The power consumption of any hardware circuit (cryptographic ASICs or processors running
cryptographic software) is a function of the switching activity at the wires inside it.
Since the switching activity (and hence, power consumption) is data dependent, it is not
surprising that the key used in a cryptographic algorithm can be inferred from the power
consumption statistics gathered over a wide range of input data. These attacks are called
power analysis attacks and have been shown to be very effective in breaking embedded
systems such as smartcards. Power analysis attacks are categorized into two main classes:
Simple Power Analysis (SPA) attacks and Differential Power Analysis (DPA) attacks.
SPA attacks rely on the observation that in some systems, the power profile of
cryptographic computations can be directly used to reveal cryptographic information. For
example, Figure 1 shows the power consumption profile for an ASIC implementing the DES
algorithm. From the profile, one can easily identify the 16 rounds of the DES algorithm.
While SPA attacks have been useful in determining higher granularity information such as
the cryptographic algorithm used, the cryptographic operations being performed, etc.,
they require reasonably high resolution to reveal the cryptographic key directly. In
practice, SPA attacks have been found be useful in augmenting or simplifying brute-force
attacks. For example, it has been shown in that the brute-force search space for a SW DES
implementation on an 8-bit processor with 7 Bytes of key data can be reduced to 2^40 keys
from 2^56 keys with the help of SPA.
Figure 1: The power consumption profile of a custom hardware implementation
of the DES algorithm
DPA attacks employ statistical analysis to infer the cryptographic key from power
consumption data. These attacks use the notion of differential traces (difference between
traces) to overcome the disadvantages of measurement error and noise associated with SPA
techniques. DPA has been shown to be highly robust and effective in extracting keys from
several embedded systems, not limited to smartcards. Recent approaches such as enhance the
effectiveness of DPA attacks by providing techniques that improve the signal to noise
ratio. While the initial DPA attacks targeted DES implementations, DPA has also been used
to break public-key cryptosystems.