Cracking The Cash Codes

Loaded with electronic cash that has been protected by an encryption scheme, a smart card represents a convenient, versatile medium for business transactions. Roughly the size of a standard credit card, it incorporates circuitry for processing information and keeping records.

That microcircuitry also makes it vulnerable to attack. Cryptographers have now identified techniques for breaking the security system built into a smart card. They cracked the codes by monitoring power consumption as the circuitry performed its cryptographic operations.

"We have implemented these attacks against a large number of smart cards and, at this point, do not believe that any cryptographic smart cards on the market are immune to these analysis techniques," says Paul Kocher of the consulting firm Cryptography Research in San Francisco.

Last week, Kocher and coworkers Joshua Jaffe and Benjamin Jun posted their report revealing the security flaw. It can be found on the World Wide Web at

"[The flaw] is indeed a serious security threat to many existing systems," says Ross Anderson of the University of Cambridge Computer Laboratory in England. "It allows relatively low-budget attackers to get at key material that previously required a moderately well-equipped lab."

The integrated circuits on smart cards consist of vast arrays of transistors, which act as voltage-controlled switches. Different microprocessor instructions initiate characteristic switching patterns. The resulting motion of electric charge consumes power and generates electromagnetic radiation, which can be detected outside the card.

Researchers have already demonstrated that it is possible to accumulate enough data to deduce secret keys - strings of 1s and 0s - required to decrypt confidential information stored on smart cards. Using sophisticated tools, they've measured the duration of cryptographic operations.

In the new threat, an attacker can use less expensive equipment to monitor a smart card's electronic responses. Fluctuations in power consumption correspond to different stages in a cryptographic process. By magnifying the signal, it is possible to detect individual microprocessor instructions and distinguish between various arithmetic operations.

A more sophisticated analysis of these data relies on the application of statistical and error-correction techniques to extract information useful for deducing secret keys. Once the secret key is found, a criminal could make a copy of the smart card and obtain unauthorized access to someone else's account or, in some systems, automatically refill the card with cash.

Such threats, however, require that criminals have special equipment attached to or physically near the card. Smart cards are safe when stored in a wallet or purse, Kocher says.

Stolen or lost smart cards are another matter, because they can be connected to a power sensor nd computer.

One approach to increased security is to recognize a smart card's vulnerability. An electronic cash system used by Visa International, for example, checks for unusual account activity. When that system was designed, Anderson says, "We did not know as much about breaking into smart cards as we do now, but we suspected that it would be done." Other companies have also started to adopt countermeasures.

Written by I. Peterson