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Fix PreIssuer detection #554

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Merged
merged 5 commits into from
Sep 9, 2025
Merged

Fix PreIssuer detection #554

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9b6ab49
Fix PreIssuer detection
phbnf Sep 8, 2025
3f70cf7
Use OIDs from rfc6962
phbnf Sep 8, 2025
d488ec4
add IsCA check, and more thorough tests.
phbnf Sep 9, 2025
08f23a7
better names
phbnf Sep 9, 2025
58094dd
add comment
phbnf Sep 9, 2025
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4 changes: 3 additions & 1 deletion internal/types/rfc6962/rfc6962.go
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Original file line number Diff line number Diff line change
Expand Up @@ -43,8 +43,10 @@ const (
TreeNodePrefix = byte(0x01)
)

// Defined in RFC 6962 s3.1.
// Defined or referenced in RFC 6962 s3.1.
var (
OIDExtAuthorityKeyId = asn1.ObjectIdentifier{2, 5, 29, 35}
OIDExtKeyUsage = asn1.ObjectIdentifier{2, 5, 29, 37}
OIDExtensionCTPoison = asn1.ObjectIdentifier{1, 3, 6, 1, 4, 1, 11129, 2, 4, 3}
OIDExtKeyUsageCertificateTransparency = asn1.ObjectIdentifier{1, 3, 6, 1, 4, 1, 11129, 2, 4, 4}
)
Expand Down
62 changes: 35 additions & 27 deletions internal/x509util/ct.go
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Original file line number Diff line number Diff line change
Expand Up @@ -25,13 +25,9 @@ import (

"github.com/transparency-dev/tessera/ctonly"
"github.com/transparency-dev/tesseract/internal/types/rfc6962"
)

var (
oidExtensionAuthorityKeyId = asn1.ObjectIdentifier{2, 5, 29, 35}
// These extensions are defined in RFC 6962 s3.1.
oidExtensionCTPoison = asn1.ObjectIdentifier{1, 3, 6, 1, 4, 1, 11129, 2, 4, 3}
oidExtensionKeyUsageCertificateTransparency = asn1.ObjectIdentifier{1, 3, 6, 1, 4, 1, 11129, 2, 4, 4}
"golang.org/x/crypto/cryptobyte"
cryptobyte_asn1 "golang.org/x/crypto/cryptobyte/asn1"
)

type tbsCertificate struct {
Expand Down Expand Up @@ -110,7 +106,7 @@ func removeExtension(tbsData []byte, oid asn1.ObjectIdentifier) ([]byte, error)
// - The precert's AuthorityKeyId is changed to the AuthorityKeyId of the
// intermediate.
func BuildPrecertTBS(tbsData []byte, preIssuer *x509.Certificate) ([]byte, error) {
data, err := removeExtension(tbsData, oidExtensionCTPoison)
data, err := removeExtension(tbsData, rfc6962.OIDExtensionCTPoison)
if err != nil {
return nil, err
}
Expand All @@ -133,28 +129,20 @@ func BuildPrecertTBS(tbsData []byte, preIssuer *x509.Certificate) ([]byte, error
// to that of the preIssuer.
var issuerKeyID []byte
for _, ext := range preIssuer.Extensions {
if ext.Id.Equal(oidExtensionAuthorityKeyId) {
if ext.Id.Equal(rfc6962.OIDExtAuthorityKeyId) {
issuerKeyID = ext.Value
break
}
}

// The x509 package does not parse CT EKU, so look for it in
// extensions directly.
seenCTEKU := false
for _, ext := range preIssuer.Extensions {
if ext.Id.Equal(oidExtensionKeyUsageCertificateTransparency) {
seenCTEKU = true
break
}
}
if !seenCTEKU {
// TODO(phbnf): is this check really necessary?
if !isPreIssuer(preIssuer) {
return nil, fmt.Errorf("issuer does not have CertificateTransparency extended key usage")
}

keyAt := -1
for i, ext := range tbs.Extensions {
if ext.Id.Equal(oidExtensionAuthorityKeyId) {
if ext.Id.Equal(rfc6962.OIDExtAuthorityKeyId) {
keyAt = i
break
}
Expand All @@ -169,7 +157,7 @@ func BuildPrecertTBS(tbsData []byte, preIssuer *x509.Certificate) ([]byte, error
} else if issuerKeyID != nil {
// PreCert did not have an auth-key-id, but the preIssuer does, so add it at the end.
authKeyIDExt := pkix.Extension{
Id: oidExtensionAuthorityKeyId,
Id: rfc6962.OIDExtAuthorityKeyId,
Critical: false,
Value: issuerKeyID,
}
Expand Down Expand Up @@ -198,7 +186,6 @@ func RemoveCTPoison(tbsData []byte) ([]byte, error) {

// EntryFromChain generates an Entry from a chain and timestamp.
// copied from certificate-transparency-go/serialization.go
// TODO(phboneff): add tests
func EntryFromChain(chain []*x509.Certificate, isPrecert bool, timestamp uint64) (*ctonly.Entry, error) {
leaf := ctonly.Entry{
IsPrecert: isPrecert,
Expand Down Expand Up @@ -256,14 +243,35 @@ func EntryFromChain(chain []*x509.Certificate, isPrecert bool, timestamp uint64)
return &leaf, nil
}

// isPreIssuer indicates whether a certificate is a pre-cert issuer with the specific
// certificate transparency extended key usage.
// isPreIssuer indicates if a certificate is a precertificate signing cert.
//
// From RFC6962 s3.1, these certs should contain:
// (CA:true, Extended Key Usage: Certificate Transparency, OID 1.3.6.1.4.1.11129.2.4.4)
func isPreIssuer(cert *x509.Certificate) bool {
// Look for the extension in the Extensions field and not ExtKeyUsage
// since crypto/x509 does not recognize this extension as an ExtKeyUsage.
if !cert.IsCA {
return false
}
// Look for the extension in the Extensions field and not in ExtKeyUsage
// since crypto/x509 does not recognize this extension as such.
// We cannot reliably check in UnknownExtKeyUsage either, since it might
// one day disappear from UnknownExtKeyUsage and make it to ExtKeyUsage.
// Given that ExtKeyUsage does not contain OIDs, we would not be able to
// detect such a move.
for _, ext := range cert.Extensions {
if rfc6962.OIDExtKeyUsageCertificateTransparency.Equal(ext.Id) {
return true
if rfc6962.OIDExtKeyUsage.Equal(ext.Id) {
der := cryptobyte.String(ext.Value)
if !der.ReadASN1(&der, cryptobyte_asn1.SEQUENCE) {
continue
}
for !der.Empty() {
var eku asn1.ObjectIdentifier
if !der.ReadASN1ObjectIdentifier(&eku) {
continue
}
if rfc6962.OIDExtKeyUsageCertificateTransparency.Equal(eku) {
return true
}
}
}
}
return false
Expand Down
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