Additional methods could be added to make those solutions robust against malicious attack; however, we restrict our discussion on honest-but-curious fashion. We also note that all documents are treated as text files the same way as search engine does. For example, if a document is a web page, the style tags will this be pruned.5.1. Scheme DefinitionThe secure-query biased preview (SecQBP) scheme contains two parties: a user U and a remote server S. U encrypts his private document d to D, generates a secure additive ranking index (SecARI) H, and then outsources them to S. S stores the document, performs the computation for the scores when queried by multiple keywords, and returns the result to U. U then selects the best snippet indexed by i and privately retrieves it from S.

Without loss of generality, we consider the construction for a single document. The scheme could be extended to a document collection with ease. Now we define the SecQBP scheme as follows.Definition (secure query-biased preview scheme) ��SecQBP scheme is a collection of six polynomial-time algorithms SecQBP = (Gen, Setup, Query, ComputeScore, DecScore, DecSnip) as follows. K��Gen(1k) is a probabilistic algorithm that takes as input a security parameter k and outputs the secret key collection K. It is run by the user, and the keys are kept secret. (D, H)��SetupK(d) is a probabilistic algorithm that takes as input a document d and outputs a encrypted document D (using any cryptosystem) and an index H. It is run by the user, and D, H are outsourced to the server.

q��QueryK(w) is a deterministic algorithm that takes as input the queried multiple keywords w = (w1,��, wn) and outputs a secret query token q. It is run by the user, and q is sent to the server. r��ComputeScore(q, H) is a deterministic algorithm that takes as input the secret query q and the index H and outputs the result r that contains the final score information about each snippet. It is run by the server. i��DecScoreK(w,d-,r) is a deterministic algorithm that takes as input the queried keywords w, the document identifier d��, and the query result r and outputs the snippet index number i. It is run by the user. si��DecSnipK(Di) is a deterministic algorithm that takes as input the ciphertext Di and outputs the recovered plaintext snippet si. It is run by the user.

Note that, if the user retrieves the entire encrypted document, he could decrypt the document by decrypting each snippet. 5.2. Security ModelInformally speaking, SecQBP must guarantee that, first, given the encrypted document c and the index H, the adversary cannot learn any partial information about the document; second, given a sequence of queries Drug_discovery q = (q1,��, qn), the adversary cannot learn any partial information about the queried keywords and the matched snippet (including the index number and the content). We now present the security definition for adaptive adversaries.