fix: check extra field lens

Author: yiweichi

consensus/system_contract/consensus.go

@@ -110,7 +110,7 @@ func (s *SystemContract) verifyHeader(chain consensus.ChainHeaderReader, header
 		return errInvalidNonce
 	}
 	// Check that the extra-data contains signature
-	if len(header.Extra) != extraSeal {
+	if header.Number != big.NewInt(0) && len(header.Extra) != extraSeal {
 		return errMissingSignature
 	}
 	// Ensure that the mix digest is zero
@@ -121,7 +121,7 @@ func (s *SystemContract) verifyHeader(chain consensus.ChainHeaderReader, header
 	if header.UncleHash != uncleHash {
 		return errInvalidUncleHash
 	}
-	// Ensure that the difficulty is zero
+	// Ensure that the difficulty is one
 	if header.Difficulty.Cmp(common.Big0) != 1 {
 		return errInvalidDifficulty
 	}

consensus/system_contract/system_contract.go

@@ -78,7 +78,6 @@ func (s *SystemContract) Start() {
 				return
 			case <-syncTicker.C:
 				blockNumber := big.NewInt(-1) // todo: get block number from L1BlocksContract (l1 block hash relay) or other source (depending on exact design)
-
 				address, err := s.client.StorageAt(s.ctx, s.config.SystemContractAddress, s.config.SystemContractSlot, blockNumber)
 				if err != nil {
 					log.Error("failed to get signer address from L1 System Contract", "err", err)