A Server-Aided Computation Protocol Revisited for Confidentiality of Cloud Service

In cloud-computing services, using the SSL/TLS protocol is not enough to ensure data confidentiality. For instance, cloud service providers can see the plaintext after the decryption at the end point of a secure channel. It is wise to introduce an encryption layer between the service client and the communication channel so the data will not be seen by the cloud service provider. The encryption/decryption process should be light for cases where a cloud-service user has a low-power device such as a smart phone. We pay attention to server-aided computation as an approach of speeding up cryptographic processing. On the other hand, for future cloud services, homomorphic encryption is a useful primitive for cryptographic protocols. In this paper, we propose a server-aided computation protocol using ElGamal encryption, which is homomorphic. The proposed protocol is secure under the discrete logarithm assumption for passive and active attacks. Furthermore, we present experimental results suggesting that the processing time of the proposed protocol is shorter than the original ElGamal encryption.