A User Centric Security Model for Tamper-Resistant Devices

6.6 Analysis of the Proposed Protocols
securely communicate with each other. One exception to this is the STCP SC . The SC generates
the Die-Hellman exponential before the SP but it does not reveal the values until
it receives the Die-Hellman exponential from the SP. In this way, they satisfy requirements
SOG-3 to SOG-5 and SOG-12 like the other two STCPs; because generating the
Die-Hellman exponential before SP is not a problem as long as it is not revealed to the SP.
All communicating parties in the STCPs use the generated session keys to securely communicate
with each other, which gives an implicit mutual key conrmation, satisfying the
SOG-6.
In the STCPs, session keys generated in one session have no link with the session keys
generated in other sessions, even when a session is established between the same entities.
This enables the protocol to provide resilience against the known-key security (SOG-7).
This unlinkability of session keys is because each entity not only generates a new Die-
Hellman exponential but also a random number, both of which are used during the STCP
to generate new session keys. Therefore, even if an adversary A nds out about the
exponentials and random numbers of a session, it would not enable him to generate past
or future session keys.
Furthermore, to provide unknown key share resilience (SOG8) the STCPs include the
Die-Hellman exponentials and random numbers along with identities of individual entities
in a message (e.g. hs and hp) that is then signed by all communicating entities. Therefore,
the receiving entity can then ascertain the identity of the entity with which it has shared
the key by verifying the signature and parameters used to generate the session keys (e.g.
Die-Hellman exponentials and random numbers).
The STCPs can be considered KCI-resilient (SOG9) protocols, as the protection against
the KCI is based on the digital signatures. In addition, the cryptographic certicates of
each signature key include its association with a particular SP or smart card. Therefore, if
A has the knowledge of the signature key of a smart card (or an SP) then it can masquerade
the smart card to other entities but not other entities to the smart card. Another point to
note is that during the STCPs, all signed messages and certicates are encrypted using the
session key. This facilitates the STCPs in meeting the requirements SOG-8 and SOG-9,
as an adversary cannot substitute the certicate or signature.
The STCPs also meet the perfect forward secrecy (SOG10) by making the key generation
process independent of any long-term keys. The session keys are generated using fresh
values of Die-Hellman exponentials and random numbers, regardless of the long-term
keys like the smart card, user, and SP signature keys. Therefore, even if A nds out the
signature key of any entity, this knowledge will not enable him to nd out past session
keys.
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