James A. Donald:
To detect and reject a double spending event in a
timely manner, one must have most past transactions
of the coins in the transaction, which, naively
implemented, requires each peer to have most past
transactions, or most past transactions that
occurred recently. If
A sim card contains a shared symmetric secret that is known to the
network operator and to rather too many people on the operator's staff,
and which could be easily discovered by the phone holder - but which is
very secure against everyone else.
This means that cell phones provide
