Recently, Yuan et al. (IEEE Infocom'13, pp.2652-2660) proposed an efficient secure nearest neighbor (SNN) search scheme on encrypted cloud database. Their scheme is claimed to be secure against the collusion attack of query clients and cloud server, because the colluding attackers cannot infer the encryption/decryption key. In this letter, we observe that the encrypted dataset in Yuan's scheme can be broken by the collusion attack without deducing the key, and present a simple but powerful attack to their scheme. Experiment results validate the high efficiency of our attacking approach. Additionally, we also indicate an upper bound of collusion-resistant ability of any accurate SNN query scheme.

A decentralized secure protocol for casting trust rating in reputation systems (StR protocol) is lately proposed by Dimitriou and Michalas, and the StR protocol is verified to be faster than the previous work providing anonymous feedback. In this letter, we present new enhanced scheme of StR. Compared with StR protocol, our new approach attains the exactly same security, but requires less processing time and about half communication overheads. Therefore, we improve the performance without sacrificing any security, especially the communication delay is dramatically reduced.