Method for information retrieval in an encrypted corpus stored on a server

A method for information retrieval in an encrypted corpus stored on a server, from a text request calculated on a customer computer device, including a succession of terms, the method comprising:

• encrypting the text request on the customer computer device;
• transmitting the encrypted text request to the server;
• performing homomorphic calculation on the server of an encrypted response to the encrypted text request recorded on the server;
• transmitting the encrypted response to the customer computer device;
• and
• decrypting the encrypted response on the customer computer device and extracting document identifiers, wherein decrypting the encrypted response comprises:
• a) calculating, on the customer computer device, when introducing a new requestable document, for each document belonging to the encrypted corpus, a first table and a second table different from the first table;
• the first table comprising, for each indexed term of the new requestable document, a number of occurrences of the indexed term in the new requestable document, and
• the second table indicating indices of each indexed term that is present in the new requestable document;
• b) encrypting the new requestable document and the second table, as well as encrypting, by a homomorphic encryption method, the first table, and transmitting the encrypted new requestable document, the encrypted second table, and the encrypted first table to the server for recording in a storage space, dedicated to a user or to a group of users;
• c) creating or updating an index on the customer computer device, the index associated with the user, for all documents accessible by the user, with the index being constituted by a table indicating for each indexed term a number of documents containing the indexed term;
• d) requesting, comprising:
• encrypting, on the customer computer device, the text request constituted by the succession of terms, by a homomorphic encryption belonging to the same cryptosystem as the encryption applied to the first table;
• transmitting the text request thus encrypted to the server for carrying out the homomorphic encryption and transmitting the encrypted response to the customer computer device and decrypting the encrypted response by the customer computer device;
• aggregating, by the customer computer device, the document identifiers of data contained in the encrypted response and in the index recorded on the customer computer device;
• and
• presenting a result in an orderly form of the documents on the customer computer device.