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Sanitizable Signcryption: Sanitization over Encrypted Data (Full Version)

We initiate the study of a new functional signcryption primitive, by exploring sanitizable signatures over encrypted data. While previous solutions for sanitizable signatures require the sanitizer to know, in clear, the original message-signature pair in order to generate the new signature, we investigate the case where it should be hidden from the sanitizer and how this can be achieved with encryption. We call this primitive sanitizable signcryption, and argue that there are two options concerning what the sanitizer learns about the sanitized output: in semi-oblivious sanitizable signcryption schemes the sanitizer may get to know the sanitized message-signature pair, while fully oblivious sanitizable signcryption schemes even protect the output data. Depending on the application, either notion may be preferable. We give feasibility results for both settings by showing that semi-oblivious sanitizable signcryption schemes can be constructed by wraping a regular sanitizable signature scheme into a multi-input functional encryption scheme, such that functional decryption corresponds to the sanitization process. Remarkably, the multi-input functional encryption scheme cannot easily be transferred to a fully oblivious sanitizable signcryption version, so we give a restricted solution based on fully homomorphic encryption for this case. We stress that we see our contribution in directing the attention the question of sanitizable signcryption and show that solutions can be constructed in principle; it yet remains to find truly practical instantiations.