Unix Password Hashing and Security

Unix Password Hashing

• MD5-based hash function is recommended for Unix
• Salt of up to 48-bits is used to create a 128-bit hash
• Password length is unlimited
• 1000 iterations of an inner loop are used to achieve slowdown

Bcrypt Hash Algorithm

• Used by OpenBSD
• Based on Blowfish block cipher
• Uses 128-bit salt to create 192-bit hash value
• Most secure version of Unix hash/salt scheme

Password Attacks

• Dictionary attacks: use a large dictionary of possible passwords to try against the password file
• Rainbow table attacks: pre-compute tables of hash values for all salts
• Can be countered by using a sufficiently large salt value and a sufficiently large hash length

Password Crackers

• John the Ripper: open-source password cracker that uses a combination of brute-force and dictionary techniques
• Exploits the fact that people choose easily guessable passwords

Defending Against Password Attacks

• Use sufficiently large and unique salt values for each user's password
• Use complex password policies to force users to pick stronger passwords

Password Selection Strategies

• User education: inform users about the importance of using hard-to-guess passwords
• Computer-generated passwords: can be difficult for users to remember
• Reactive password checking: system periodically runs its own password cracker to find guessable passwords
• Complex password policy: system checks to ensure the password is allowable

Proactive Password Checking

• Rule enforcement: specific rules that passwords must adhere to
• Password checker: compile a large dictionary of passwords not to use
• Bloom filter: used to build a table based on hash values

Token-Based Authentication

• Card-based tokens: hardware tokens
• Two primary ways to authenticate using mobile phones: SMS messages and software apps