We specify OCH, the first authenticated encryption with associated data scheme built to provide 128-bit multi-user AE security, 128-bit context commitment security, and 256-bit nonces with optional nonce privacy. It therefore addresses pressing limitations of currently widely-deployed schemes. We construct and formally analyze the security of OCH in a modular fashion, with transforms that are of broader applicability. On Intel Raptor Lake CPUs, OCH using the Areion permutation family has a peak encryption speed of 0.62 cycles per byte (cpb), not far off from AES128-GCM (0.38cpb) and outperforming both ChaCha20/Poly1305 (1.63cpb) and TurboSHAKE128-Wrap (3.52cpb).

Existing tamper-evident logging systems suffer from high overhead and severe data loss in high-load settings, yet only provide coarse-grained tamper detection. Moreover, installing such systems requires recompiling kernel code. To address these challenges, we present Nitro, a high-performance, tamper-evident audit logging system that supports fine-grained detection of log tampering. Even better, our system avoids kernel recompilation by using the eBPF technology. To formally justify the security of Nitro, we provide a new definitional framework for logging systems, and give a practical cryptographic construction meeting this new goal. Unlike prior work that focus only on the cryptographic processing, we codesign the cryptographic part with the pre- and post-processing of the logs to exploit all system-level optimizations. Our evaluations demonstrate Nitro's superior performance, achieving 10X-25X improvements in high-stress conditions and 2X-10X in real-world scenarios while maintaining near-zero data loss. We also provide an advanced variant, Nitro-R that introduces in-kernel log reduction techniques to reduce runtime overhead even further.

As more High-Performance Computing (HPC) applications that process sensitive data are moving to run on the public cloud, there is a need for the cloud infrastructure to provide privacy and integrity support. In this work, we investigate how to add encryption to all-gather to protect internode communication. This task is challenging since encryption is often more expensive than communication in contemporary HPC systems. We derive performance bounds for encrypted allgather, and develop new algorithms that meet the theoretical lower bounds. Our empirical evaluation on production systems demonstrates that the new algorithms achieve substantially better performance than the naive approach.

We analyze the multi-user security of the streaming encryption in Google's Tink library via an extended version of the framework of nonce-based online authenticated encryption of Hoang et al. (CRYPTO'15) to support random-access decryption. We show that Tink's design choice of using random nonces and a nonce-based key-derivation function indeed improves the concrete security bound. We then give two better alternatives that are more robust against randomness failure. In addition, we show how to efficiently instantiate the key-derivation function via AES, instead of relying on HMAC-SHA256 like the current design in Tink. To accomplish this we give a multi-user analysis of the XOR-of-permutation construction of Bellare, Krovetz, and Rogaway (EUROCRYPT'98).

As High Performance Computing (HPC) applications with data security requirements are increasingly moving to execute in the public cloud, there is a demand that the cloud infrastructure for HPC should support privacy and integrity. Incorporating privacy and integrity mechanisms in the communication infrastructure of today's public cloud is challenging because recent advances in the networking infrastructure in data centers have shifted the communication bottleneck from the network links to the network end points and because encryption is computationally intensive. In this work, we consider incorporating encryption to support privacy and integrity in the Message Passing Interface (MPI) library, which is widely used in HPC applications. We empirically study four contemporary cryptographic libraries, OpenSSL, BoringSSL, Libsodium, and CryptoPP using micro-benchmarks and NAS parallel benchmarks to evaluate their overheads for encrypting MPI messages on two different networking technologies, 10Gbps Ethernet and 40Gbps InfiniBand. The results indicate that (1) the performance differs drastically across cryptographic libraries, and (2) effectively supporting privacy and integrity in MPI communications on high speed data center networks is challenging-even with the most efficient cryptographic library, encryption can still introduce very significant overheads in some scenarios such as a single MPI communication operation on InfiniBand, but (3) the overall overhead may not be prohibitive for practical uses since there can be multiple concurrent communications.

Multi-user (mu) security considers large-scale attackers (e.g., state actors) that given access to a number of sessions, attempt to compromise at least one of them. Mu security of authenticated encryption (AE) was explicitly considered in the development of TLS 1.3. This paper revisits the mu security of GCM, which remains to date the most widely used dedicated AE mode. We provide new concrete security bounds which improve upon previous work by adopting a refined parameterization of adversarial resources that highlights the impact on security of (1) nonce re-use across users and of (2) re-keying. As one of the main applications, we give tight security bounds for the nonce-randomization mechanism adopted in the record protocol of TLS 1.3 as a mitigation of large-scale multi-user attacks. We provide tight security bounds that yield the first validation of this method. In particular, we solve the main open question of Bellare and Tackmann (CRYPTO '16), who only considered restricted attackers which do not attempt to violate integrity, and only gave non-tight bounds.

We introduce identity-based format-preserving encryption (IB-FPE) as a way to localize and limit the damage to format-preserving encryption (FPE) from key exposure. We give definitions, relations between them, generic attacks and two transforms of FPE schemes to IB-FPE schemes. As a special case, we introduce and cover identity-based tweakable blockciphers. We apply all this to analyze DFF, an FPE scheme proposed to NIST for standardization.

We give attacks on Feistel-based format-preserving encryption (FPE) schemes that succeed in message recovery (not merely distinguishing scheme outputs from random) when the message space is small. For $4$-bit messages, the attacks fully recover the target message using $2^{21}$ examples for the FF3 NIST standard and $2^{25}$ examples for the FF1 NIST standard. The examples include only three messages per tweak, which is what makes the attacks non-trivial even though the total number of examples exceeds the size of the domain. The attacks are rigorously analyzed in a new definitional framework of message-recovery security. The attacks are easily put out of reach by increasing the number of Feistel rounds in the standards.

Authenticated encryption (AE) schemes are symmetric-key encryption schemes ensuring strong notions of confidentiality and integrity. Although various AE schemes are known, there remains significant interest in developing schemes that are more efficient, meet even stronger security notions (e.g., misuse-resistance), or satisfy certain non-cryptographic properties (e.g., being patent-free).

We advocate schemes based on fixed-key AES as the best route to highly efficient circuit-garbling. We provide such schemes making only one AES call per garbled-gate evaluation. On the theoretical side, we justify the security of these methods in the random-permutation model, where parties have access to a public random permutation. On the practical side, we provide the Just Garble system, which implements our schemes. Just Garble evaluates moderate-sized garbled-circuits at an amortized cost of 23.2 cycles per gate (7.25 nsec), far faster than any prior reported results.

Garbled circuits, a classical idea rooted in the work of Yao, have long been understood as a cryptographic technique, not a cryptographic goal. Here we cull out a primitive corresponding to this technique. We call it a garbling scheme. We provide a provable-security treatment for garbling schemes, endowing them with a versatile syntax and multiple security definitions. The most basic of these, privacy, suffices for two-party secure function evaluation (SFE) and private function evaluation (PFE). Starting from a PRF, we provide an efficient garbling scheme achieving privacy and we analyze its concrete security. We next consider obliviousness and authenticity, properties needed for private and verifiable outsourcing of computation. We extend our scheme to achieve these ends. We provide highly efficient blockcipher-based instantiations of both schemes. Our treatment of garbling schemes presages more efficient garbling, more rigorous analyses, and more modularly designed higher-level protocols.

Consider a set of labels $L$ and a set of trees ${mathcal T} = { {mathcal T}^{(1), {mathcal T}^{(2), ldots, {mathcal T}^{(k) $ where each tree ${mathcal T}^{(i)$ is distinctly leaf-labeled by some subset of $L$. One fundamental problem is to find the biggest tree (denoted as supertree) to represent $mathcal T}$ which minimizes the disagreements with the trees in ${mathcal T}$ under certain criteria. This problem finds applications in phylogenetics, database, and data mining. In this paper, we focus on two particular supertree problems, namely, the maximum agreement supertree problem (MASP) and the maximum compatible supertree problem (MCSP). These two problems are known to be NP-hard for $k geq 3$. This paper gives the first polynomial time algorithms for both MASP and MCSP when both $k$ and the maximum degree $D$ of the trees are constant.