The first 6 digits of a credit card are the issuer identifier; your personal account number only starts at the 7th digit. Whether or not digits 7 and 8 are uniformly distributed over 00-99 depends on how that issuer makes up its card numbers. The fact that you and your friend's match is a pretty strong indicator that they don't choose them randomly. (There is indeed a checksum digit, but that doesn't affect your question at all.) --Michael On Wed, Jun 24, 2009 at 11:30 AM, James Propp <jpropp@cs.uml.edu> wrote:
I learned (through entirely licit means) that a friend of mine who lives not far from me has the same first eight digits on her Visa card as mine.
(She's probably the only person whose credit card number I know, other than myself and my wife.)
How unlikely is this? Does anyone know much about the schemes used in devising credit card numbers? I know that there is at least one check- digit, so the naive estimate 1/10^8 needs to be replaced by 1/10^7 or 1/10^6. Is there any region-coding involved? That would further raise the probability.
Jim Propp
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