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historytree.go
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historytree.go
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/*
Package historytree implements part of Crosby's and Wallach's history tree data
structure, as presented in "Efficient Data Structures for Tamper-Evident Logging",
available at http://static.usenix.org/event/sec09/tech/full_papers/crosby.pdf .
We provide membership proofs and omit incremental proofs and signing roots.
We also support computing the updated roots on the history tree based on a
membership proof that fixes the rest of the tree. See the Balloon paper for details.
*/
package historytree
import (
"errors"
"math"
"strconv"
"github.com/mndrix/ps"
"github.com/pylls/balloon/util"
)
var prefixZero = []byte{0x0}
var prefixOne = []byte{0x1}
// Tree is a history tree.
type Tree struct {
//frozen map[Position][]byte
//events map[int][]byte
frozen ps.Map
events ps.Map
size int
}
// Position is a position in the tree.
type Position struct {
Index, Layer int
}
func (p *Position) toString() string {
return strconv.Itoa(p.Index) + "." + strconv.Itoa(p.Layer)
}
// ProofPosition is a position and its hash in the tree.
type ProofPosition struct {
Hash []byte
Position
}
// MembershipProof is a proof of membership of an event.
type MembershipProof struct {
Nodes []ProofPosition
Index, Version int
Event, Root []byte
}
// NewTree returns a new history tree.
func NewTree() (t *Tree) {
t = new(Tree)
t.size = 0
t.frozen = ps.NewMap()
t.events = ps.NewMap()
return
}
// Clone creates a copy of the history tree.
func (t *Tree) Clone() (clone *Tree) {
clone = new(Tree)
clone.size = t.size
clone.frozen = t.frozen
clone.events = t.events
return
}
// layeR, Index, Version
// See Figure 5 in "Efficient Data Structures for Tamper-Evident Logging"
func (t *Tree) getHashedNode(index, layer, version int, proofMode bool) (value []byte, err error) {
// always prefer frozen hashes, if we have calculated them
if proofMode || version >= index+util.Pow(2, layer)-1 {
if t.isFrozenHash(index, layer) {
return t.getFrozenHash(index, layer)
}
}
// special case for child nodes
if layer == 0 && version >= index {
event, err := t.getEvent(index)
if err != nil {
return nil, errors.New("no event with the provided index")
}
value = util.Hash(prefixZero, event)
// have version determine if the right node is there or not
} else if version >= index+util.Pow(2, layer-1) {
a1, err := t.getHashedNode(index, layer-1, version, proofMode)
if err != nil {
return nil, errors.New("failed to get internal node with index " + strconv.Itoa(index))
}
a2, err := t.getHashedNode(index+util.Pow(2, layer-1), layer-1, version, proofMode)
if err != nil {
return nil, errors.New("failed to get internal node with index " + strconv.Itoa(index))
}
value = util.Hash(prefixOne, a1, a2)
} else {
a, err := t.getHashedNode(index, layer-1, version, proofMode)
if err != nil {
return nil, errors.New("failed to get internal node with index " + strconv.Itoa(index))
}
value = util.Hash(prefixOne, a)
}
// should we add this to the frozen hash cache?
if version >= index+util.Pow(2, layer)-1 {
t.setFrozenHash(index, layer, value)
}
return
}
func (t *Tree) isFrozenHash(index, layer int) bool {
p := new(Position)
p.Index = index
p.Layer = layer
_, exists := t.frozen.Lookup(p.toString())
return exists
}
func (t *Tree) setFrozenHash(index, layer int, value []byte) {
p := new(Position)
p.Index = index
p.Layer = layer
t.frozen = t.frozen.Set(p.toString(), value)
}
func (t *Tree) getFrozenHash(index, layer int) (value []byte, err error) {
p := new(Position)
p.Index = index
p.Layer = layer
v, exists := t.frozen.Lookup(p.toString())
if !exists {
return nil, errors.New("no such frozen hash")
}
return v.([]byte), nil
}
// MembershipProof generates a membership proof.
func (t *Tree) MembershipProof(index, version int) (proof MembershipProof, err error) {
if index < 0 || index >= t.Size() || index > version {
return proof, errors.New("invalid index, has to be: 0 <= index <= version < size")
}
proof.Index = index
proof.Version = version
proof.Event, err = t.getEvent(index)
if err != nil {
return
}
proof.Root, err = t.getHashedNode(0, t.calculateDepth(version+1), version, false)
if err != nil {
return
}
// we know that the biggest possible proof is one node per layer
proof.Nodes = make([]ProofPosition, 0, t.getDepth())
err = t.membershipProof(index, 0, t.getDepth(), version, &proof)
return
}
// the game is: walk the tree from the root to the target leaf
func (t *Tree) membershipProof(target, index, layer, version int, proof *MembershipProof) (err error) {
if layer == 0 {
return
}
// the number of events to the left of the node
n := index + util.Pow(2, layer-1)
if target < n {
// go left, but should we save right first? We need to save right if there are any leaf nodes
// fixed by the right node (otherwise we know it's hash is nil), dictated by the version of the
// tree we are generating
if version >= n {
p := new(ProofPosition)
p.Index = n
p.Layer = layer - 1
p.Hash, err = t.getHashedNode(p.Index, p.Layer, version, false)
if err != nil {
return
}
proof.Nodes = append(proof.Nodes, *p)
}
return t.membershipProof(target, index, layer-1, version, proof)
}
// go right, once we have saved the left node
p := new(ProofPosition)
p.Index = index
p.Layer = layer - 1
p.Hash, err = t.getHashedNode(p.Index, p.Layer, version, false)
if err != nil {
return
}
proof.Nodes = append(proof.Nodes, *p)
return t.membershipProof(target, n, layer-1, version, proof)
}
// Verify verifies a membership proof
func (p *MembershipProof) Verify() (correct bool) {
if p.Root == nil || p.Event == nil || p.Index < 0 ||
p.Version < 0 {
return false
}
proofTree := NewTree()
for _, n := range p.Nodes {
proofTree.frozen = proofTree.frozen.Set(n.Position.toString(), n.Hash)
}
proofTree.events = proofTree.events.Set(strconv.Itoa(p.Index), p.Event)
c, err := proofTree.getHashedNode(0, proofTree.calculateDepth(p.Version+1), p.Version, true)
if err != nil {
return false
}
return util.Equal(c, p.Root)
}
func (t *Tree) getEvent(index int) (event []byte, err error) {
e, exists := t.events.Lookup(strconv.Itoa(index))
if !exists {
return nil, errors.New("no such event")
}
return e.([]byte), nil
}
func (t *Tree) setEvent(index int, event []byte) (err error) {
_, exists := t.events.Lookup(strconv.Itoa(index))
if exists {
return errors.New("there is already an event with that index")
}
t.events = t.events.Set(strconv.Itoa(index), event)
t.size++
return
}
func (t *Tree) getDepth() (depth int) {
return t.calculateDepth(t.size)
}
func (t *Tree) calculateDepth(n int) (depth int) {
if n == 0 {
return 0
}
return int(math.Ceil(math.Log2(float64(n))))
}
// Size returns the number of events in the tree.
func (t *Tree) Size() int {
return t.size
}
// LatestVersion returns the latest version of the tree.
func (t *Tree) LatestVersion() int {
return t.size - 1
}
// Add adds an event to the history tree.
func (t *Tree) Add(event []byte) (root []byte, err error) {
err = t.setEvent(t.Size(), event)
if err != nil {
return
}
// Since we use getDepth() here, inside it, the depth is already +1 due to
// len(t.events) being +1 due to the SetEvent above. This means that we get
// a growing tree that supports an arbitrary number of events, as noted by
// Crosby and Wallach on page 6.
return t.getHashedNode(0, t.getDepth(), t.Size()-1, false)
}
// Update calculates the updated history tree, represented by the valid membership
// proof for the latest event inserted into a history tree, by inserting the provided
// events. Returns the updated root, or an error.
func (p *MembershipProof) Update(events [][]byte) (root []byte, version int, err error) {
proofTree := NewTree()
// set all the nodes in the proof, we know they are all frozen
// due to the membership proof being on the last inserted event
for _, n := range p.Nodes {
proofTree.frozen = proofTree.frozen.Set(n.Position.toString(), n.Hash)
}
// check if we are updating an empty history tree or not
var startIndex int
if len(p.Event) == 0 {
proofTree.size = 0
startIndex = 0
} else {
proofTree.size = p.Version + 1
startIndex = p.Index + 1
// set the event we are proving to and the size of the pruned tree
// to the size of the actual tree
proofTree.events = proofTree.events.Set(strconv.Itoa(p.Index), p.Event)
}
// add all events in order of the slice
for i := 0; i < len(events); i++ {
err = proofTree.setEvent(startIndex+i, events[i])
if err != nil {
return
}
}
return proofTree.Root(), proofTree.Size() - 1, nil
}
// Root returns the current root of the tree.
func (t *Tree) Root() (c []byte) {
if t.Size() > 0 {
c, _ = t.getHashedNode(0, t.getDepth(), t.Size()-1, false)
}
return
}
// Size returns the size in bytes of a membership proof.
func (p *MembershipProof) Size() (bytes int) {
bytes = 8 + 8 + len(p.Root) + len(p.Event)
bytes = bytes + len(p.Nodes)*(16+util.HashOutputLen)
return
}