Notes for February 19, 1997

1.	Hello

a.	Turn back midterms. mean was 70.

b.	When HW 2 returned (by Friday, I hope!) Iºll also hand out a note saying how 
youºre doing.

2.	Puzzle of the day

a.	Key problem: how do you know itºs really the student taking the test? Cameras, 
keystroke identification, etc.

3.	Malicious logic

a.	Quickly review Trojan horses, viruses, bacteria

b.	Logic Bombs

c.	Worms (Schoch and Hupp)

4.	Review trust and TCB

a.	Notion is informal

b.	Assume trusted components called by untrusted programs

5.	Ideal: program to detect malicious logic

a.	Can be shown: not possible to be precise in most general case

b.	Can detect all such programs if willing to accept false positives

c.	Can constrain case enough to locate specific malicious logic

d.	Can use: writing, structural detection (patterns in code), common code analyzers, 
coding style analyzers, instruction analysis (duplicting OS), dynamic analysis (run 
it in controlled environment and watch)

6.	Best approach: data, instruction typing

a.	On creation, itºs type Ñdataæ

b.	Trusted certifier must move it to type Ñexecutableæ

c.	UNIX idea: executable bit is Ñcertified as executableæ and must be set by trusted 
user (Duff)

7.	Practise: blocking writing to communicate information or do damage

a.	Limit writing (use of MAC if available; show how to arrange system executables)

b.	Isolation

c.	Quarantine

8.	Trust

a.	Untrusted software: what is it, example (USENET)

b.	Check source, programs (what to look for); C examples

c.	Limit who has access to what

d.	Your environment (how do you know what youºre executing); UNIX examples

e.	Least privilege; above with root

9.	Practise: detecting writing

a.	Integrity check files a la binaudit, tripwire; go through signature block

b.	LOCUS approach: encipher program, decipher as you execute.

c.	Co-processors: checksum each sequence of instructions, compute checksum as 
you go;  on difference, complain