<P> The problem with guessable PINs surprisingly worsens when customers are forced to use additional digits, moving from about a 25% probability with fifteen numbers to more than 30% (not counting 7 - digits with all those phone numbers). In fact, about half of all 9 - digit PINs can be reduced to two dozen possibilities, largely because more than 35% of all people use the all too tempting 123456789 . As for the remaining 64%, there's a good chance they're using their Social Security Number, which makes them vulnerable . (Social Security Numbers contain their own well - known patterns .) </P> <P> In 2002 two PhD students at Cambridge University, Piotr Zieliński and Mike Bond, discovered a security flaw in the PIN generation system of the IBM 3624, which was duplicated in most later hardware . Known as the decimalization table attack, the flaw would allow someone who has access to a bank's computer system to determine the PIN for an ATM card in an average of 15 guesses . </P> <P> Rumours have been in e-mail circulation claiming that in the event of entering a PIN into an ATM backwards, police will be instantly alerted as well as money being ordinarily issued as if the PIN had been entered correctly . The intention of this scheme would be to protect victims of muggings; however, despite the system being proposed for use in some US states, there are no ATMs currently in existence that employ this software . </P> <P> A mobile phone may be PIN protected . If enabled, the PIN (also called a passcode) for GSM mobile phones can be between four and eight digits and is recorded in the SIM card . If such a PIN is entered incorrectly three times, the SIM card is blocked until a personal unblocking code (PUC or PUK), provided by the service operator, is entered . If the PUC is entered incorrectly ten times, the SIM card is permanently blocked, requiring a new SIM card from the mobile carrier service . </P>

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