A practical distinguisher on the full Skyscraper permutation

ePrint Report: A practical distinguisher on the full Skyscraper permutation
Antoine Bak

Skyscraper is a cryptographic permutation published in TCHES 2025, optimized for use in proof systems such as PlonK. This primitive is based on a 10-round Feistel network combining $x^2$ monomials and lookup-based functions to achieve competitive plain performances and efficiency in proof systems supporting lookups. In terms of security, the $x^2$ monomials are supposed to provide security against statistical attacks, while lookups are supposed to provide security against algebraic attacks.

In this note, we show that this primitive has a much lower security margin than expected. Using a rebound attack, we find practical truncated differentials on the full permutation. As a corollary, we also find a practical collision attack on the compression function based on a 9-round Skyscraper permutation, which significantly reduces the security margin of the primitive. All of these attacks have been implemented and work in practice.

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