Concept - Password Cracking
- Offline dictionary attack (s.16): a large dictionary of possible passwords; each is hashed with each available salt and compared with the password file; common variations (backward spelling, added numbers, special characters) raise the chance.
- Rainbow table attack (s.16): precompute tables of hash values for all salts for a large dictionary; “a mammoth table”: a 1.4 GB table cracks 99.9% of alphanumeric Windows passwords in 13.8 seconds; not feasible if larger salt values are used.
- Why users make it easy (s.17): Purdue study of 7000 accounts, 3% of passwords were 3 characters or less (fix: reject too-short choices); a study of 14,000 encrypted passwords guessed nearly 1/4 of them; about 1 hour on the fastest systems to compute all variants, “and only need 1 break”.
- Case study (s.18): analysing real password structure to build an intelligent cracker; with 25,000 students’ passwords over 10% were recovered after guesses.
| Offline dictionary | Rainbow table | |
|---|---|---|
| Idea | hash each dictionary word with each salt, compare (s.16) | precompute tables of hash values for all salts (s.16) |
| Made harder by | salt (s.13); keeping the file out of reach (s.8, 19) | larger salt values: “not feasible” (s.16) |
Related: Concept - Password Hashing and Salt, Concept - Better Passwords, Concept - Password Vulnerabilities and Countermeasures. Appears in 03 User Authentication - Chunk 01 (s.1-33).