1. A method comprising the steps: conversion by a conversion device of a first ciphertext into a second ciphertext, the first ciphertext corresponding to a result of a symmetrical probabilistic encryption of a plaintext message element by means of a product of a first secret matrix and of a random vector, the second ciphertext corresponding to a result of a symmetrical probabilistic encryption of the plaintext message element by means of a product of a second secret matrix and of the random vector; and calculation, by the conversion device, of the second ciphertext by encryption of the first ciphertext by means of a product of a secret conversion matrix, the secret conversion matrix being obtained by the conversion device from a confidence entity which calculates the conversion matrix by means of the first and second secret matrices respectively transmitted by the first and the second communication devices, said second secret matrix being different from the first secret matrix.