Providing secure indexes for searching encrypted data

Providing an encrypted search index for performing searches on encrypted documents, the method comprising: (i) providing a set of documents, the documents comprising a plurality of unencrypted phrases; (ii) providing a master key; (iii) providing, based on the master key, for each phrase a set of encryption keys comprising one or more encryption keys; (iv) selecting, for each phrase, one encryption key of the set of encryption keys; (v) encrypting each phrase with the selected encryption key; and (vi) building an index based on the encrypted phrases, the index comprising information regarding which encrypted phrase is comprised within a certain document.