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// package merkle provides hashing operations that can be used to verify a
// Sigsum log's Merkle tree. The exact hash strategy is defined in RFC 6962.
package merkle
import (
"bytes"
"crypto/sha256"
"fmt"
)
type Prefix uint8
const (
PrefixLeafNode Prefix = iota
PrefixInteriorNode
)
var (
prefixLeafNode = []byte{byte(PrefixLeafNode)}
prefixInteriorNode = []byte{byte(PrefixInteriorNode)}
)
const (
HashSize = sha256.Size
)
type Hash [HashSize]byte
func HashFn(b []byte) *Hash {
var ret Hash
h := sha256.Sum256(b)
copy(ret[:], h[:])
return &ret
}
func HashLeafNode(leaf []byte) *Hash {
var ret Hash
h := sha256.New()
h.Write(prefixLeafNode)
h.Write(leaf)
copy(ret[:], h.Sum(nil))
return &ret
}
func HashInteriorNode(left, right Hash) *Hash {
var ret Hash
h := sha256.New()
h.Write(prefixInteriorNode)
h.Write(left[:])
h.Write(right[:])
copy(ret[:], h.Sum(nil))
return &ret
}
// VerifyConsistency verifies that a Merkle tree is consistent. The algorithm
// used is in RFC 9162, §2.1.4.2. It is the same proof technique as RFC 6962.
func VerifyConsistency(oldSize, newSize uint64, oldRoot, newRoot Hash, path []Hash) error {
// Step 1
if len(path) == 0 {
return fmt.Errorf("proof input is malformed: no path")
}
// Step2
if pow2(oldSize) {
path = append([]Hash{oldRoot}, path...)
}
// Step 3
fn := oldSize - 1
sn := newSize - 1
// Step 4
for lsb(fn) {
fn = rshift(fn)
sn = rshift(sn)
}
// Step 5
fr := path[0]
sr := path[0]
// Step 6
for _, c := range path[1:] {
// Step 6(a)
if sn == 0 {
return fmt.Errorf("proof input is malformed: reached root too soon")
}
// Step 6(b)
if lsb(fn) || fn == sn {
// Step 6(b), i
fr = *HashInteriorNode(c, fr)
// Step 6(b), ii
sr = *HashInteriorNode(c, sr)
// Step 6(b), iii
if !lsb(fn) {
for {
fn = rshift(fn)
sn = rshift(sn)
if lsb(fn) || fn == 0 {
break
}
}
}
} else {
// Step 6(b), i
sr = *HashInteriorNode(sr, c)
}
// Step 6(c)
fn = rshift(fn)
sn = rshift(sn)
}
// Step 7
if sn != 0 {
return fmt.Errorf("proof input is malformed: never reached the root")
}
if !bytes.Equal(fr[:], oldRoot[:]) {
return fmt.Errorf("invalid proof: old root mismatch")
}
if !bytes.Equal(sr[:], newRoot[:]) {
return fmt.Errorf("invalid proof: new root mismatch")
}
return nil
}
// VerifyInclusion verifies that something is in a Merkle tree. The algorithm
// used is in RFC 9162, §2.1.3.2. It is the same proof technique as RFC 6962.
func VerifyInclusion(leaf Hash, index, size uint64, root Hash, path []Hash) error {
// Step 1
if index >= size {
return fmt.Errorf("proof input is malformed: index out of range")
}
// Step 2
fn := index
sn := size - 1
// Step 3
r := leaf
// Step 4
for _, p := range path {
// Step 4(a)
if sn == 0 {
return fmt.Errorf("proof input is malformed: reached root too soon")
}
// Step 4(b)
if lsb(fn) || fn == sn {
// Step 4(b), i
r = *HashInteriorNode(p, r)
// Step 4(b), ii
if !lsb(fn) {
for {
fn = rshift(fn)
sn = rshift(sn)
if lsb(fn) || fn == 0 {
break
}
}
}
} else {
// Step 4(b), i
r = *HashInteriorNode(r, p)
}
// Step 4(c)
fn = rshift(fn)
sn = rshift(sn)
}
// Step 5
if sn != 0 {
return fmt.Errorf("proof input is malformed: never reached the root")
}
if !bytes.Equal(r[:], root[:]) {
return fmt.Errorf("invalid proof: root mismatch")
}
return nil
}
// lsb outputs true if the least significant bit is set
func lsb(num uint64) bool {
return (num & 1) != 0
}
// pow2 outputs true if the number is a power of 2
func pow2(num uint64) bool {
return (num & (num - 1)) == 0
}
// rshift returns the right-shifted number
func rshift(num uint64) uint64 {
return num >> 1
}
|
