International Association for Cryptologic Research

International Association
for Cryptologic Research


Atom: A Stream Cipher with Double Key Filter

Subhadeep Banik , LASEC, École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland
Andrea Caforio , LASEC, École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland
Takanori Isobe , University of Hyogo, Kobe, Japan; NICT, Tokyo, Japan; PRESTO, Japan Science and Technology Agency, Tokyo, Japan
Fukang Liu , University of Hyogo, Kobe, Japan; East China Normal University, Shanghai, China
Willi Meier , University of Applied Sciences and Arts Northwestern Switzerland, Windisch, Switzerland
Kosei Sakamoto , University of Hyogo, Kobe, Japan
Santanu Sarkar , Indian Institute of Technology, Madras, India
DOI: 10.46586/tosc.v2021.i1.5-36
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Abstract: 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.
Video from TOSC 2021
  title={Atom: A Stream Cipher with Double Key Filter},
  journal={IACR Transactions on Symmetric Cryptology},
  publisher={Ruhr-Universität Bochum},
  volume={2021, Issue 1},
  author={Subhadeep Banik and Andrea Caforio and Takanori Isobe and Fukang Liu and Willi Meier and Kosei Sakamoto and Santanu Sarkar},