Dynamic Collusion Bounded Functional Encryption from Identity-Based Encryption

Functional Encryption is a powerful notion of encryption in which each decryption key is associated with a function f such that decryption recovers the function evaluation f (m). Informally, security states that a user with access to function keys sk f1 , sk f2 , . . . (and so on) can only learn f 1 (m), f 2 (m), . . . (and so on) but nothing more about the message. The system is said to be q-bounded collusion resistant if the security holds as long as an adversary gets access to at most q = q(λ) function keys. A major drawback of such statically bounded collusion systems is that the collusion bound q must be declared at setup time and is fixed for the entire lifetime of the system.

We initiate the study of dynamically bounded collusion resistant functional encryption systems which provide more flexibility in terms of selecting the collusion bound, while reaping the benefits of statically bounded collusion FE systems (such as quantum resistance, simulation security, and general assumptions). Briefly, the virtues of a dynamically bounded scheme can be summarized as:

Fine-grained individualized selection. It lets each encryptor select the collusion bound by weighing the trade-off between performance overhead and the amount of collusion resilience.