Storing and retrieving ciphertext in data storage can include determining a first ciphertext value for a first data chunk to be saved to a client-server data storage system using an encrypted chunk hash value associated with the first data chunk as an initial value, and storing the first data chunk on a server in the client-server data storage system in response to determining that the first ciphertext value is a unique ciphertext value. Also, storing and retrieving ciphertext in data storage can include decrypting a ciphertext value for a second data chunk received from a client in the client-server data storage system and based on an encrypted chunk hash value associated with the second data chunk, and sending the second data chunk to the client in response to determining that the decrypted ciphertext value corresponds to an original data chunk saved to the server by the client.