RFID Guardian Security Protocols

From RFID Guardian

RFID Guardian and RFID readers talk to each other over broadcast radio waves which can be heard by any nearby receivers. Therefore, it is important they use cryptographic services that prevent others from reading their messages. Another benefit of cryptography is that it can provide authentication between each participants. Generally speaking, cryptographic techniques are divided into two categories: Asymmetric Key Protocols and Symmetric Key Protocols. These techniques are reviewed below.here and here

The page disussing Conceptual User Interface of RFID Guardian could be found Here

Asymmetric vs Symmetric Key Protocols

Asymmetric Key Protocols

The advantage of using certificate based authentication protocols is that the public keys and certificates can be sent over insecure medium, even if adversaries may be listening. The reason is that certificates are affixed with Digital Signature by the CA. Therefore, it is generally impossible for an attacker to create bogus certificates.

Another advantage of using asymmetric key is that the two participants do not have to share any secret before they communicate. This feature is especially useful when the Guardian user is concerning about privacy problems. Consider this scenario. A supermarket places an RFID reader at its check out desk. Since the market cannot predict who its customers will be, asymmetric key protocols naturally becomes an option for them. What the shop should do is to publish their public key (possibly on its website), and ask its customers to install that key.

The disadvantage of using asymmetric key protocols, however, is that you have to know other's public keys in advance. Of course, with the current implementation based on SSL, the two communication participants can exchange their public keys at the beginning of communication (handshake phase). But this introduces another problem --- what if one party does not trust this public key or the certificate that signs that key? Otherwise, the two parties have to use either plain text or one-time symmetric key.

Symmetric Key Protocols

The advantage of strong symmetric key authentication is that all the nounces are fresh and it is difficult for eavesdroppers to perform dictionary attacks.

The disadvantage is that there are always extra requirements: if the protocol is N-S, there has to be an active on-line server for authentication and key generation; if the protocol is EKE, there has to be a shared secret beforehand.

Another disadvantage is that there is a potential threat against the Guardian holder that the reader holder could deliberately lend this shared symmetric key to hostiles. Even though Guardian's context feature could help its user to a certain extent, it could not solve all problems. For example, when the Guardian user enters a supermarket, her Guardian could already be switched to a context that is ready to exchange messages with the reader; Then, a hostile with the symmetric key from the supermarket can now make a conversation to the Guardian. Therefore, for security reasons, I recommend that RFID Guardian could give user a warning, i.e. a beep, so that the user knows that the Guardian is going to change information with some readers. It is better that the user can see and recognize that RFID reader with her own eyes, so that she knows for sure which company is querying her tags. After that, RFID Guardian may ask the user for permission for communication, i.e. click a ``OK button or enter a password on the pin pad. Of course, if the reader holder and Guardian holder knows each other and trusts each other, they can change symmetric keys even ahead of time.

Some people may argue that by using symmetric key protocols, there is also a threat that the reader holder will record Guardian user's habit. For example, there is already some reports saying that some inns are eavesdropping their customers on their favourite wines by RFID technology. But this also holds true for asymmetric key protocols --- by sending the certificate to the Guardian, asymmetric-key-readers could also authenticate themselves to the Guardian. And that is the reason why researchers introduce context feature into RFID Guardian.

User Case Discussion

Contactless Smart Card

Most contactless smart cards are produced by big banks, and such banks usually have many branches therefore many card readers distributed in different areas. This means asymmetric key protocols are their first choice because the Guardian holder can easily download the public key from its webpages. Of course the Guardian and the card reader can still use their public key to exchange a short-term symmetric key, and use that symmetric key for further communication.

If they indeed agree on asymmetric key protocols, then the authentication process will be based on certificates. As explained earlier, in order to simply things, it is best for the bank to choose a commercial CA. After all, such a successful and well-known bank should not care about some charges for certificate signing!

Of course, if the reader-holding organization instead chooses to use a symmetric key, or the certificate verification fails (i.e. the certificate expires), then the Guardian and the reader have to use short-term secret keys.

Supermarket

As discussed before, the cryptographic protocols for shop readers could be either asymmetric or symmetric. In case of asymmetric key protocols, a two or three layered certificate chain signed by a well-known CA would be convenient for the Guardian holder. But if the shop instead decides to set up a local CA then if its customers are Guardian holders, they will have to perform extra step of installing that certificate themselves. If the customers can be sure that the supermarket will not leak symmetric key information to others, then they could also choose to use EKE symmetric key protocols.

Library

People usually prefer to go to the nearby libraries, so the customers of a library is almost fixed. Nobody will fly across the Pacific Ocean if he could find exactly the same book in a library down the street --- this is a matter of convenience. Therefore, the libraries could possibly start with a local CA, and asks its customer to accept its root certificate when the customer registers at the library.

E-passport

Passports are held by the total population, and the country will have many airports and police stations. So perhaps symmetric key protocols are not fit for this scenario, and instead asymmetric key protocols and certificate should be used for authentication. It is obvious that a centralized CA could help in this scenario. But some citizens or governments may not like their identification document information to be accessed by a business, so they may prefer for the national security department (or whatever government department) to set up a local CA.

Car Keyless Entry System

Although the RFID Reader in this scenario is owned by the user (actually the car, not the reader), it is impossible for the user to operate the reader directly, because the reader is usually firmly attached inside the car. So a symmetric key protocol is probably more appropriate in this user case. The reason is that the reader is not publicly reachable, and there is no need for the automobile company to publish a public key for a particular reader, then asks all Guardians to communicate with that reader. So it may be more convenient for the automobile company to generate a secrete key for each car, and pass that key to the Guardian holder when the Guardian holder becomes the owner of the car.

Home Appliance

By placing RFID readers at home, one can

1) Use intelligent household appliance inside the house

2)Have separate readers to do triggering. For example, by placing an RFID reader at the front door, one's Guardian could be triggered to switch to another context when the user leaves home.

Since the Guardian holder controls both the Guardian and the reader, there is no need to implement a PKI and use a certificate for authentication. But a symmetric key could be used with long-term keys for both encryption and authentication.

Animal Identification

Most animal identification systems have their shelters distributed across the country, or even across the continent. So it is easy for them to implement asymmetric key protocols and use certificate for authentication. Perhaps they can construct a certificate hierarchy by geographic locations. For example, the top layer represents the whole country, while the second layer represents provinces or states, then the third layer represents cities or towns. This means a centralized CA is best suited for this scenario.

Military and Commercial Supply Chains

The size of a commercial supply chain varies, so it cannot simply say whether asymmetric key or symmetric key would fit all cases. For a small chain (for example only three suppliers), a symmetric key could be used. They could exchange the secret key as soon as the managers sign the contract. On the other hand if the chain is large, they could perhaps better choose a asymmetric key protocol. For the certificate-based authentication process, it may be better for the customer (i.e. Wal Mart) to set up a local CA. The reason is that all the suppliers are stake holders within this chain and their profits are restricted by the business contract.

The military supply chains always require a higher security level, so convenience could be sacrificed. For example, a new symmetric key could be generated for each stop, which could then be securely stored in all Guardians and readers through the internal military network. In this way, any other readers that do not know the correct symmetric key can be detected.