That finding comes from a research paper written by Ashwini Rao, a Ph.D. student at Carnegie Mellon University, and two colleagues, titled "Effect of Grammar on Security of Long Passwords." Rao will present the paper at next month's Association for Computing Machinery's Conference on Data and Application Security and Privacy (CODASPY 2013) in San Antonio, Texas.
"Use of long sentence-like or phrase-like passwords such as 'abiggerbetterpassword' and 'thecommunistfairy' is increasing," the researchers said in their paper. But could some types of passphrases be inherently less secure than others?
To find out, they built a grammar-aware passphrase cracker and preloaded it with a dictionary for speech, as well as an algorithm for recognizing the types of sequences that are typically used to generate passphrases, such as "noun-verb-adjective-adverb."
Passphrases are an often-recommended technique for creating complex passwords. Instead of creating a password like "sheep," for example, security experts recommend a passphrase such as "[email protected]" Simply put, passphrases can add complexity -- making passwords harder to crack or guess for an attacker -- while still being easy to remember.
[ For the latest on Java's security vulnerabilities, see Java Security Work Remains, Bug Hunter Says. ]
But as is always the case with passwords, some types of passphrase complexity are better than others. The researchers tested their proof-of-concept software against 1,434 passwords, each containing 16 or more characters, and found that by taking grammar into consideration, they were able to crack three times as many passwords as current state-of-the-art cracking tools. In addition, their tool alone cracked 10% of the password data set.
Cue their resulting finding: The strength of long passwords does not increase uniformly with length.
Based on their research, good grammar comes at a security cost because some grammatical structures are scarcer than others, which in passphrases makes them easier to crack. In terms of scarcity, the researchers noted that pronouns are far fewer in number than verbs, verbs fewer than adjectives, and adjectives fewer than nouns.
The bottom line: Emphasize nouns and adjectives in passphrases. For example, the researchers found that the five-word passphrase "Th3r3 can only b3 #1!" was easier to crack that this three-word passphrase: "Hammered asinine requirements." Meanwhile, they found that the passphrase "My passw0rd is $uper str0ng!" was 100 times stronger than "Superman is $uper str0ng!" and in turn that phrase was 10,000 times stronger than "Th3r3 can only b3 #1!"
The researchers said their findings should be applied to help users select more secure passwords. They also said the research may apply not just to passphrases but to securing other types of structures such as postal addresses, email addresses and URLs present within long passwords -- or perhaps even encrypted sets of data.
"I've seen password policies that say, 'Use five words,'" said report author Rao in a statement. "Well, if four of those words are pronouns, they don't add much security."
But in cases where attackers obtained stored passwords that have been hashed or encrypted, the more complex the password, the longer it may take for an attacker to decrypt the password, if it can be decoded at all. Any delay can buy website owners time to spot the related breach, notify customers and immediately expire all current passwords, thus helping to contain the breach.
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