1. A method for conversion by a conversion entity of a first ciphertext into a second ciphertext, the first ciphertext corresponding to the result of a symmetrical probabilistic encryption of a plaintext message element by means of a first secret matrix whose parameters are generated by a random vector, the second ciphertext corresponding to the result of a symmetrical probabilistic encryption of the plaintext message element by means of a second secret matrix whose parameters are generated by the random vector, the method comprising the step: calculation of the second ciphertext by encryption of the first ciphertext by means of a secret conversion matrix being a function of the first and second secret matrices, and whose parameters are generated by the random vector.