Affiliation: LASEC, École Polytechnique Fédérale de Lausanne, Switzerland
Atom: A Stream Cipher with Double Key Filter
It has been common knowledge that for a stream cipher to be secure against generic TMD tradeoff attacks, the size of its internal state in bits needs to be at least twice the size of the length of its secret key. In FSE 2015, Armknecht and Mikhalev however proposed the stream cipher Sprout with a Grain-like architecture, whose internal state was equal in size with its secret key and yet resistant against TMD attacks. Although Sprout had other weaknesses, it germinated a sequence of stream cipher designs like Lizard and Plantlet with short internal states. Both these designs have had cryptanalytic results reported against them. In this paper, we propose the stream cipher Atom that has an internal state of 159 bits and offers a security of 128 bits. Atom uses two key filters simultaneously to thwart certain cryptanalytic attacks that have been recently reported against keystream generators. In addition, we found that our design is one of the smallest stream ciphers that offers this security level, and we prove in this paper that Atom resists all the attacks that have been proposed against stream ciphers so far in literature. On the face of it, Atom also builds on the basic structure of the Grain family of stream ciphers. However, we try to prove that by including the additional key filter in the architecture of Atom we can make it immune to all cryptanalytic advances proposed against stream ciphers in recent cryptographic literature.
Beyond Security and Efficiency: On-Demand Ratcheting with Security Awareness 📺
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.
The Area-Latency Symbiosis: Towards Improved Serial Encryption Circuits
The bit-sliding paper of Jean et al. (CHES 2017) showed that the smallest-size circuit for SPN based block ciphers such as AES, SKINNY and PRESENT can be achieved via bit-serial implementations. Their technique decreases the bit size of the datapath and naturally leads to a significant loss in latency (as well as the maximum throughput). Their designs complete a single round of the encryption in 168 (resp. 68) clock cycles for 128 (resp. 64) bit blocks. A follow-up work by Banik et al. (FSE 2020) introduced the swap-and-rotate technique that both eliminates this loss in latency and achieves even smaller footprints.In this paper, we extend these results on bit-serial implementations all the way to four authenticated encryption schemes from NIST LWC. Our first focus is to decrease latency and improve throughput with the use of the swap-and-rotate technique. Our block cipher implementations have the most efficient round operations in the sense that a round function of an n-bit block cipher is computed in exactly n clock cycles. This leads to implementations that are similar in size to the state of the art, but have much lower latency (savings up to 20 percent). We then extend our technique to 4- and 8-bit implementations. Although these results are promising, block ciphers themselves are not end-user primitives, as they need to be used in conjunction with a mode of operation. Hence, in the second part of the paper, we use our serial block ciphers to bootstrap four active NIST authenticated encryption candidates: SUNDAE-GIFT, Romulus, SAEAES and SKINNY-AEAD. In the wake of this effort, we provide the smallest block-cipher-based authenticated encryption circuits known in the literature so far.