Check BLS_TO_EXECUTION_CHANGE as if they were from Capella (#11762)

In the event we receive a BLS_TO_EXECUTION_CHANGE and our head state is
before Capella, verify it's signature with the Capella fork version.

Co-authored-by: Raul Jordan <raul@prysmaticlabs.com>

beacon-chain/core/blocks/withdrawals.go

@@ -176,7 +176,17 @@ func BLSChangesSignatureBatch(
 		Descriptions: make([]string, len(changes)),
 	}
 	epoch := slots.ToEpoch(st.Slot())
-	domain, err := signing.Domain(st.Fork(), epoch, params.BeaconConfig().DomainBLSToExecutionChange, st.GenesisValidatorsRoot())
+	var fork *ethpb.Fork
+	if st.Version() < version.Capella {
+		fork = &ethpb.Fork{
+			PreviousVersion: params.BeaconConfig().BellatrixForkVersion,
+			CurrentVersion:  params.BeaconConfig().CapellaForkVersion,
+			Epoch:           params.BeaconConfig().CapellaForkEpoch,
+		}
+	} else {
+		fork = st.Fork()
+	}
+	domain, err := signing.Domain(fork, epoch, params.BeaconConfig().DomainBLSToExecutionChange, st.GenesisValidatorsRoot())
 	if err != nil {
 		return nil, err
 	}

beacon-chain/core/blocks/withdrawals_test.go

@@ -785,3 +785,73 @@ func TestBLSChangesSignatureBatch(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, true, verify)
 }
+
+func TestBLSChangesSignatureBatchFromBellatrix(t *testing.T) {
+	spb := &ethpb.BeaconStateBellatrix{
+		Fork: &ethpb.Fork{
+			CurrentVersion:  params.BeaconConfig().AltairForkVersion,
+			PreviousVersion: params.BeaconConfig().BellatrixForkVersion,
+		},
+	}
+	numValidators := 10
+	validators := make([]*ethpb.Validator, numValidators)
+	blsChanges := make([]*ethpb.BLSToExecutionChange, numValidators)
+	spb.Balances = make([]uint64, numValidators)
+	privKeys := make([]common.SecretKey, numValidators)
+	maxEffectiveBalance := params.BeaconConfig().MaxEffectiveBalance
+	executionAddress := []byte{0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f, 0x10, 0x11, 0x12, 0x13}
+
+	for i := range validators {
+		v := &ethpb.Validator{}
+		v.EffectiveBalance = maxEffectiveBalance
+		v.WithdrawableEpoch = params.BeaconConfig().FarFutureEpoch
+		v.WithdrawalCredentials = make([]byte, 32)
+		priv, err := bls.RandKey()
+		require.NoError(t, err)
+		privKeys[i] = priv
+		pubkey := priv.PublicKey().Marshal()
+
+		message := &ethpb.BLSToExecutionChange{
+			ToExecutionAddress: executionAddress,
+			ValidatorIndex:     types.ValidatorIndex(i),
+			FromBlsPubkey:      pubkey,
+		}
+
+		hashFn := ssz.NewHasherFunc(hash.CustomSHA256Hasher())
+		digest := hashFn.Hash(pubkey)
+		digest[0] = params.BeaconConfig().BLSWithdrawalPrefixByte
+		copy(v.WithdrawalCredentials, digest[:])
+		validators[i] = v
+		blsChanges[i] = message
+	}
+	spb.Validators = validators
+	st, err := state_native.InitializeFromProtoBellatrix(spb)
+	require.NoError(t, err)
+
+	signedChanges := make([]*ethpb.SignedBLSToExecutionChange, numValidators)
+	spc := &ethpb.BeaconStateCapella{
+		Fork: &ethpb.Fork{
+			CurrentVersion:  params.BeaconConfig().CapellaForkVersion,
+			PreviousVersion: params.BeaconConfig().BellatrixForkVersion,
+			Epoch:           params.BeaconConfig().CapellaForkEpoch,
+		},
+	}
+	stc, err := state_native.InitializeFromProtoCapella(spc)
+	require.NoError(t, err)
+
+	for i, message := range blsChanges {
+		signature, err := signing.ComputeDomainAndSign(stc, time.CurrentEpoch(stc), message, params.BeaconConfig().DomainBLSToExecutionChange, privKeys[i])
+		require.NoError(t, err)
+
+		signed := &ethpb.SignedBLSToExecutionChange{
+			Message:   message,
+			Signature: signature,
+		}
+		signedChanges[i] = signed
+	}
+	batch, err := blocks.BLSChangesSignatureBatch(st, signedChanges)
+	require.NoError(t, err)
+	verify, err := batch.Verify()
+	require.NoError(t, err)
+	require.Equal(t, true, verify)
+}