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Mobile IoT: Article

Playing the Smart Card

Playing the Smart Card

Cryptography is a wonderful thing. Long keys and well-designed algorithms mean that even the most determined government is unlikely to be able to break your encrypted messages. However, every encryption system has one weak point: Where and how do you store your keys? Most encryption software will store your keys on your hard disk (if your device has one) or somewhere safe in memory, carefully encrypted so no one can read it. But the problem with this approach is that it denotes trust in the operating system, and secure applications frequently have to live in the most hostile of environments.

Take the example of a desktop computer running Microsoft Windows; environments don't get much more hostile than that. Imagine a virus. It scans your system looking for an appropriate file (say, secring.skr), and waits for any other application to access that file. As soon as it notices such access it scans memory in the hope of finding your decrypted private keys!

Of course, there's no reason to be quite so clever. The virus can just wait for you to type in your pass phrase and catch the key presses, then collect both pass phrase and file and send them off to its grateful author. This isn't fiction; such virus code already exists, but where can we put our keys if we don't trust the operating system?

Even more extreme is the situation where you want to issue a key to someone, but don't trust him or her to keep it secret. A good example of this is pay TV, where you want your subscribers to have access to encoded content, but don't want them to actually know the key being used to decrypt the signal. Otherwise they'll tell their mates and your revenue stream starts falling apart.

The solution to both these problems is to keep the keys, and anything else important, on another very small computer. Your main machine can pass things to be decrypted to the other computer, which can use the keys and pass back the decrypted data, so the keys are never vulnerable. Such a device can be embedded in plastic, and is called a Smart Card.

Smart Cards are descended from European phone cards, which, thanks to our monopoly phone companies, were mechanical things to be slotted into a phone booth to make calls. The most basic ones actually cut grooves into the side of the card to indicate how much credit had been used (only to have them built up again with Crazy Glue!). As they got more advanced, the intelligence inside the plastic card started to attract other industries, with pay TV being an early adopter, and every European credit card company quickly following. Now Smart Cards are everywhere, providing unprecedented security from both physical and logical attack.

What's in a Card?
If you dismantle a Smart Card, such as the American Express Blue Card, you'll find something like the cross section shown in Figure 1, with chips glued to the back of the contacts and sitting in a depression cut in the card. You'll also annoy American Express quite a lot, so remember to use someone else's card. Don't be misled by the diminutive size of the chips, there's a whole computer in there, with an 8-bit processor, some ROM and RAM, and a dedicated cryptographic chip, all coated in resin to make it hard to see what's there. The basic internal architecture is shown in Figure 2.

At its most basic a Smart Card records information in its flash memory, though the amount of space is normally very limited (up to 16KB), but more complex cards can do anything a microcomputer can do. Communication with the outside world is via the contacts on the outside of the card (see Figure 3), which include a clock signal and power for the computer on the card. Serial communication (at a maximum of 9600 baud) might seem basic, but it's more than enough for cryptographic functions. You just pass in an encrypted message and it returns the decrypted version, without the keys ever leaving the safe environment of the card.

To ensure interoperability between cards and readers there's an international standard known as ISO7816, which specifies not only the physical size of the card and contacts, but also a basic set of commands for retrieving and storing information in the flash memory and performing cryptographic functions. ISO7816 also makes demands on the robustness of the cards, many of which will have to survive in the back pocket of a pair of jeans for years. Flexing and twisting the card must not break it (to a point), and contacts must be conductive enough to work with a layer of grime on them.

The Wireless Connection
Putting a card into a reader doesn't require much effort, but sometimes it's too much. Many applications want Smart Card functionality without having to be removed from the wallet or purse, so we come to the wireless Smart Card. Powered through an induction loop that also serves as an aerial (see Figure 4), wireless Smart Cards have a range of about 3 inches - not a lot, but enough for passengers boarding a bus or subway train. The sight of passengers on the Hong Kong subway wiggling as they pass through gates, to get their back pockets within 3 inches of the reader, will remain with me for years (at least until the London Underground adopts the same system). This is one wireless link where security isn't a problem. As the communication is used only to authenticate encrypted packets, the open nature of radio communications isn't a problem (not to mention any attacker is going to have to be snuggled up real close!).

In addition to being in millions of credit cards, Smart Cards can also be found in every GSM mobile telephone. The Subscriber Identity Module (or SIM) provides the cryptographic backbone for the GSM telephone network, as well as some additional benefits. SIMs are smaller than Smart Cards (see Figure 5), but conform to much the same specifications as well as being constructed the same way.

Each SIM relates to a specific phone number, and GSM users can switch phone handsets simply by moving their SIM into another device (it's not rare, on finding that your mobile battery is dead, to borrow someone else's and just pop your SIM in). Modern SIMs also hold your phone book, and Internet Service Provider details if you use your mobile for data access, allowing all these to be transferred to a new phone easily and simply. Not everything is stored on the SIM. WAP bookmarks and customized ringtones (very fashionable here) are lost when you change handsets, but the SIM is a secure computing environment and not limited to making phone calls.

Applications developed to run on a SIM include services such as home banking, shopping, and share dealing, all of which have already been ported to mobile telephones, predating and providing a better interface than WAP technology. SIM applications can send and receive SMS messages that can be encrypted by the SIM for secure service access, and are compatible with every GSM mobile handset. In the UK at least one mobile network has given up providing handsets, just selling replacement SIM chips to users who already own a handset (the cost of which was probably subsidized by another network!).

Pay TV services have also been quick to see the value in Smart Cards. By providing customers with Smart Cards for decoding television, they can control which subscribers have access to which channels, all in a very secure manner. Early systems relied on a single key embedded in every card, relying on the defenses in the card to protect the key from attack. This approach was flawed and, over the years, keys have been compromised, ultimately by dismantling the card and looking at the positions of the memory gates under an electron microscope.

While this is clearly beyond the reach of most consumers, the problem was that once the key is compromised it's relatively easy to reproduce forged cards, with associated financial rewards. Modern systems are more complex in that every card has its own public/private key pair, and the video encryption key is sent to each subscriber individually encoded with their public key. Should a card become compromised, the network operator can simply turn off that subscription as soon as they become aware that forged cards are in circulation.

Credit card companies can also see the value in proper encryption for their transactions, and the vast majority of European credit cards now sport the familiar pattern of contacts. Credit card fraud is, of course, massive and the reliance on a signature has shown itself to be ineffective (though I was shocked to see how ineffective it is in the U.S. on my last visit). By embedding a chip into the card, it becomes almost impossible to forge (depending on the technology used) and offers much more functionality.

For example, the magnetic strip on a normal credit card can hold 66 bytes of information, while a Smart Card can easily hold a photograph (though the amount of security provided by the inclusion of a photograph is very controversial) or a record of recent transactions. But it's in online transactions where the Smart Card can really revolutionize security. By providing a link from the card to the vendor, rather than from the desktop PC to the vendor, security can be enhanced massively, especially if another Smart Card is used at the other end of the transaction. But such innovations will have to wait until every PC has a Smart Card reader, which is going to take a while.

Applying the Card
Developing applications to run on a Smart Card used to be an arcane affair, with applications developed in machine code and downloaded onto the card through the use of special keys, but increases in processing power and available memory have led to a plethora of development environments. Java Card offers a very basic subset of the Java programming language that can run on a Smart Card.

While there's no room for a real Java Virtual Machine on the card, companies provide cross compilers that will convert your Java code into card-specific code before installing. MULTOS is another standard operating system for Smart Cards, allowing C development in a very comfortable environment. Most surprising of all is Microsoft's Windows for Smart Cards (Windows not being known for its small size and fast execution speed), which generates applications from a familiar Windows interface. However, most Smart Card applications are actually very simple, with the work being done by the surrounding system rather than on the card itself (which is restricted to cryptographic functions).

Increased storage and functionality is endemic in the IT industry, though in the Smart Card industry it's hampered by the constant need for security, with chips embedded in resin and surrounded by detectors (to wipe the content should the resin be removed). There's also a limit to how much data you can usefully store on a device whose only communications with the outside world is at 9,600 bits per second. Smart Cards are relatively expensive, ranging in cost from 10¢ to $6 for the most advanced cryptographic cards, while magnetic strip cards can still be produced for a few cents each.

Chips are going to replace the magnetic strip on the back of our credit cards. The additional security and features make a convincing case, and if fraud can be reduced by a small percentage it will more than cover the cost of the cards and readers. Credit card companies are offering free readers to encourage their use online, and everyone will benefit from decreased fraud through reduced interest rates (except the forgers, but I'm sure they'll find work elsewhere). But the Smart Card provides only the middle of a secure application. It's the surrounding system that's complex and potentially vulnerable. Worldwide adoption of Smart Cards is inevitable, and as they get smarter, the range of applications will increase to ideas not yet thought of. I was recently beaten at chess by a Smart Card, and I'm not sure I like the idea that my credit card is smarter than I am.


More Stories By Bill Ray

Bill Ray, former editor-in-chief (and continuing distinguished contributor to) Wireless Business & Technology magazine, has been developing wireless applications for over 20 ears on just about every platform available. Heavily involved in Java since its release, he developed some of the first cryptography applications for Java and was a founder of JCP Computer Services, a company later sold to Sun Microsystems. At Swisscom he was responsible for the first Java-capable DTV set-top box, and currently holds the position of head of Enabling Software at 02, a UK network operator.

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