Understanding Security APIs - CrySyS Lab

• Single security officer (in an m-of-n scheme)
As a team, the security officers are all-powerful. A single security officer is
already part way to exercising this total power, which would straight away
yield an attack. He or she is in a good position to deceive the other security
officers, and is likely to be trained and experienced in operation of the module.
However, he is under scrutiny from all levels – management, colleagues and
users – as they are aware that deviation from established procedures gives
him full privilege. He will have good host access, but is unlikely to be able to
operate the host every day.
• Single operator (in an m-of-n scheme)
Like the security officer, an operator holding an access card is already part
way authorised to perform sensitive actions. He or she is in a good position to
deceive other operators, and it is likely that the training and skill of the other
operators will be lower than that of a security officer. A card-holding operator
would regularly have access to the host in the normal line of business.
• Operator (access to host, no cards)
An operator who does not have rights over any of the access tokens still has
scope to perform attacks. He is in a good position to subvert the host or
harvest passwords and PINs from other operators. As a colleague he would
be in a position to deceive card-holding operators, or possibly even security
officers.
4.4 Prepayment Electricity Meters
HSMs are an important and integral part of the prepayment electricity meter systems
used to sell electric power to students in halls of residence, to the third-world poor,
and to poor customers in rich countries [5]. They are typical of the many systems
that once used coin-operated vending, but have now switched to tokens such as
magnetic cards or smartcards. The principle of operation is simple: the meter
will supply a certain quantity of energy on receipt of an encrypted instruction –
a ‘credit token’, then interrupt the supply. These credit tokens are created in a
token vending machine, which contains an HSM that knows the secret key in each
local meter. The HSM is designed to limit the loss if a vending machine is stolen or
misused; this enables the supplier to entrust vending machines to marginal economic
players ranging from student unions to third-world village stores.
The HSM inside the vending machine thus needs to be tamper-resistant, and protect
the meter keys and a value counter. The value counter enforces a credit limit; after
that much electricity has been sold, the machine stops working until it is reloaded.
This requires an encrypted message from a controller one step up higher the chain of
37