Practical Aspects of Vertical Side-Channel Analyses on HMAC-SHA-2

Cryptographic hashing with secret key is widely used for message authentication, e.g., in the popular HMAC protocol. This algorithm is a suitable target for vertical Side-Channel Analyses. In practice, such analyses involve various attacks on intermediate computations to recover security critical information. If the hash function is SHA-2, an attacker compares an expected leakage model of modular addition and logical AND operations, with the side-channel activity. In both cases, there are practical constraints that lead to errors in correlation-based analyses and make security assessment difficult. This becomes even more apparent when the target of evaluation is not theoretical values but a real device. We discuss these practical aspects and show a real evaluation on an embedded implementation.