feat(consensus): require the exact 7-byte Shasta extraData layout (#221)

The drivers only ever produce the 7-byte [pctg | proposalId(6)] layout,
the live chains carry it, and the proposalId consumers already need the
7-byte tail. Header validation previously enforced only the generic
32-byte cap, so any 0-32-byte extraData would import; pin the rule to
the exact layout so a misbehaving block producer fails at import.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: davidtaikocha

crates/consensus/src/validation/mod.rs

@@ -22,7 +22,7 @@ use crate::eip4396::{
     calculate_next_block_eip4396_base_fee,
 };
 use alethia_reth_chainspec::{TAIKO_MAINNET, hardfork::TaikoHardforks, spec::TaikoChainSpec};
-use alethia_reth_primitives::transaction::is_allowed_tx_type;
+use alethia_reth_primitives::{SHASTA_EXTRA_DATA_LEN, transaction::is_allowed_tx_type};
 
 /// Anchor transaction selectors, gas rules, and validation functions.
 mod anchor;
@@ -152,6 +152,20 @@ where
         }
 
         validate_header_extra_data(header, MAXIMUM_EXTRA_DATA_SIZE)?;
+
+        // Shasta extraData must be the 7-byte [pctg | proposalId(6)] layout — the only shape the
+        // drivers produce and the live chains carry. Reject anything else at import so a
+        // misbehaving block producer fails loudly here instead of minting headers whose embedded
+        // proposalId consumers cannot decode.
+        if self.chain_spec.is_shasta_active(header.timestamp()) &&
+            header.extra_data().len() != SHASTA_EXTRA_DATA_LEN
+        {
+            return Err(ConsensusError::Other(format!(
+                "invalid Shasta extra-data length: have {}, want {SHASTA_EXTRA_DATA_LEN}",
+                header.extra_data().len()
+            )));
+        }
+
         validate_header_gas(header)?;
         validate_header_base_fee(header, &self.chain_spec)
     }

crates/consensus/src/validation/tests.rs

@@ -174,6 +174,7 @@ fn unzen_header_allows_nonzero_difficulty() {
         difficulty: U256::from(7_u64),
         base_fee_per_gas: Some(1),
         gas_limit: 30_000_000,
+        extra_data: shasta_extra_data(),
         ..Default::default()
     };
 
@@ -182,6 +183,56 @@ fn unzen_header_allows_nonzero_difficulty() {
         .expect("Unzen headers should allow nonzero difficulty");
 }
 
+#[test]
+fn shasta_header_requires_exact_extra_data_len() {
+    let consensus = test_consensus(devnet_chain_spec());
+    let base = Header {
+        timestamp: 1,
+        base_fee_per_gas: Some(1),
+        gas_limit: 30_000_000,
+        ..Default::default()
+    };
+
+    consensus
+        .validate_header(&SealedHeader::new_unhashed(Header {
+            extra_data: shasta_extra_data(),
+            ..base.clone()
+        }))
+        .expect("the 7-byte extraData layout should validate");
+
+    for (name, extra) in [
+        ("empty", Bytes::new()),
+        ("2 bytes", Bytes::from_static(&[75, 0])),
+        ("3 bytes", Bytes::from_static(&[75, 0, 1])),
+        ("12 bytes", Bytes::from_static(&[0; 12])),
+    ] {
+        let err = consensus
+            .validate_header(&SealedHeader::new_unhashed(Header {
+                extra_data: extra,
+                ..base.clone()
+            }))
+            .expect_err(name);
+        assert!(matches!(err, ConsensusError::Other(_)), "{name}: unexpected error {err:?}");
+    }
+}
+
+#[test]
+fn pre_shasta_header_has_no_extra_data_len_rule() {
+    let mut chain_spec = devnet_chain_spec();
+    chain_spec.inner.hardforks.insert(TaikoHardfork::Shasta, ForkCondition::Never);
+    let consensus = test_consensus(chain_spec);
+    let header = Header {
+        timestamp: 1,
+        base_fee_per_gas: Some(1),
+        gas_limit: 30_000_000,
+        ..Default::default()
+    };
+
+    consensus
+        .validate_header(&SealedHeader::new_unhashed(header))
+        .expect("pre-Shasta headers have no extraData length rule");
+}
+
 #[test]
 fn unzen_post_execution_rejects_body_past_truncation_point() {
     let consensus = test_consensus(unzen_chain_spec());
@@ -273,6 +324,10 @@ fn test_consensus(chain_spec: TaikoChainSpec) -> TaikoBeaconConsensus {
     TaikoBeaconConsensus::new(Arc::new(chain_spec), Arc::new(NullBlockReader))
 }
 
+fn shasta_extra_data() -> Bytes {
+    Bytes::from_static(&[75, 0, 0, 0, 0, 0x4c, 0x81])
+}
+
 fn devnet_chain_spec() -> TaikoChainSpec {
     (*TAIKO_DEVNET).as_ref().clone()
 }

crates/primitives/src/extra_data.rs

@@ -1,6 +1,7 @@
 //! Helpers for decoding Taiko-specific block `extraData` fields.
 
-/// Minimum number of bytes required for Shasta extra data.
+/// Exact length of the Shasta extra data layout: `[basefeeSharingPctg | proposalId(6)]`.
+/// Header validation rejects Shasta headers whose extra data has any other length.
 pub const SHASTA_EXTRA_DATA_LEN: usize = 7;
 
 /// Returns the base fee sharing percentage encoded in Shasta extra data.