Shadowing Practice: How does Tap to Pay Work? - Learn English Speaking with Video

Let's say you're at your favorite coffee shop.

You've just finished ordering and you tap your credit card on the terminal.

Within a second, the screen lights up.

Approved.

Tap to pay may seem incredibly simple.

However, behind that one little tap is an entire world of physics, engineering, communication protocols, cryptography, and more than two dozen steps.

And all of this seamlessly happens in less time than it takes to say, thanks for the coffee.

So, in this video, we're going to take that one second when you tap your credit card, slow time way down,

and then dive into the physics and engineering of how the point -of -sale terminal wirelessly powers your credit card, how the two communicate back and forth,

what those messages actually say, the exact cryptographic algorithms used to securely transmit your bank details,

and everything else involved in near -field communication, or NFC, which is the technology behind contactless payments.

Additionally, we'll branch into related topics such as RFID badges,

hotel key cards, wireless charging for your smartphone, and so much more.

We'll explore all of it in intricate detail, because if you're anything like us, You've probably used any one of these technologies and wondered,

how on earth does that actually work?

So, let's begin our journey.

One second at the checkout counter, spread across the next 30 minutes of accurate, in -depth 3D animation.

This video is sponsored by Brilliant.

Okay, to be real, sometimes contactless payments can take two,

three, or more seconds, especially if you have no clue where to place your credit card on the terminal.

But we're optimistic, and we'll use a total transaction time of one second.

So, let's begin.

Time, zero.

There's no battery in the credit card, so how is power wirelessly sent from the terminal to the card?

To see how it works, let's do a quick teardown of the terminal at the checkout counter.

Inside, we find the display, a magnetic stripe reader on the side, a thermal receipt printer, a Wi -Fi antenna with its networking chips,

and the main PCB with spring leaf contacts for the chip reader on the underside.

And on the top side, push button contacts under each key.

the important part is over here two connectors from the pcb

lead to a coil of wire this coil is the nfc antenna it powers up

and communicates with your credit card

and it sits directly beneath the wireless payment symbol on the

terminal's face in this case the symbol is at one end of the terminal

but its location varies across different terminals as we get into

the physics you'll see why that's the exact spot to tap your card.

To power the card, the terminal energizes its coil with an alternating current of 13 .56 MHz,

creating an oscillating magnetic field that extends a few centimeters above the terminal's surface.

So when you tap your card on the tap -to -pay symbol, you're placing it directly inside that invisible field, and that's what powers the card.

But how does the card actually capture that power?

Well, let's take a look inside it.

The largest element is the antenna coil, made from two intricate flat sheets of die -cut metal, sandwiched inside thin plastic layers.

Two sets of vias pass through the plastic and electrically connect the two metal sheets, forming a single continuous path.

Let's modify the 3D model a little, so we can better see how the path winds into multiple loops forming an inductor.

When these loops sit inside the terminal's oscillating magnetic field, the field induces a voltage as described by Faraday's law of induction,

thereby driving an alternating current back and forth through the loops.

However, there's more to the intricate metal than just the large loops.

Over here are several linear strips along with an intricate pattern of rectangular patches separated by the same thin plastic layer.

Together, these form a capacitor, and when paired with the inductor loops,

we get an LC circuit tuned to resonate at exactly 13 .56 MHz,

thereby matching the terminal's frequency and maximizing the power transferred into the card.

Another element in the metal sheets is a second, smaller coil, and the credit card's chip sits right in the middle of it.

This is called an EMV chip, short for Europay, MasterCard, and Visa, which are the three companies that originally developed the contactless credit card specifications.

The EMV chip has its own set of coils

that inductively couples across the air gap to the smaller coil in the metal sheets.

The reason for having this small pair of inductively coupled coils is

that it's far easier

and cheaper to first mass -produce the layers of laminated plastic with the die -cut metal sheets in the middle.

and then in a separate semiconductor fab build a smaller emv chip

and finally glue the two together with no electrical connections required

let's zoom in even further on the emv chip connecting to

the ends of the emv chip coil are hair thin bond wires

that carry the induced ac voltage into the integrated circuit

which is then sent through a voltage rectifier

and regulator converting it down to a steady 1 .8 volts dc

which is used to power up the digital circuitry there are other sections inside the integrated circuit

that we'll get to later

but for now let's further explore the emv chip the ic is protected by a drop of epoxy

and inside are more hair thin bond wires

and they connect through more vias to the set of metal contact pads on the other side As you probably figured,

these pads directly connect the chip to the terminal when it's inserted.

But also, they're used during manufacturing to program the card and store the bank details and cryptographic keys.

Now that we've explored the terminal, card, and power transfer between the two, there are a couple of quick notes.

First, RFID -blocking wallets work by lining the wallet with conductive materials

that block or reflect the magnetic fields before they can reach the card.

And second, this type of wireless power transfer is closely related to wireless charging in your smartphone and to induction cooktops,

the key differences of the operating frequency and the amount of power.

While NFC credit cards use only a few milliwatts of power, wireless phone charging is closer to 5 to 15 watts,

and then induction stoves deliver 1 ,500 to 5 ,000 watts of power.

So, don't put your credit card or phone on an active induction stove,

though actually, placing your devices on any type of active stove is just as bad.

Let's move on and jump forward a few milliseconds, and by this point, the chip is fully powered up.

While the terminal waits for a payment method, the NFC antenna repeatedly broadcasts a message, asking whether any cards are present.

The card, now sitting in that field, receives the message and responds with its unique 7 -byte ID.

Following these messages is a complex series of exchanges that eventually results in the card sending its encrypted bank details.

Before we get to that, let's first thoroughly explore the physics of how messages are actually transmitted between the two devices.

They each use different methods to transmit data to the other, so we'll start with the terminal to card direction.

The terminal is already producing a localized oscillating magnetic field to deliver power, so it might as well use that same field to carry data.

And here's how it does it.

The 13 .56 MHz signal is divided into equal time slots, each 128 waves wide.

Note that there are too many waves to fit cleanly on the screen, so when scrunched up, the waves look more like blocks of lines.

One bit of data is sent in each time slot, and therefore, 106 ,000 bits can be sent each second.

This data is sent by varying the strength of each time slot or group of 128 waves, which is called amplitude modulation.

The simple approach would be to have 100 % amplitude of a time slot represent a 1,

and a reduced amplitude, say 50 % or 0%, represent a 0.

However, this has a problem.

A long run of zeros would yield a reduced magnetic field power and starve the card of energy.

Therefore, to keep power flowing, NFC uses a clever scheme called modified Miller encoding.

Within each 128 -wave time slot, the terminal briefly drops the signal amplitude for a few microseconds,

creating a short pause or blip in the signal and the position of that pause within the time slot encodes the data.

100

Dropping it near the start of the time slot encodes a zero, whereas dropping it in the middle encodes a one.

101

However, to add a little more complexity, when a zero immediately follows a one, the pause or blip is skipped entirely.

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Therefore, a binary message from the terminal asking for the card's ID number looks like this.

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So then, how does the credit card send messages back? well the car doesn't have its own transmitter

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and no battery to power one

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so instead of generating its own signal it cleverly manipulates the field the terminal is already emitting this means

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that only the terminal or card can transmit at a time

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and therefore while the card is transmitting the terminal is in listening mode

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and maintains a steady amplitude in its oscillating magnetic field to

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understand how the card sends data imagine you're firmly holding a large neodymium magnet

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and a friend waves a block of steel over the magnet

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each time the steel comes close you'd feel the magnet pull towards it

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and then when the steel moves away the force vanishes

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so how does this relate to the credit card well the terminal's emitted magnetic field is like the neodymium magnet

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and then inside the card are loops of metal similar to the block of steel

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however instead of physically moving the card as we did with

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the steel the card uses a method called load modulation inside

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the cards integrated circuit we find a transistor controlling a resistor

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which is connected to the coil

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when the transistor is on the resistor is connected causing the coil to draw more energy from the field

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and this is called a high load however

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when the transistor is off the resistor and

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that extra path is removed resulting in the card drawing less energy from the field

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and this is called a low load

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when the terminal is in listening mode it carefully monitors the magnetic field it generates

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and the power drawn by the card as a result the terminal can tell

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when the card connects or disconnects the resistor and changes between high and low loads.

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This load modulation is how the card sends data to the terminal.

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But what scheme does the card use?

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Well, the card also divides the 13 .56 MHz signal up into time slots,

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each 128 waves long and similar to before.

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An obvious scheme would be that a high load is a 1 and a low load is a 0.

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But the problem is that a string of 1s or 0s results in nothing changing.

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Instead, the card uses a method called Manchester encoding, ensuring that there is always a change in the card's load at the middle of each time slot.

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Sending a binary 1 corresponds to a transition from a low load to a high load,

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whereas sending a binary 0 corresponds to a transition from a high load to a low load.

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The resulting binary message looks like this.

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Note that there can also be transitions outside the middle section of the time slot, but only the middle region is what matters for sending data.

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Now that we've covered how the terminal and card physically communicate,

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let's explore the messages they exchange and the cryptography they use to keep your card and bank details safe.

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But first… To make this video, we bought this payment terminal, took it apart, photographed every component,

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and then spent about 120 hours meticulously modeling every single component in Blender.

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Hi, I'm Mike and I work with a team of real artists

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and animators to create this video and all the others on branch education.

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When we add research, script writing, modeling everything else in the video and animation, a 30 -minute video like this one took took about 1400 hours.

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It's a ton of work, but that's what it takes to create a technically accurate deep dive video on the tech you use on a daily basis.

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We will never make AI slop, and we will keep doing the research, script writing, teardowns, 3D modeling, and animating with human artists and animators.

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The dream of branch education is to create an in -depth, accurate, entirely free engineering curriculum on a wide range of topics,

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and change the way engineering is taught and learned.

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If you want to support us, the best thing you can do is like this video, subscribe, comment below, and share it with others.

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YouTube uses those signals to sort AI slob from authentic content so thank you let's get back to the video Okay,

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so here's the full transaction process that takes place between the card, terminal, and bank.

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And as you can see, it's dense with protocols and acronyms, but once we strip away the jargon, it boils down to four key steps.

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Step one, the terminal repeatedly sends out a ping, and your credit card replies with its unique ID.

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Step two, we're now about 50 milliseconds in, and the card and terminal decide which applications and protocols they'll use moving forward.

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Each card issuer, such as Visa,

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MasterCard or Amex, follows a slightly different procedure and may have rules about when the terminal should require a signature or PIN.

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But in general, this step is mostly handshakes and protocol negotiations.

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So we'll move on to step three.

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We're about a tenth of a second in

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and the terminal encounters a critical security snag how does it

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know it's talking to an authentic bank issued credit card

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and not an imposter like a fraudulent card

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or a hacking device well before sending you the card in

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the mail the bank programmed the card's chip with a secret

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numeric key that's unique to your card this secret key proves

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that the card is authentic however it can't be wirelessly transmitted to the terminal

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because if hackers were listening they could copy

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and reuse it to make fraudulent purchases

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so the question is how does the card prove

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that it knows the secret key without ever transmitting it

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or giving it away the solution is a method called asymmetric encryption

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or what's also known as public key cryptography let's explore how

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it works to start in addition to the secret key stored in the card

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which is referred to as the private key asymmetric encryption also uses a second numeric key known as the public key.

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The public key is unique to your card, forming a pair with its private key, and can be shared freely without any risk of revealing the value of the private key.

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These two keys are mathematically related so that if you encrypt a piece of data using the private key,

180

that encryption can only be undone using the public key and vice versa.

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Essentially, whatever one key encrypts, the other can decrypt.

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So, how are these two keys used?

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Well, first, the terminal generates a random number and sends it to the card.

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The card encrypts that random number using its private key and sends the resulting encrypted value back to the terminal.

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Finally, the terminal uses the unique public key to decrypt the message

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and if the result is the same random number it originally generated the terminal knows

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that the corresponding private or secret key is stored in the card proving its authenticity

188

which is great however this introduces a new problem the terminal

189

can't feasibly store the unique public key for every one of the billions of credit cards in the world

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so how does it get your card's public key well your

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card actually sends its own unique public key to the terminal itself.

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This might seem a bit circular.

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Like, if your card provides its own public key, how does the terminal know that public key isn't also a forgery?

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The solution is to use another layer of asymmetric cryptography, which is confusing, but stay with us.

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When your card was manufactured and programmed, the bank sent your card's unique public key to the payment network, such as Visa.

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Visa has its own set of private and public keys, and therefore they took your card's unique public key encrypted it using visa's own private key

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and then programmed it into the chip meanwhile visa's public key is shared with every payment terminal

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that supports visa

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so to amend our previous sequence during a transaction the card first sends its unique public key

200

which was encrypted using visa's private key to the terminal next

201

the terminal uses visa's public key to decrypt the card's public key

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if the decryption works the terminal knows

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that the public key stored in the card was vouched for

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or digitally signed by visa finally the terminal can use the

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card's decrypted public key with the steps we discussed earlier to check

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if the card also contains the corresponding private key proving

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that the card is authentic the takeaway behind asymmetric cryptography is

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that neither visa's private key nor your card's private key ever leads the security of visa's servers

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or the emv chip in your credit card greatly reducing the risk

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that your card will be cloned before we move on there

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are two quick notes first we discuss the general steps behind

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using asymmetric encryption for authentication however the math behind these cryptographic algorithms is considerably more complex.

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In general, they utilize trapdoor functions, which are math problems that are easy to solve in one direction.

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but nearly impossible in reverse.

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RSA, for example, uses the fact that it's easy to multiply two prime numbers, each around 250 digits long.

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However, if you're only given the resultant 500 -digit long number, figuring out which two prime numbers it's made of is simply impossible by today's computers.

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That said, there are other exploits of wireless payments, but in general, these cards are far more secure than simply having your bank

218

account printed on a magnetic stripe the second note is

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that cryptography is a deep deep technical rabbit hole

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and there are additional details such as hashing algorithms more layers of asymmetric cryptography digital signatures elliptic curves

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and so much more our explanation simplified a lot of things

222

but let us know in the comments below

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if you're interested in a video dedicated to cryptography so now

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that the terminal knows the card is authentic let's move on to step four

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which starts at around a quarter of a second in

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and uses up the remainder of our one second timer in

227

this step the terminal sends the transaction details such as the

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purchase amount your name credit card account number date the store's location

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and more across the internet

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and payment network to the bank to ask for approval however

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transmitting data across the internet introduces another major security problem how does the card ensure

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that hackers haven't tampered with the transaction details on their way to the bank for example a hacked

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or compromised terminal could display a 17 purchase on the screen while secretly requesting $1 ,000 from the bank.

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Well, to protect the transaction details from tampering, the card uses symmetric cryptography, which uses yet another secret key unique to your card,

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but entirely separate from the asymmetric keys from earlier.

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There are only two copies of this symmetric key.

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One is stored inside the credit card, and the other is on the bank's servers.

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So then, how is this symmetric key used?

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Well, first, the terminal sends the transaction details over to the card, which encrypts them using its copy of the symmetric key,

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producing a unique block of data called an Authorization Request Cryptogram, or ARQC, which it sends back to the terminal.

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The terminal then sends the ARQC, along with an unencrypted copy of the transaction details, over the Internet and payment network to the bank.

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The bank takes the unencrypted transaction details and uses its own copy of the symmetric key to encrypt it, producing its own version of the ARQC.

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If the ARQC generated by the bank matches the one created by your card,

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the bank knows it must be seeing the same transaction details your card saw unencrypted, thereby proving that none of the transaction details were modified in transit.

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Next, the bank processes the transaction, and if you're not over your balance and everything else looks good,

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the bank sends an approval message back to the terminal through the Internet.

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Finally, once the message arrives, the terminal beeps, the screen displays approved, and the transaction is complete.

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It's worth mentioning that both asymmetric and symmetric cryptography involve rather computationally intensive algorithms

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that can only be completed using an advanced processor.

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Well, the EMV chip is exactly that.

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And rather interestingly, it's computationally on par with an entire PlayStation 1 video game console.

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Here's a brief description of the various sections of the chip's integrated circuit.

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Let's move on to discussing tap -to -pay in smartphones and other RFID applications.

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But first, if you've made it this far, you probably know the future will be filled with even more complicated engineering and technology,

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and we need to prepare the next generation for what's to come.

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that we've covered tap to pay in credit cards let's quickly

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discuss how smartphones do tap to pay inside your smartphone we find a dedicated nfc antenna coil

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and microprocessor these look a bit different but they're designed to emulate the coils

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and chips in a credit card by drawing energy from a payment terminal's magnetic field

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and sending data back via load modulation the transaction process is rather similar

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but with some extra layers of security additionally your phone can

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act as a payment terminal generating its own magnetic field to power and communicate with credit cards

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the final topic we'll discuss is the broader family of rfid or radio frequency identification

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of which the NFC standard and protocols are a part.

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RFID includes hundreds of other technologies and applications, so we'll explore a few that you're most likely familiar with.

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First, ID badges and hotel key cards typically operate on the same 13 .56 MHz frequency as NFC credit cards,

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have similar antenna designs, receive power through inductive coupling, and communicate using load modulation.

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What makes these cards different is that when you tap one to a door access pad, instead of a complex back -and -forth conversation,

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the card simply transmits its unique ID, which the security system uses to check your access privileges or room number.

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This simpler protocol makes these ID badges and hotel cards easier to program, but also makes them more susceptible to hacking or cloning.

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That said, some cards use more security protocols and encryption than others.

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Moving on, the security tags used in library books function similarly to ID badges,

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except that the exit gate and antenna coil are vertical and cover a larger volume.

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When the book passes through the gate, the tag transmits its unique ID,

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and the gate sets off an alarm if the book hasn't been marked as checked out in the library servers.

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The tags used in retail security, on the other hand, typically don't contain a microchip or processor at all.

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Instead, they're just an RF coil and a capacitor, tuned to resonate at 8 .2 MHz.

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The gate at the exit emits an oscillating magnetic field at this frequency, and when an active security tag passes through, the tag's LC circuit resonates,

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reflecting energy back to the gate, which detects it and triggers the alarm.

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To disable a tag at checkout, a deactivator in the counter emits a strong 8 .2 MHz field that damages the security tag's capacitor,

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preventing it from resonating and responding to the gate's frequency.

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The last type of RFID device we'll discuss are the ones that operate over long distances,

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such as tags used for container or warehouse inventory tracking, or for drive -thru automatic tollbooths.

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These distances are outside the near -field inductive coupling range, so the RFID readers and tags operate on different principles.

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Instead of an antenna coil, readers or sensors use a dipole antenna and operate at 860 to 960 megahertz,

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producing self -propagating radio waves.

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When these waves hit a tag, the tag harvests energy from them using its own dipole antenna

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and then sends messages back to the reader by changing how much of the incoming wave it reflects, which is a technique called backscatter modulation.

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Functionally, you can picture the reader

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or sensor shining a bright light to power the tag

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and the tag replying by switching between a mirror and a non -reflective object.

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That said, each of these technologies has dozens of variations.

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There are far more frequencies and nuances than we can fit here, but you've got the fundamentals now, so we'll leave it there.

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We're thankful to all our Patreon and YouTube membership sponsors for supporting our videos.

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If you want to financially support our work, You can find the links in the description below.

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This is Branch Education, and we create 3D animations that dive deeply into the technology that drives our modern world.

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Watch another Branch video by clicking one of these cards, or click here to subscribe.

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Thanks for watching to the end.

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