Repository: celestiaorg/nmt (go)

Issue:
feat: Handling Panics in HashNode Method
# Problem
Identified as part of this EPIC: https://github.com/celestiaorg/celestia-app/issues/1296.
The HashNode method of the NMT may potentially cause a panic if it receives invalid or malformed inputs (as mentioned by @evan-forbes in this comment https://github.com/celestiaorg/nmt/pull/102#discussion_r1110338821). For example, if the left or right children do not conform to the namespace hash format, meaning their length is less than the namespace ID size, or if the relation between the left and right children namespace range is invalid. If an invalid node is sent to a light node, it can result in a crash.

It is important to note that returning an error from this method is not possible as it would violate the interface it is intended to fulfill i.e., `NodeHasher` and `Hash`.

# Acceptance Criteria
This issue is to implement proper error handling mechanism  for `HashNode`.

Pull request description:
feat: implements input validation functions for HashNode and HashLeaf to prevent panics
## Overview
An incremental PR toward #109 
**Next**: The validation utility functions developed in this PR must be further integrated into both the tree construction and proof verification process to detect invalid inputs and notify the caller of any errors. This will close #109  and  is in line with the objectives outlined in issues #99 and #97. I will address this in a subsequent pull request.
## Checklist

- [x] New and updated code has appropriate documentation
- [x] New and updated code has new and/or updated testing
- [x] Required CI checks are passing
- [x] Visual proof for any user facing features like CLI or documentation updates
- [x] Linked issues closed with keywords


Patch:
diff --git a/hasher.go b/hasher.go
index d87ded9..91db108 100644
--- a/hasher.go
+++ b/hasher.go
@@ -2,6 +2,8 @@ package nmt
 
 import (
 	"bytes"
+	"errors"
+	"fmt"
 	"hash"
 
 	"github.com/celestiaorg/nmt/namespace"
@@ -14,6 +16,11 @@ const (
 
 var _ hash.Hash = (*Hasher)(nil)
 
+var (
+	ErrUnorderedSiblings = errors.New("NMT sibling nodes should be ordered lexicographically by namespace IDs")
+	ErrInvalidNodeLen    = errors.New("invalid NMT node size")
+)
+
 type Hasher struct {
 	baseHasher   hash.Hash
 	NamespaceLen namespace.IDSize
@@ -113,26 +120,94 @@ func (n *Hasher) EmptyRoot() []byte {
 	return digest
 }
 
-// HashLeaf hashes leaves to:
-// ns(leaf) || ns(leaf) || hash(leafPrefix || leaf), where ns(leaf) is the namespaceID
-// inside the leaf's data namely leaf[:n.NamespaceLen]).
-// Hence, the input length has to be greater or equal to the
-// size of the underlying namespace.ID.
-//
-// Note that for leaves minNs = maxNs = ns(leaf) = leaf[:NamespaceLen].
+// IsNamespacedData checks whether data is namespace prefixed.
+func (n *Hasher) IsNamespacedData(data []byte) (err error) {
+	nidSize := int(n.NamespaceSize())
+	lenData := len(data)
+	if lenData < nidSize {
+		return fmt.Errorf("%w: got: %v, want >= %v", ErrMismatchedNamespaceSize, lenData, nidSize)
+	}
+	return nil
+}
+
+// HashLeaf computes namespace hash of the namespaced data item `ndata` as
+// ns(ndata) || ns(ndata) || hash(leafPrefix || ndata), where ns(ndata) is the
+// namespaceID inside the data item namely leaf[:n.NamespaceLen]). Note that for
+// leaves minNs = maxNs = ns(leaf) = leaf[:NamespaceLen]. HashLeaf can panic if
+// the input is not properly namespaced. To avoid panic, call IsNamespacedData
+// on the input data `ndata` before invoking HashLeaf method.
 //
 //nolint:errcheck
-func (n *Hasher) HashLeaf(leaf []byte) []byte {
+func (n *Hasher) HashLeaf(ndata []byte) []byte {
 	h := n.baseHasher
 	h.Reset()
 
-	nID := leaf[:n.NamespaceLen]
+	if err := n.IsNamespacedData(ndata); err != nil {
+		panic(err)
+	}
+
+	nID := ndata[:n.NamespaceLen]
 	resLen := int(2*n.NamespaceLen) + n.baseHasher.Size()
-	res := append(append(make([]byte, 0, resLen), nID...), nID...)
-	// h(0x00, leaf)
-	data := append(append(make([]byte, 0, len(leaf)+1), LeafPrefix), leaf...)
-	h.Write(data)
-	return h.Sum(res)
+	minMaxNIDs := make([]byte, 0, resLen)
+	minMaxNIDs = append(minMaxNIDs, nID...) // nID
+	minMaxNIDs = append(minMaxNIDs, nID...) // nID || nID
+
+	// add LeafPrefix to the ndata
+	leafPrefixedNData := make([]byte, 0, len(ndata)+1)
+	leafPrefixedNData = append(leafPrefixedNData, LeafPrefix)
+	leafPrefixedNData = append(leafPrefixedNData, ndata...)
+	h.Write(leafPrefixedNData)
+
+	// compute h(LeafPrefix || ndata) and append it to the minMaxNIDs
+	nameSpacedHash := h.Sum(minMaxNIDs) // nID || nID || h(LeafPrefix || ndata)
+	return nameSpacedHash
+}
+
+// validateNodeFormat checks whether the supplied node conforms to the
+// namespaced hash format.
+func (n *Hasher) validateNodeFormat(node []byte) (err error) {
+	totalNamespaceLen := 2 * n.NamespaceLen
+	nodeLen := len(node)
+	if nodeLen < int(totalNamespaceLen) {
+		return fmt.Errorf("%w: got: %v, want >= %v", ErrInvalidNodeLen, nodeLen, totalNamespaceLen)
+	}
+	return nil
+}
+
+// validateSiblingsNamespaceOrder checks whether left and right as two sibling
+// nodes in an NMT have correct namespace IDs relative to each other, more
+// specifically, the maximum namespace ID of the left sibling should not exceed
+// the minimum namespace ID of the right sibling. Note that the function assumes
+// that the left and right nodes are in correct format, i.e., they are
+// namespaced hash values.
+func (n *Hasher) validateSiblingsNamespaceOrder(left, right []byte) (err error) {
+	// each NMT node has two namespace IDs for the min and max
+	totalNamespaceLen := 2 * n.NamespaceLen
+	leftMaxNs := namespace.ID(left[n.NamespaceLen:totalNamespaceLen])
+	rightMinNs := namespace.ID(right[:n.NamespaceLen])
+
+	// check the namespace range of the left and right children
+	if rightMinNs.Less(leftMaxNs) {
+		return fmt.Errorf("%w: the maximum namespace of the left child %x is greater than the min namespace of the right child %x", ErrUnorderedSiblings, leftMaxNs, rightMinNs)
+	}
+	return nil
+}
+
+// ValidateNodes is helper function to be called prior to HashNode to verify the
+// validity of the inputs of HashNode and avoid panics. It verifies whether left
+// and right comply by the namespace hash format, and are correctly ordered
+// according to their namespace IDs.
+func (n *Hasher) ValidateNodes(left, right []byte) error {
+	if err := n.validateNodeFormat(left); err != nil {
+		return err
+	}
+	if err := n.validateNodeFormat(right); err != nil {
+		return err
+	}
+	if err := n.validateSiblingsNamespaceOrder(left, right); err != nil {
+		return err
+	}
+	return nil
 }
 
 // HashNode calculates a namespaced hash of a node using the supplied left and
@@ -140,15 +215,16 @@ func (n *Hasher) HashLeaf(leaf []byte) []byte {
 // values with the format "minNID || maxNID || hash." The HashNode function may
 // panic if the inputs provided are invalid, i.e., when left and right are not
 // in the namespaced hash format or when left.maxNID is greater than
-// right.minNID. To prevent panicking, it is advisable to check these criteria
-// before calling the HashNode function. By default, the normal namespace hash
-// calculation is followed, which is "res = min(left.minNID, right.minNID) ||
-// max(left.maxNID, right.maxNID) || H(NodePrefix, left, right)". "res" refers
-// to the return value of the HashNode. However, if the "ignoreMaxNs" property
-// of the Hasher is set to true, the calculation of the namespace ID range of
-// the node slightly changes. In this case, when setting the upper range, the
-// maximum possible namespace ID (i.e., 2^NamespaceIDSize-1) should be ignored
-// if possible. This is achieved by taking the maximum value among the namespace
+// right.minNID. To avoid causing panic, it is recommended to first call
+// ValidateNodes(left, right) to check if the criteria are met before invoking
+// the HashNode function. By default, the normal namespace hash calculation is
+// followed, which is "res = min(left.minNID, right.minNID) || max(left.maxNID,
+// right.maxNID) || H(NodePrefix, left, right)". "res" refers to the return
+// value of the HashNode. However, if the "ignoreMaxNs" property of the Hasher
+// is set to true, the calculation of the namespace ID range of the node
+// slightly changes. In this case, when setting the upper range, the maximum
+// possible namespace ID (i.e., 2^NamespaceIDSize-1) should be ignored if
+// possible. This is achieved by taking the maximum value among the namespace
 // IDs available in the range of its left and right children (i.e.,
 // max(left.minNID, left.maxNID , right.minNID, right.maxNID)), which is not
 // equal to the maximum possible namespace ID value. If such a namespace ID does
@@ -158,19 +234,24 @@ func (n *Hasher) HashNode(left, right []byte) []byte {
 	h := n.baseHasher
 	h.Reset()
 
+	if err := n.validateNodeFormat(left); err != nil {
+		panic(err)
+	}
+	if err := n.validateNodeFormat(right); err != nil {
+		panic(err)
+	}
+
+	// check the namespace range of the left and right children
+	if err := n.validateSiblingsNamespaceOrder(left, right); err != nil {
+		panic(err)
+	}
+
 	// the actual hash result of the children got extended (or flagged) by their
 	// children's minNs || maxNs; hence the flagLen = 2 * NamespaceLen:
 	flagLen := 2 * n.NamespaceLen
 	leftMinNs, leftMaxNs := left[:n.NamespaceLen], left[n.NamespaceLen:flagLen]
 	rightMinNs, rightMaxNs := right[:n.NamespaceLen], right[n.NamespaceLen:flagLen]
 
-	// check the namespace range of the left and right children
-	rightMinNID := namespace.ID(rightMinNs)
-	leftMaxNID := namespace.ID(leftMaxNs)
-	if rightMinNID.Less(leftMaxNID) {
-		panic("nodes are out of order: the maximum namespace of the left child is greater than the min namespace of the right child")
-	}
-
 	minNs := min(leftMinNs, rightMinNs)
 	var maxNs []byte
 	if n.ignoreMaxNs && n.precomputedMaxNs.Equal(leftMinNs) {


Test patch:
diff --git a/hasher_test.go b/hasher_test.go
index 6f025bc..653980d 100644
--- a/hasher_test.go
+++ b/hasher_test.go
@@ -3,6 +3,7 @@ package nmt
 import (
 	"crypto"
 	"crypto/sha256"
+	"errors"
 	"reflect"
 	"testing"
 
@@ -232,3 +233,194 @@ func TestHashNode_ChildrenNamespaceRange(t *testing.T) {
 		})
 	}
 }
+
+func TestValidateSiblingsNamespaceOrder(t *testing.T) {
+	type children struct {
+		l []byte // namespace hash of the left child with the format of MinNs||MaxNs||h
+		r []byte // namespace hash of the right child with the format of MinNs||MaxNs||h
+	}
+
+	tests := []struct {
+		name     string
+		nidLen   namespace.IDSize
+		children children
+		wantErr  bool
+	}{
+		{
+			"left.maxNs>right.minNs", 2,
+			children{[]byte{0, 0, 1, 1}, []byte{0, 0, 1, 1}},
+			true,
+		},
+		{
+			"left.maxNs=right.minNs", 2,
+			children{[]byte{0, 0, 1, 1}, []byte{1, 1, 2, 2}},
+			false,
+		},
+		{
+			"left.maxNs<right.minNs", 2,
+			children{[]byte{0, 0, 1, 1}, []byte{2, 2, 3, 3}},
+			false,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			n := NewNmtHasher(sha256.New(), tt.nidLen, false)
+			err := n.validateSiblingsNamespaceOrder(tt.children.l, tt.children.r)
+			assert.Equal(t, tt.wantErr, err != nil)
+		})
+	}
+}
+
+func TestValidateNodeFormat(t *testing.T) {
+	tests := []struct {
+		name    string
+		nIDLen  namespace.IDSize
+		minNID  []byte
+		maxNID  []byte
+		hash    []byte
+		wantErr bool
+		errType error
+	}{
+		{ // valid node
+			"valid node",
+			2,
+			[]byte{0, 0},
+			[]byte{1, 1},
+			[]byte{1, 2, 3, 4},
+			false,
+			nil,
+		},
+		{ // mismatched namespace size
+			"invalid node: length",
+			2,
+			[]byte{0},
+			[]byte{1},
+			[]byte{0},
+			true,
+			ErrInvalidNodeLen,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			n := NewNmtHasher(sha256.New(), tt.nIDLen, false)
+			err := n.validateNodeFormat(append(append(tt.minNID, tt.maxNID...), tt.hash...))
+			assert.Equal(t, tt.wantErr, err != nil)
+			if tt.wantErr {
+				assert.True(t, errors.Is(err, tt.errType))
+			}
+		})
+	}
+}
+
+func TestIsNamespacedData(t *testing.T) {
+	tests := []struct {
+		name    string
+		data    []byte
+		nIDLen  namespace.IDSize
+		wantErr bool
+	}{
+		{
+			"valid namespaced data",
+			[]byte{0, 0},
+			2,
+			false,
+		},
+		{
+			"non-namespaced data",
+			[]byte{1},
+			2,
+			true,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			n := NewNmtHasher(sha256.New(), tt.nIDLen, false)
+			assert.Equal(t, tt.wantErr, n.IsNamespacedData(tt.data) != nil)
+		})
+	}
+}
+
+func TestHashLeafWithIsNamespacedData(t *testing.T) {
+	tests := []struct {
+		name    string
+		data    []byte
+		nIDLen  namespace.IDSize
+		wantErr bool
+	}{
+		{
+			"valid namespaced data",
+			[]byte{0, 0},
+			2,
+			false,
+		},
+		{
+			"non-namespaced data",
+			[]byte{1},
+			2,
+			true,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			n := NewNmtHasher(sha256.New(), tt.nIDLen, false)
+			if tt.wantErr {
+				require.Error(t, n.IsNamespacedData(tt.data))
+				require.Panics(t, func() {
+					n.HashLeaf(tt.data)
+				})
+			}
+		})
+	}
+}
+
+func TestHashNodeWithValidateNodes(t *testing.T) {
+	type children struct {
+		l []byte // namespace hash of the left child with the format of MinNs||MaxNs||h
+		r []byte // namespace hash of the right child with the format of MinNs||MaxNs||h
+	}
+
+	tests := []struct {
+		name     string
+		nidLen   namespace.IDSize
+		children children
+		wantErr  bool
+	}{
+		{
+			"left.maxNs<right.minNs", 2,
+			children{[]byte{0, 0, 1, 1}, []byte{2, 2, 3, 3}},
+			false,
+		},
+		{
+			"left.maxNs=right.minNs", 2,
+			children{[]byte{0, 0, 1, 1}, []byte{1, 1, 2, 2}},
+			false,
+		},
+		{
+			"left.maxNs>right.minNs", 2,
+			children{[]byte{0, 0, 1, 1}, []byte{0, 0, 1, 1}},
+			true,
+		},
+		{
+			"len(left)<NamespaceLen", 2,
+			children{[]byte{0, 0, 1}, []byte{2, 2, 3, 3}},
+			true,
+		},
+		{
+			"len(right)<NamespaceLen", 2,
+			children{[]byte{0, 0, 1, 1}, []byte{2, 2, 3}},
+			true,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			n := NewNmtHasher(sha256.New(), tt.nidLen, false)
+			if tt.wantErr {
+				require.Error(t, n.ValidateNodes(tt.children.l, tt.children.r))
+				require.Panics(t, func() {
+					n.HashNode(tt.children.l, tt.children.r)
+				})
+			}
+		})
+	}
+}