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Let us briefly dwell on the physical foundations of contactless card technology - the phenomena of electromagnetic induction and resonance.
The meaning of the law of electromagnetic induction, discovered and mathematically described by M. Faraday, is that an alternating electric current is generated in a closed conducting loop placed in an alternating magnetic field. Moreover, if the magnetic field changes according to a sinusoidal law with a certain frequency, then the alternating current also changes according to the same law (accurate to phase) and with the same frequency. Well, if the conducting circuit is a resonant EC circuit with a natural frequency that coincides with the frequency of the external alternating magnetic field, then in this case the current in the circuit increases in accordance with the phenomenon of resonance. In this case, the amplification of the current (the scale of the increase in the current strength) is determined by the Q-factor of the resonant circuit.
In the attachment to the cards, the case looks like this (Fig. 7.4). The card contains an antenna that acts as an inductor of the oscillating circuit (the antenna is 3 to 10 turns (usually 4 turns) of wire or conductive ink, usually placed along the perimeter of the card). In turn, the coil is connected in series with a capacitor. The parameters of the inductance of the coil and the capacitance of the capacitor are such that the corresponding natural frequency of the card's oscillatory circuit is close to the oscillation frequency of the external magnetic field.
For example, when using 4 turns of wire along the perimeter of the card (area of the turn S = 75 mm x 43 mm), the antenna inductance will be approximately L = 4 μH. The characteristic value of the capacitance of a capacitor, which can be realized in a microcircuit, is C - 30-100 pcf. Hence, we find that the natural frequency of the oscillatory circuit at L = 4 μH and C = 30 pcf according to Thompson's formula is v = = 14.5 MHz.
1 2kVlC
Moreover, according to Faraday's law, the EMF of induction arising in the oscillatory circuit of the U (t) card is determined by the equality:
Comrades ,. Lt si (g) dB (t) S AT _ ". ,.
U (t) = -N ---- = -N ----- = NSwB n sm (wt),
dt dt
where w = 2nv;
B (t) = p o pH (t) "p 0 H (t);
B (t) is the vector of the magnetic induction of the magnetic field (measured
in teslas);
H (t) - magnetic field strength (A / m);
c is the magnetic permeability of the medium, which in our case has a value close to 1;
p 0 = 4l • 10 7 Gn / m;
n = 4 is the number of wire turns in the oscillatory circuit;
S - 75 mm x 43 mm - the area of one antenna turn.
Then the maximum value of the induction EMF is U max = NSwB 0 = = 2np 0 NSH 0 = 5.8 V, and the value of the voltage supplied to the microcircuit after rectifying the current is U DC = U max a / 2 = 4.1 V, which very close to the characteristic values of the voltage supplied to the microcircuit of the microprocessor card (1.8 V, 3 V, 5 V).
The external magnetic field, in turn, is created by the resonant LC-circuit of the reader (Fig. 7.5), to the input of which an alternating voltage is applied. The oscillation frequency of the external voltage, the natural frequency of the resonant circuit of the reader and the natural frequency of the resonant circuit of the card take close values.
As a result, when an external AC voltage is applied to the reader circuit, an alternating current appears in the resonant circuit of the card. This current
Contactless Smart Card
Card Body (Front)) Card Body
Rice. 7.4. General understanding of the structure of a contactless card
Reader
That Host Computer
Rice. 7.5. Interaction of the card with the reader
charges a special capacitor connected in parallel to the resonant circuit of the card. The energy stored in the capacitor is used to carry out various operations of the microcircuit card. It turns out that in order for the card's energy to be sufficient to implement various functions of the EMV protocol, it is necessary that the amplitude of the magnetic field strength in the region of the microcircuit is 1.5-7.5 A / m. In this case, the energy consumed by the card to perform its functions is replenished with the energy of the reader's magnetic field.
As shown by the research of Gemalto specialists, the most power-consuming procedures are the procedures for processing the INTERNAL AUTHENTICATE command by the card and the VERIFY command with the encrypted PIN-code value, which require execution of the RSA algorithm on a long exponent for their implementation. During the execution of these commands, the card consumes on average about 30 mW of power. The GENERATE АС command, which requires the execution of the 3DES algorithm, takes the second place in terms of power consumption (Table 7.1).
Tab. 7.1 Power consumed by the microcircuit when executing various commands
The measurement results are given in table. 7.1, are made for a dual-interface card with the following parameters: RAM = 4.5 KB, EEPROM - 32 KB, ROM = 136 KB, processor clock frequency is 10 MHz. The card supports Java Card 2.1, the ISO 14443 Type B radio interface, has an RSA coprocessor and an SDES accelerator, and implements the Dynamic Data Authentication method.
It is important to understand the following: despite the fact that the resonant circuit of the reader emits electromagnetic waves, at characteristic distances between the card and the reader (in the applicable standards, this distance is no more than 4-6 cm and certainly does not exceed 10 cm) at the frequencies used (13.56 MHz) “ripple” effect is not felt. In the close environment of the reader (in the so-called Fresnel zone, the radius of which in our case is about 3.53 m), an electromagnetic wave is equivalent to an alternating magnetic field with sufficient accuracy. In other words, we can assume that the resonant circuits of the card and reader create a common alternating magnetic field around them.
In this case, the amplitude of the magnetic field strength decreases in inverse proportion to the cube of the distance from the center of the reader circuit (Bio-Savart-Laplace law), and therefore, the energy of the magnetic field created by the reader decreases as the sixth power of the distance from the center of the reader circuit. This rapid decrease in magnetic field strength is an important element of contactless payment technology. It allows you to achieve a situation when in the working area of the reader (the area where the magnetic field strength is high enough to initiate the operation on the card) at the time of the operation there will be a single card (there will be no several cards competing for the right to work with the reader).
Obviously, the reader's magnetic field is used not only to "energize" the card. The exchange of information between the card and the reader is carried out using the same magnetic field. For this, pre-agreed standards for signal interfaces are used, which determine the method of modulating a discrete signal (amplitude and phase modulation is used) and a method for coding a bit with a discrete signal (modified Miller's code, Manchester code, NRZ coding are widely used).
Obviously, there are many ways to transfer information (bits "O" and "1") from the reader to the card. To do this, the reader must only use one of the previously agreed-upon methods to change the parameters of the alternating magnetic field (modulate the signal), the source of which it is. How to transfer information from a passive card to a reader?
For this, the so-called load modulation is used. In the resonant circuit of the card, either the value of the capacitance of the capacitor changes, or a load resistance is connected to it. As a result, the natural frequency of the circuit changes, and, as a result, the current in the circuit of the card decreases (the frequency of the external field does not change in this case). Changing the current strength in the circuit of the card leads to a decrease in the magnetic field created by it in the area of the reader circuit. As a result, the voltage at the reader's antenna changes. It changes insignificantly - by only 10 mV at a voltage on the antenna of about 100 V (such a high voltage on the antenna is caused by the phenomenon of resonance). Detection of such a weak signal (the signal-to-noise ratio in our case is of the order of -80 dB) requires a rather complex circuit on the reader's side,
The methods of modulation and signal coding used are described in a little more detail in the next section.
The meaning of the law of electromagnetic induction, discovered and mathematically described by M. Faraday, is that an alternating electric current is generated in a closed conducting loop placed in an alternating magnetic field. Moreover, if the magnetic field changes according to a sinusoidal law with a certain frequency, then the alternating current also changes according to the same law (accurate to phase) and with the same frequency. Well, if the conducting circuit is a resonant EC circuit with a natural frequency that coincides with the frequency of the external alternating magnetic field, then in this case the current in the circuit increases in accordance with the phenomenon of resonance. In this case, the amplification of the current (the scale of the increase in the current strength) is determined by the Q-factor of the resonant circuit.
In the attachment to the cards, the case looks like this (Fig. 7.4). The card contains an antenna that acts as an inductor of the oscillating circuit (the antenna is 3 to 10 turns (usually 4 turns) of wire or conductive ink, usually placed along the perimeter of the card). In turn, the coil is connected in series with a capacitor. The parameters of the inductance of the coil and the capacitance of the capacitor are such that the corresponding natural frequency of the card's oscillatory circuit is close to the oscillation frequency of the external magnetic field.
For example, when using 4 turns of wire along the perimeter of the card (area of the turn S = 75 mm x 43 mm), the antenna inductance will be approximately L = 4 μH. The characteristic value of the capacitance of a capacitor, which can be realized in a microcircuit, is C - 30-100 pcf. Hence, we find that the natural frequency of the oscillatory circuit at L = 4 μH and C = 30 pcf according to Thompson's formula is v = = 14.5 MHz.
1 2kVlC
Moreover, according to Faraday's law, the EMF of induction arising in the oscillatory circuit of the U (t) card is determined by the equality:
Comrades ,. Lt si (g) dB (t) S AT _ ". ,.
U (t) = -N ---- = -N ----- = NSwB n sm (wt),
dt dt
where w = 2nv;
B (t) = p o pH (t) "p 0 H (t);
B (t) is the vector of the magnetic induction of the magnetic field (measured
in teslas);
H (t) - magnetic field strength (A / m);
c is the magnetic permeability of the medium, which in our case has a value close to 1;
p 0 = 4l • 10 7 Gn / m;
n = 4 is the number of wire turns in the oscillatory circuit;
S - 75 mm x 43 mm - the area of one antenna turn.
Then the maximum value of the induction EMF is U max = NSwB 0 = = 2np 0 NSH 0 = 5.8 V, and the value of the voltage supplied to the microcircuit after rectifying the current is U DC = U max a / 2 = 4.1 V, which very close to the characteristic values of the voltage supplied to the microcircuit of the microprocessor card (1.8 V, 3 V, 5 V).
The external magnetic field, in turn, is created by the resonant LC-circuit of the reader (Fig. 7.5), to the input of which an alternating voltage is applied. The oscillation frequency of the external voltage, the natural frequency of the resonant circuit of the reader and the natural frequency of the resonant circuit of the card take close values.
As a result, when an external AC voltage is applied to the reader circuit, an alternating current appears in the resonant circuit of the card. This current
Contactless Smart Card
Card Body (Front)) Card Body
Rice. 7.4. General understanding of the structure of a contactless card
Reader
That Host Computer
Rice. 7.5. Interaction of the card with the reader
charges a special capacitor connected in parallel to the resonant circuit of the card. The energy stored in the capacitor is used to carry out various operations of the microcircuit card. It turns out that in order for the card's energy to be sufficient to implement various functions of the EMV protocol, it is necessary that the amplitude of the magnetic field strength in the region of the microcircuit is 1.5-7.5 A / m. In this case, the energy consumed by the card to perform its functions is replenished with the energy of the reader's magnetic field.
As shown by the research of Gemalto specialists, the most power-consuming procedures are the procedures for processing the INTERNAL AUTHENTICATE command by the card and the VERIFY command with the encrypted PIN-code value, which require execution of the RSA algorithm on a long exponent for their implementation. During the execution of these commands, the card consumes on average about 30 mW of power. The GENERATE АС command, which requires the execution of the 3DES algorithm, takes the second place in terms of power consumption (Table 7.1).
Tab. 7.1 Power consumed by the microcircuit when executing various commands
| Command | Power consumption (mW) |
| SELECT | 12 |
| GET PROCESSING OPTIONS | 19 |
| READ RECORD | 21 |
| GET DATA | 21 |
| VERIFY | 30 |
| INTERNAL AUTHENTICATE | 30 |
| 1ST GENERATE AC | 24 |
| 2ND GENERATE AC | 24 |
The measurement results are given in table. 7.1, are made for a dual-interface card with the following parameters: RAM = 4.5 KB, EEPROM - 32 KB, ROM = 136 KB, processor clock frequency is 10 MHz. The card supports Java Card 2.1, the ISO 14443 Type B radio interface, has an RSA coprocessor and an SDES accelerator, and implements the Dynamic Data Authentication method.
It is important to understand the following: despite the fact that the resonant circuit of the reader emits electromagnetic waves, at characteristic distances between the card and the reader (in the applicable standards, this distance is no more than 4-6 cm and certainly does not exceed 10 cm) at the frequencies used (13.56 MHz) “ripple” effect is not felt. In the close environment of the reader (in the so-called Fresnel zone, the radius of which in our case is about 3.53 m), an electromagnetic wave is equivalent to an alternating magnetic field with sufficient accuracy. In other words, we can assume that the resonant circuits of the card and reader create a common alternating magnetic field around them.
In this case, the amplitude of the magnetic field strength decreases in inverse proportion to the cube of the distance from the center of the reader circuit (Bio-Savart-Laplace law), and therefore, the energy of the magnetic field created by the reader decreases as the sixth power of the distance from the center of the reader circuit. This rapid decrease in magnetic field strength is an important element of contactless payment technology. It allows you to achieve a situation when in the working area of the reader (the area where the magnetic field strength is high enough to initiate the operation on the card) at the time of the operation there will be a single card (there will be no several cards competing for the right to work with the reader).
Obviously, the reader's magnetic field is used not only to "energize" the card. The exchange of information between the card and the reader is carried out using the same magnetic field. For this, pre-agreed standards for signal interfaces are used, which determine the method of modulating a discrete signal (amplitude and phase modulation is used) and a method for coding a bit with a discrete signal (modified Miller's code, Manchester code, NRZ coding are widely used).
Obviously, there are many ways to transfer information (bits "O" and "1") from the reader to the card. To do this, the reader must only use one of the previously agreed-upon methods to change the parameters of the alternating magnetic field (modulate the signal), the source of which it is. How to transfer information from a passive card to a reader?
For this, the so-called load modulation is used. In the resonant circuit of the card, either the value of the capacitance of the capacitor changes, or a load resistance is connected to it. As a result, the natural frequency of the circuit changes, and, as a result, the current in the circuit of the card decreases (the frequency of the external field does not change in this case). Changing the current strength in the circuit of the card leads to a decrease in the magnetic field created by it in the area of the reader circuit. As a result, the voltage at the reader's antenna changes. It changes insignificantly - by only 10 mV at a voltage on the antenna of about 100 V (such a high voltage on the antenna is caused by the phenomenon of resonance). Detection of such a weak signal (the signal-to-noise ratio in our case is of the order of -80 dB) requires a rather complex circuit on the reader's side,
The methods of modulation and signal coding used are described in a little more detail in the next section.
