Functional encryption from (small) hardware tokens

Abstract. Functional encryption (FE) enables fine-grained access con-trol of encrypted data while promising simplified key management. In the past few years substantial progress has been made on functional encryp-tion and a weaker variant called predicate encryption. Unfortunately, fundamental impossibility results have been demonstrated for construct-ing FE schemes for general functions satisfying a simulation-based defi-nition of security. We show how to use hardware tokens to overcome these impossibility re-sults. In our envisioned scenario, an authority gives a hardware token and some cryptographic information to each authorized user; the user com-bines these to decrypt received ciphertexts. Our schemes rely on stateless tokens that are identical for all users. (Requiring a different token for each user trivializes the problem, and would be a barrier to practical deploy-ment.) 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 func-tional encryption, function privacy, and more.