Understanding Security APIs - CrySyS Lab

Because the ATM network is link based architecture, rather than end-to-end, the
increasing user-base increases the encryption workload several fold.
4.1.1 Targets of Attack
The ATM system security in banks has a quite conventional threat model which
shares aspects with that of many other data-processing and service-providing corporations,
because there are no secrets which are absolutely mission-critical. Whilst
the keys used for deriving PINs from account numbers are extremely valuable, even
a theft of tens of millions of pounds is not enough to collapse a bank for financial
reasons (their brand name, however, can certainly be considered mission-critical).
The crucial secret is the customer PIN. Sometimes PIN information exists as explicit
data which must be kept secret from an attacker, and sometimes it is kept as a PIN
derivation key. The authorisation responses sent to ATMs, although not secret, are
also valuable. If the integrity of these responses can be compromised, and a no
turned to a yes, money can be withdrawn from a particular cash machine, without
knowledge of the correct PIN. Service codes for disabling ATM tamper-resistance,
or for test dispensing of cash are also of course valuable and must be kept secret.
It is actually quite easy to quantify the financial loss associated with PIN theft.
‘Velocity checking’ limits mean that maybe a maximum of £300 can be withdrawn
per calendar day, and if monthly statements to the customer are checked, then the
fraud can perpetuate for at most one month. Thus each stolen PIN is worth up
to £9300 to the attacker – maybe on average £5000. There have been cases where
velocity checking was bypassed, or not even present [20]; in these circumstances, as
a rough guide, one person can withdraw about £25000 per day working full time.
Unfortunately for the bank, because their prime business of storing money for people
is strongly built upon trust, the effects of fraud on the bank’s image and the trust
of its customer base have to be factored in. These are very difficult for an outsider
to assess, let alone consider quantitatively.
If handled shrewdly, the loss may not be costly at all, for instance if the bank
declares the customer to be liable. If the amount is small enough, the customer
will be tempted to give up rather than wasting time and effort to retrieve a small
sum. However, the potential loss of revenue from bad handling of a fraud worth,
say £10000, could be in the tens of millions in terms of long-term business lost.
The primary motive for attacking a bank is obviously financial gain. Some attackers
seek short-term financial gain, for instance by extracting PINs for several hundred
accounts and looting them in a couple of weeks. Others may plan for mid-term
financial gain, for instance by selling a PIN extraction service for stolen cards at
£50 per card; this slow trickle of cash may be easier for the attacker to conceal in
his finances.
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