fix EIP-7044 implementation when using batch verification (#5953)

In #5120, EIP-7044 support got added to the state transition function to
force `CAPELLA_FORK_VERSION` to be used when validiting `VoluntaryExit`
messages, irrespective of their `epoch`.

In #5637, similar logic was added when batch verifying BLS signatures,
which is used during gossip validation (libp2p gossipsub, and req/resp).
However, that logic did not match the one introduced in #5120, and only
uses `CAPELLA_FORK_VERSION` when a `VoluntaryExit`'s `epoch` was set to
a value `>= CAPELLA_FORK_EPOCH`. Otherwise, `BELLATRIX_FORK_VERSION`
would still be used when validating `VoluntaryExit`, e.g., with `epoch`
set to `0`, as is the case in this Holesky block:

- https://holesky.beaconcha.in/slot/1076985#voluntary-exits

Extracting the correct logic from #5120 into a function, and reusing it
when verifying BLS signatures fixes this issue, and also leverages the
exhaustive EF test suite that covers the (correct) #5120 logic.

This fix only affects networks that have EIP-7044 applied (post-Deneb).

Without the fix, Deneb blocks with a `VoluntaryExit` with `epoch` set to
`< CAPELLA_FORK_EPOCH` incorrectly fail to validate despite being valid.

Incorrect blocks that contain a malicious `VoluntaryExit` with `epoch`
set to `< CAPELLA_FORK_EPOCH` and signed using `BELLATRIX_FORK_VERSION`
_would_ pass the BLS verification stage, but subsequently fail the state
transition logic. Such blocks would still correctly be labeled invalid.

beacon_chain/consensus_object_pools/block_clearance.nim

@@ -285,7 +285,7 @@ proc addHeadBlockWithParent*(
     var sigs: seq[SignatureSet]
     if (let e = sigs.collectSignatureSets(
         signedBlock, dag.db.immutableValidators,
-        dag.clearanceState, dag.cfg.genesisFork(), dag.cfg.capellaFork(),
+        dag.clearanceState, dag.cfg.genesisFork(), dag.cfg.CAPELLA_FORK_VERSION,
         cache); e.isErr()):
       # A PublicKey or Signature isn't on the BLS12-381 curve
       info "Unable to load signature sets",

beacon_chain/spec/signatures.nim

@@ -216,6 +216,20 @@ func compute_voluntary_exit_signing_root*(
       fork, DOMAIN_VOLUNTARY_EXIT, epoch, genesis_validators_root)
   compute_signing_root(voluntary_exit, domain)
 
+func voluntary_exit_signature_fork*(
+    consensusFork: static ConsensusFork,
+    state_fork: Fork,
+    capella_fork_version: Version): Fork =
+  when consensusFork >= ConsensusFork.Deneb:
+    # Always use Capella fork version, disregarding `VoluntaryExit` epoch
+    # [Modified in Deneb:EIP7044]
+    Fork(
+      previous_version: capella_fork_version,
+      current_version: capella_fork_version,
+      epoch: GENESIS_EPOCH)  # irrelevant when current/previous identical
+  else:
+    state_fork
+
 func get_voluntary_exit_signature*(
     fork: Fork, genesis_validators_root: Eth2Digest,
     voluntary_exit: VoluntaryExit,

beacon_chain/spec/signatures_batch.nim

@@ -235,7 +235,7 @@ proc collectSignatureSets*(
        validatorKeys: openArray[ImmutableValidatorData2],
        state: ForkedHashedBeaconState,
        genesis_fork: Fork,
-       capella_fork: Fork,
+       capella_fork_version: Version,
        cache: var StateCache): Result[void, cstring] =
   ## Collect all signature verifications that process_block would normally do
   ## except deposits, in one go.
@@ -385,25 +385,24 @@ proc collectSignatureSets*(
   #   SSZ deserialization guarantees that blocks received from random sources
   #   including peer or RPC
   #   have at most MAX_VOLUNTARY_EXITS voluntary exits.
-  for i in 0 ..< signed_block.message.body.voluntary_exits.len:
-    # don't use "items" for iterating over large type
-    # due to https://github.com/nim-lang/Nim/issues/14421
-    # fixed in 1.4.2
-    template volex: untyped = signed_block.message.body.voluntary_exits[i]
-    let key = validatorKeys.load(volex.message.validator_index).valueOr:
-      return err("collectSignatureSets: invalid voluntary exit")
+  if signed_block.message.body.voluntary_exits.len > 0:
+    let voluntary_exit_fork = withConsensusFork(state.kind):
+      consensusFork.voluntary_exit_signature_fork(fork, capella_fork_version)
+    for i in 0 ..< signed_block.message.body.voluntary_exits.len:
+      # don't use "items" for iterating over large type
+      # due to https://github.com/nim-lang/Nim/issues/14421
+      # fixed in 1.4.2
+      template volex: untyped = signed_block.message.body.voluntary_exits[i]
+      let key = validatorKeys.load(volex.message.validator_index).valueOr:
+        return err("collectSignatureSets: invalid voluntary exit")
 
-    sigs.add voluntary_exit_signature_set(
-      # https://eips.ethereum.org/EIPS/eip-7044
-      # https://github.com/ethereum/consensus-specs/blob/v1.4.0-beta.4/specs/deneb/beacon-chain.md#modified-process_voluntary_exit
-      (if state.kind >= ConsensusFork.Capella:
-         capella_fork
-       else:
-         fork),
-      genesis_validators_root, volex.message, key,
-      volex.signature.load.valueOr do:
-        return err(
-          "collectSignatureSets: cannot load voluntary exit signature"))
+      sigs.add voluntary_exit_signature_set(
+        # https://eips.ethereum.org/EIPS/eip-7044
+        # https://github.com/ethereum/consensus-specs/blob/v1.4.0-beta.7/specs/deneb/beacon-chain.md#modified-process_voluntary_exit
+        voluntary_exit_fork, genesis_validators_root, volex.message, key,
+        volex.signature.load.valueOr do:
+          return err(
+            "collectSignatureSets: cannot load voluntary exit signature"))
 
   block:
     when signed_block is phase0.SignedBeaconBlock:

beacon_chain/spec/state_transition_block.nim

@@ -379,16 +379,11 @@ proc check_voluntary_exit*(
 
   # Verify signature
   if skipBlsValidation notin flags:
-    let exitSignatureFork =
-      when typeof(state).kind >= ConsensusFork.Deneb:
-        Fork(
-          previous_version: cfg.CAPELLA_FORK_VERSION,
-          current_version: cfg.CAPELLA_FORK_VERSION,
-          epoch: cfg.CAPELLA_FORK_EPOCH)
-      else:
-        state.fork
+    const consensusFork = typeof(state).kind
+    let voluntary_exit_fork = consensusFork.voluntary_exit_signature_fork(
+      state.fork, cfg.CAPELLA_FORK_VERSION)
     if not verify_voluntary_exit_signature(
-        exitSignatureFork, state.genesis_validators_root, voluntary_exit,
+        voluntary_exit_fork, state.genesis_validators_root, voluntary_exit,
         validator[].pubkey, signed_voluntary_exit.signature):
       return err("Exit: invalid signature")