International Association for Cryptologic Research

International Association
for Cryptologic Research


F. Betül Durak


Beyond Security and Efficiency: On-Demand Ratcheting with Security Awareness 📺
Andrea Caforio F. Betül Durak Serge Vaudenay
Secure asynchronous two-party communication applies ratcheting to strengthen privacy, in the presence of internal state exposures. Security with ratcheting is provided in two forms: forward security and post-compromise security. There have been several such secure protocols proposed in the last few years. However, they come with a high cost. In this paper, we propose two generic constructions with favorable properties. Concretely, our first construction achieves security awareness. It allows users to detect non-persistent active attacks, to determine which messages are not safe given a potential leakage pattern, and to acknowledge for deliveries. In our second construction, we define a hybrid system formed by combining two protocols: typically, a weakly secure "light" protocol and a strongly secure "heavy" protocol. The design goals of our hybrid construction are, first, to let the sender decide which one to use in order to obtain an efficient protocol with ratchet on demand; and second, to restore the communication between honest participants in the case of a message loss or an active attack. We can apply our generic constructions to any existing protocol.
FAST: Secure and High Performance Format-Preserving Encryption and Tokenization 📺
We propose a new construction for format-preserving encryption. Our design provides the flexibility for use in format-preserving encryption (FPE) and for static table-driven tokenization. Our algorithm is a substitution-permutation network based on random Sboxes. Using pseudorandom generators and pseudorandom functions, we prove a strong adaptive security based on the super-pseudorandom permutation assumption of our core design. We obtain empirical parameters to reach this assumption. We suggest parameters for quantum security. Our design accommodates very small domains, with a radix $a$ from 4 to the Unicode alphabet size and a block length $l$ starting 2. The number of Sbox evaluations per encryption is asymptotically $l^{\frac32}$, which is also the number of bytes we need to generate using AES in CTR mode for each tweak setup. For instance, we tokenize 10 decimal digits using 29 (parallel) AES computations to be done only once, when the tweak changes.
Cryptanalysis of LowMC instances using single plaintext/ciphertext pair
Arguably one of the main applications of the LowMC family ciphers is in the post-quantum signature scheme PICNIC. Although LowMC family ciphers have been studied from a cryptanalytic point of view before, none of these studies were directly concerned with the actual use case of this cipher in PICNIC signature scheme. Due to the design paradigm of PICNIC, an adversary trying to perform a forgery attack on the signature scheme instantiated with LowMC would have access to only a single given plaintext/ciphertext pair, i.e. an adversary would only be able to perform attacks with data complexity 1 in a known-plaintext attack scenario. This restriction makes it impossible to employ classical cryptanalysis methodologies such as differential and linear cryptanalysis. In this paper we introduce two key-recovery attacks, both in known-plaintext model and of data complexity 1 for two variants of LowMC, both instances of the LowMC cryptanalysis challenge.
Misuse Attacks on Post-quantum Cryptosystems 📺
Many post-quantum cryptosystems which have been proposed in the National Institute of Standards and Technology (NIST) standardization process follow the same meta-algorithm, but in different algebras or different encoding methods. They usually propose two constructions, one being weaker and the other requiring a random oracle. We focus on the weak version of nine submissions to NIST. Submitters claim no security when the secret key is used several times. In this paper, we analyze how easy it is to run a key recovery under multiple key reuse. We mount a classical key recovery under plaintext checking attacks (i.e., with a plaintext checking oracle saying if a given ciphertext decrypts well to a given plaintext) and a quantum key recovery under chosen ciphertext attacks. In the latter case, we assume quantum access to the decryption oracle.