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[ISN] Passphrases only marginally more secure than passwords because of poor choices
By Dan Goodin
March 14, 2012
Passwords that contain multiple words aren't as resistant as some
researchers expected to certain types of cracking attacks, mainly
because users frequently pick phrases that occur regularly in everyday
speech, a recently published paper concludes.
Security managers have long regarded passphrases as an easy-to-remember
way to pack dozens of characters into the string that must be entered to
access online accounts or to unlock private encryption keys. The more
characters, the thinking goes, the harder it is for attackers to guess
or otherwise crack the code, since there are orders of magnitude more
But a pair of computer scientists from Cambridge University has found
that a significant percentage of passphrases used in a real-world
scenario were easy to guess. Using a dictionary containing 20,656
phrases of movie titles, sports team names, and other proper nouns, they
were able to find about 8,000 passphrases chosen by users of Amazon's
now-defunct PayPhrase system. That's an estimated 1.13 percent of the
available accounts. The promise of passphrases' increased entropy, it
seems, was undone by many users' tendency to pick phrases that are
staples of the everyday lexicon.
"Our results suggest that users aren't able to choose phrases made of
completely random words, but are influenced by the probability of a
phrase occurring in natural language," researchers Joseph Bonneau and
Ekaterina Shutova wrote in the paper (PDF), which is titled "Linguistic
properties of multi-word passphrases." "Examining the surprisingly weak
distribution of phrases in natural language, we can conclude that even
4-word phrases probably provide less than 30 bits of security which is
insufficient against offline attack," the paper says.
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