Paper 2025/058

Skyscraper-v2: Fast Hashing on Big Primes

Abstract

Arithmetic hash functions defined over prime fields have been actively developed and used in verifiable computation (VC) protocols. Among those, elliptic-curve-based SNARKs require large (256-bit and higher) primes. Such hash functions are notably slow, losing a factor of up to 1000 compared to regular constructions like SHA-2/3. In this paper, we present the hash function Skyscraper-v2, which is aimed at large prime fields and provides major improvements compared to Reinforced Concrete and Monolith. First, the design is exactly the same for all large primes, which simplifies analysis and deployment. Secondly, it achieves a performance comparable to cryptographic hash standards by using low-degree non-invertible transformations and minimizing modulo reductions. Concretely, it hashes two 256-bit prime field (BLS12-381 curve scalar field) elements in 256 nanoseconds, which makes it faster than Poseidon2 by a factor of more than 15, and faster than the current performance-leader Reinforced Concrete by a factor of more than 5 in this scenario. The low circuit complexity of Skyscraper-v2, together with its high native speed, should allow a substantial reduction in many VC scenarios, particularly in recursive proofs.

Note: This updated version of Skyscraper addresses recent cryptanalysis results by adding rounds at the beginning and at the end of the permutation. A detailed list of changes is provided in the Appendix.