From c21d12efb52077a7dfe54c48d30f289311d4940e Mon Sep 17 00:00:00 2001
From: Daniel Lubarov <daniel@lubarov.com>
Date: Tue, 27 Feb 2024 23:01:36 -0700
Subject: [PATCH] fix: S-box in Poseidon2 config

x^5 works with Mersenne31, but x^7 is the smallest permutation monomial over BabyBear. I think this mistake originates in some Plonky3 example code which had the same issue.
---
 core/src/utils/prove.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/core/src/utils/prove.rs b/core/src/utils/prove.rs
index f1db1b207..1c0664708 100644
--- a/core/src/utils/prove.rs
+++ b/core/src/utils/prove.rs
@@ -183,7 +183,7 @@ pub(super) mod baby_bear_poseidon2 {
 
     pub type Challenge = BinomialExtensionField<Val, 4>;
 
-    pub type Perm = Poseidon2<Val, DiffusionMatrixBabybear, 16, 5>;
+    pub type Perm = Poseidon2<Val, DiffusionMatrixBabybear, 16, 7>;
     pub type MyHash = PaddingFreeSponge<Perm, 16, 8, 8>;
 
     pub type MyCompress = TruncatedPermutation<Perm, 2, 8, 16>;