Constructing Committing and Leakage-Resilient Authenticated Encryption

The main goal of this work is to construct authenticated encryption (AE) that is both committing and leakage-resilient. As a first approach for this we consider generic composition as a well-known method for constructing AE schemes. While the leakage resilience of generic composition schemes has already been analyzed by Barwell et al. (AC\u2717), for committing security this is not the case. We fill this gap by providing a separate analysis of the generic composition paradigms with respect to committing security, giving both positive and negative results: By means of a concrete attack, we show that Encrypt-then-MAC is not committing. Furthermore, we prove that Encrypt-and-MAC is committing, given that the underlying schemes satisfy security notions we introduce for this purpose. We later prove these new notions achievable by providing schemes that satisfy them. MAC-then-Encrypt turns out to be more difficult due to the fact that the tag is not outputted alongside the ciphertext as it is done for the other two composition methods. Nevertheless, we give a detailed heuristic analysis of MAC-then-Encrypt with respect to committing security, leaving a definite result as an open task for future work. Our results, in combination with the fact that only Encrypt-then-MAC yields leakage-resilient AE schemes, show that one cannot obtain AE schemes that are both committing and leakage-resilient via generic composition. As a second approach for constructing committing and leakage-resilient AE, we develop a generic transformation that turns an arbitrary AE scheme into one that fulfills both properties. The transformation relies on a keyed function that is both binding, i.e., it is hard to find key-input pairs that result in the same output, and leakage-resilient pseudorandom

Constructing Committing and Leakage-Resilient Authenticated Encryption

The main goal of this work is to construct authenticated encryption (AE) hat is both committing and leakage-resilient. As a first approach for this we consider generic composition as a well-known method for constructing AE schemes. While the leakage resilience of generic composition schemes has already been analyzed by Barwell et al. (Asiacrypt’17), for committing security this is not the case. We fill this gap by providing a separate analysis of the generic composition paradigms with respect to committing security, giving both positive and negative results: By means of a concrete attack, we show that Encrypt-then-MAC is not committing. Furthermore, we prove that Encrypt-and-MAC is committing, given that the underlying schemes satisfy security notions we introduce for this purpose. We later prove these new notions achievable by providing schemes that satisfy them. MAC-then-Encrypt turns out to be more difficult due to the fact that the tag is not outputted alongside the ciphertext as it is done for the other two composition methods. Nevertheless, we give a detailed heuristic analysis of MAC-then-Encrypt with respect to committing security, leaving a definite result as an open task for future work. Our results, in combination with the fact that only Encrypt-then-MAC yields leakage-resilient AE schemes, show that one cannot obtain AE schemes that are both committing and leakage-resilient via generic composition. As a second approach for constructing committing and leakage-resilient AE, we develop a generic transformation that turns an arbitrary AE scheme into one that fulfills both properties. The transformation relies on a keyed function that is both binding, i.e., it is hard to find key-input pairs that result in the same output, and leakage-resilient pseudorandom

Committing Authenticated Encryption: Sponges vs. Block-Ciphers in the case of the NIST LWC Finalists

Committing security has gained considerable attention in the field of authenticated encryption (AE). This can be traced back to a line of recent attacks, which suggests that AE schemes used in practice should not only provide confidentiality and authenticity, but also committing security. Roughly speaking, a committing AE scheme guarantees that ciphertexts will decrypt only for one key. Despite the recent research effort in this area, the finalists of the NIST lightweight cryptography standardization process have not been put under consideration yet. We close this gap by providing an analysis of these schemes with respect to their committing security. Despite the structural similarities the finalists exhibit, our results are of a quite heterogeneous nature: We break four of the schemes with effectively no costs, while for two schemes our attacks are costlier, yet still efficient. For the remaining three schemes ISAP, Ascon, and (a slightly modified version of) Schwaemm, we give formal security proofs. Our analysis reveals that sponges—due to their large states—are more favorable for committing security compared to block-ciphers

Hash your Keys before Signing: BUFF Security of the Additional NIST PQC Signatures

In this work, we analyze the so-called Beyond UnForgeability Features (BUFF) security of the submissions to the current standardization process of additional signatures by NIST. The BUFF notions formalize security against maliciously generated keys and have various real-world use cases, where security can be guaranteed despite misuse potential on a protocol level. Consequently, NIST declared the security against the BUFF notions as desirable features. Despite NIST\u27s interest, only $6$ out of $40$ schemes consider BUFF security at all, but none give a detailed analysis. We close this gap by analyzing the schemes based on codes, isogenies, lattices, and multivariate equations. The results vary from schemes that achieve neither notion (e.g., Wave) to schemes that achieve all notions (e.g., PROV). In particular, we dispute certain claims by SQUIRRELS and VOX regarding their BUFF security. Resulting from our analysis, we observe that three schemes (CROSS, HAWK and PROV) achieve BUFF security without having the hash of public key and message as part of the signature, as BUFF transformed schemes would have. HAWK and PROV essentially use the lighter PS-3 transform by Pornin and Stern (ACNS\u2705). We further point out whether this transform suffices for the other schemes to achieve the BUFF notions, with both positive and negative results