Ensuring High Testability without Degrading Security

International audience ; Cryptographic algorithms are used to protect sensitive information from untrusted parties when the communication medium is not secure. Many secure systems such as smartcards include hardware implementation of symmetric cryptographic algorithms such as (Triple) Data Encryption Standard and Advanced Encryption Standard. The secret keys used to encrypt the data with these algorithms are large enough to prevent any brute force attack that consists in exploring the whole solution space. However, the hardware implementation of these cryptographic algorithms allows the hackers to measure the observable characteristics of the physical implementation and deduce the secret key (side‐channel attacks). The key can even be discovered by applying a side‐channel attack on scan chains. These scan chains, which aim to provide full controllability and observability of internal states, represent nevertheless the most popular design‐for‐testability scheme. Because crypto‐processors and others cores in a secure system must pass through high‐quality test procedures to ensure that data are correctly processed, testing of crypto chips faces a dilemma: how to develop a design‐for‐testability scheme that provides high testability (high controllability and observability) while maintaining high security (minimal controllability and observability)? This tutorial presents the security weaknesses generated by scan designs on hardware AES and DES implementations. It also discusses the pros and cons of security‐dedicated DFT, BIST and Fault tolerance solutions taken from the literature.