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Four-digit banking PINs are almost as insecure as website passwords, according to a study by Cambridge University computer scientists.

The first-ever quantitative analysis of the difficulty of guessing four-digit banking PINs estimates the widespread practice of using a date of birth as a PIN code and other factor means that opportunistic thieves will be able to correctly guess a PIN before a card is blocked between 8-9 per cent of the time.

Most people are significantly more careful choosing PINs than online passwords, with a majority using an effectively random sequence of digits. However a few weak choices – and using birthdays in particular – provide hope for opportunistic thief.

In a blog post, Cambridge University researcher Joseph Bonneau explains:

About a quarter stick with their bank-assigned random PIN and over a third choose their PIN using an old phone number, student ID, or other sequence of numbers which is, at least to a guessing attack, statistically random.

In total, 63.7 per cent use a pseudo-random PIN, much more than the 23 to 27 per cent we estimated for our base datasets. Another 5 per cent use a numeric pattern (like 4545) and 9 per cent use a pattern on the entry keypad, also lower than the other two datasets.

Altogether, this gives an attacker with six guesses (three at an ATM and three with a CAP [hand-held card] reader) less than a 2 per cent chance of success.

Unfortunately, the final group of 23 per cent of users chose a PIN representing a date, and nearly a third of these used their own birthday. This is a game-changer because over 99 per cent of customers reported that their birth date is listed somewhere in the wallet or purse where they keep their cards. If an attacker knows the cardholder’s date of birth and guesses optimally, the chances of successfully guessing jump to around 9 per cent.

If customers use their wallet then it is very likely to have a ID document with a birth date printed on it. That makes choosing a date of birth for a bank card PIN a terrible idea. Other not-so-random codes include PINs representing dates, years, repeated digits, ascending digits or ending in 69. More bad practices in the area include sharing and reusing PINs.

The researchers suggest that blacklisting the top 100 PINs can drive the guessing rate down to around 0.2 per cent in the general, though not if a user's birthday is known, where the rate stays at 5 per cent. Dropping all dates that can be considered as birthdays doesn't work because there are too many.

Even so, some blacklisting can be done. Shamefully some banks in both the US and Europe fail to prohibit obviously guessable PINs, such as 1234, the Cambridge boffins report.

The team's research paper, A birthday present every eleven wallets? The security of customer-chosen banking PINs – by Joseph Bonneau, Sören Preibusch and Ross Anderson, can be found here (PDF). ®

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