Updatable Encryption with Post-Compromise Security

An updatable encryption scheme allows to periodically rotate the encryption key and move already existing ciphertexts from the old to the new key. These ciphertext updates are done with the help of a so-called update token and can be performed by an untrusted party, as the update never decrypts the data. Updatable encryption is particularly useful in settings where encrypted data is outsourced, e.g., stored on a cloud server. The data owner can produce an update token, and the cloud server can update the ciphertexts.

We provide a comprehensive treatment of ciphertext-independent schemes, where a single token is used to update all ciphertexts. We show that the existing ciphertext-independent schemes and models by Boneh et al. (CRYPTO’13) and Everspaugh et al. (CRYPTO’17) do not guarantee the post-compromise security one would intuitively expect from key rotation. In fact, the simple scheme recently proposed by Everspaugh et al. allows to recover the current key upon corruption of a single old key. Surprisingly, none of the models so far reflects the timely aspect of key rotation which makes it hard to grasp when an adversary is allowed to corrupt keys. We propose strong security models that clearly capture post-compromise and forward security under adaptive attacks. We then analyze various existing schemes and show that none of them is secure in this strong model, but we formulate the additional constraints that suffice to prove their security in a relaxed version of our model. Finally, we propose a new updatable encryption scheme that achieves our strong notions while being (at least) as efficient as the existing solutions.