CFE: Secure Filtered Words in End-to-End Encrypted Messaging System

We introduce a new lightweight Symmetric Threshold Predicate Encryption (STPE) scheme, which expands the definition of Predicate Encryption. In STPE, the recipient’s private key evaluates only k predicates instead of all predicates on the sender’s encrypted data. The recipient can decrypt the data if at least k predicates are satisfied. As a new building block, we design a Content-Filtered Encryption (CFE) scheme based on STPE, which allows the sender to encrypt the message with the extracted words and the recipient to generate a filter with abusive words. The recipient can decrypt the message if the evaluation of extracted words and abusive words does not intersect more than a threshold k, where k is a flexible limit of sensitive words the recipient can accept. Otherwise, the recipient can refuse to read the message. It is essential for the recipient to generate a filter of abusive words beforehand; the incoming encrypted message will be delivered only if it bypasses this filter. Therefore, our proposed scheme enables secure filtering of words in the end-to-end encryption messaging protocol, which achieves selective security and efficiency for all communication devices. We prove that our STPE and CFE schemes are secure under the selected security assumptions. Furthermore, by utilizing the Pseudo-Random Function and XOR gate, our construction achieves lightweight computation, which benefits from the primitives of symmetric crypto mechanisms. We experimented on multiple devices, such as PCs and mobile devices. Additionally, our work demonstrates the feasibility across heterogeneous devices.