Holy Hash!

What are NFC cards and how are they protected?

Ever since I posted an initial article “Hack NFC Door Locks” I see a steady stream of people that come with queries like “what’s the protection of an NFC card” and “how do you hack a protected NFC card”. Obviously, there is something out there interesting enough for people to begin inquiring.

What is an “NFC card”? As opposed to an “NFC device”, an NFC card is simply a contactless smart card. The NFC protocol allows for a great flexibility in choosing what you may name an NFC card and nearly anything in the vicinity and proximity card world can be termed an NFC card.

Most of the time though you will be dealing with the good old Type A and Type B cards from the ISO 14443 standard. Unless you are in Asia and then the chances are high you will be facing a Sony FeliCa card. There is nothing NFC about any of them except the new name. They are all good old contactless smart cards.

Now, to the question that actually interests most of the people seeking enlightenment, the protection of those smart cards can vary. What kind of protection is used depends more on the system that specified what kind of a card will be used there. So if we are talking about door locks we are likely to see the cheapest MiFare cards that can actually be broken comparatively easily. When we are in some banking applications, we are likely to see high-end smart cards with seriously mean security features.

Since NFC cards are “just” smart cards, you must be looking for the information on how to deal with the smart cards and all of that will be applicable to the NFC cards. The low end is fairly simple, often the system does not use encryption, the cards may be read out and copied with very little effort. In more serious systems the cards usually do not let themselves to investigation erasing the content at the least suspicion of a break-in.

The protection mechanisms may include (and this is not an exhaustive list, just off the top of my head):

• Constant time execution of all routines
• Checks of the execution state at regular intervals and at critical operation beginning and end
• Encryption of all of the memory content or sensitive areas like key storage
• Encryption of input and output, sometimes double encryption
• No debug and error output, just lock up in case of an error
• Sensors for temperature, light, voltage, current
• Protective mesh over and in between layers of the chip with cut sensors
• Stabilizers of current consumption and noise generators
• Scrambled and encrypted buses and memory content
• Parallel execution to compare results against tampering
• Randomized circuit layout

Basically, there are two things there: (1) protection of the hardware against tampering and side channel analysis and (2) protection of the software against induced faults and side channel analysis. Typically, the designers work hard to make sure you have to defeat both to get any meaningful results. So to get a go at the smart card security, you are better off to search for a security lab that does smart card security evaluations and ask them to work for you.

I always assumed there are tons of literature on the subject although right now a quick search on Amazon proved me wrong, there is only a handful of books. Maybe I should write more on smart card security?..

Philosophy of door locks

When working on security, there is something extremely important to keep in mind at all times. We are not trying to make systems impenetrable. We are trying to make it real, real hard for the attacker, that’s all.

If an attacker has physical access to your system, you lost. All measures, passwords, firewalls, everything is there to deter an attacker that is attacking remotely. But the only thing that actually stands between your system and a determined attacker is your door lock. Never thought of that, did you? The security of your computer at home is only as good as your door lock.

Yes, there are smart cards that are physically secure computers. But their application is limited and most if the time we have to deal with systems that we protect in the “virtual world” while in the real world they are basically defenseless. So we make it harder for the attackers with door locks, security guards and CCTV cameras.

Again, we are just making it harder, not impossible. Impossible would be impossible, not to mention prohibitively expensive. Given that an attack is always possible and there are many venues of attack, the attacker will always tend to choose a path that is most economical – the cheapest way to break into your system.

My task as I see it is to convince you to use such security measures that it becomes cheaper for the attacker to break into your house than to attack your computer through the software. Once we are at that point, you start looking into the well-understood world of physical security and my task is done. But we are far from there.