A METHOD FOR A COMMUNICATION DEVICE TO RETRIEVE A PLAINTEXT FROM A CIPHERTEXT MEMORIZED IN A REMOTE DATABASE

摘要

This invention relates to a method for a first communication device (100) to retrieve a plaintext P from a first ciphertext C memorized in a first server called remote database (102), the first ciphertext C being generated by a second communication device (101) by applying a first encryption function E() in order to perform an homomorphic encryption of plaintext P by combining it with a first encryption key eHE provisioned in said second communication device. The method comprises the following steps carried out by the first communication device (100): receiving (200) from the remote database (102) the first ciphertext C; generating (201) a second ciphertext C' by applying a second encryption function F() that combines a single-use encryption key K generated by a random number generator with the first ciphertext C; transmitting (202) the second ciphertext C' to a second remote server (103) provisioned with a first decryption key dHE corresponding to the first encryption key eHE; receiving (203) from the second remote server (103) a third ciphertext M generated by applying a first decryption function D() corresponding to the first encryption function E() in order to combine the second ciphertext C' with the first decryption key dHE; retrieving (204) the plaintext P by applying a second decryption function G() corresponding to the second encryption function F() in order to combine the single-use encryption key K with the third ciphertext M.