936: Password Strength

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Password Strength

Title text: To anyone who understands information theory and security and is in an infuriating argument with someone who does not (possibly involving mixed case), I sincerely apologize.

Explanation

This explanation may be incomplete or incorrect. If you see a way to improve it, edit it! Thanks.

Computer security consultant Mark Burnett has posted a good discussion and analysis of this comic on his blog.

This comic is saying that the password in the top frames "Tr0ub4dor&3" is easier for password cracking software to guess than "correcthorsebatterystaple". And this is absolutely true that people make passwords hard to remember because that means that they are "safer".

It is certainly true that length, all other things being equal, tends to make for very strong passwords and this can confirmed by using rumkin.com's password strength checker. Even if the individual characters are all limited to [a-z], the exponent implied in "we added another lowercase character, so multiply by 26 again" tends to dominate the results.

Now, what is not clearly addressed:

• Will these passwords have to be entered manually? And if so, how difficult is it, mechanically, to enter an each character of the password? On a keyboard it's easy, but on a smartphone or console... not so much.
• How easy are these passwords to remember?
• How sophisticated are the password attacks? In other words, will they actually attempt common schemes like "dictionary words separated by spaces", or "a complete sentence with punctuation", or "leet-speak numb3r substitution" as implied by xkcd?

Steve Gibson from the Security Now podcast did a lot of work in this arena and found that this password D0g..................... (24 characters long) is stronger than PrXyc.N(n4k77#L!eVdAfp9 (23 characters long) because both have at least one uppercase letter, lowercase letter, number, and "special" character, so length trumps perceived complexity. Steve Gibson makes this very clear in his password haystack reference guide and tester:

"Once an exhaustive password search begins, the most important factor is password length!"

The important thing to take away from this comic is that longer passwords are better because each additional character adds much more time to the breaking of the password. That's what Randall is trying to get through here. Complexity does not matter unless you have length in passwords. Complexity is more difficult for humans to remember. Length is not.

Transcript

The comic illustrates the relative strength of passwords assuming basic knowledge of the system used to generate them. A set of boxes is used to indicate how many bits of entropy a section of the password provides. The comic is laid out with 6 panels arranged in a 3x2 grid. On each row, the first panel explains the breakdown of a password, the second panel shows how long it would take for a computer to guess, and the third panel provides an example scene showing someone trying to remember the password.

[The password "Tr0ub4dor&3" is shown in the center of the panel. A line from each annotation indicates the word section the comment applies to.]

Uncommon (non-gibberish) base word

[Highlighting the base word - 16 bits of entropy.]

Caps?

[Highlighting the first letter - 1 bit of entropy.]

Common Substitutions

[Highlighting the letters 'a' (substituted by '4') and both 'o's (the first of which is substituted by '0') - 3 bits of entropy.]

Punctuation

[Highlighting the symbol appended to the word - 4 bits of entropy.]

Numeral

[Highlighting the number appended to the word - 3 bits of entropy.]

Order unknown

[Highlighting the appended characters - 1 bit of entropy.]

(You can add a few more bits to account for the fact that this is only one of a few common formats.)

~28 bits of entropy

2^28 = 3 days at 1000 guesses sec

(Plausible attack on a weak remote web service. Yes, cracking a stolen hash is faster, but it's not what the average user should worry about.)

Difficulty to guess: Easy.

[Cueball stands scratching his head trying to remember the password.]

Cueball: Was it trombone? No, Troubador. And one of the O's was a zero?

Cueball: And there was some symbol...

Difficulty to remember: Hard.

[The passphrase "correct horse battery staple" is shown in the center of the panel.]

Four random common words {Each word has 11 bits of entropy.}

~44 bits of entropy

2^44 = 550 years at 1000 guesses sec

Difficulty to guess: Hard.

[Cueball is thinking, in his thought bubble a horse is standing to one side talking to an off-screen observer. An arrow points to a staple attached to the side of a battery.]

Horse: That's a battery staple.

Observer: Correct!

Difficulty to remember: You've already memorized it

The caption below the comic reads: Through 20 years of effort, we've successfully trained everyone to use passwords that are hard for humans to remember, but easy for computers to guess.

External links

• Some info was used from the highest voted answer given to the question of "how accurate is this XKCD comic" at StackExchange [1]
• Similarly, a question of "how right this comic is" was made at AskMetaFilter [2]
• Also the Wikipedia article on 'Passphrase' is useful [3]
• In case you missed it in the explanation, GRC's Steve Gibson has a fantastic page [4] about this (and may have prompted this comic, as his podcast [5] about this was posted the month before this comic).

 add a comment!

Discussion

You still have to vary the words with a bit of capitalization, punctuation and numbers a bit, or hackers can just run a dictionary attack against your string of four words. Davidy²²[talk] 09:12, 9 March 2013 (UTC)

No you don't. Hackers cannot run a dictionary attack against a string of four randomly picked words. Look at the number of bits displayed in the image: 11 bits for each word. That means he's assuming a dictionary of 2048 words, from which each word is picked randomly. The assumption is that the cracker knows your password scheme. 86.81.151.19 20:17, 28 April 2013 (UTC)

Willem