))$ to Bob

     • Only Bob can decryptL $K_{B,priv}(K_{B,pub}(M, K_{A,priv}(M))$

     • Decrypts using $K_{A,pub}$ proving Alice signed it