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sct.go
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sct.go
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// Package sct verifies Signed Certificate Timestamp in TLS connections.
// See [RFC 6962](https://datatracker.ietf.org/doc/rfc6962/).
package sct
import (
"context"
"crypto/tls"
"errors"
"fmt"
"sync"
"time"
ct "github.com/google/certificate-transparency-go"
"github.com/google/certificate-transparency-go/loglist2"
ctx509 "github.com/google/certificate-transparency-go/x509"
ctx509util "github.com/google/certificate-transparency-go/x509util"
)
var (
defaultCheckerOnce sync.Once
defaultChecker *checker
)
// checker performs SCT checks.
type checker struct {
ll *loglist2.LogList
}
// getDefaultChecker returns the default Checker, initializing it if needed.
func getDefaultChecker() *checker {
defaultCheckerOnce.Do(func() {
defaultChecker = &checker{
ll: newDefaultLogList(),
}
})
return defaultChecker
}
// CheckConnectionState examines SCTs (both embedded and in the TLS extension) and returns
// nil if at least one of them is valid.
func CheckConnectionState(state *tls.ConnectionState) error {
return getDefaultChecker().checkConnectionState(state)
}
func (c *checker) checkConnectionState(state *tls.ConnectionState) error {
if state == nil {
return errors.New("no TLS connection state")
}
if len(state.PeerCertificates) == 0 {
return errors.New("no peer certificates in TLS connection state")
}
chain, err := buildCertificateChain(state.PeerCertificates)
if err != nil {
return err
}
lastError := errors.New("no Signed Certificate Timestamps found")
// SCTs provided in the TLS handshake.
if err = c.checkTLSSCTs(state.SignedCertificateTimestamps, chain); err != nil {
lastError = err
} else {
return nil
}
// Check SCTs embedded in the leaf certificate.
if err = c.checkCertSCTs(chain); err != nil {
lastError = err
} else {
return nil
}
// TODO(mberhault): check SCTs in OSCP response.
return lastError
}
// Check SCTs provided with the TLS handshake. Returns an error if no SCT is valid.
func (c *checker) checkTLSSCTs(scts [][]byte, chain []*ctx509.Certificate) error {
if len(scts) == 0 {
return errors.New("no SCTs in SSL handshake")
}
merkleLeaf, err := ct.MerkleTreeLeafFromChain(chain, ct.X509LogEntryType, 0)
if err != nil {
return err
}
for _, sct := range scts {
x509SCT := &ctx509.SerializedSCT{Val: sct}
err := c.checkOneSCT(x509SCT, merkleLeaf)
if err == nil {
// Valid: return early.
return nil
}
}
return errors.New("no valid SCT in SSL handshake")
}
// Check SCTs embedded in the leaf certificate. Returns an error if no SCT is valid.
func (c *checker) checkCertSCTs(chain []*ctx509.Certificate) error {
leaf := chain[0]
if len(leaf.SCTList.SCTList) == 0 {
return errors.New("no SCTs in leaf certificate")
}
if len(chain) < 2 {
// TODO(mberhault): optionally fetch issuer from IssuingCertificateURL.
return errors.New("no issuer certificate in chain")
}
issuer := chain[1]
merkleLeaf, err := ct.MerkleTreeLeafForEmbeddedSCT([]*ctx509.Certificate{leaf, issuer}, 0)
if err != nil {
return err
}
for _, sct := range leaf.SCTList.SCTList {
err := c.checkOneSCT(&sct, merkleLeaf)
if err == nil {
// Valid: return early.
return nil
}
}
return errors.New("no valid SCT in SSL handshake")
}
func (c *checker) checkOneSCT(x509SCT *ctx509.SerializedSCT, merkleLeaf *ct.MerkleTreeLeaf) error {
sct, err := ctx509util.ExtractSCT(x509SCT)
if err != nil {
return err
}
ctLog := c.ll.FindLogByKeyHash(sct.LogID.KeyID)
if ctLog == nil {
return fmt.Errorf("no log found with KeyID %x", sct.LogID)
}
logInfo, err := newLogInfoFromLog(ctLog)
if err != nil {
return fmt.Errorf("could not create client for log %s", ctLog.Description)
}
err = logInfo.VerifySCTSignature(*sct, *merkleLeaf)
if err != nil {
return err
}
_, err = logInfo.VerifyInclusion(context.Background(), *merkleLeaf, sct.Timestamp)
if err != nil {
age := time.Since(ct.TimestampToTime(sct.Timestamp))
if age >= logInfo.MMD {
return fmt.Errorf("failed to verify inclusion in log %q", ctLog.Description)
}
// TODO(mberhault): option to fail on timestamp too recent.
return nil
}
return nil
}