1. Cpr E 308 Spring 2004 Security Unix Passwords Security across a network Cryptography – encryption and decryption

2. Cpr E 308 Spring 2004 Passwords in UNIX Login: snt Password: cpre308 How does the system check if the password is correct? One solution: –Password file has (username, password) pairs –Store [snt, cpre308] in /etc/passwd –Password file readable only by privileged user Privileged users can get your password –Why is this a problem?

3. Cpr E 308 Spring 2004 Solution: One-Way Functions f(x) is easy to compute f -1 (x) is extremely difficult, if not impossible, to compute Password file can now be world-readable –Unix password file contains image of each password »/etc/passwd contains snt:y »snt logs in, supplies x »if f(x) == y, then ok Copyright © 2002 Thomas W. Doeppner. All rights reserved.

4. Cpr E 308 Spring 2004 Dictionary Attack (Morris and Thompson) For all words in dictionary, compute f(word) Find word such that f(word) == y Many users use simple passwords Copyright © 2002 Thomas W. Doeppner. All rights reserved.

5. Cpr E 308 Spring 2004 Counterattack Salting –for each password, create random “salt” value –Password file contains (f(append(word, salt)), salt) –12-bit salt values in Unix –attacker must do dictionary attack 4096 times, for each salt value »Not secure enough.. »Feldmeier and Karn produced list of 732,000 most common passwords concatenated with each of 4096 salt values covers ~30% of all passwords Copyright © 2002 Thomas W. Doeppner. All rights reserved. For additional security: systems make password file unreadable

6. Cpr E 308 Spring 2004 Networks Copyright © 2002 Thomas W. Doeppner. All rights reserved.

7. Cpr E 308 Spring 2004 “Trust Me” ACDE Copyright © 2002 Thomas W. Doeppner. All rights reserved. B

8. Cpr E 308 Spring 2004 “Trust Everyone”: The Internet Email usually unauthenticated and unencrypted Passwords are (sometimes) passed unencrypted – telnet Copyright © 2002 Thomas W. Doeppner. All rights reserved.