I'm not sure how hard or not it would be to do. I'm presuming it uses
some form of encryption though given any reader can show you how much
you have left and where you've been then theres either some basic
system wide common key encryption going on or none at all which makes
me wonder how easy it would be to extract the details if you had a
suitable reader yourself or even up the amount of cash on a pre-pay.
Apparently the MIFARE system can work at much greater ranges than the
one TfL uses so I'm also wondering if it would be possible to grab
details from travellers as they walk past an office building and so
forth. Anyone heard any rumours of anything like this happening?

B2003