test: add fuzz testing + replace get_* with try_get_* to avoid panics (#165)

This adds fuzz testing via cargo-fuzz for three categories:

- Parser targets: every Read impl in summit-types. Arbitrary bytes must parse to Ok / Err without panicking, and successful decodes roundtrip to byte-identical output.
- Non-codec targets: ssz_tree_key::parse_key, derive_child_public, SszProof::verify with adversarial inputs.
- Property-based targets: WithdrawalQueue op-sequence invariants, ExtPrivateKey sign-verify roundtrip, SSZ incremental-update-vs-rebuild root equality, and generate→verify proof roundtrip across all proof kinds.

Also replaces calls to get_u64 (and similar) with try_get_u64 to avoid panics if there aren't enough bytes to fill the type.

Author: matthias-wright

.gitignore

@@ -3,3 +3,9 @@
 testnet/
 .vscode/*
 .nvim.lua
+
+# cargo-fuzz build artifacts + generated corpus / crashes
+fuzz/target/
+fuzz/corpus/
+fuzz/artifacts/
+fuzz/coverage/

docs/testing.md

@@ -115,6 +115,16 @@ Tests a new validator joining by syncing from genesis with no checkpoint.
 - **Flow**: First withdraws one validator (modifying the peer set from genesis config), then generates new keys for a joining validator, sends a deposit, creates a `bootstrappers.toml` for peer discovery, and starts node4 with no checkpoint.
 - **Verifies**: The new node syncs the entire chain from genesis, catches up to the current epoch, and joins the active validator set.
 
+## Fuzz Testing
+
+Coverage-guided fuzz testing lives in the `fuzz/` crate and runs under nightly Rust via [`cargo-fuzz`](https://github.com/rust-fuzz/cargo-fuzz). Three categories:
+
+- **Parser targets** — every `Read` impl in `summit-types`. Arbitrary bytes must parse to `Ok` / `Err` without panicking, and successful decodes roundtrip to byte-identical output.
+- **Non-codec targets** — `ssz_tree_key::parse_key`, `derive_child_public`, `SszProof::verify` with adversarial inputs.
+- **Property-based targets** — `WithdrawalQueue` op-sequence invariants, `ExtPrivateKey` sign-verify roundtrip, SSZ incremental-update-vs-rebuild root equality, and generate→verify proof roundtrip across all proof kinds.
+
+See [`fuzz/README.md`](../fuzz/README.md) for the full target list, running instructions, crash reproduction, and how to add new targets.
+
 ## Benchmarks
 
 ### Consensus State Benchmark

finalizer/src/db.rs

@@ -341,9 +341,11 @@ impl<V: Variant> Read for Value<V> {
     type Cfg = ();
 
     fn read_cfg(buf: &mut impl Buf, _cfg: &Self::Cfg) -> Result<Self, Error> {
-        let value_type = buf.get_u8();
+        let value_type = buf.try_get_u8().map_err(|_| Error::EndOfBuffer)?;
         match value_type {
-            0x01 => Ok(Self::U64(buf.get_u64())),
+            0x01 => Ok(Self::U64(
+                buf.try_get_u64().map_err(|_| Error::EndOfBuffer)?,
+            )),
             0x05 => Ok(Self::ConsensusState(Box::new(ConsensusState::read_cfg(
                 buf,
                 &(),
@@ -388,6 +390,7 @@ impl<V: Variant> Write for Value<V> {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use commonware_codec::ReadExt;
     use commonware_consensus::simplex::types::{Finalization, Proposal};
     use commonware_consensus::types::{Epoch, Round, View};
     use commonware_cryptography::bls12381::primitives::{
@@ -405,6 +408,26 @@ mod tests {
     use rand::rngs::StdRng;
     use summit_types::Block;
 
+    #[test]
+    fn test_value_read_truncated_input_returns_err() {
+        // Empty buffer — must not panic.
+        let empty: &[u8] = &[];
+        assert!(matches!(
+            Value::<MinPk>::read(&mut empty.as_ref()),
+            Err(Error::EndOfBuffer)
+        ));
+
+        // Tag 0x01 (U64) with 0..8 payload bytes — all truncated.
+        for n in 0..8 {
+            let mut buf = vec![0x01u8];
+            buf.extend(std::iter::repeat_n(0u8, n));
+            assert!(matches!(
+                Value::<MinPk>::read(&mut buf.as_ref()),
+                Err(Error::EndOfBuffer)
+            ));
+        }
+    }
+
     async fn create_test_db_with_context<
         E: Clock + Storage + Metrics + commonware_runtime::BufferPooler,
         V: Variant,