A method for securing communications between a point-of-sale terminal and a transaction control server using a mobile terminal, said method comprising: - generating by the point-of-sale terminal a second random chain which is transferred from the point-of-sale terminal to the mobile terminal and from the mobile terminal to the transaction control server; - building, by the transaction control server, according to a first cryptography algorithm, a first encrypted message comprising a first random chain generated by the transaction control server, a secret code, and said second random chain; - inputting the secret code on the point-of-sale terminal; - transmitting by the transaction control server to the point-of-sale terminal said first random chain; - building, by the point-of-sale terminal, according to the same said first cryptography algorithm, a second encrypted message comprising the secret code inputted, said first random chain and said second random chain; this method further comprising - building, by said point-of-sale terminal, according to a second ciphering algorithm, a third ciphered message from at least one third datum, using said second encrypted message as a ciphering key; - transmitting, by said point-of-sale terminal, said third ciphered message to said transaction control server; - deciphering, by said transaction control server, according to said second ciphering algorithm, said third ciphered message, by using said first encrypted message as a deciphering key; - comparing, by said transaction control server, the content of said third deciphered message with said third datum to check whether the secret code inputted on the point-of-sale terminal is correct, the third datum being known from the transaction control server; - building, by said transaction control server, according to a third ciphering algorithm, a fourth ciphered message as a function of the content of said third deciphered message, using the first encrypted message as a ciphering key; - transmitting, by said transaction control server, said fourth ciphered message to said point-of-sale terminal; - deciphering, by said point-of-sale terminal, according to said third ciphering algorithm, said fourth ciphered message, using said second encrypted message as a deciphering key. - generating by the point-of-sale terminal a second random chain which is transferred from the point-of-sale terminal to the mobile terminal and from the mobile terminal to the transaction control server; - building, by the transaction control server, according to a first cryptography algorithm, a first encrypted message comprising a first random chain generated by the transaction control server, a secret code, and said second random chain; - inputting the secret code on the point-of-sale terminal; - transmitting by the transaction control server to the point-of-sale terminal said first random chain; - building, by the point-of-sale terminal, according to the same said first cryptography algorithm, a second encrypted message comprising the secret code inputted, said first random chain and said second random chain; this method further comprising - building, by said point-of-sale terminal, according to a second ciphering algorithm, a third ciphered message from at least one third datum, using said second encrypted message as a ciphering key; - transmitting, by said point-of-sale terminal, said third ciphered message to said transaction control server; - deciphering, by said transaction control server, according to said second ciphering algorithm, said third ciphered message, by using said first encrypted message as a deciphering key; - comparing, by said transaction control server, the content of said third deciphered message with said third datum to check whether the secret code inputted on the point-of-sale terminal is correct, the third datum being known from the transaction control server; - building, by said transaction control server, according to a third ciphering algorithm, a fourth ciphered message as a function of the content of said third deciphered message, using the first encrypted message as a ciphering key; - transmitting, by said transaction control server, said fourth ciphered message to said point-of-sale terminal; - deciphering, by said point-of-sale terminal, according to said third ciphering algorithm, said fourth ciphered message, using said second encrypted message as a deciphering key.