htree.go

/*
Copyright 2022 CodeNotary, Inc. 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.
You may obtain a copy of the License at

	http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/
package htree

import (
	"crypto/sha256"
	"errors"
	"math/bits"
)

var ErrMaxWidthExceeded = errors.New("max width exceeded")
var ErrIllegalArguments = errors.New("illegal arguments")
var ErrIllegalState = errors.New("illegal state")

const LeafPrefix = byte(0)
const NodePrefix = byte(1)

type HTree struct {
	levels   [][][sha256.Size]byte
	maxWidth int
	width    int
	root     [sha256.Size]byte
}

type InclusionProof struct {
	Leaf  int
	Width int
	Terms [][sha256.Size]byte
}

func New(maxWidth int) (*HTree, error) {
	if maxWidth < 1 {
		return nil, ErrIllegalArguments
	}

	lw := 1
	for lw < maxWidth {
		lw = lw << 1
	}

	height := bits.Len64(uint64(maxWidth-1)) + 1

	levels := make([][][sha256.Size]byte, height)
	for l := 0; l < height; l++ {
		levels[l] = make([][sha256.Size]byte, lw>>l)
	}

	return &HTree{
		levels:   levels,
		maxWidth: maxWidth,
	}, nil
}

func (t *HTree) BuildWith(digests [][sha256.Size]byte) error {
	if len(digests) > t.maxWidth {
		return ErrMaxWidthExceeded
	}

	if len(digests) == 0 {
		return ErrIllegalArguments
	}

	for i, d := range digests {
		leaf := [1 + sha256.Size]byte{LeafPrefix}
		copy(leaf[1:], d[:])
		t.levels[0][i] = sha256.Sum256(leaf[:])
	}

	l := 0
	w := len(digests)

	for w > 1 {
		b := [1 + 2*sha256.Size]byte{NodePrefix}

		wn := 0

		for i := 0; i+1 < w; i += 2 {
			copy(b[1:], t.levels[l][i][:])
			copy(b[1+sha256.Size:], t.levels[l][i+1][:])
			t.levels[l+1][wn] = sha256.Sum256(b[:])
			wn++
		}

		if w%2 == 1 {
			t.levels[l+1][wn] = t.levels[l][w-1]
			wn++
		}

		l++
		w = wn
	}

	t.width = len(digests)
	t.root = t.levels[l][0]

	return nil
}

func (t *HTree) Root() (root [sha256.Size]byte, err error) {
	if t.width == 0 {
		err = ErrIllegalState
		return
	}
	return t.root, nil
}

// InclusionProof returns the shortest list of additional nodes required to compute the root
// It's an adaption from the algorithm for proof construction at github.com/codenotary/merkletree
func (t *HTree) InclusionProof(i int) (proof *InclusionProof, err error) {
	if i >= t.width {
		return nil, ErrIllegalArguments
	}

	m := i
	n := t.width

	var offset int
	var l int
	var r int

	proof = &InclusionProof{
		Leaf:  i,
		Width: t.width,
	}

	if t.width == 1 {
		return
	}

	for {
		d := bits.Len(uint(n - 1))
		k := 1 << (d - 1)
		if m < k {
			l, r = offset+k, offset+n-1
			n = k
		} else {
			l, r = offset, offset+k-1
			m = m - k
			n = n - k
			offset += k
		}

		layer := bits.Len(uint(r - l))
		index := l / (1 << layer)

		proof.Terms = append([][sha256.Size]byte{t.levels[layer][index]}, proof.Terms...)

		if n < 1 || (n == 1 && m == 0) {
			return
		}
	}
}

func VerifyInclusion(proof *InclusionProof, digest, root [sha256.Size]byte) bool {
	if proof == nil {
		return false
	}

	leaf := [1 + sha256.Size]byte{LeafPrefix}
	copy(leaf[1:], digest[:])

	calcRoot := sha256.Sum256(leaf[:])
	i := proof.Leaf
	r := proof.Width - 1

	for _, t := range proof.Terms {
		b := [1 + 2*sha256.Size]byte{NodePrefix}

		if i%2 == 0 && i != r {
			copy(b[1:], calcRoot[:])
			copy(b[1+sha256.Size:], t[:])
		} else {
			copy(b[1:], t[:])
			copy(b[1+sha256.Size:], calcRoot[:])
		}

		calcRoot = sha256.Sum256(b[:])
		i /= 2
		r /= 2
	}

	return i == r && root == calcRoot
}