Functional encryption (FE) enables fine-grained access control of encrypted data while promising simplified key management. In the past few years substantial progress has been made on functional encryption and a weaker variant called predicate encryption. Unfortunately, fundamental impossibility results have been demonstrated for constructing FE schemes for general functions satisfying a simulation-based definition of security.
We show how to use \emph{hardware tokens} to overcome these impossibility results. In our envisioned scenario, an authority gives a hardware token and some cryptographic information to each authorized user; the user combines these to decrypt received ciphertexts. Our schemes rely on \emph{stateless} tokens that are \emph{identical} for all users. (Requiring a different token for each user trivializes the problem, and would be a barrier to practical deployment.) The tokens can implement relatively ``lightweight'' computation relative to the functions supported by the scheme.
Our token-based approach can be extended to support hierarchal functional encryption, function privacy, and more.
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