Defend encryption systems against side-channel attacks

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by Pankaj Rohatgi, Technical Director, Cryptography Research Division, Rambus, Mar. 17, 2015 – 

From its ancient origin as a tool for protecting sensitive wartime or espionage-related messages, cryptography has become a foundational building-block for securing the systems, protocols, and infrastructure that underpin our modern interconnected world. But the physical mechanisms used in performing encryption and decryption can leak information, making it possible to bypass this security. Protecting designs against such side-channel attacks starts with understanding how such attacks operate.

At its very essence, cryptography is a branch of mathematics dealing with efficiently computable transforms that convert inputs to outputs using additional data known as a cryptographic key. These transforms have the property that, despite observing many input/output pairs, it remains infeasible to compute or invert the transform without the knowledge of the key.

An example of a cryptographic transformation is the symmetric-key based Advanced Encryption Standard (AES-256). An AES-256 encryption device that has access to a 256-bit secret cryptographic key, can transform any sensitive message - known as plaintext - into an unintelligible form known as the ciphertext. Anyone observing the ciphertext, without knowing the plaintext or the key, cannot recover the plaintext or the key. Further, even an observer who knows or can choose the plaintext and can observe the corresponding ciphertext can still not recover the secret key being using within the encryption device. However, any AES decryption device that has access to the same 256-bit secret key as the encrypting device, can readily recover the plaintext from the ciphertext.

Another example of a cryptographic transformation is a public-key based RSA (Rivest-Shamir-Adelman) digital signature algorithm. This algorithm uses pairs of cryptographic keys consisting of a non-secret public key and a secret private key. A signing device that has access to a secret private key can attach a "tag" or digital signature to any message. This RSA signature has the property that without knowledge of the private key, it is infeasible to calculate the digital signature to a message. Anyone who receives a message with a digital signature on that message can use the corresponding public key to establish the authenticity of the message by verifying that the digital signature corresponds to that message.

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