Streaming Functional Encryption.

We initiate the study of streaming functional encryption (sFE) which is designed for scenarios in which data arrives in a streaming manner and is computed on in an iterative manner as the stream arrives. Unlike in a standard functional encryption (FE) scheme, in an sFE scheme, we (1) do not require the entire data set to be known at encryption time and (2) allow for partial decryption given only a prefix of the input. More specifically, in an sFE scheme, we can sequentially encrypt each data point x i in a stream of data x = x 1 … x n as it arrives, without needing to wait for all n values. We can then generate function keys for streaming functions which are stateful functions that take as input a message x i and a state st i and output a value y i and the next state st i + 1 . For any k ≤ n , a user with a function key for a streaming function f can learn the first k output values y 1 … y k where ( y i , st i + 1 ) = f ( x i , st i ) and st 1 = ⊥ given only ciphertexts for the first k elements x 1 … x k . In this work, we introduce the notion of sFE and show how to construct it from FE. In particular, we show how to achieve a secure sFE scheme for P / Poly from a compact, secure FE scheme for P / Poly , where our security notion for sFE is similar to standard FE security except that we require all function queries to be made before the challenge ciphertext query. Furthermore, by combining our result with the FE construction of Jain, Lin, and Sahai (STOC, 2022), we show how to achieve a secure sFE scheme for P / Poly from the polynomial hardness of well-studied assumptions.