Protection and Security. Sarah Diesburg Operating Systems CS 3430

Transcription

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 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 Uses one-way transformations Encrypts a typed password and compares encrypted passwords

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

8 Original UNIX Required only lower-case, 5-lettered passwords 26 5 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 6 letters of upper, lower cases, numbers, and special characters 70 6 or 100 billion combinations Unfortunately, people still pick common words

10 Partial Solutions Delay every login by 1 second Assign very long passwords Give everyone a password calculator (credit card) Requires a physical theft to steal the password

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

13 Kerberos Protocol Key xy is needed to talk between x and y Server S Encrypt(Key AS, I want Key AB ) Client A Client B Key AS Key BS

14 Kerberos Protocol Key xy is needed to talk between x and y Server S Encrypt(Key AS, Here is Key AB and a message to B ) Client A Client B Key AS Key BS

15 Kerberos Protocol Key xy is needed to talk between x and y Server S Client A Client B Key AS Key BS message Encrypt(Key BS, use Key AB to talk to A )

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 ) Key AS and Key BS are renewed periodically to reduce their exposures

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

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

19 Public Key Encryption Encrypt(Key my_public, Hi, Sarah ) Anyone can create it, but only I can read it (secrecy) Encrypt(Key my_private, I m Sarah ) Everyone can read it, but only I can create it (authentication)

20 Public Key Encryption Encrypt(Key your_public, Encrypt(Key my_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 File 1 Bart read,write read Lisa Maggie Lisa s diary File3 read, write -The matrix tends to be sparse

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 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 combinations

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

38 Tenex 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