Previous Table of Contents Next


Physical Security

Central to the overall security architecture is the concept of physical security. The smart card figures very prominently in this. From the cardholder’s standpoint, being able to have the smart card computer platform in physical possession is a large step toward overall security. In this case, attacks against the security of the overall system have to be made against the system components while in operation or through examination of information gained while the system was in operation. This means, for example, that attacking encryption algorithms used by the smart card must typically proceed from captured cyphertext, not from active examination of the card while in use.

Conversely, the overall security architecture of the smart card-enabled system must be such that if a card is no longer in the cardholder’s possession, the damage to the system through a security attack can be limited through the knowledge that the card is no longer in the cardholder’s possession. Further, the vulnerability to the entire system must be minimized if the information related to a single cardholder is compromised.

Processor and Memory Architecture

An adjunct to physical security, at least in the case of the smart card, is the enhanced security architecture of the microprocessor-based computer installed in the card and the tamper-resistant packaging of the card itself. Chapter 2, “Physical Characteristics of Smart Cards,” examines the architecture of the smart card’s computer. Packaging the processor, memory, and I/O support in a single integrated circuit chip enhances the security of the entire configuration. It is difficult, though certainly not impossible, to connect electrical probes to lines internal to an integrated circuit chip. The equipment to insert such probes is reasonably expensive. Consequently, for an attacker to extract information directly from a chip requires physical possession of the card, costly equipment, and detailed knowledge of both the hardware architecture of the chip and the software loaded onto the chip.

Tamper-Resistant Packaging

The packaging of the integrated circuit chip into a smart card is typically viewed as being tamper-resistant as well as tamper-apparent. Tamper-resistant refers to the characteristic that, given physical possession of a smart card, it’s a nontrivial task to get to the chip and even more nontrivial to extract information from the chip. Tamper-apparent, or tamper-evident, refers to the characteristic that to do so will typically leave an obvious trail that the card has been tampered with. Thus, it is difficult to learn the secrets contained within a smart card without the cardholder knowing that the card has been compromised.

Authentication

The field of cryptography is dedicated to the development of mechanisms through which information can be shared in a secure fashion. A variety of mechanisms have thus been developed through which the security concepts discussed earlier can actually be realized. Several different mechanisms have been developed to support the authentication of identity among widely diverse participants in a transaction. A few of the more prevalent of these mechanisms are described in the following sections.

Symmetric Key Authentication

Most, if not all, authentication mechanisms involve the sharing of a secret among all the participants in a transaction. Two such mechanisms involved distinct forms for encryption and decryption of information; the first makes use of a symmetric key encryption algorithm and the second a public-key encryption algorithm. Both of these mechanisms involve a shared secret; however, the manner in which the secret is shared in each case makes the two mechanisms preferable in different situations. Specifically, symmetric key algorithms are most useful in providing bulk encryption of information since they are less processor intensive than public-key algorithms.

Symmetric encryption algorithms make use of a single key for both the encryption and the decryption of information. This is illustrated in Figure 9.1.


Figure 9.1.  Symmetric key encryption.

In a symmetric key approach, the same key is fed into the encryption algorithm to both encrypt information and decrypt information. Plain text information is passed into the encryption process, where it is modified through the application of the key value. The resulting cyphertext contains all the information present in the original plain text; however, due to the manipulation of the encryption algorithm, the information is in a form not understandable by a reader that does not possess the key. When the cyphertext is passed back through the encryption algorithm with the same key applied (as was used for the encryption process), the plain text is recovered.

It is apparent that this approach can be used to keep secret the plain text information from anyone who does not have the required key. The approach can also be used, however, to allow each side of a pair-wise transaction to confirm that the other side holds the same key and thereby authenticate a known identity.

This symmetric key identity authentication for a smart card environment is illustrated in Figure 9.2.


Figure 9.2.  Authentication via shared secret.

In the case shown in Figure 9.2, the application spans both the terminal-side environment and the card-side environment. In most common instances today, the application is created by the card issuer, who installs the shared secret (the key) in both environments. It should also be pointed out that the case shown in Figure 9.2 could be extended to make use of two distinct authentication operations, each using a different key. This approach would be quite useful if, for example, each of many different cards with different cardholders simply needed to authenticate a single identity for the terminal-side application; in this scenario, the terminal-side application would need to authenticate the unique identity of each individual card.


Previous Table of Contents Next