On the Physical Security of Cryptographic Implementations

In modern cryptography, an encryption system is usually studied in the so-called black-box model. In this model, the cryptosystem is seen as an oracle replying to message encryption (and/or decryption) queries according to a secret value: the key. The security of the cryptosystem is then defined following a simple game. An adversary questions the oracle about the encryption (and/or decryption) of messages of its choice and, depending on the answers, attempts to recover the value of the secret key (or to encrypt/decrypt a message for which he did not query the oracle). If by following an optimal strategy the adversary only has a negligible chance of winning, the system is considered as secure. Several cryptosystems have been proved secure in the black-box model. However, this model is not always sufficient to ensure the security of a cryptosystem in practice. Let us consider the example of smart cards which are used as platforms for cryptosystems in various applications such as banking, access control, mobile telephony, pay TV, or electronic passport. By the very nature of these applications, a cryptosystem embedded on a smart card is physically accessible [...]