fix absent keys in proofs (#401)

* fix absent keys in proofs

Signed-off-by: Ignacio Hagopian <jsign.uy@gmail.com>

* add POA stub marker

Signed-off-by: Ignacio Hagopian <jsign.uy@gmail.com>

* defensive case

Signed-off-by: Ignacio Hagopian <jsign.uy@gmail.com>

* fixes

Signed-off-by: Ignacio Hagopian <jsign.uy@gmail.com>

* improving comment

Signed-off-by: Ignacio Hagopian <jsign.uy@gmail.com>

---------

Signed-off-by: Ignacio Hagopian <jsign.uy@gmail.com>

doc.go

@@ -35,6 +35,7 @@ var (
 	errInsertIntoOtherStem    = errors.New("insert splits a stem where it should not happen")
 	errUnknownNodeType        = errors.New("unknown node type detected")
 	errMissingNodeInStateless = errors.New("trying to access a node that is missing from the stateless view")
+	errIsPOAStub              = errors.New("trying to read/write a proof of absence leaf node")
 )
 
 const (

proof_test.go

@@ -32,6 +32,8 @@ import (
 	"fmt"
 	"reflect"
 	"testing"
+
+	"github.com/crate-crypto/go-ipa/common"
 )
 
 func TestProofVerifyTwoLeaves(t *testing.T) {
@@ -472,7 +474,7 @@ func TestProofOfAbsenceOtherMultipleLeaves(t *testing.T) {
 	if err := root.Insert(key, testValue, nil); err != nil {
 		t.Fatalf("could not insert key: %v", err)
 	}
-	root.Commit()
+	rootC := root.Commit()
 
 	ret1, _ := hex.DecodeString("0303030303030303030303030303030303030303030303030303030303030300")
 	ret2, _ := hex.DecodeString("0303030303030303030303030303030303030303030303030303030303030301")
@@ -485,6 +487,45 @@ func TestProofOfAbsenceOtherMultipleLeaves(t *testing.T) {
 	if len(proof.PoaStems) > 1 {
 		t.Fatalf("invalid number of proof-of-absence stems: %d", len(proof.PoaStems))
 	}
+
+	deserialized, err := PreStateTreeFromProof(proof, rootC)
+	if err != nil {
+		t.Fatalf("error deserializing %v", err)
+	}
+
+	got, err := deserialized.Get(ret1, nil)
+	if err != nil {
+		t.Fatalf("error while trying to read missing value: %v", err)
+	}
+	if got != nil {
+		t.Fatalf("should have returned nil, got: %v", got)
+	}
+
+	// simulate the execution of a tx that creates a leaf at an address that isn't the one that is
+	// proven for absence, but needs to be inserted in the proof-of-absence stem.
+	// It differs from the poa stem here: 🠃
+	ret3, _ := hex.DecodeString("0303030304030303030303030303030303030303030303030303030303030300")
+	err = deserialized.Insert(ret3, testValue, nil)
+	if err != nil {
+		t.Fatalf("error inserting value in proof-of-asbsence stem: %v", err)
+	}
+
+	// check that there are splits up to depth 4
+	node := deserialized.(*InternalNode)
+	for node.depth < 4 {
+		child, ok := node.children[ret3[node.depth]].(*InternalNode)
+		if !ok {
+			t.Fatalf("expected Internal node at depth %d, trie = %s", node.depth, ToDot(deserialized))
+		}
+		node = child
+	}
+
+	if _, ok := node.children[ret3[4]].(*LeafNode); !ok {
+		t.Fatalf("expected leaf node at depth 5, got %v", node.children[ret3[4]])
+	}
+	if ln, ok := node.children[key[4]].(*LeafNode); !ok || !ln.isPOAStub {
+		t.Fatalf("expected unknown node at depth 5, got %v", node.children[key[4]])
+	}
 }
 
 func TestProofOfAbsenceNoneMultipleStems(t *testing.T) {
@@ -990,4 +1031,28 @@ func TestGenerateProofWithOnlyAbsentKeys(t *testing.T) {
 	if ok, err := VerifyVerkleProof(dproof, pe.Cis, pe.Zis, pe.Yis, cfg); !ok || err != nil {
 		t.Fatalf("reconstructed proof didn't verify: %v", err)
 	}
+
+	// Double-check that if we try to access any key in 40000000000000000000000000000000000000000000000000000000000000{XX}
+	// in the reconstructed tree, we get an error. This LeafNode is only supposed to prove
+	// the absence of 40100000000000000000000000000000000000000000000000000000000000{YY}, so
+	// we don't know anything about any value for slots XX.
+	for i := 0; i < common.VectorLength; i++ {
+		var key [32]byte
+		copy(key[:], presentKey)
+		key[31] = byte(i)
+		if _, err := droot.Get(key[:], nil); err != errIsPOAStub {
+			t.Fatalf("expected ErrPOALeafValue, got %v", err)
+		}
+	}
+
+	// The same applies to trying to insert values in this LeafNode, this shouldn't be allowed since we don't know
+	// anything about C1 or C2 to do a proper updating.
+	for i := 0; i < common.VectorLength; i++ {
+		var key [32]byte
+		copy(key[:], presentKey)
+		key[31] = byte(i)
+		if err := droot.Insert(key[:], zeroKeyTest, nil); err != errIsPOAStub {
+			t.Fatalf("expected ErrPOALeafValue, got %v", err)
+		}
+	}
 }

tree.go

@@ -183,6 +183,11 @@ type (
 		c1, c2     *Point
 
 		depth byte
+
+		// IsPOAStub indicates if this LeafNode is a proof of absence
+		// for a steam that isn't present in the tree. This flag is only
+		// true in the context of a stateless tree.
+		isPOAStub bool
 	}
 )
 
@@ -380,10 +385,15 @@ func (n *InternalNode) InsertStem(stem []byte, values [][]byte, resolver NodeRes
 		// splits.
 		return n.InsertStem(stem, values, resolver)
 	case *LeafNode:
-		n.cowChild(nChild)
 		if equalPaths(child.stem, stem) {
+			// We can't insert any values into a POA leaf node.
+			if child.isPOAStub {
+				return errIsPOAStub
+			}
+			n.cowChild(nChild)
 			return child.insertMultiple(stem, values)
 		}
+		n.cowChild(nChild)
 
 		// A new branch node has to be inserted. Depending
 		// on the next word in both keys, a recursion into
@@ -439,6 +449,15 @@ func (n *InternalNode) CreatePath(path []byte, stemInfo stemInfo, comms []*Point
 			n.children[path[0]] = Empty{}
 		case extStatusAbsentOther:
 			// insert poa stem
+			newchild := &LeafNode{
+				commitment: comms[0],
+				stem:       stemInfo.stem,
+				values:     nil,
+				depth:      n.depth + 1,
+				isPOAStub:  true,
+			}
+			n.children[path[0]] = newchild
+			comms = comms[1:]
 		case extStatusPresent:
 			// insert stem
 			newchild := &LeafNode{
@@ -518,6 +537,11 @@ func (n *InternalNode) GetStem(stem []byte, resolver NodeResolverFn) ([][]byte,
 		return n.GetStem(stem, resolver)
 	case *LeafNode:
 		if equalPaths(child.stem, stem) {
+			// We can't return the values since it's a POA leaf node, so we know nothing
+			// about its values.
+			if child.isPOAStub {
+				return nil, errIsPOAStub
+			}
 			return child.values, nil
 		}
 		return nil, nil
@@ -1014,6 +1038,10 @@ func (n *InternalNode) touchCoW(index byte) {
 }
 
 func (n *LeafNode) Insert(key []byte, value []byte, _ NodeResolverFn) error {
+	if n.isPOAStub {
+		return errIsPOAStub
+	}
+
 	if len(key) != StemSize+1 {
 		return fmt.Errorf("invalid key size: %d", len(key))
 	}
@@ -1268,6 +1296,10 @@ func (n *LeafNode) Delete(k []byte, _ NodeResolverFn) (bool, error) {
 }
 
 func (n *LeafNode) Get(k []byte, _ NodeResolverFn) ([]byte, error) {
+	if n.isPOAStub {
+		return nil, errIsPOAStub
+	}
+
 	if !equalPaths(k, n.stem) {
 		// If keys differ, return nil in order to
 		// signal that the key isn't present in the
@@ -1368,19 +1400,36 @@ func (n *LeafNode) GetProofItems(keys keylist, _ NodeResolverFn) (*ProofElements
 	if err := StemFromBytes(&poly[1], n.stem); err != nil {
 		return nil, nil, nil, fmt.Errorf("error serializing stem '%x': %w", n.stem, err)
 	}
-	if err := banderwagon.BatchMapToScalarField([]*Fr{&poly[2], &poly[3]}, []*Point{n.c1, n.c2}); err != nil {
-		return nil, nil, nil, fmt.Errorf("batch mapping to scalar fields: %s", err)
-	}
 
 	// First pass: add top-level elements first
 	var hasC1, hasC2 bool
 	for _, key := range keys {
-		hasC1 = hasC1 || (key[31] < 128)
-		hasC2 = hasC2 || (key[31] >= 128)
-		if hasC2 {
-			break
+		// Note that keys might contain keys that don't correspond to this leaf node.
+		// We should only analize the inclusion of C1/C2 for keys corresponding to this
+		// leaf node stem.
+		if equalPaths(n.stem, key) {
+			hasC1 = hasC1 || (key[31] < 128)
+			hasC2 = hasC2 || (key[31] >= 128)
+			if hasC2 {
+				break
+			}
+		}
+	}
+
+	// If this tree is a full tree (i.e: not a stateless tree), we know we have c1 and c2 values.
+	// Also, we _need_ them independently of hasC1 or hasC2 since the prover needs `Fis`.
+	if !n.isPOAStub {
+		if err := banderwagon.BatchMapToScalarField([]*Fr{&poly[2], &poly[3]}, []*Point{n.c1, n.c2}); err != nil {
+			return nil, nil, nil, fmt.Errorf("batch mapping to scalar fields: %s", err)
 		}
+	} else if hasC1 || hasC2 || n.c1 != nil || n.c2 != nil {
+		// This LeafNode is a proof of absence stub. It must be true that
+		// both c1 and c2 are nil, and that hasC1 and hasC2 are false.
+		// Let's just check that to be sure, since the code below can't use
+		// poly[2] or poly[3].
+		return nil, nil, nil, fmt.Errorf("invalid proof of absence stub")
 	}
+
 	if hasC1 {
 		pe.Cis = append(pe.Cis, n.commitment)
 		pe.Zis = append(pe.Zis, 2)