A password store that lets attackers in every time may not sound very useful, but researchers at Germany's Fraunhofer Institute for Secure Information Technology beg to disagree.
Traditional password stores, even those with the strongest encryption, have a weakness, according to Ruben Wolf: They help attackers trying to guess the master password by telling them when their guess is wrong.
Wolf, an expert in cloud computing, identity and privacy at the Institute, thinks it's better to let attackers in every time, but replace the real passwords and PINs in the password store with fake ones when the master password is entered incorrectly.
That's the approach taken with iMobileSitter, an app for the iPhone and iPad developed by researchers at the institute and now available through the iTunes Store. An Android version is in the works and, when that's complete, a PC version will follow. The encrypted password store can be exported in an email to share between devices, all using the same master password, or as a backup.
The app uses the master password to generate a long sequence of bits using a cryptographic algorithm. To save a password in the store, it takes each character of the password in turn, finds an occurrence of the character in the generated sequence of bits, and then notes the position.
To retrieve a password from the store, the app generates characters corresponding to the patterns of bits at each of the positions noted. If the master password entered is the correct one, the long sequence of bits will be the same as when the password was stored, and the retrieved password too will be the same. If the master password is incorrect, the sequence of bits will differ, and so too will the retrieved password.
The app is able to generate fake passwords using only the set of characters valid for the original password, so if you store your bank PIN, the fake will still be a four-digit code and not, say, XLYT, which would be a dead giveaway to an attacker that something was wrong.
It's possible -- with a chance of one in 10,000 -- that a fake PIN could be the same as the real PIN, Wolf said. That's necessary because otherwise an attacker could enter millions of fake passwords looking for the one PIN that never came up.
While iMobileSitter gives attackers no useful feedback about their password guesses, it does offer the user a little clue if they mistype the master password (a distinct possibility on a virtual keyboard): a small symbol derived from the password is displayed in one of four slots at the top of the login screen. A given password will always correspond to the same symbol, color combination and position -- a black circle on a pink background in the second slot, say, or a red-on-yellow mushroom in the third. If you don't see your usual symbol, you mistyped. But an attacker can't work back from the symbol to the correct password, as there are only around 8,000 or so symbol-color-position combinations, and billions upon billions of possible passwords: It's a sort of visual hash, said Wolf.
The truly paranoid (with good memories) can even deliberately use multiple master passwords to separate passwords stored in the same file into different domains: If one master password is compromised, at least the passwords stored using other master passwords are still safe.
There is one downside to iMobileSitter's hidden-in-plain-sight approach. Only the passwords are hidden: The websites and usernames are not. That means anyone with access to your iPhone will not only see that you're a secret Twitter addict, they'll also know your handle -- they just won't be able to log in as you.
The app is in the iTunes Store now and costs €4.99.
Peter Sayer covers open source software, European intellectual property legislation and general technology breaking news for IDG News Service. Send comments and news tips to Peter at firstname.lastname@example.org.