Key-Aggregate Proxy Re-Encryption with Dynamic Condition Generation Using Multilinear Map

With the growing accessibility of the internet, many people store files in the cloud storage. In consideration of privacy preserving, users tent to encrypt their files before uploading them to the cloud. Under certain conditions, a file owner may want to share the encrypted files directly on the cloud. In such cases, the file owner will delegate the re-encryption ability to a proxy with re-encryption keys. However, Chen et al. found that the size of the re-encryption key (or condition key) would increase with the number of the conditions, which motivated them in proposing the concept of key-aggregate proxy re-encryption. Unfortunately, we found that the conditions cannot be changed arbitrarily once we set the conditions at the beginning. To solve the problem, we apply the multilinear maps to the key-aggregate proxy re-encryption to extend the number of conditions to be exponentially large, while keeping the size of public parameters small and practical.