Side channel attacks against cryptographic systems involve identifying ways in which their physical implementations leak useful information. A cryptographic algorithm may be secure on paper but when implemented on physical hardware some of the secret data, such as key bits, may potentially be recovered by an attacker by measuring various physical properties whilst encryption or decryption is being performed. This essay reviews the most successful countermeasures that can be used to make different classes of side channel attacks as difficult as possible. An understanding of basic principles of cryptography is assumed.
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Source: https://packetstormsecurity.com/files/142457/Side-Channel-Attack-Countermeasures-In-Cryptographic-Systems.html