In this chapter, side-channel analysis and fault analysis on block ciphers are introduced. They belong to the physical attacks or the implementation attacks, in which the attackers attempt to recover the secret information from a cryptographic algorithm implemented in a physical device. A straightforward implementation of the block ciphers can be easily broken by physical attacks without leaving any attack evidence.
In cryptanalysis, it is assumed that the intermediate values, that is, the intermediate calculation result during the cryptographic operation is unavailable to the attackers. In contrast, in the physical attacks, the attackers can obtain information such as the Hamming weight (HW) of an intermediate value by physically manipulating and observing the device. Such information enables the attackers to efficiently recover the secret key of a block cipher.
5.1.1 Intrusion Degree of Physical Attacks
The category of the physical attacks is shown in Figure 5.1. According to the physical intrusion degree to the target device, the physical attacks are sorted into three categories; invasive, semi-invasive, and noninvasive attacks. Generally speaking, the higher the intrusion degree is, the more powerful the attack becomes. However, a deep intrusion is likely to link to a high cost in implementing the attack and may result in leaving much evidence of the performed attacks.