Fix eviction of absent participant

share/dkg/dkg.go

@@ -552,6 +552,7 @@ func (d *DistKeyGenerator) ProcessResponses(bundles []*ResponseBundle) (*Result,
 		return res, nil, err
 	}
 
+	var validAuthors []Index
 	var foundComplaint bool
 	for _, bundle := range bundles {
 		if bundle == nil {
@@ -566,8 +567,7 @@ func (d *DistKeyGenerator) ProcessResponses(bundles []*ResponseBundle) (*Result,
 		}
 
 		if bytes.Compare(bundle.SessionID, d.c.Nonce) != 0 {
-			// XXX for the moment we continue,
-			// TODO: fix it with a proper eviction list of share holders
+			d.evictedHolders = append(d.evictedHolders, bundle.ShareIndex)
 			continue
 		}
 
@@ -591,8 +591,28 @@ func (d *DistKeyGenerator) ProcessResponses(bundles []*ResponseBundle) (*Result,
 			if response.Status == Complaint {
 				foundComplaint = true
 			}
+			validAuthors = append(validAuthors, bundle.ShareIndex)
 		}
 	}
+	// In case of fast sync, we want to make sure all share holders have sent a
+	// valid response. All share holders that did not will be evicted from the
+	// final group. Since we are using a broadcast channel, if a node is honest,
+	// its response will be received by all honest nodes.
+	if d.c.FastSync {
+		// we only need to look at the nodes that did not sent any response,
+		// since the invalid one are already markes as evicted
+		allSent := append(validAuthors, d.evictedHolders...)
+		isPresent := isEvicted
+		for _, n := range d.c.NewNodes {
+			if d.canReceive && d.nidx == n.Index {
+				continue // we dont evict ourself
+			}
+			if !isPresent(allSent, n.Index) {
+				d.evictedHolders = append(d.evictedHolders, n.Index)
+			}
+		}
+	}
+
 	if !foundComplaint {
 		// there is no complaint !
 		if d.canReceive && d.statuses.CompleteSuccess() {
@@ -883,7 +903,6 @@ func (d *DistKeyGenerator) computeResharingResult() (*Result, error) {
 			qual = append(qual, newNode)
 		}
 	}
-
 	if len(qual) < d.c.Threshold {
 		return nil, fmt.Errorf("dkg: too many uncompliant new participants %d/%d", len(qual), d.c.Threshold)
 	}

share/dkg/dkg_test.go

@@ -1,6 +1,7 @@
 package dkg
 
 import (
+	"fmt"
 	"testing"
 
 	"github.com/drand/kyber"
@@ -215,6 +216,112 @@ func TestDKGFull(t *testing.T) {
 	testResults(t, suite, thr, n, results)
 }
 
+func TestDKGResharing(t *testing.T) {
+	n := 5
+	thr := 4
+	var suite = bn256.NewSuiteG2()
+	var sigSuite = bn256.NewSuiteG1()
+	tns := GenerateTestNodes(suite, n)
+	list := NodesFromTest(tns)
+	conf := Config{
+		Suite:     suite,
+		NewNodes:  list,
+		Threshold: thr,
+		Auth:      schnorr.NewScheme(suite),
+	}
+
+	results := RunDKG(t, tns, conf, nil, nil, nil)
+	for i, t := range tns {
+		t.res = results[i]
+	}
+	testResults(t, suite, thr, n, results)
+
+	// create a partial signature with the share now and make sure the partial
+	// signature is verifiable and then *not* verifiable after the resharing
+	oldShare := results[0].Key.Share
+	msg := []byte("Hello World")
+	scheme := tbls.NewThresholdSchemeOnG1(sigSuite)
+	oldPartial, err := scheme.Sign(oldShare, msg)
+	require.NoError(t, err)
+	poly := share.NewPubPoly(suite, suite.Point().Base(), results[0].Key.Commits)
+	require.NoError(t, scheme.VerifyPartial(poly, msg, oldPartial))
+
+	// we setup now the second group with higher node count and higher threshold
+	// and we remove one node from the previous group
+	newN := n + 5
+	newT := thr + 4
+	var newTns = make([]*TestNode, newN)
+	// remove the last node from the previous group
+	offline := 1
+	copy(newTns, tns[:n-offline])
+	// + offline because we fill the gap of the offline nodes by new nodes
+	newNode := newN - n + offline
+	for i := 0; i < newNode; i++ {
+		//  new node can have the same index as a previous one, separation is made
+		newTns[n-1+i] = NewTestNode(suite, n-1+i)
+	}
+	newList := NodesFromTest(newTns)
+	newConf := &Config{
+		Suite:        suite,
+		NewNodes:     newList,
+		OldNodes:     list,
+		Threshold:    newT,
+		OldThreshold: thr,
+		Auth:         schnorr.NewScheme(suite),
+	}
+
+	SetupReshareNodes(newTns, newConf, tns[0].res.Key.Commits)
+
+	var deals []*DealBundle
+	for _, node := range newTns {
+		if node.res == nil {
+			// new members don't issue deals
+			continue
+		}
+		d, err := node.dkg.Deals()
+		require.NoError(t, err)
+		deals = append(deals, d)
+	}
+
+	var responses []*ResponseBundle
+	for _, node := range newTns {
+		resp, err := node.dkg.ProcessDeals(deals)
+		require.NoError(t, err)
+		if resp != nil {
+			// last node from the old group is not present so there should be
+			// some responses !
+			responses = append(responses, resp)
+		}
+	}
+	require.True(t, len(responses) > 0)
+
+	results = nil
+	for _, node := range newTns {
+		res, just, err := node.dkg.ProcessResponses(responses)
+		require.NoError(t, err)
+		require.Nil(t, res)
+		// since the last old node is absent he can't give any justifications
+		require.Nil(t, just)
+	}
+
+	for _, node := range newTns {
+		res, err := node.dkg.ProcessJustifications(nil)
+		require.NoError(t, err)
+		require.NotNil(t, res)
+		results = append(results, res)
+	}
+	testResults(t, suite, newT, newN, results)
+
+	// test a tbls signature is correct
+	newShare := results[0].Key.Share
+	newPartial, err := scheme.Sign(newShare, msg)
+	require.NoError(t, err)
+	newPoly := share.NewPubPoly(suite, suite.Point().Base(), results[0].Key.Commits)
+	require.NoError(t, scheme.VerifyPartial(newPoly, msg, newPartial))
+	// test we can not verify the old partial with the new public polynomial
+	require.Error(t, scheme.VerifyPartial(poly, msg, newPartial))
+}
+
 func TestDKGThreshold(t *testing.T) {
 	n := 5
 	thr := 4
@@ -280,8 +387,8 @@ func TestDKGThreshold(t *testing.T) {
 	testResults(t, suite, thr, n, filtered)
 }
 
-func TestDKGResharing(t *testing.T) {
-	n := 5
+func TestDKGResharingFast(t *testing.T) {
+	n := 6
 	thr := 4
 	var suite = bn256.NewSuiteG2()
 	var sigSuite = bn256.NewSuiteG1()
@@ -346,13 +453,26 @@ func TestDKGResharing(t *testing.T) {
 		newTns[n-1+i] = NewTestNode(suite, n-1+i)
 	}
 	newList := NodesFromTest(newTns)
+	// key from the previous and new group which is registered in the
+	// group but wont participate
+	p := 1
+	skipKey := list[p].Public
+	var skipNew Index
+	for _, n := range newList {
+		if n.Public.Equal(skipKey) {
+			skipNew = n.Index
+		}
+	}
+	fmt.Println("skipping old index: ", list[p].Index, "public key", skipKey, "newIdx", skipNew)
+
 	newConf := &Config{
 		Suite:        suite,
 		NewNodes:     newList,
 		OldNodes:     list,
 		Threshold:    newT,
 		OldThreshold: thr,
 		Auth:         schnorr.NewScheme(suite),
+		FastSync:     true,
 	}
 
 	SetupReshareNodes(newTns, newConf, tns[0].res.Key.Commits)
@@ -363,15 +483,22 @@ func TestDKGResharing(t *testing.T) {
 			// new members don't issue deals
 			continue
 		}
+		if node.Public.Equal(skipKey) {
+			continue
+		}
 		d, err := node.dkg.Deals()
 		require.NoError(t, err)
 		deals = append(deals, d)
 	}
 
 	var responses []*ResponseBundle
 	for _, node := range newTns {
+		if node.Public.Equal(skipKey) {
+			continue
+		}
 		resp, err := node.dkg.ProcessDeals(deals)
 		require.NoError(t, err)
+
 		if resp != nil {
 			// last node from the old group is not present so there should be
 			// some responses !
@@ -381,20 +508,39 @@ func TestDKGResharing(t *testing.T) {
 	require.True(t, len(responses) > 0)
 
 	results = nil
+	var justifs []*JustificationBundle
 	for _, node := range newTns {
+		if node.Public.Equal(skipKey) {
+			continue
+		}
 		res, just, err := node.dkg.ProcessResponses(responses)
 		require.NoError(t, err)
 		require.Nil(t, res)
-		// since the last old node is absent he can't give any justifications
-		require.Nil(t, just)
+		if node.res == nil {
+			// new members don't issue justifications
+			continue
+		}
+		require.NotNil(t, just.Justifications)
+		fmt.Println(just)
+		require.Equal(t, just.Justifications[0].ShareIndex, skipNew)
+		justifs = append(justifs, just)
 	}
 
 	for _, node := range newTns {
-		res, err := node.dkg.ProcessJustifications(nil)
+		if node.Public.Equal(skipKey) {
+			continue
+		}
+		res, err := node.dkg.ProcessJustifications(justifs)
 		require.NoError(t, err)
 		require.NotNil(t, res)
 		results = append(results, res)
 	}
+
+	for _, res := range results {
+		for _, n := range res.QUAL {
+			require.False(t, n.Public.Equal(skipKey))
+		}
+	}
 	testResults(t, suite, newT, newN, results)
 
 	// test a tbls signature is correct