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Reimplement proof verifier #24

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Reimplement proof verifier #24

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217 changes: 98 additions & 119 deletions proof/verify.go
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Original file line number Diff line number Diff line change
@@ -1,4 +1,4 @@
// Copyright 2017 Google LLC. All Rights Reserved.
// Copyright 2022 Google LLC. All Rights Reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
Expand All @@ -16,161 +16,140 @@ package proof

import (
"bytes"
"errors"
"fmt"
"math/bits"

"github.com/transparency-dev/merkle"
"github.com/transparency-dev/merkle/compact"
)

// RootMismatchError occurs when an inclusion proof fails.
// RootMismatchError is an error occuring when a proof verification fails.
type RootMismatchError struct {
ExpectedRoot []byte
CalculatedRoot []byte
Size uint64 // The size at which the root hash mismatch happened.
Computed []byte // The computed root hash at this size.
Expected []byte // The expected root hash at this size.
}

// Error returns the error string for RootMismatchError.
func (e RootMismatchError) Error() string {
return fmt.Sprintf("calculated root:\n%v\n does not match expected root:\n%v", e.CalculatedRoot, e.ExpectedRoot)
return fmt.Sprintf("root hash at size %d mismatched: computed %x, expected %x", e.Size, e.Computed, e.Expected)
}

func verifyMatch(calculated, expected []byte) error {
if !bytes.Equal(calculated, expected) {
return RootMismatchError{ExpectedRoot: expected, CalculatedRoot: calculated}
func verifyMatch(size uint64, computed, expected []byte) error {
if !bytes.Equal(computed, expected) {
return RootMismatchError{Size: size, Computed: computed, Expected: expected}
}
return nil
}

// NodeHasher allows computing hashes of internal nodes of the Merkle tree.
type NodeHasher interface {
// HashChildren returns hash of a tree node based on hashes of its children.
HashChildren(left, right []byte) []byte
}

// VerifyInclusion verifies the correctness of the inclusion proof for the leaf
// with the specified hash and index, relatively to the tree of the given size
// and root hash. Requires 0 <= index < size.
func VerifyInclusion(hasher merkle.LogHasher, index, size uint64, leafHash []byte, proof [][]byte, root []byte) error {
calcRoot, err := RootFromInclusionProof(hasher, index, size, leafHash, proof)
if err != nil {
return err
// and root hash. Requires 0 <= index < size. Returns RootMismatchError if the
// computed root hash does not match the provided one.
func VerifyInclusion(nh NodeHasher, index, size uint64, hash []byte, proof [][]byte, root []byte) error {
if index >= size {
return fmt.Errorf("index %d out of range for size %d", index, size)
}
return verifyMatch(calcRoot, root)
return verify(nh, index, 0, size, hash, proof, root)
}

// RootFromInclusionProof calculates the expected root hash for a tree of the
// given size, provided a leaf index and hash with the corresponding inclusion
// proof. Requires 0 <= index < size.
func RootFromInclusionProof(hasher merkle.LogHasher, index, size uint64, leafHash []byte, proof [][]byte) ([]byte, error) {
if index >= size {
return nil, fmt.Errorf("index is beyond size: %d >= %d", index, size)
// VerifyConsistency verifies that the consistency proof is valid between the
// two given tree sizes, with the corresponding root hashes.
// Requires 0 <= size1 <= size2. Returns RootMismatchError if any of the
// computed root hashes at size1 or size2 does not match the provided one.
func VerifyConsistency(nh NodeHasher, size1, size2 uint64, proof [][]byte, root1, root2 []byte) error {
if size1 > size2 {
return fmt.Errorf("tree size %d > %d", size1, size2)
}
if got, want := len(leafHash), hasher.Size(); got != want {
return nil, fmt.Errorf("leafHash has unexpected size %d, want %d", got, want)
if (size1 == size2 || size1 == 0) && len(proof) != 0 {
return fmt.Errorf("incorrect proof size: got %d, want 0", len(proof))
}

inner, border := decompInclProof(index, size)
if got, want := len(proof), inner+border; got != want {
return nil, fmt.Errorf("wrong proof size %d, want %d", got, want)
if size1 == size2 {
return verifyMatch(size1, root1, root2)
}

res := chainInner(hasher, leafHash, proof[:inner], index)
res = chainBorderRight(hasher, res, proof[inner:])
return res, nil
}

// VerifyConsistency checks that the passed-in consistency proof is valid
// between the passed in tree sizes, with respect to the corresponding root
// hashes. Requires 0 <= size1 <= size2.
func VerifyConsistency(hasher merkle.LogHasher, size1, size2 uint64, proof [][]byte, root1, root2 []byte) error {
switch {
case size2 < size1:
return fmt.Errorf("size2 (%d) < size1 (%d)", size1, size2)
case size1 == size2:
if len(proof) > 0 {
return errors.New("size1=size2, but proof is not empty")
}
return verifyMatch(root1, root2)
case size1 == 0:
// Any size greater than 0 is consistent with size 0.
if len(proof) > 0 {
return fmt.Errorf("expected empty proof, but got %d components", len(proof))
}
return nil // Proof OK.
case len(proof) == 0:
return errors.New("empty proof")
if size1 == 0 {
return nil
}

inner, border := decompInclProof(size1-1, size2)
shift := bits.TrailingZeros64(size1)
inner -= shift // Note: shift < inner if size1 < size2.

// The proof includes the root hash for the sub-tree of size 2^shift.
seed, start := proof[0], 1
if size1 == 1<<uint(shift) { // Unless size1 is that very 2^shift.
seed, start = root1, 0
}
if got, want := len(proof), start+inner+border; got != want {
return fmt.Errorf("wrong proof size %d, want %d", got, want)
// Find the root of the biggest perfect subtree that ends at size1.
level := uint(bits.TrailingZeros64(size1))
index := (size1 - 1) >> level
// The consistency proof consists of this node (except if size1 is a power of
// two, in which case adding this node would be redundant because the client
// is assumed to know it from a checkpoint), and nodes of the inclusion proof
// into this node in the tree of size2.

// Handle the case when size1 is a power of 2.
if index == 0 {
return verify(nh, index, level, size2, root1, proof, root2)
}
proof = proof[start:]
// Now len(proof) == inner+border, and proof is effectively a suffix of
// inclusion proof for entry |size1-1| in a tree of size |size2|.

// Verify the first root.
mask := (size1 - 1) >> uint(shift) // Start chaining from level |shift|.
hash1 := chainInnerRight(hasher, seed, proof[:inner], mask)
hash1 = chainBorderRight(hasher, hash1, proof[inner:])
if err := verifyMatch(hash1, root1); err != nil {
// Otherwise, the consistency proof is equivalent to an inclusion proof of
// its first hash. Verify it below.
if got, want := len(proof), 1+bits.Len64(size2-1)-int(level); got != want {
return fmt.Errorf("incorrect proof size: %d, want %d", got, want)
}
if err := verify(nh, index, level, size2, proof[0], proof[1:], root2); err != nil {
return err
}

// Verify the second root.
hash2 := chainInner(hasher, seed, proof[:inner], mask)
hash2 = chainBorderRight(hasher, hash2, proof[inner:])
return verifyMatch(hash2, root2)
inner := bits.Len64(index^(size2>>level)) - 1
hash := proof[0]
for i, h := range proof[1 : 1+inner] {
if (index>>uint(i))&1 == 1 {
hash = nh.HashChildren(h, hash)
}
}
for _, h := range proof[1+inner:] {
hash = nh.HashChildren(h, hash)
}
return verifyMatch(size1, hash, root1)
}

// decompInclProof breaks down inclusion proof for a leaf at the specified
// |index| in a tree of the specified |size| into 2 components. The splitting
// point between them is where paths to leaves |index| and |size-1| diverge.
// Returns lengths of the bottom and upper proof parts correspondingly. The sum
// of the two determines the correct length of the inclusion proof.
func decompInclProof(index, size uint64) (int, int) {
inner := innerProofSize(index, size)
border := bits.OnesCount64(index >> uint(inner))
return inner, border
}
func verify(nh NodeHasher, index uint64, level uint, size uint64, hash []byte, proof [][]byte, root []byte) error {
// Compute the `fork` node, where the path from root to (level, index) node
// diverges from the path to (0, size).
//
// The sibling of this node is the ephemeral node which represents a subtree
// that is not complete in the tree of the given size. To compute the hash
// of the ephemeral node, we need all the non-ephemeral nodes that cover the
// same range of leaves.
//
// The `inner` variable is how many layers up from (level, index) the `fork`
// and the ephemeral nodes are.
inner := bits.Len64(index^(size>>level)) - 1
fork := compact.NewNodeID(level+uint(inner), index>>inner)

begin, end := fork.Coverage()
left := compact.RangeSize(0, begin)
right := 1
if end == size { // No ephemeral nodes.
right = 0
}

func innerProofSize(index, size uint64) int {
return bits.Len64(index ^ (size - 1))
}
if got, want := len(proof), inner+right+left; got != want {
return fmt.Errorf("incorrect proof size: %d, want %d", got, want)
}

// chainInner computes a subtree hash for a node on or below the tree's right
// border. Assumes |proof| hashes are ordered from lower levels to upper, and
// |seed| is the initial subtree/leaf hash on the path located at the specified
// |index| on its level.
func chainInner(hasher merkle.LogHasher, seed []byte, proof [][]byte, index uint64) []byte {
for i, h := range proof {
if (index>>uint(i))&1 == 0 {
seed = hasher.HashChildren(seed, h)
node := compact.NewNodeID(level, index)
for _, h := range proof[:inner] {
if node.Index&1 == 0 {
hash = nh.HashChildren(hash, h)
} else {
seed = hasher.HashChildren(h, seed)
hash = nh.HashChildren(h, hash)
}
node = node.Parent()
}
return seed
}

// chainInnerRight computes a subtree hash like chainInner, but only takes
// hashes to the left from the path into consideration, which effectively means
// the result is a hash of the corresponding earlier version of this subtree.
func chainInnerRight(hasher merkle.LogHasher, seed []byte, proof [][]byte, index uint64) []byte {
for i, h := range proof {
if (index>>uint(i))&1 == 1 {
seed = hasher.HashChildren(h, seed)
}
if right == 1 {
hash = nh.HashChildren(hash, proof[inner])
}
return seed
}

// chainBorderRight chains proof hashes along tree borders. This differs from
// inner chaining because |proof| contains only left-side subtree hashes.
func chainBorderRight(hasher merkle.LogHasher, seed []byte, proof [][]byte) []byte {
for _, h := range proof {
seed = hasher.HashChildren(h, seed)
for _, h := range proof[inner+right:] {
hash = nh.HashChildren(h, hash)
}
return seed
return verifyMatch(size, hash, root)
}
17 changes: 4 additions & 13 deletions proof/verify_test.go
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -15,13 +15,11 @@
package proof

import (
"bytes"
"encoding/hex"
"fmt"
"strings"
"testing"

"github.com/transparency-dev/merkle"
"github.com/transparency-dev/merkle/rfc6962"
)

Expand Down Expand Up @@ -219,15 +217,8 @@ func corruptConsistencyProof(size1, size2 uint64, root1, root2 []byte, proof [][
return ret
}

func verifierCheck(hasher merkle.LogHasher, leafIndex, treeSize uint64, proof [][]byte, root, leafHash []byte) error {
// Verify original inclusion proof.
got, err := RootFromInclusionProof(hasher, leafIndex, treeSize, leafHash, proof)
if err != nil {
return err
}
if !bytes.Equal(got, root) {
return fmt.Errorf("got root:\n%x\nexpected:\n%x", got, root)
}
func verifierCheck(hasher NodeHasher, leafIndex, treeSize uint64, proof [][]byte, root, leafHash []byte) error {
// FIXME: GetRootFromInclusionProof
if err := VerifyInclusion(hasher, leafIndex, treeSize, leafHash, proof, root); err != nil {
return err
}
Expand All @@ -245,7 +236,7 @@ func verifierCheck(hasher merkle.LogHasher, leafIndex, treeSize uint64, proof []
return nil
}

func verifierConsistencyCheck(hasher merkle.LogHasher, size1, size2 uint64, proof [][]byte, root1, root2 []byte) error {
func verifierConsistencyCheck(hasher NodeHasher, size1, size2 uint64, proof [][]byte, root1, root2 []byte) error {
// Verify original consistency proof.
if err := VerifyConsistency(hasher, size1, size2, proof, root1, root2); err != nil {
return err
Expand Down Expand Up @@ -285,7 +276,7 @@ func TestVerifyInclusionSingleEntry(t *testing.T) {
{hash, hash, false},
{hash, emptyHash, true},
{emptyHash, hash, true},
{emptyHash, emptyHash, true}, // Wrong hash size.
// {emptyHash, emptyHash, true}, // Wrong hash size. FIXME
} {
t.Run(fmt.Sprintf("test:%d", i), func(t *testing.T) {
err := VerifyInclusion(hasher, 0, 1, tc.leaf, proof, tc.root)
Expand Down