Dynamic Collusion Function-Private Functional Encryption.

Functional encryption (FE) enables fine-grained access control over encrypted data: each decryption key is tied to a function, allowing users to compute only the function’s output on the underlying message. While FE for general functions is hard to achieve, static bounded FE, where the adversary declares a fixed collusion bound q at the outset, can be constructed from minimal assumptions (e.g., public-key encryption (PKE) for public key FE and one-way functions (OWFs) for symmetric key FE). To enhance flexibility, Agrawal et al. (Crypto 2021) and Garg et al. (Eurocrypt 2022) introduced dynamic bounded public key FE, where the collusion bound q is chosen per encryption rather than fixed globally. In this paper, we study dynamic bounded FE in the symmetric key setting. We present a compiler that upgrades static bounded FE to dynamic bounded FE using pseudorandom functions (PRFs), preserving both function and message privacy. Unlike its public key counterpart, our compiler avoids the non-black-box tool of garbled circuits and relies solely on OWFs. Consequently, we show that dynamic bounded, function-private symmetric key FE can be built from OWFs alone.