Encrypted keyword search allows a server to perform a search over a set of encrypted documents on be- half of a client without learning the contents of the documents or the words being searched for. Design- ing a practical system is challenging because the pri- vacy constraint thwarts standard indexing and rank- ing techniques. We present Mafdet, an encrypted keyword search system we have implemented. Our system makes the search practical even for large data sets. We evaluated Mafdets performance on a set of queries and a large collection of documents. In these queries, Mafdets accuracy is within 6% of Google Desktop, and the search time is on the order of sec- onds for document sets as large as 2.6 GB. ... Controlled searching. The server cannot learn anything about the contents of documents, ex- cept when the client performs a search. Hidden queries. The client can search for a set of documents containing a keyword without revealing the keyword to the server. Query isolation. From the query result, the server learns nothing about the plain text other than the set of documents that match the query (and possibly the limited statistical information used to perform ranking). Update isolation. The server learns nothing more from updates than it would if we main- tained no additional metadata for the purpose of performing searches.