Paper 2019/1187

Adapting Rigidity to Symmetric Cryptography: Towards "Unswerving" Designs

Orr Dunkelman and Léo Perrin


While designers of cryptographic algorithms are rarely considered as potential adversaries, past examples, such as the standardization of the Dual EC PRNG highlights that the story might be more complicated. To prevent the existence of backdoors, the concept of rigidity was introduced in the specific context of curve generation. The idea is to first state a strict scope statement for the properties that the curve needs to have and then pick e.g. the one with the smallest parameters. The aim is to ensure that the designers did not have the degrees of freedom that allows the addition of a trapdoor. In this paper, we apply this approach to symmetric algorithms. The task is challenging because the corresponding primitives are more complex: they consist of several sub-components of different types, and the properties required by these sub-components to achieve the desired security level are not as clearly defined. Furthermore, security often comes in this case from the interplay between these components rather than from their individual properties. In this paper, we argue that it is nevertheless necessary to demand that symmetric algorithms have a similar but, due to their different nature, more complex property which we call "unswervingness". We motivate this need via a study of the literature on symmetric "kleptography" and via the study of some real-world standards. We then suggest some guidelines that could be used to leverage the unswervingness of a symmetric algorithm to standardize a highly trusted and equally safe variant of it.

Available format(s)
Secret-key cryptography
Publication info
Published elsewhere. Security Standardisation Research Conference 2019
Contact author(s)
leo perrin @ inria fr
orrd @ cs haifa ac il
2019-10-15: received
Short URL
Creative Commons Attribution


      author = {Orr Dunkelman and Léo Perrin},
      title = {Adapting Rigidity to Symmetric Cryptography: Towards "Unswerving" Designs},
      howpublished = {Cryptology ePrint Archive, Paper 2019/1187},
      year = {2019},
      note = {\url{}},
      url = {}
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