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

© Yingjiu Li 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?

© Yingjiu Li 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

© Yingjiu Li 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

© Yingjiu Li 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…

© Yingjiu Li 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

© Yingjiu Li 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

© Yingjiu Li 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

© Yingjiu Li 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

© Yingjiu Li 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

© Yingjiu Li 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

© Yingjiu Li 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  8+8 bytes 32 hexes=128 bits

© Yingjiu Li 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)

© Yingjiu Li 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

© Yingjiu Li Password Attacks Brute force attack Dictionary attack

© Yingjiu Li 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?

© Yingjiu Li 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?

© Yingjiu Li 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.

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

© Yingjiu Li Change pwd frequently? Arguable

© Yingjiu Li 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 bits 3.To thwart brute-force attack, we need to choose 1.Strong passwords 2.Long enough passwords 3.Strong authentication of passwords

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