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xkcdpass is yet another tool for generating passphrases by the xkcd-936 method. It generates passphrases by picking several words randomly from a word list, optionally modifies them using a standard bag of tricks such as change case, introduce digits, substitute symbols for letters, etc. It then estimates the strength of the passphrases in terms of how long they would stand up against simple brute force attack or a modern dictionary based attack. This estimate is done after each type of modification, making clear how much stronger the passphrase actually becomes at each stage.

Bruce Schneier's objection

Bruce Schneier doesn't like xkcd-936. Well, it turns out even Bruce Schneier can be wrong, and this is why:

Say you have a word list of 50,000 words. We cannot rely on security through obscurity, so we assume that the attacker has the same word list and knows how you generate your passphrases from it. We also assume that the attacker can try 1 billion candidate passphrases per second.

Pick one random word from that list as your passphrase. The attacker has to try each word in the list in order to crack your passphrase. On average he gets half-way through the list before he finds the word you used, giving 25,000 tries, which at a billion tries per second takes him 0.000025 seconds. No good.

Now let's try with two random words. The attacker now has to try each possible pair of words in the list. There are 50,000 squared = 2.5 billion such pairs. On average he has to try half of these before he gets a hit, so 1.25 billion tries which takes 1.25 seconds. Still no good.

Try with three random words. The number of combinations is now 50,000 cubed or 125 trillion. Using the same math we can compute that it will take 17.36 hours to crack. Things are starting to look up a little bit.

What about four words picked at random? The number of combinations is now 50,000 to the fourth power, which is 6.25e+18, or 625 with sixteen zeroes after it. At a billion tries per second, it will take on average 99 years to crack this passphrase.

So with no security through obscurity (the attacker has the word list and the details of how the passphrase was generated), no mis-spellings or special characters, and an unrelenting cracking effort at 1 billion attempts per second, a four word passphrase like "exasperated profusions homeliest crags" would last on average 99 years. In case you're wondering, for 5, 6 or 7 words, the corresponding values are 4,954 millenia, 247,732,749 millenia and 12,386,637,493,658 millenia.

Dos and don'ts

Here are some simple dos and don'ts for how to manage passwords for most accounts

  • DON'T memorize most of your passphrases.
  • DO use a password manager to generate and store passphrases. Keepass and LastPass are good alternatives for offline and online storage, respectively.
  • DO use different passphrases for each of your accounts.
  • DO use long random passphrases such as $3Fzli2Bl")'AjZYm0,,Pz% all acconts that you don't need to memorize.
  • DO use strong passphrases for the few accounts that you do need to memorize, probably your main email account, your password manager and not much else.

Passphrase strenght estimation

xkcdpass can be used to help settle that debate. It has been implemented with the goal of generating passphrases and estimate their strenght. In addition to computing the strength in bits, which can be hard for non-specialists to really understand, it also represents the strength of passphrases in terms of their longevity, i.e. how long it would take to break them, assuming a given number of attacks per second. The default rate of attacks is 1 billion passwords per second, which seems to be the ballpark figure for one off the shelf GPU hardware these days.

Two methods are currently used to estimate the passphrase strength. First, as the passphrase is generated, the program keeps count of how many random numbers have been used, and the range for each. So a random number in the range from 0 to 100 contributes log2(100) bits to the entropy while a random number in the range from 0 to 5 contributes log2(5) bits. This is the entropy of the passphrase. Secondly, the finished passphrase is analysed for brute force or "haystack" complexity.

Each of these two complexity measures imply a different attack model. The brute force complexity assumes a particular form of brute force attack, where every passphrase of length N is tried before any of length N+1. Furthermore, every passphrase containing lower case letters only is tried before any with lower+upper case letters, then lower+digits, then lower+symbols, then lower+upper+digits, etc. This approach assumes that the attacker knows almost nothing about how your passphrases are generated.

The entropy measure reflects the challenge faced by an attacker who knows everything about your passphrase generation method, including this program, the options that were passed to it, and the word list that was used. The only information not available to this hypothetical attacker is the values of random numbers used to generate the passphrase. A real attacker will fall somewhere between these extremes. Both measures are in the units of bits of information, and the entropy of a passphrase will always be less than its brute force complexity.

I believe neither of these attack models are very realistic, and I'd be interested to figure out how to model likely attacks better in order to improve estimates of passphrase strengths.

Passphrase generation

The passphrase generation starts by picking words at random from a word list. This gives a starting entropy of N*log2(M) bits, where N is the number of words in the passphrase and M is the number of words in the word list. The basic passphrase may then be modified in various ways, with the goal of packinng more entropy into it, while not making it too hard to remember. The possible variations are endless, the following methods are currently supported by xkcdpass:

  • Insert arbitrary (user suplied, not random) strings between each word.
  • Change the case of each word, randomly or deterministically.
  • Substitute symbols for characters, the usual 3 for e, @ for a, etc.
  • Insert random numbers inside or between words.
  • Repeat syllables randomly to intrduce stutter patters that some may find easy to remember.

Word lists

There are several wordlists included, generated from the SCOWL project. The longer the word list, the more bits you get for each word. However, a very large word list will likely contain words that you don't know and therefore can't easily remember. Ideally you should use a word list that matches our vocabulary:

word list word count bits per word longevity for a 4 word passphrase
american-40.txt 44562 15.4 125.0 years
american-50.txt 72071 16.1 855.5 years
british-40.txt 44614 15.4 125.6 years
british-50.txt 72130 16.1 858.3 years
canadian-40.txt 44586 15.4 125.3 years
canadian-50.txt 72100 16.1 856.9 years

Example of words from each word list:

  • american-40.txt: blindly snowballs spoons immigrate chunky concentration taxation gamest replicating slummer kid poisons peeves cleaner pus cackled dwarfed summons acoustic earplugs
  • american-50.txt: is Garibaldi bestiality chatty adjudicators precisely greats snorkeling briefcase captor metaphysical deft skewing purged Curtis indentation harden appeaser predilections timider
  • british-40.txt: keened capsules crackdowns appeasement keynote excising rustiest inattention mewed inattention unlike stray gentile requesting courtships bookkeeping catcall chipmunk handcuffing sparks
  • british-50.txt: Jerry appointee Brahmaputra Brunelleschi Irrawaddy slum Maseru tricepses gladiolas socialism elongate assimilates uneasiness promiscuous clownishness swigs outstretches symbioses mussels Kirby
  • canadian-40.txt: reciprocates losses dearly stubborn frostbiting varieties toasting fezzes swivelling bakery stated homering purposing untruer alerts kinder reincarnates amounts idioms logger
  • canadian-50.txt: auspiciously tutors fates Lakshmi elm theme involvements mama means algebraically qualification disaffects discommoding discords drastic arborvitae sunnier intermarriages Safeway meteorite


Here's an example of a short passphrase from a small word list, but with heavy modification:

$ ./xkcdpass.rb --stutter_count 2 --number_count 1 --numbers inside --substitution lots -substitution_count 3 --case random --verbose verbose --file wordlists/canadian-30.txt --separator $ --word_count 3
Wordlist contains 10932 words, giving 13.4 bits per word
Assuming 1.0 billion attacks per second when estimating longevity

Stage: Pick words
Phrase: responsibilities pushing functionality
Dictionary attack:  40.2 bits (longevity: 21.8 minutes)
Brute force attack: 178.6 bits (longevity: forever)

Stage: Add stutter
Phrase: responsibilities pushishing functionctionctionality
Dictionary attack:  47.8 bits (longevity: 2.9 days)
Brute force attack: 239.7 bits (longevity: forever)

Stage: Change case
Phrase: responsibilities PUSHISHING functionctionctionality
Dictionary attack:  52.6 bits (longevity: 2.6 months)
Brute force attack: 479.4 bits (longevity: forever)

Stage: Add digits
Phrase: res43ponsibilities PUSHISHING functionctionctionality
Dictionary attack:  64.8 bits (longevity: 1.0 millenia)
Brute force attack: 674.3 bits (longevity: forever)

Stage: Add separator '$'
Phrase: res43ponsibilities$PUSHISHING$functionctionctionality
Dictionary attack:  64.8 bits (longevity: 1.0 millenia)
Brute force attack: 934.4 bits (longevity: forever)


Here's an example of a longer passphrase from a larger word list:

$ ./xkcdpass.rb --number_count 1 --numbers between --substitution lots --substitution_count 2 --case random --verbose verbose --file wordlists/canadian-40.txt --separator $ --word_count 4 --stutter_count 1 
Wordlist contains 44586 words, giving 15.4 bits per word
Assuming 1.0 billion attacks per second when estimating longevity

Stage: Pick words
Phrase: accommodated playthings handgun junkie
Dictionary attack:  61.8 bits (longevity: 125.3 years)
Brute force attack: 178.6 bits (longevity: forever)

Stage: Add stutter
Phrase: accommodated playthings handgun jujujunkie
Dictionary attack:  64.8 bits (longevity: 1.0 millenia)
Brute force attack: 197.4 bits (longevity: forever)

Stage: Change case
Phrase: ACCOMMODATED PLAYTHINGS Handgun jujujunkie
Dictionary attack:  71.1 bits (longevity: 81.2 millenia)
Brute force attack: 394.8 bits (longevity: forever)

Stage: Change letters
Phrase: @((OMMOD@TE) PLAYTHINGS Handgun jujujunkie
Dictionary attack:  82.7 bits (longevity: forever)
Brute force attack: 600.9 bits (longevity: forever)

Stage: Add digits
Phrase: 75 @((OMMOD@TE) PLAYTHINGS Handgun jujujunkie
Dictionary attack:  91.3 bits (longevity: forever)
Brute force attack: 793.3 bits (longevity: forever)

Stage: Add separator '$'
Phrase: 75$@((OMMOD@TE)$PLAYTHINGS$Handgun$jujujunkie
Dictionary attack:  91.3 bits (longevity: forever)
Brute force attack: 793.3 bits (longevity: forever)



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