refactor(stm): separated key cache from srs degree

Author: damrobi

mithril-stm/src/circuits/test_utils/file_mutex.rs

@@ -166,21 +166,16 @@ mod tests {
         assert_ne!(baseline.directory(), other_depth.directory());
         assert_ne!(baseline.directory(), other_seed.directory());
 
-        // The IVC setup cache additionally folds in the SRS degree and both production verifying keys
-        // as a circuit-version salt; a change in either must also resolve to a different directory.
+        // The IVC setup cache folds in both production verifying keys as a circuit-version salt, like
+        // the certificate-key cache and it omits the SRS degree for the same reason.
         let ivc_baseline = FileMutex::for_shared_cache(
             "ivc-setup",
-            &[b"parameters-a", b"depth-4", b"degree-19", b"circuit-vk-1"],
-        );
-        let other_degree = FileMutex::for_shared_cache(
-            "ivc-setup",
-            &[b"parameters-a", b"depth-4", b"degree-20", b"circuit-vk-1"],
+            &[b"parameters-a", b"depth-4", b"circuit-vk-1"],
         );
         let other_circuit_version = FileMutex::for_shared_cache(
             "ivc-setup",
-            &[b"parameters-a", b"depth-4", b"degree-19", b"circuit-vk-2"],
+            &[b"parameters-a", b"depth-4", b"circuit-vk-2"],
         );
-        assert_ne!(ivc_baseline.directory(), other_degree.directory());
         assert_ne!(ivc_baseline.directory(), other_circuit_version.directory());
     }
 }

mithril-stm/src/circuits/trusted_setup.rs

@@ -192,9 +192,10 @@ impl Default for TrustedSetupProvider {
 }
 
 /// Seed for the deterministic unsafe SRS used by the tests; it pins the SRS's tau. Test key caches
-/// fold in this seed so they stay correct if it ever changes. The IVC setup cache also folds in the
-/// SRS degree; the certificate-key cache omits it, since keygen downsizes the seed-pinned SRS to the
-/// certificate circuit's own degree, so the oversized degree never affects the certificate key.
+/// fold in this seed so they stay correct if it ever changes. Both the certificate-key and IVC setup
+/// caches omit the SRS degree: keygen always downsizes the seed-pinned SRS to the target circuit
+/// degree before deriving keys, so the oversized starting degree never affects the derived keys —
+/// only `TrustedSetupProvider::with_unsafe_srs`'s own file layout, which nests by degree.
 #[cfg(any(test, feature = "benchmark-internals"))]
 pub(crate) const UNSAFE_SRS_SEED: u64 = 42;
 

mithril-stm/src/proof_system/ivc_halo2_snark/prover_setup.rs

@@ -156,9 +156,11 @@ impl IvcSnarkProverSetup {
 
     /// Builds an [`IvcSnarkProverSetup`] from a deterministic, oversized unsafe SRS, exercising the
     /// real `load` path without the production SRS. Shared by the slow IVC tests through a
-    /// content-keyed cache keyed by the protocol parameters, Merkle-tree depth, the unsafe SRS identity
-    /// (degree and seed), and the production verifying keys as a circuit-version salt, so the recursive
-    /// keys — the dominant cost — are computed once and reused across tests and runs.
+    /// content-keyed cache keyed by the protocol parameters, Merkle-tree depth, the unsafe SRS seed,
+    /// and the production verifying keys as a circuit-version salt — not the SRS degree, since `load`
+    /// always downsizes to `RECURSIVE_CIRCUIT_DEGREE` before deriving keys, so any `unsafe_srs_degree`
+    /// reproduces the same recursive keys — the dominant cost — letting calls at different degrees
+    /// share one already-computed key cache instead of each paying for keygen separately.
     #[cfg(test)]
     pub(crate) fn build_for_test(
         parameters: &crate::Parameters,
@@ -182,7 +184,6 @@ impl IvcSnarkProverSetup {
     ) -> StmResult<Self> {
         let parameters_bytes = parameters.to_bytes()?;
         let depth_bytes = merkle_tree_depth.to_le_bytes();
-        let degree_bytes = unsafe_srs_degree.to_le_bytes();
         let seed_bytes = UNSAFE_SRS_SEED.to_le_bytes();
         let cache = FileMutex::for_shared_cache(
             "ivc-setup",
@@ -191,7 +192,6 @@ impl IvcSnarkProverSetup {
                 RECURSIVE_CIRCUIT_VERIFICATION_KEY_FOR_PRODUCTION,
                 &parameters_bytes,
                 &depth_bytes,
-                &degree_bytes,
                 &seed_bytes,
             ],
         );