Cryptographic hardware is now widely deployed in everything from pay TV units to cell phones and Smart cards. Smart cards are most often used as cryptographic devices to provide strong authentication of users and to store secret information securely. The compromise of this private data or the hardware which guards it can have disastrous implications including loss of privacy, forged access, or direct monetary theft. These cryptographic devices are easily obtainable and attackers can carry out attacks on the system without directly attacking the mathematics of the algorithms. Even when totally secure algorithms and protocols are employed, the attacker might still be able to learn valuable secret information without having detailed knowledge implementation of the hardware. This book talks about a class of such practical "implementation attacks" on cryptographic hardware devices called "Differential Power Attacks". Differential Power Analysis (DPA) attacks area a powerful side channel attack that can be mounted easily to reveal secret key. It exploits data dependency of cryptographic circuits on power consumption with statistical analysis to figure out the secret key.