These attacks use take advantage of the normal use of the card and monitor current, voltage changes, or even changes in emmissions to determine a secret key. While a power analysis attack will not be able to recover an entire 56 bit key, it can be used to guess enough key bits to make a brute force attack feasible. Another form of non-invasive attack, the glitch attack can be used to trick the smartcard into skipping a portion of an algorithm, and or even missing an instruction branch, simply by putting a little hiccup into the normally steady clock input. Smartcard manufacturers have fought back by making power analysis more difficult, introducing layers of shielding over the circuitry, adding cryptographic algorithms into bus decoding for memory accesses, and building the circuits so that they do not sit on a single plane, but lie at different depths. With enough effort, an attacker can glean the information protected by the smartcard.
Most attackers focus on smartcards holding shared secrets, such as symmetric keys.
Breaking a single smartcard provides a single key that has widespread use. But the most common use of smartcards for network security will not involve a single secret, but individual, password-like secrets, public key certificates, and assymmetric keys. In these cases, the amount of effort to recover a single key will generally be much greater than the value of that key. The University of Michigan uses Kerberos V for authenticating students into computer systems. Kerberos V permits students to login once, and then use many network facilities, and has a greater level of security than the single-login facility used in NT 4.