What is a dictionary attack?

Question

When we say dictionary attack, we don't really mean a real dictionary, do we? My guess is we mean a hacker's dictionary i.e. rainbow tables, right?

My point is we're not talking about someone keying different passwords into the login box, we're talking about someone who has full access to your database (which has hashed passwords, not plain passwords) and this person is reversing the hashes, right?

Answer 1

Since passwords are oft-times the easiest-to-attack part of cryptography it's actually sort of a real dictionary. The assumption is that people are lazy and choose proper words as password or construct passphrases out of them. The dictionary can include other things, though, such as commonly used non-words or letter/number combination. Essentially everything that is likely to be a poor-chosen password.

There are programs out there which will take an entire hard drive and build a dictionary out of every typable string on it on the assumption that the user's password was at some point in time put in plaintext into memory (and then into the pagefile) or that it simply exists in the corpus if text stored on the drive¹:

Even so, none of this might actually matter. AccessData sells another program, Forensic Toolkit, that, among other things, scans a hard drive for every printable character string. It looks in documents, in the Registry, in e-mail, in swap files, in deleted space on the hard drive ... everywhere. And it creates a dictionary from that, and feeds it into PRTK.

And PRTK breaks more than 50 percent of passwords from this dictionary alone.

Actually, you can make dictionaries more effective even if you include knowledge on how people usually build passwords. Schneier talks about this lengthily¹:

• Common word dictionary: 5,000 entries
• Names dictionary: 10,000 entries
• Comprehensive dictionary: 100,000 entries
• Phonetic pattern dictionary: 1/10,000 of an exhaustive character search

The phonetic pattern dictionary is interesting. It's not really a dictionary; it's a Markov-chain routine that generates pronounceable English-language strings of a given length. For example, PRTK can generate and test a dictionary of very pronounceable six-character strings, or just-barely pronounceable seven-character strings. They're working on generation routines for other languages.

PRTK also runs a four-character-string exhaustive search. It runs the dictionaries with lowercase (the most common), initial uppercase (the second most common), all uppercase and final uppercase. It runs the dictionaries with common substitutions: "$" for "s," "@" for "a," "1" for "l" and so on. Anything that's "leet speak" is included here, like "3" for "e."

The appendage dictionaries include things like:

• All two-digit combinations
• All dates from 1900 to 2006
• All three-digit combinations
• All single symbols
• All single digit, plus single symbol
• All two-symbol combinations

---

¹ Bruce Schneier: Choosing Secure Passwords. In: Schneier on Security. (URL)

Answer 2

Well, if I threw a dictionary at you, it would hurt right?

But yes, a dictionary attack uses a list of words. They might be derived from a dictionary, or lists of common phrases or passwords ('123456' for example).

A rainbow table is different to a dictionary though - it is a reverse lookup for a given hash function, so that if you know a hash, you can identify a string which would generate that hash. For example, if I knew your password had an unsalted MD5 hash of e10adc3949ba59abbe56e057f20f883e, I could use a rainbow table to determine that 123456 hashes to that value.

Answer 3

A 'dictionary attack' usually refers to an attempt to guess a password using a 'dictionary'; that is, a long list of commonly-used passwords, usually corresponding to words or combination of words that people may lazily set as their password. Rainbow tables would be used if, instead of trying to guess the password by specifying the actual plaintext password, you had a password hash, and you wanted to guess the password. You'd specify the hashes of the commonly-used passwords and try to match them against the password hash you had, in order to try and get a match to determine what the password is.