A method for converting, by means of a conversion entity, a first digit into a second digit, the first cipher corresponding to the result of a symmetric probabilistic encryption of an plain message element using a first secret matrix parameterized by a random vector, the second digit corresponding to the result of a symmetric probabilistic encryption of the plain message element using a second secret matrix that is parameterized by the random vector, characterized in that the method includes a step of: calculating the second digit by encrypting the first digit using a secret conversion matrix which is a function of the first and second secret matrices, and which is parameterized by the random vector.