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

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

Mallory Alice, I’m Bob Alice, I’m Bob Who are you? Bob Alice Who are you really? • Impersonation in cyber-world • How does Bob prove he is Bob?

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… Mallory Alice, I’m Bob. Here are my sig and cert Bob Alice

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 Mallory Alice, I’m Bob. I know our secret S Bob Alice

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…

Password based authentications • The most popular user authentication technique • Weak authentication based on password  this week • Strong authentication based on password week 9 Alice, I’m Bob, and I know my pw Bob Alice

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 id, Bob password Bob Alice

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 Password file Bob id – Bob password ..... Bob id, Bob password Bob Alice

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 id – h(Bob password) ....... Bob id, Bob password Bob Alice

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 id, DES25 (Bob pwd , zeros) ... Bob id, Bob password Bob Alice

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

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

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)

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

Password Attacks • Brute force attack • Dictionary attack

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?

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?

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.

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)

Change pwd frequently? • Arguable

Review • How long is unix password when stored • 12 bits • 56 bits • 64 bits • How long is LM hash or NT hash • 14 letters • 64 bits • 128 bits • To thwart brute-force attack, we need to choose • Strong passwords • Long enough passwords • Strong authentication of passwords

Notice • Project draft (hard copy) due during week 9 class • It will not be graded

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