TEDT, a Leakage-Resilient AEAD mode for High (Physical) Security Applications

We propose TEDT, a new Authenticated Encryption with Associated Data (AEAD) mode leveraging Tweakable Block Ciphers (TBCs). TEDT provides the following features: (i) It offers asymptotically optimal security in the multi-user setting. (ii) It offers nonce misuse-resilience, that is, the repetition of nonces does not impact the security of ciphertexts produced with fresh nonces. (iii) It offers KDM security in the multi-user setting, that is, its security is maintained even if key-dependent messages are encrypted. (iv) It offers full leakage-resilience, that is, it limits the exploitability of physical leakages via side-channel attacks, even if these leakages happen during every message encryption and decryption operation. (v) It can be implemented with a remarkably low energy cost when strong resistance to side-channel attacks is needed, supports online encryption and handles static & incremental associated data efficiently. Concretely, TEDT encourages leveled implementations, in which two TBCs are implemented: one needs strong and energy demanding protections against side-channel attacks but is used in a limited way, while the other only requires weak and energy efficient protections and performs the bulk of the computation. As a result, TEDT leads to considerably more energy efficient implementations compared to traditional AEAD schemes, whose side-channel security requires to uniformly protect every (T)BC execution

Strong Authenticity with Leakage under Weak and Falsifiable Physical Assumptions

Authenticity can be compromised by information leaked via side-channels (e.g., power consumption). Examples of attacks include direct key recoveries and attacks against the tag verification which may lead to forgeries. At FSE 2018, Berti et al. described two authenticated encryption schemes which provide authenticity assuming a “leak-free implementation” of a Tweakable Block Cipher (TBC). Precisely, security is guaranteed even if all the intermediate computations of the target implementation are leaked in full but the TBC long-term key. Yet, while a leak-free implementation reasonably models strongly protected implementations of a TBC, it remains an idealized physical assumption that may be too demanding in many cases, in particular, if hardware engineers mitigate the leakage to a good extent but (due to performance constraints) do not reach leak-freeness. In this paper, we get rid of this important limitation by introducing the notion of “Strong Unpredictability with Leakage” for BC\u27s and TBC\u27s. It captures the hardness for an adversary to provide a fresh and valid input/output pair for a (T)BC, even having oracle access to the (T)BC, its inverse and their leakages. This definition is game-based and may be verified/falsified by laboratories. Based on it, we then provide two Message Authentication Codes (MAC) which are secure if the (T)BC on which they rely are implemented in a way that maintains a sufficient unpredictability. Thus, we improve the theoretical foundations of leakage-resilient MAC and extend them towards engineering constraints that are easier to achieve in practice