fix(boojum): compute the correct number of PoW seed challenges (#43)

This PR fixes a bug in PoW seed challenge count calculation.
matter-labs · Nov 18, 2024 · 8d2f5f7 · 8d2f5f7
1 parent 9ecb5cd
commit 8d2f5f7
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Showing 3 changed files with 6 additions and 12 deletions.
diff --git a/crates/boojum/src/cs/implementations/prover.rs b/crates/boojum/src/cs/implementations/prover.rs
@@ -2111,11 +2111,9 @@ impl<
 
             let now = std::time::Instant::now();
 
+            const SEED_BITS: usize = 256;
             // pull enough challenges from the transcript
-            let mut num_challenges = 256 / F::CHAR_BITS;
-            if num_challenges % F::CHAR_BITS != 0 {
-                num_challenges += 1;
-            }
+            let num_challenges = SEED_BITS.next_multiple_of(F::CHAR_BITS) / F::CHAR_BITS;
             let challenges = transcript.get_multiple_challenges(num_challenges);
             let pow_challenge = POW::run_from_field_elements(challenges, new_pow_bits, worker);
 

diff --git a/crates/boojum/src/cs/implementations/verifier.rs b/crates/boojum/src/cs/implementations/verifier.rs
@@ -1958,11 +1958,9 @@ impl<F: SmallField, EXT: FieldExtension<2, BaseField = F>> Verifier<F, EXT> {
             log!("Doing PoW verification for {} bits", new_pow_bits);
             log!("Prover gave challenge 0x{:016x}", proof.pow_challenge);
 
+            const SEED_BITS: usize = 256;
             // pull enough challenges from the transcript
-            let mut num_challenges = 256 / F::CHAR_BITS;
-            if num_challenges % F::CHAR_BITS != 0 {
-                num_challenges += 1;
-            }
+            let num_challenges = SEED_BITS.next_multiple_of(F::CHAR_BITS) / F::CHAR_BITS;
             let challenges = transcript.get_multiple_challenges(num_challenges);
             let pow_challenge = proof.pow_challenge;
 

diff --git a/crates/boojum/src/gadgets/recursion/recursive_verifier.rs b/crates/boojum/src/gadgets/recursion/recursive_verifier.rs
@@ -1493,11 +1493,9 @@ impl<F: SmallField, EXT: FieldExtension<2, BaseField = F>, CS: ConstraintSystem<
             log!("Doing PoW verification for {} bits", new_pow_bits);
             // log!("Prover gave challenge 0x{:016x}", proof.pow_challenge);
 
+            const SEED_BITS: usize = 256;
             // pull enough challenges from the transcript
-            let mut num_challenges = 256 / F::CHAR_BITS;
-            if num_challenges % F::CHAR_BITS != 0 {
-                num_challenges += 1;
-            }
+            let num_challenges = SEED_BITS.next_multiple_of(F::CHAR_BITS) / F::CHAR_BITS;
             let _challenges: Vec<_> = transcript.get_multiple_challenges(cs, num_challenges);
 
             todo!()