• Journal of Digital Convergence
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• v.11 no.10
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• pp.737-746
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• 2013

This research aim is to investigate the rate and pattern of re-using the four-digit personal identification numbers(PINs). 1313 types of PINs were observed by 224 students who took this author's classes from last 2006 to 2011 at A-university. Some students used PINs as few as 3-4 and as many as 12-13. The average is 5.86 per person. The rates of re-using PINs were calculated by each student. 87%(195/224) of students reused PINs and 64% of them reused with just only one type of PINs, 20% reused with 2 types, and about 3% reused with 3-4 types. With respect to PINs, 884 out of the total 1313 PINs were reused, that is around 64.3%. In a broad sense, a pair of slight modification of PINs were also observed, that is, new PINs were partly matched in position or size of numbers of previous PINs. And if the reuse rate falling under the slight modification of PINs, 10.4% is added, about 75% of the PINs were reused in a broad sense. The re-using rate of male students is higher than the one of female students. This paper's results may provide to make plan for hacks of passwords.