Ci Poseidon Hash Aims For Cleaner Constants In Zero Knowledge Proofs

The short version

Zero-knowledge rollups need hash functions that are cheap in arithmetic circuits. Ci Poseidon derives round constants from the rational number Ci = 85/27 instead of opaque randomness. Authors report avalanche near fifty percent on BN254, BLS12-381, and Goldilocks fields in an IACR ePrint 2026/109784 preprint.

What the researchers did

The ci-Poseidon repository documents a Poseidon2-style hash tuned for arithmetization-friendly fields. Constants come deterministically from Ci = 85/27. The team benchmarks against Poseidon2 on BN254, BLS12-381, and Goldilocks, measuring avalanche and implementation cost. Paper status is listed as IACR ePrint 2026/109784 pending full catalog indexing.

What they found

Reported avalanche on BN254 is 50.02 percent, described as statistically ideal, with BLS12-381 at 49.93 percent and Goldilocks at similar balance in the readme tables. The design goal is reproducible constants that auditors can trace, which matters when upgrading rollup hash gadgets.

Why it matters for you

Protocol engineers choosing hash functions for zkSNARK circuits care about security margins and auditability. Deterministic constant generation can speed review when multiple chains must align on the same permutation.

What to keep in mind

Preprint on IACR ePrint, not yet peer-reviewed at a conference. Production deployment still needs independent cryptanalysis and cross-client tests.

Key takeaways

• Rational constant Ci = 85/27 seeds round constants.
• Poseidon2-style layout targets ZK-friendly fields.
• Mixing spread reports cluster near 50% on major curves.
• ePrint 2026/109784 listed in project readme.
• Open code supports reproducible benchmarks.

Sources

• Primary — IACR ePrint, ci-Poseidon rational constants for proof friendly hashes (2026). Mixing spread benchmarks and construction.
• Secondary — GitHub, ci-Poseidon implementation (2026). Code and readme metrics.