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checkpoint.go
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checkpoint.go
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// Copyright 2023 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found at
// https://go.googlesource.com/go/+/refs/heads/master/LICENSE.
package sunlight
import (
"crypto"
"crypto/ecdsa"
"crypto/rsa"
"crypto/sha256"
"crypto/x509"
"encoding/base64"
"encoding/binary"
"errors"
"fmt"
"strconv"
"strings"
"unicode"
"unicode/utf8"
ct "github.com/google/certificate-transparency-go"
"golang.org/x/crypto/cryptobyte"
"golang.org/x/mod/sumdb/note"
"golang.org/x/mod/sumdb/tlog"
)
const maxCheckpointSize = 1e6
// A Checkpoint is a tree head to be formatted according to c2sp.org/checkpoint.
//
// A checkpoint looks like this:
//
// example.com/origin
// 923748
// nND/nri/U0xuHUrYSy0HtMeal2vzD9V4k/BO79C+QeI=
//
// It can be followed by extra extension lines.
type Checkpoint struct {
Origin string
tlog.Tree
// Extension is empty or a sequence of non-empty lines,
// each terminated by a newline character.
Extension string
}
func ParseCheckpoint(text string) (Checkpoint, error) {
// This is an extended version of tlog.ParseTree.
if strings.Count(text, "\n") < 3 || len(text) > maxCheckpointSize {
return Checkpoint{}, errors.New("malformed checkpoint")
}
if !strings.HasSuffix(text, "\n") {
return Checkpoint{}, errors.New("malformed checkpoint")
}
lines := strings.SplitN(text, "\n", 4)
n, err := strconv.ParseInt(lines[1], 10, 64)
if err != nil || n < 0 || lines[1] != strconv.FormatInt(n, 10) {
return Checkpoint{}, errors.New("malformed checkpoint")
}
h, err := base64.StdEncoding.DecodeString(lines[2])
if err != nil || len(h) != tlog.HashSize {
return Checkpoint{}, errors.New("malformed checkpoint")
}
rest := lines[3]
for rest != "" {
before, after, found := strings.Cut(rest, "\n")
if before == "" || !found {
return Checkpoint{}, errors.New("malformed checkpoint")
}
rest = after
}
var hash tlog.Hash
copy(hash[:], h)
return Checkpoint{lines[0], tlog.Tree{N: n, Hash: hash}, lines[3]}, nil
}
func FormatCheckpoint(c Checkpoint) string {
return fmt.Sprintf("%s\n%d\n%s\n%s",
c.Origin, c.N, base64.StdEncoding.EncodeToString(c.Hash[:]), c.Extension)
}
// NewRFC6962Verifier constructs a new [note.Verifier] that verifies a RFC 6962
// TreeHeadSignature formatted according to c2sp.org/sunlight.
//
// tf, if not nil, is called with the timestamp extracted from any valid
// verified signature.
func NewRFC6962Verifier(name string, key crypto.PublicKey, tf func(uint64)) (note.Verifier, error) {
if !isValidName(name) {
return nil, fmt.Errorf("invalid name %q", name)
}
pkix, err := x509.MarshalPKIXPublicKey(key)
if err != nil {
return nil, err
}
keyID := sha256.Sum256(pkix)
v := &verifier{}
v.name = name
v.hash = keyHash(name, append([]byte{0x05}, keyID[:]...))
v.verify = func(msg, sig []byte) (ok bool) {
c, err := ParseCheckpoint(string(msg))
if err != nil {
return false
}
if c.Extension != "" {
return false
}
// Parse the RFC6962NoteSignature.
var timestamp uint64
var hashAlg, sigAlg uint8
var signature []byte
s := cryptobyte.String(sig)
if !s.ReadUint64(×tamp) ||
!s.ReadUint8(&hashAlg) || hashAlg != 4 || !s.ReadUint8(&sigAlg) ||
!s.ReadUint16LengthPrefixed((*cryptobyte.String)(&signature)) ||
!s.Empty() {
return false
}
defer func() {
if ok && tf != nil {
tf(timestamp)
}
}()
sth := ct.SignedTreeHead{
Version: ct.V1,
TreeSize: uint64(c.N),
Timestamp: timestamp,
SHA256RootHash: ct.SHA256Hash(c.Hash),
}
sthBytes, err := ct.SerializeSTHSignatureInput(sth)
if err != nil {
return false
}
digest := sha256.Sum256(sthBytes)
switch key := key.(type) {
case *rsa.PublicKey:
if sigAlg != 1 {
return false
}
return rsa.VerifyPKCS1v15(key, crypto.SHA256, digest[:], sig) == nil
case *ecdsa.PublicKey:
if sigAlg != 3 {
return false
}
return ecdsa.VerifyASN1(key, digest[:], signature)
default:
return false
}
}
return v, nil
}
type verifier struct {
name string
hash uint32
verify func(msg, sig []byte) bool
}
func (v *verifier) Name() string { return v.name }
func (v *verifier) KeyHash() uint32 { return v.hash }
func (v *verifier) Verify(msg, sig []byte) bool { return v.verify(msg, sig) }
func isValidName(name string) bool {
return name != "" && utf8.ValidString(name) &&
strings.IndexFunc(name, unicode.IsSpace) < 0 &&
!strings.Contains(name, "+")
}
func keyHash(name string, key []byte) uint32 {
h := sha256.New()
h.Write([]byte(name))
h.Write([]byte("\n"))
h.Write(key)
sum := h.Sum(nil)
return binary.BigEndian.Uint32(sum)
}