fix: correct threshold for light client state update (#1461)

No issue related.

### This PR:
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- Let n be the total stake. We only need n/3+1 signatures (at least 1
signature from the honest party) to update the light client contract.
- Correct the comment/documentation for `use_mock_contract` option.

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hotshot-state-prover/src/mock_ledger.rs

@@ -16,7 +16,9 @@ use hotshot_contract_adapter::jellyfish::{field_to_u256, open_key, u256_to_field
 use hotshot_contract_adapter::light_client::ParsedLightClientState;
 use hotshot_stake_table::vec_based::StakeTable;
 
-use crate::{generate_state_update_proof, preprocess, Proof, VerifyingKey};
+use crate::{
+    generate_state_update_proof, preprocess, service::one_honest_threshold, Proof, VerifyingKey,
+};
 use hotshot_types::traits::stake_table::StakeTableScheme;
 use hotshot_types::{
     light_client::{GenericLightClientState, GenericPublicInput, LightClientState},
@@ -86,7 +88,8 @@ impl MockLedger {
             key_archive.insert(qc_keys[i], state_keys[i].0.clone());
         }
         let st = stake_table_for_testing(&qc_keys, &state_keys);
-        let threshold = st.total_stake(SnapshotVersion::LastEpochStart).unwrap() * 2 / 3;
+        let threshold =
+            one_honest_threshold(st.total_stake(SnapshotVersion::LastEpochStart).unwrap());
 
         // arbitrary commitment values as they don't affect logic being tested
         let block_comm_root = F::from(1234);
@@ -120,12 +123,11 @@ impl MockLedger {
         {
             self.epoch += 1;
             self.st.advance();
-            self.threshold = self
-                .st
-                .total_stake(SnapshotVersion::LastEpochStart)
-                .unwrap()
-                * 2
-                / 3;
+            self.threshold = one_honest_threshold(
+                self.st
+                    .total_stake(SnapshotVersion::LastEpochStart)
+                    .unwrap(),
+            );
         }
 
         let new_root = self.new_dummy_comm();

hotshot-state-prover/src/service.rs

@@ -79,6 +79,12 @@ pub struct StateProverConfig {
     pub stake_table_capacity: usize,
 }
 
+#[inline]
+/// A helper function to compute the quorum threshold given a total amount of stake.
+pub fn one_honest_threshold(total_stake: U256) -> U256 {
+    total_stake / 3 + 1
+}
+
 pub fn init_stake_table(
     bls_keys: &[BLSPubKey],
     state_keys: &[StateVerKey],
@@ -156,7 +162,7 @@ pub async fn light_client_genesis(
     let (bls_comm, schnorr_comm, stake_comm) = st
         .commitment(SnapshotVersion::LastEpochStart)
         .expect("Commitment computation shouldn't fail.");
-    let threshold = st.total_stake(SnapshotVersion::LastEpochStart)? * 2 / 3;
+    let threshold = one_honest_threshold(st.total_stake(SnapshotVersion::LastEpochStart)?);
 
     let pi = vec![
         u256_to_field(threshold),
@@ -297,7 +303,7 @@ pub async fn sync_state<Ver: StaticVersionType>(
     tracing::debug!("Old state: {old_state:?}");
     tracing::debug!("New state: {:?}", bundle.state);
 
-    let threshold = st.total_stake(SnapshotVersion::LastEpochStart)? * 2 / 3;
+    let threshold = one_honest_threshold(st.total_stake(SnapshotVersion::LastEpochStart)?);
     tracing::info!("Threshold before syncing state: {}", threshold);
     let entries = st
         .try_iter(SnapshotVersion::LastEpochStart)

sequencer/src/bin/deploy.rs

@@ -95,7 +95,7 @@ struct Options {
     #[clap(flatten)]
     contracts: DeployedContracts,
 
-    /// If toggled, launch a mock prover contract that does not do any proof verification.
+    /// If toggled, launch a mock prover contract with a smaller verification key.
     #[clap(short, long)]
     pub use_mock_contract: bool,
 

sequencer/src/bin/state-relay-server.rs

@@ -1,6 +1,8 @@
 use async_compatibility_layer::logging::{setup_backtrace, setup_logging};
 use clap::Parser;
 use es_version::SEQUENCER_VERSION;
+use ethers::types::U256;
+use hotshot_state_prover::service::one_honest_threshold;
 use sequencer::state_signature::relay_server::run_relay_server;
 
 #[derive(Parser)]
@@ -31,9 +33,12 @@ async fn main() {
     setup_backtrace();
 
     let args = Args::parse();
-    let threshold = ((2 * args.total_stake) / 3) + 1;
+    let threshold = one_honest_threshold(U256::from(args.total_stake));
 
-    tracing::info!(port = args.port, threshold, "starting state relay server");
+    tracing::info!(
+        port = args.port,
+        "starting state relay server, quorum threshold: {threshold}"
+    );
     run_relay_server(
         None,
         threshold,

sequencer/src/state_signature/relay_server.rs

@@ -224,7 +224,7 @@ where
 
 pub async fn run_relay_server<Ver: StaticVersionType + 'static>(
     shutdown_listener: Option<OneShotReceiver<()>>,
-    threshold: u64,
+    threshold: U256,
     url: Url,
     bind_version: Ver,
 ) -> std::io::Result<()> {
@@ -234,7 +234,6 @@ pub async fn run_relay_server<Ver: StaticVersionType + 'static>(
 
     // We don't have a stake table yet, putting some temporary value here.
     // Related issue: [https://github.com/EspressoSystems/espresso-sequencer/issues/1022]
-    let threshold = U256::from(threshold);
     let state =
         State::new(StateRelayServerState::new(threshold).with_shutdown_signal(shutdown_listener));
     let mut app = App::<State, Error>::with_state(state);