1. Protection and Security
Sarah Diesburg
Operating Systems
CS 3430

2. Definitions Security: policy of authorizing accesses
Prevents intentional misuses of a system
Protection: the actual mechanisms implemented to enforce the specialized policy
Prevents either accidental or intentional misuses

3. Security Goals Data confidentiality: secret data remains secret
Data integrity: unauthorized users should not be able to modify data
System availability: nobody can make a system unusable

4. Security Components Authentication determines who the user is
Authorization determines who is allowed to do what
Enforcement makes it so people can do only what they are allowed to do

5. Authentication The most common approach: passwords Problems:
If I know the secret, the machine can assume that I’m the user
Problems:
1. Password storage
2. Poor passwords

6. Password Storage Encryption
Uses a key to transform the data
Difficult to reverse without the key
UNIX stores encrypted passwords in /etc/passwd (/etc/shadow)
Uses one-way transformations
Encrypts a typed password and compares encrypted passwords

7. Poor Passwords Short passwords Long passwords Easy to crack
Tend to be written down somewhere

8. Original UNIX Required only lower-case, 5-lettered passwords
265 or 1 million combinations
In 1975, it would take one day to crack one password
Today, we can go through all those combinations < 1 second

9. Partial Solutions Extend password with a unique number
Require more complex passwords
8 letters of upper, lower cases, numbers, and special characters
708 or 576 trillion combinations
Unfortunately, people still pick common words

10. Partial Solutions Delay every login by >=1 second
Assign very long passwords
2-factor authentication
Login requires passphrase and code sent to your phone
Requires a physical theft to steal the password
3-factor authentication is best
What are the 3 factors?

11. Authentication in Distributed Systems
Private key encryption of data
Encrypt(Key, Plaintext) = Cipher text
Decrypt(Key, Cipher text) = Plaintext
Hard to reverse without the key
With the plaintext and the cipher text, one cannot derive the key
Provides secrecy and authentication, as long as the key stays secret

12. How to distribute the keys?
Authentication server
Keeps a list of keys
Kerberos is a network authentication protocol that works on the basis of tickets to allow nodes communicating over a non-secure network to prove their identity to each other

13. Kerberos Protocol Keyxy is needed to talk between x and y Server S
Encrypt(KeyAS, “I want KeyAB”)
Client B
Client A
KeyBS
KeyAS

14. Kerberos Protocol Keyxy is needed to talk between x and y Server S
Encrypt(KeyAS,“Here is KeyAB and
a message to B”)
Client B
Client A
KeyBS
KeyAS

15. Encrypt(KeyBS, “use KeyAB to talk to A”)
Kerberos Protocol
Keyxy is needed to talk between x and y
Server S
Client B
Client A
message
Encrypt(KeyBS, “use KeyAB to talk to A”)
KeyBS
KeyAS

16. Additional Details Expiration timestamp for a key
Prevents a machine from replaying messages (e.g., “deposit $100”)
Checksum for an encrypted message
Prevents modifications to a message (e.g., “deposit $1000”)
KeyAS and KeyBS are renewed periodically to reduce their exposures

17. Public Key Encryption Separates authentication from secrecy
Involves a public key and private key
Encrypt(Keypublic, plaintext) = cipher text
Decrypt(Keyprivate, cipher text) = plaintext
Encrypt(Keyprivate, plaintext) = cipher text
Decrypt(Keypublic, cipher text) = plaintext

18. Public Key Encryption Idea: Private key is kept secret
Public key is advertised

19. Public Key Encryption Encrypt(Keymy_public, “Hi, Sarah”)
Anyone can create it, but only I can read it (secrecy)
Encrypt(Keymy_private, “I’m Sarah”)
Everyone can read it, but only I can create it (authentication)

20. Public Key Encryption Encrypt(Keyyour_public, Encrypt(Keymy_private,
“I know your secret”))
Only I can create it, and only you can read it

21. Authorization Access matrix describes who can do what
-The matrix tends to be sparse
File 1
Lisa’s diary
File3
Bart
read,write
read
Lisa
read, write
Maggie

22. Access Control List
Stores all permissions for all users with each object
Analogy: a guard in front of a door
Checks for a list of people allowed to enter
UNIX: permission of each file is specified according to its owner, group, and the world

23. Capability List Stores all objects a process can touch Analogy: Keys
A key owner has the right of entry
Example: page tables
Each process has a list of pages that it can access

24. Access Control List vs. Capability List
Access control list (commonly used)
Easy to know who can access the object
Hard to know which objects a user can access
Capability list
A user knows the list of objects to access
Hard to know who can access an object
More difficult to revoke capabilities

25. Enforcement
Enforcer programs check passwords, access control lists, and so on…
In UNIX, enforcers are run as superuser
If there is a bug, you are hosed!

26. The State of the World in Security
Authentication
Poor passwords
Nobody encrypts s
Authorization
Coarse-grained access control list
Often turned off for sharing
Enforcement
Buggy operating systems

27. Classes of Security Problems
Eavesdropping is the listener approach
Tap into the Ethernet and see everything
Countermeasure: pressurized cabled or encryption
Abuse of privilege
If the superuser is evil, there is nothing you can do

28. Classes of Security Problems
Imposter breaks into the system by pretending to be someone else
Recorded voice and facial image
Countermeasure: behavioral monitoring to look for suspicious activities
Overwriting the boot block

29. Classes of Security Problems
A Trojan horse is a seemingly innocent program that performs an unexpected function
Countermeasure: integrity checking
Periodically, check binaries against their checksums

30. Classes of Security Problems
Salami attack builds up an attack, one-bit at a time
Example: send partial pennies to a bank account
Countermeasure: code reviews

31. Classes of Security Problems
Logic bombs: a programmer may secretly insert a piece of code into the production system
A programmer feeds the system password periodically
If the programmer is fired, the logic bomb goes off
Countermeasure: code reviews

32. Classes of Security Problems
Denial-of-service attacks aim to reduce system availability
A handful of machines can flood a victim machine to disrupt its normal use
Countermeasure: open

33. Pentagon Traffic Analysis
Before the 1991 Persian Gulf War
Foreign intelligence tried to predict the starting date of the war
time

34. Pentagon Traffic Analysis
So much for the element of surprise…

35. Tenex Used to be the most popular system at universities before UNIX
Thought to be very secure

36. Tenex Source code for the password check: for (j = 0; j < 8; j++) {
if (input[j] != pw[j]) {
// go to error; }
Need to go through 2568 combinations

37. Tenex Unfortunately, Tenex used virtual memory
A fast password check means that the first character is wrong (error)
A slow check means that the first character is correct (page fault)
password
in memory
on disk

38. Tenex
2568 checks to crack a password is reduced down to 256 * 8 checks

39. The Internet Worm
In 1988, a Cornell graduate student, RTM, released a worm into the Internet
The worm used three attacks
rsh
fingerd
sendmail

40. The Internet Worm
Some machines trust other machines, the use of rsh was sufficient to get into a remote machine without authentication

41. The Internet Worm finger command did not check the input buffer size
finger
Overflow the buffer
Overwrite the return address of a procedure
Jump and execute a shell (under root privilege)

42. The Internet Worm
sendmail allowed the worm to mail a copy of the code and get it executed
The worm was caught due to multiple infections
People noticed the high CPU load