Method for conversion of a first encryption into a second encryption

A conversion device comprising:

• a processing unit;
• a non-transitory memory for storage of a secret conversion matrix;
• and
• means for calculating the second ciphertext, designed to calculate the second ciphertext by encryption of the first received ciphertext by means of a product of the secret conversion matrix, said 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;
• and
• means for converting 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.

A conversion system comprising:

• a conversion device designed to convert the first ciphertext into the second ciphertext, the conversion device comprising a processing unit, a non-transitory memory for storage of a secret conversion key, and means for calculating the second ciphertext, wherein means are designed to calculate the second ciphertext by encryption of the first received ciphertext by means of a product of a 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;
• and means for converting 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
• a confidence entity designed to calculate the conversion matrix starting from the secret matrices.