Key Rotation for Authenticated Encryption
A common requirement in practice is to periodically rotate the keys used to encrypt stored data. Systems used by Amazon and Google do so using a hybrid encryption technique which is eminently practical but has questionable security in the face of key compromises and does not provide full key rotation. Meanwhile, symmetric updatable encryption schemes (introduced by Boneh et al. CRYPTO 2013) support full key rotation without performing decryption: ciphertexts created under one key can be rotated to ciphertexts created under a different key with the help of a re-encryption token. By design, the tokens do not leak information about keys or plaintexts and so can be given to storage providers without compromising security. But the prior work of Boneh et al. addresses relatively weak confidentiality goals and does not consider integrity at all. Moreover, as we show, a subtle issue with their concrete scheme obviates a security proof even for confidentiality against passive attacks.