An electromagnetic transponder comprising: an oscillating circuit; means for receiving from a terminal generating an electromagnetic field, a first ratio ((I1 off-load /I1 R20) MES) between a first data representative of a value of a current in an oscillating circuit of the off-load terminal (I1 off-load) and a second data representative of said current (I1 R20) for a first value of resistive load constituted by the transponder; means for measuring a voltage representative of the voltage (V C2) across the terminals of said oscillating circuit with a first value of resistive load; means for modifying the resistive load to a second value; means for measuring said voltage with the second value and for storing the results; at least one of the two following functionalities: the first functionality comprising means for calculating a first expected ratio ((I1 off-load /I1 R20) EVAL) between said first data and a second data representative of said current (I1 R20) for said first value of resistive load, and means for comparing said first expected ratio with said first ratio received from the terminal, the terminal being authenticated if said ratios are identical; the second functionality comprising means for evaluating (CALC(V C2]R21)EVAL) the value of said voltage with the second value of said resistive load and means for comparing said evaluated value of the measured value with the second value of said voltage, the terminal being authenticated if said values are identical, means for calculating a second expected ratio ((I1 off-load /I1 R21) EVAL) between said first data and a third data representative of said current (I1 R21) for said second value of resistive load; and means for transmitting said second expected ratio to the terminal, wherein the means for modifying the resistive load are formed of a switchable resistive circuit, comprising two resistors (R43 and R45), parallel connected, and each being in series with a switch, respectively; or comprising a single switchable resistor, or are means with which the consumption of the transponder circuits can be modified. an oscillating circuit; means for receiving from a terminal generating an electromagnetic field, a first ratio ((I1 off-load /I1 R20) MES) between a first data representative of a value of a current in an oscillating circuit of the off-load terminal (I1 off-load) and a second data representative of said current (I1 R20) for a first value of resistive load constituted by the transponder; means for measuring a voltage representative of the voltage (V C2) across the terminals of said oscillating circuit with a first value of resistive load; means for modifying the resistive load to a second value; means for measuring said voltage with the second value and for storing the results; at least one of the two following functionalities: the first functionality comprising means for calculating a first expected ratio ((I1 off-load /I1 R20) EVAL) between said first data and a second data representative of said current (I1 R20) for said first value of resistive load, and means for comparing said first expected ratio with said first ratio received from the terminal, the terminal being authenticated if said ratios are identical; the second functionality comprising means for evaluating (CALC(V C2]R21)EVAL) the value of said voltage with the second value of said resistive load and means for comparing said evaluated value of the measured value with the second value of said voltage, the terminal being authenticated if said values are identical, means for calculating a second expected ratio ((I1 off-load /I1 R21) EVAL) between said first data and a third data representative of said current (I1 R21) for said second value of resistive load; and means for transmitting said second expected ratio to the terminal, wherein the means for modifying the resistive load are formed of a switchable resistive circuit, comprising two resistors (R43 and R45), parallel connected, and each being in series with a switch, respectively; or comprising a single switchable resistor, or are means with which the consumption of the transponder circuits can be modified. the first functionality comprising means for calculating a first expected ratio ((I1 off-load /I1 R20) EVAL) between said first data and a second data representative of said current (I1 R20) for said first value of resistive load, and means for comparing said first expected ratio with said first ratio received from the terminal, the terminal being authenticated if said ratios are identical; the second functionality comprising means for evaluating (CALC(V C2]R21)EVAL) the value of said voltage with the second value of said resistive load and means for comparing said evaluated value of the measured value with the second value of said voltage, the terminal being authenticated if said values are identical, means for calculating a second expected ratio ((I1 off-load /I1 R21) EVAL) between said first data and a third data representative of said current (I1 R21) for said second value of resistive load; and means for transmitting said second expected ratio to the terminal, wherein the means for modifying the resistive load are formed of a switchable resistive circuit, comprising two resistors (R43 and R45), parallel connected, and each being in series with a switch, respectively; or comprising a single switchable resistor, or are means with which the consumption of the transponder circuits can be modified.