Password Cracking: Case I

Password Cracking: Case I

• Attack 1 specific password without using a dictionary
• E.g., administrator’s password
• Must try 256/2 = 255 on average
• Like exhaustive key search
• Does salt help in this case?

Password Cracking: Case II

• Attack 1 specific password with dictionary
• With salt
• Expected work: 1/4 (219) + 3/4 (255) ≈ 254.6
• In practice, try all pwds in dictionary…
• …then work is at most 220 and probability of success is 1/4
• What if no salt is used?
• One-time work to compute dictionary: 220
• Expected work is of same order as above
• But with precomputed dictionary hashes, the “in practice” attack is essentially free…

Password Cracking: Case III

• Any of 1024 pwds in file, without dictionary
• Assume all 210 passwords are distinct
• Need 255 comparisons before expect to find pwd
• If no salt is used
• Each computed hash yields 210 comparisons
• So expected work (hashes) is 255/210 = 245
• If salt is used
• Expected work is 255
• Each comparison requires a hash computation

Password Cracking: Case IV

• Any of 1024 pwds in file, with dictionary
• Prob. one or more pwd in dict.: 1 – (3/4)1024 ≈ 1
• So, we ignore case where no pwd is in dictionary
• If salt is used, expected work less than 222
• See book, or slide notes for details
• Work ≈ size of dictionary / P(pwd in dictionary)
• What if no salt is used?
• If dictionary hashes not precomputed, work is about 219/210 = 29

Other Password Issues

• Too many passwords to remember
• Results in password reuse
• Why is this a problem?
• Who suffers from bad password?
• Login password vs ATM PIN
• Failure to change default passwords
• Social engineering
• Error logs may contain “almost” passwords
• Bugs, keystroke logging, spyware, etc.

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