Method for authenticating at least one piece of data, the method being implemented during a payment transaction taking place between a merchant's communications terminal and a user device, the method being of the type comprising the transmission, by the communications terminal, of at least one piece of data to be signed to the user device by means of a near field communications wireless data link, the method being characterized in that it comprises: - a step for obtaining said piece of data to be signed ( DaS) - a step for obtaining an identifier of said communications terminal ( UiD); - a step of signing, within a secured processing unit of said communications terminal, by means of a private key (sk) belonging to a {private key; public key} pair of said communications terminal, of said piece of data to be signed ( DaS) and of said identifier of the communications terminal (UiD), delivering a pair of signed pieces of data, the step of signing comprising: - a step of transmission of said piece of data to be signed and said identifier of the communications terminal to said secured processing unit of said communications terminal by a general processing unit of said communications terminal; - a step of signing, by said secured processing unit, using said private key of the communications terminal, of said piece of data to be signed and said identifier of the communications terminal, respectively delivering a piece of signed data and a signed identifier; - a step of transmission of said piece of signed data and of the signed identifier to said general processing unit of said communications terminal; - a step of transmission of the pair of signed pieces of data to said user device; and - a step of reception, from said user device, of a piece of encrypted data establishing the authentication of said pair of signed pieces of data; - a step of decryption, by means of a secured processing unit of said communications terminal, of said piece of encrypted data delivering a piece of signed data; - a step of verification of the validity of said piece of signed data in relation to a piece of reference data, said piece of reference data being equal to said piece of data to be signed ( DaS); and - when the step of verification delivers a positive result, a step of transmission, by the communications terminal, of said piece of signed data to a payment transaction processing system (STTP). - a step for obtaining said piece of data to be signed ( DaS) - a step for obtaining an identifier of said communications terminal ( UiD); - a step of signing, within a secured processing unit of said communications terminal, by means of a private key (sk) belonging to a {private key; public key} pair of said communications terminal, of said piece of data to be signed ( DaS) and of said identifier of the communications terminal (UiD), delivering a pair of signed pieces of data, the step of signing comprising: - a step of transmission of said piece of data to be signed and said identifier of the communications terminal to said secured processing unit of said communications terminal by a general processing unit of said communications terminal; - a step of signing, by said secured processing unit, using said private key of the communications terminal, of said piece of data to be signed and said identifier of the communications terminal, respectively delivering a piece of signed data and a signed identifier; - a step of transmission of said piece of signed data and of the signed identifier to said general processing unit of said communications terminal; - a step of transmission of said piece of data to be signed and said identifier of the communications terminal to said secured processing unit of said communications terminal by a general processing unit of said communications terminal; - a step of signing, by said secured processing unit, using said private key of the communications terminal, of said piece of data to be signed and said identifier of the communications terminal, respectively delivering a piece of signed data and a signed identifier; - a step of transmission of said piece of signed data and of the signed identifier to said general processing unit of said communications terminal; - a step of transmission of the pair of signed pieces of data to said user device; and - a step of reception, from said user device, of a piece of encrypted data establishing the authentication of said pair of signed pieces of data; - a step of decryption, by means of a secured processing unit of said communications terminal, of said piece of encrypted data delivering a piece of signed data; - a step of verification of the validity of said piece of signed data in relation to a piece of reference data, said piece of reference data being equal to said piece of data to be signed ( DaS); and - when the step of verification delivers a positive result, a step of transmission, by the communications terminal, of said piece of signed data to a payment transaction processing system (STTP).