1. IS 302: Information Security and Trust Week 7: User Authentication (part I) 2012

2. © Yingjiu Li 20072 Who are you really? Impersonation in cyber-world How does Bob prove he is Bob? Bob Alice Mallory Alice, I’m Bob Who are you?

3. © Yingjiu Li 20073 Asymmetric solution with certificate Bob: Hi, Alice, I am Bob. Here is my signature and certificate. Alice: Ok, let me verify your signature and certificate… Bob Alice Mallory Alice, I’m Bob. Here are my sig and cert

4. © Yingjiu Li 20074 Symmetric solution with shared secret Bob: Hi, Alice, I am Bob. I know our shared secret S –Weak authentication: reveal S itself –Strong authentication: Bob does not reveal S itself Bob Alice Mallory Alice, I’m Bob. I know our secret S

5. © Yingjiu Li 20075 What is shared secret? What Bob knows –Password, PIN, mother’s maiden name… What Bob possesses –Physical key, token, smart card, passport… Who Bob is –Fingerprint, retina, voice, face, signature dynamics, DNA…

6. © Yingjiu Li 20076 Password based authentications The most popular user authentication technique –Weak authentication based on password  this week –Strong authentication based on password  week 9 Bob Alice Alice, I’m Bob, and I know my pw

7. © Yingjiu Li 20077 Weak authentication based on password It is subject to eavesdropping attack when a Bob sends pwd across network to a remote server It can be used when Bob logins into a local computer Bob Alice Bob id, Bob password

8. © Yingjiu Li 20078 Store pwd directly Non-cryptographic technique –Alice: stores “Bob id – Bob password” in a password file –Alice: authenticates Bob by comparing received password to the password stored in password file Bob Alice Bob id – Bob password..... Bob id, Bob password Password file

9. © Yingjiu Li 20079 Store hashed or encrypted pwd “hashed or encrypted” password file –Alice: stores hash or cipher of Bob’s password –Alice: authenticates Bob by hashing (or encrypting) received password and comparing it to the corresponding entry in password file. Bob Alice Bob id – h(Bob password )....... Bob id, Bob password

10. © Yingjiu Li 200710 Example I: Unix pwd Unix pwd –DES is repeatedly used 25 times to encrypt 64 bit zeros –Encryption key: user password –How many possible pwds? Bob Alice Bob id, DES25 (Bob pwd, zeros )... Bob id, Bob password

11. © Yingjiu Li 200711 Example II: Windows LM Hash LAN Manager (LM) –Advanced network OS (MS and 3Com) LM hash –Windows 9X  Windows Me: store pwd in LM hash –Windows 2000, NT, and XP: also store LM hash by default for backwards compatibility (can be disabled) –Windows Vista onwards: eliminates LM hash  store NT(LM) hash only

12. © Yingjiu Li 200712 LM Hash Security of LM hash –Passwords >7 chars  two 7-char halves are hashed independently –Upper case only (26+10 for alphabets and numbers) 36^7=2^36 for each half, 2^37 possible pwds –Modern desktop can brute-force any LM hash (14-char pw) in a few hours. User pwd  uppercase Null-padded or truncated to 14 bytes  7+7 bytes 1st 7 bytes  DES key1; 2nd 7 bytes  DES key 2 Each DES key enc. string “KGS!@#$%”  8+8 bytes 32 hexes=128 bits

13. © Yingjiu Li 200713 NT(LM) Hash MD4 hash value of password –16 bytes=128 bits (the same length as LM hash) Security of NTLM hash –not half-half, not upper case only (52+10 for alphabets and numbers) –62^14 =2^84 possible pwds –(compare to 2^37 pwds in LM and 2^56 pwds in UNIX)

14. © Yingjiu Li 200714 SAM File Where does windows store LM hash and/or NTLM hash? –C:\Windows\System32\config\SAM –Can you read/copy it? –How to get access to it? –Password cracking test/lab in week 11

15. © Yingjiu Li 200715 Password Attacks Brute force attack Dictionary attack

16. © Yingjiu Li 200716 Brute Force Attack Mallory –Get access to a hashed/encrypted password file –Hash/encrypt every possible password and compare it to password file How to thwart brute force attack?

17. © Yingjiu Li 200717 Dictionary Attack Mallory –Create a dictionary of commonly used passwords –Pre-compute a password file for pwd dictionary –Look for a match between pre-computed password file and real password file How to thwart dictionary attack?

18. © Yingjiu Li 200718 Choose strong pwd –DO NOT use anyone’s name as your password. –DO NOT use words in common dictionary as your password. –DO NOT use birth date as your password. –DO use a combination of alphabets, digits and special characters.

19. © Yingjiu Li 200719 Choose long pwd Using pass-phrase –Easy to remember –Longer, thus harder to crack Examples –Redskin is My Favorite @ SMU (to login at SMU) –Redskin is My Favorite @ gmail (to login at gmail)

20. © Yingjiu Li 200720 Change pwd frequently? Arguable

21. © Yingjiu Li 200721 Review 1.How long is unix password when stored 1.12 bits 2.56 bits 3.64 bits 2.How long is LM hash or NT hash 1.14 letters 2.64 bits 3.128 bits 3.To thwart brute-force attack, we need to choose 1.Strong passwords 2.Long enough passwords 3.Strong authentication of passwords

22. © Yingjiu Li 200722 Notice Project draft (hard copy) due during week 9 class –It will not be graded