Anonymity of prepaid phone chip-cards

[Thomas Shaddack]

Local cops busted somebody who threatened to derail some trains if he
won't get paid. That's a common news.

Less common, and more important, detail that the TV news reported confirms
the suspicion I had from the beginning of deployment of the prepaid cards
technology for local payphones.

Each prepaid "Trick" phone card has its unique serial number. The payphone
reads it from the card. The busted person (let's call him "target") used
the same card for multiple phone calls, thus becoming the card's number
known as the target's temporary identity.

The interesting part was that the phone company knew in realtime when the
card was used - enough in real time to dispatch a police patrol car to the
location.

Hence, the Trick cards can't be considered as anonymous as coins used to
be; at best, they can be used only as pseudonymous-identity tokens.

I strongly suspect the usage logs exist for individual cards, allowing to
back-trace the phonecalls done with the given card, thus tracing the
identity of the card's owner by the call patterns.


Suggested countermeasure: When true anonymity is requested, use the card
ONLY ONCE, then destroy it. Makes the calls rather expensive, but less
risky. Make sure you can't be traced back by other means, ranging from
surveillance cameras in the vicinity of the phone booths to the location
data from cellphones (because, as it's well-known but often overlooked,
the cellphone networks know the location of every active phone).


Wondering if there are any records of the UIDs for the cards paired with
the locations of the vendor outlets the cards were shipped to.