There are two major power analysis techniques – simple power analysis (SPA) and differential power analysis (DPA). SPA involves direct observation of the power consumption during cryptographic or other security sensitive operations. SPA can reveal information about the device’s operation as well as the key material. If the attacker knows the cryptographic algorithm (and especially its implementation in the tested device) he can easily work out some bits of information by observing the sequences of CPU instructions, especially rotation and conditional branches. If the result of an arithmetic or logic operation can be observed as well, i.e. the state of carry, zero or negative flags, more information can be obtained. DPA is a more powerful technique, because the attacker does not have to know as many details about how the cryptographic algorithm was implemented. It uses statistical analysis to extract hidden information from a large sample of power traces obtained during cryptographic computations with known ciphertexts. The statistical methods identify small differences in power consumption which can be used to recover individual bits in a secret key.