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// Copyright The Notary Project Authors.
// 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 x509
import (
"crypto/x509"
"errors"
"fmt"
"github.com/notaryproject/notation-core-go/internal/oid"
)
// ValidateTimestampingCertChain takes an ordered time stamping certificate
// chain and validates issuance from leaf to root
// Validates certificates according to this spec:
// https://github.com/notaryproject/notaryproject/blob/main/specs/signature-specification.md#certificate-requirements
func ValidateTimestampingCertChain(certChain []*x509.Certificate) error {
if len(certChain) < 1 {
return errors.New("certificate chain must contain at least one certificate")
}
// For self-signed signing certificate (not a CA)
if len(certChain) == 1 {
cert := certChain[0]
if err := cert.CheckSignature(cert.SignatureAlgorithm, cert.RawTBSCertificate, cert.Signature); err != nil {
return fmt.Errorf("invalid self-signed certificate. subject: %q. Error: %w", cert.Subject, err)
}
if err := validateTimestampingLeafCertificate(cert); err != nil {
return fmt.Errorf("invalid self-signed certificate. Error: %w", err)
}
return nil
}
for i, cert := range certChain {
if i == len(certChain)-1 {
selfSigned, selfSignedError := isSelfSigned(cert)
if selfSignedError != nil {
return fmt.Errorf("root certificate with subject %q is invalid or not self-signed. Certificate chain must end with a valid self-signed root certificate. Error: %v", cert.Subject, selfSignedError)
}
if !selfSigned {
return fmt.Errorf("root certificate with subject %q is not self-signed. Certificate chain must end with a valid self-signed root certificate", cert.Subject)
}
} else {
// This is to avoid extra/redundant multiple root cert at the end
// of certificate-chain
selfSigned, selfSignedError := isSelfSigned(cert)
// not checking selfSignedError != nil here because we expect
// a non-nil err. For a non-root certificate, it shouldn't be
// self-signed, hence CheckSignatureFrom would return a non-nil
// error.
if selfSignedError == nil && selfSigned {
if i == 0 {
return fmt.Errorf("leaf certificate with subject %q is self-signed. Certificate chain must not contain self-signed leaf certificate", cert.Subject)
}
return fmt.Errorf("intermediate certificate with subject %q is self-signed. Certificate chain must not contain self-signed intermediate certificate", cert.Subject)
}
parentCert := certChain[i+1]
issuedBy, issuedByError := isIssuedBy(cert, parentCert)
if issuedByError != nil {
return fmt.Errorf("invalid certificates or certificate with subject %q is not issued by %q. Error: %v", cert.Subject, parentCert.Subject, issuedByError)
}
if !issuedBy {
return fmt.Errorf("certificate with subject %q is not issued by %q", cert.Subject, parentCert.Subject)
}
}
if i == 0 {
if err := validateTimestampingLeafCertificate(cert); err != nil {
return err
}
} else {
if err := validateTimestampingCACertificate(cert, i-1); err != nil {
return err
}
}
}
return nil
}
func validateTimestampingCACertificate(cert *x509.Certificate, expectedPathLen int) error {
if err := validateCABasicConstraints(cert, expectedPathLen); err != nil {
return err
}
return validateTimestampingCAKeyUsage(cert)
}
func validateTimestampingLeafCertificate(cert *x509.Certificate) error {
if err := validateLeafBasicConstraints(cert); err != nil {
return err
}
if err := validateTimestampingLeafKeyUsage(cert); err != nil {
return err
}
if err := validateTimestampingExtendedKeyUsage(cert); err != nil {
return err
}
return validateSignatureAlgorithm(cert)
}
func validateTimestampingCAKeyUsage(cert *x509.Certificate) error {
if err := validateTimestampingKeyUsagePresent(cert); err != nil {
return err
}
if cert.KeyUsage&x509.KeyUsageCertSign == 0 {
return fmt.Errorf("certificate with subject %q: key usage must have the bit positions for key cert sign set", cert.Subject)
}
return nil
}
func validateTimestampingLeafKeyUsage(cert *x509.Certificate) error {
if err := validateTimestampingKeyUsagePresent(cert); err != nil {
return err
}
return validateLeafKeyUsage(cert)
}
func validateTimestampingKeyUsagePresent(cert *x509.Certificate) error {
var hasKeyUsageExtension bool
for _, ext := range cert.Extensions {
if ext.Id.Equal(oid.KeyUsage) {
hasKeyUsageExtension = true
break
}
}
if !hasKeyUsageExtension {
return fmt.Errorf("certificate with subject %q: key usage extension must be present", cert.Subject)
}
return nil
}
func validateTimestampingExtendedKeyUsage(cert *x509.Certificate) error {
// RFC 3161 2.3: The corresponding certificate MUST contain only one
// instance of the extended key usage field extension. And it MUST be
// marked as critical.
if len(cert.ExtKeyUsage) != 1 ||
cert.ExtKeyUsage[0] != x509.ExtKeyUsageTimeStamping ||
len(cert.UnknownExtKeyUsage) != 0 {
return fmt.Errorf("timestamp signing certificate with subject %q must have and only have %s as extended key usage", cert.Subject, ekuToString(x509.ExtKeyUsageTimeStamping))
}
// check if Extended Key Usage extension is marked critical
for _, ext := range cert.Extensions {
if ext.Id.Equal(oid.ExtKeyUsage) {
if !ext.Critical {
return fmt.Errorf("timestamp signing certificate with subject %q must have extended key usage extension marked as critical", cert.Subject)
}
break
}
}
return nil
}
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