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August 10, 2011

#936: Password Strength

Password Strength

((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 centre 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


(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.

[[A person stands scratching their head trying to remember the password.]]

Person: Was it trombone? No, Troubador. And one of the Os was a zero?

Person: And there was some symbol…

Difficulty to remember: Hard.

[[The passphrase “correct horse battery staple” is shown in the centre 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


Difficulty to guess: Hard.

[[A person is thinking, in their 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.))