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// Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package x509
import (
"fmt"
"net"
"runtime"
"strings"
"time"
"unicode/utf8"
)
type InvalidReason int
const (
// NotAuthorizedToSign results when a certificate is signed by another
// which isn't marked as a CA certificate.
NotAuthorizedToSign InvalidReason = iota
// Expired results when a certificate has expired, based on the time
// given in the VerifyOptions.
Expired
// CANotAuthorizedForThisName results when an intermediate or root
// certificate has a name constraint which doesn't include the name
// being checked.
CANotAuthorizedForThisName
// TooManyIntermediates results when a path length constraint is
// violated.
TooManyIntermediates
// IncompatibleUsage results when the certificate's key usage indicates
// that it may only be used for a different purpose.
IncompatibleUsage
)
// CertificateInvalidError results when an odd error occurs. Users of this
// library probably want to handle all these errors uniformly.
type CertificateInvalidError struct {
Cert *Certificate
Reason InvalidReason
}
func (e CertificateInvalidError) Error() string {
switch e.Reason {
case NotAuthorizedToSign:
return "x509: certificate is not authorized to sign other certificates"
case Expired:
return "x509: certificate has expired or is not yet valid"
case CANotAuthorizedForThisName:
return "x509: a root or intermediate certificate is not authorized to sign in this domain"
case TooManyIntermediates:
return "x509: too many intermediates for path length constraint"
case IncompatibleUsage:
return "x509: certificate specifies an incompatible key usage"
}
return "x509: unknown error"
}
// HostnameError results when the set of authorized names doesn't match the
// requested name.
type HostnameError struct {
Certificate *Certificate
Host string
}
func (h HostnameError) Error() string {
c := h.Certificate
var valid string
if ip := net.ParseIP(h.Host); ip != nil {
// Trying to validate an IP
if len(c.IPAddresses) == 0 {
return "x509: cannot validate certificate for " + h.Host + " because it doesn't contain any IP SANs"
}
for _, san := range c.IPAddresses {
if len(valid) > 0 {
valid += ", "
}
valid += san.String()
}
} else {
if len(c.DNSNames) > 0 {
valid = strings.Join(c.DNSNames, ", ")
} else {
valid = c.Subject.CommonName
}
}
return "x509: certificate is valid for " + valid + ", not " + h.Host
}
// UnknownAuthorityError results when the certificate issuer is unknown
type UnknownAuthorityError struct {
cert *Certificate
// hintErr contains an error that may be helpful in determining why an
// authority wasn't found.
hintErr error
// hintCert contains a possible authority certificate that was rejected
// because of the error in hintErr.
hintCert *Certificate
}
func (e UnknownAuthorityError) Error() string {
s := "x509: certificate signed by unknown authority"
if e.hintErr != nil {
certName := e.hintCert.Subject.CommonName
if len(certName) == 0 {
if len(e.hintCert.Subject.Organization) > 0 {
certName = e.hintCert.Subject.Organization[0]
}
certName = "serial:" + e.hintCert.SerialNumber.String()
}
s += fmt.Sprintf(" (possibly because of %q while trying to verify candidate authority certificate %q)", e.hintErr, certName)
}
return s
}
// SystemRootsError results when we fail to load the system root certificates.
type SystemRootsError struct{}
func (SystemRootsError) Error() string {
return "x509: failed to load system roots and no roots provided"
}
// VerifyOptions contains parameters for Certificate.Verify. It's a structure
// because other PKIX verification APIs have ended up needing many options.
type VerifyOptions struct {
DNSName string
Intermediates *CertPool
Roots *CertPool // if nil, the system roots are used
CurrentTime time.Time // if zero, the current time is used
// KeyUsage specifies which Extended Key Usage values are acceptable.
// An empty list means ExtKeyUsageServerAuth. Key usage is considered a
// constraint down the chain which mirrors Windows CryptoAPI behaviour,
// but not the spec. To accept any key usage, include ExtKeyUsageAny.
KeyUsages []ExtKeyUsage
}
const (
leafCertificate = iota
intermediateCertificate
rootCertificate
)
// isValid performs validity checks on the c.
func (c *Certificate) isValid(certType int, currentChain []*Certificate, opts *VerifyOptions) error {
now := opts.CurrentTime
if now.IsZero() {
now = time.Now()
}
if now.Before(c.NotBefore) || now.After(c.NotAfter) {
return CertificateInvalidError{c, Expired}
}
if len(c.PermittedDNSDomains) > 0 {
ok := false
for _, domain := range c.PermittedDNSDomains {
if opts.DNSName == domain ||
(strings.HasSuffix(opts.DNSName, domain) &&
len(opts.DNSName) >= 1+len(domain) &&
opts.DNSName[len(opts.DNSName)-len(domain)-1] == '.') {
ok = true
break
}
}
if !ok {
return CertificateInvalidError{c, CANotAuthorizedForThisName}
}
}
// KeyUsage status flags are ignored. From Engineering Security, Peter
// Gutmann: A European government CA marked its signing certificates as
// being valid for encryption only, but no-one noticed. Another
// European CA marked its signature keys as not being valid for
// signatures. A different CA marked its own trusted root certificate
// as being invalid for certificate signing. Another national CA
// distributed a certificate to be used to encrypt data for the
// country’s tax authority that was marked as only being usable for
// digital signatures but not for encryption. Yet another CA reversed
// the order of the bit flags in the keyUsage due to confusion over
// encoding endianness, essentially setting a random keyUsage in
// certificates that it issued. Another CA created a self-invalidating
// certificate by adding a certificate policy statement stipulating
// that the certificate had to be used strictly as specified in the
// keyUsage, and a keyUsage containing a flag indicating that the RSA
// encryption key could only be used for Diffie-Hellman key agreement.
if certType == intermediateCertificate && (!c.BasicConstraintsValid || !c.IsCA) {
return CertificateInvalidError{c, NotAuthorizedToSign}
}
if c.BasicConstraintsValid && c.MaxPathLen >= 0 {
numIntermediates := len(currentChain) - 1
if numIntermediates > c.MaxPathLen {
return CertificateInvalidError{c, TooManyIntermediates}
}
}
return nil
}
// Verify attempts to verify c by building one or more chains from c to a
// certificate in opts.Roots, using certificates in opts.Intermediates if
// needed. If successful, it returns one or more chains where the first
// element of the chain is c and the last element is from opts.Roots.
//
// If opts.Roots is nil and system roots are unavailable the returned error
// will be of type SystemRootsError.
//
// WARNING: this doesn't do any revocation checking.
func (c *Certificate) Verify(opts VerifyOptions) (chains [][]*Certificate, err error) {
// Use Windows's own verification and chain building.
if opts.Roots == nil && runtime.GOOS == "windows" {
return c.systemVerify(&opts)
}
if len(c.UnhandledCriticalExtensions) > 0 {
return nil, UnhandledCriticalExtension{}
}
if opts.Roots == nil {
opts.Roots = systemRootsPool()
if opts.Roots == nil {
return nil, SystemRootsError{}
}
}
err = c.isValid(leafCertificate, nil, &opts)
if err != nil {
return
}
if len(opts.DNSName) > 0 {
err = c.VerifyHostname(opts.DNSName)
if err != nil {
return
}
}
candidateChains, err := c.buildChains(make(map[int][][]*Certificate), []*Certificate{c}, &opts)
if err != nil {
return
}
keyUsages := opts.KeyUsages
if len(keyUsages) == 0 {
keyUsages = []ExtKeyUsage{ExtKeyUsageServerAuth}
}
// If any key usage is acceptable then we're done.
for _, usage := range keyUsages {
if usage == ExtKeyUsageAny {
chains = candidateChains
return
}
}
for _, candidate := range candidateChains {
if checkChainForKeyUsage(candidate, keyUsages) {
chains = append(chains, candidate)
}
}
if len(chains) == 0 {
err = CertificateInvalidError{c, IncompatibleUsage}
}
return
}
func appendToFreshChain(chain []*Certificate, cert *Certificate) []*Certificate {
n := make([]*Certificate, len(chain)+1)
copy(n, chain)
n[len(chain)] = cert
return n
}
func (c *Certificate) buildChains(cache map[int][][]*Certificate, currentChain []*Certificate, opts *VerifyOptions) (chains [][]*Certificate, err error) {
possibleRoots, failedRoot, rootErr := opts.Roots.findVerifiedParents(c)
for _, rootNum := range possibleRoots {
root := opts.Roots.certs[rootNum]
err = root.isValid(rootCertificate, currentChain, opts)
if err != nil {
continue
}
chains = append(chains, appendToFreshChain(currentChain, root))
}
possibleIntermediates, failedIntermediate, intermediateErr := opts.Intermediates.findVerifiedParents(c)
nextIntermediate:
for _, intermediateNum := range possibleIntermediates {
intermediate := opts.Intermediates.certs[intermediateNum]
for _, cert := range currentChain {
if cert == intermediate {
continue nextIntermediate
}
}
err = intermediate.isValid(intermediateCertificate, currentChain, opts)
if err != nil {
continue
}
var childChains [][]*Certificate
childChains, ok := cache[intermediateNum]
if !ok {
childChains, err = intermediate.buildChains(cache, appendToFreshChain(currentChain, intermediate), opts)
cache[intermediateNum] = childChains
}
chains = append(chains, childChains...)
}
if len(chains) > 0 {
err = nil
}
if len(chains) == 0 && err == nil {
hintErr := rootErr
hintCert := failedRoot
if hintErr == nil {
hintErr = intermediateErr
hintCert = failedIntermediate
}
err = UnknownAuthorityError{c, hintErr, hintCert}
}
return
}
func matchHostnames(pattern, host string) bool {
host = strings.TrimSuffix(host, ".")
pattern = strings.TrimSuffix(pattern, ".")
if len(pattern) == 0 || len(host) == 0 {
return false
}
patternParts := strings.Split(pattern, ".")
hostParts := strings.Split(host, ".")
if len(patternParts) != len(hostParts) {
return false
}
for i, patternPart := range patternParts {
if i == 0 && patternPart == "*" {
continue
}
if patternPart != hostParts[i] {
return false
}
}
return true
}
// toLowerCaseASCII returns a lower-case version of in. See RFC 6125 6.4.1. We use
// an explicitly ASCII function to avoid any sharp corners resulting from
// performing Unicode operations on DNS labels.
func toLowerCaseASCII(in string) string {
// If the string is already lower-case then there's nothing to do.
isAlreadyLowerCase := true
for _, c := range in {
if c == utf8.RuneError {
// If we get a UTF-8 error then there might be
// upper-case ASCII bytes in the invalid sequence.
isAlreadyLowerCase = false
break
}
if 'A' <= c && c <= 'Z' {
isAlreadyLowerCase = false
break
}
}
if isAlreadyLowerCase {
return in
}
out := []byte(in)
for i, c := range out {
if 'A' <= c && c <= 'Z' {
out[i] += 'a' - 'A'
}
}
return string(out)
}
// VerifyHostname returns nil if c is a valid certificate for the named host.
// Otherwise it returns an error describing the mismatch.
func (c *Certificate) VerifyHostname(h string) error {
// IP addresses may be written in [ ].
candidateIP := h
if len(h) >= 3 && h[0] == '[' && h[len(h)-1] == ']' {
candidateIP = h[1 : len(h)-1]
}
if ip := net.ParseIP(candidateIP); ip != nil {
// We only match IP addresses against IP SANs.
// https://tools.ietf.org/html/rfc6125#appendix-B.2
for _, candidate := range c.IPAddresses {
if ip.Equal(candidate) {
return nil
}
}
return HostnameError{c, candidateIP}
}
lowered := toLowerCaseASCII(h)
if len(c.DNSNames) > 0 {
for _, match := range c.DNSNames {
if matchHostnames(toLowerCaseASCII(match), lowered) {
return nil
}
}
// If Subject Alt Name is given, we ignore the common name.
} else if matchHostnames(toLowerCaseASCII(c.Subject.CommonName), lowered) {
return nil
}
return HostnameError{c, h}
}
func checkChainForKeyUsage(chain []*Certificate, keyUsages []ExtKeyUsage) bool {
usages := make([]ExtKeyUsage, len(keyUsages))
copy(usages, keyUsages)
if len(chain) == 0 {
return false
}
usagesRemaining := len(usages)
// We walk down the list and cross out any usages that aren't supported
// by each certificate. If we cross out all the usages, then the chain
// is unacceptable.
NextCert:
for i := len(chain) - 1; i >= 0; i-- {
cert := chain[i]
if len(cert.ExtKeyUsage) == 0 && len(cert.UnknownExtKeyUsage) == 0 {
// The certificate doesn't have any extended key usage specified.
continue
}
for _, usage := range cert.ExtKeyUsage {
if usage == ExtKeyUsageAny {
// The certificate is explicitly good for any usage.
continue NextCert
}
}
const invalidUsage ExtKeyUsage = -1
NextRequestedUsage:
for i, requestedUsage := range usages {
if requestedUsage == invalidUsage {
continue
}
for _, usage := range cert.ExtKeyUsage {
if requestedUsage == usage {
continue NextRequestedUsage
} else if requestedUsage == ExtKeyUsageServerAuth &&
(usage == ExtKeyUsageNetscapeServerGatedCrypto ||
usage == ExtKeyUsageMicrosoftServerGatedCrypto) {
// In order to support COMODO
// certificate chains, we have to
// accept Netscape or Microsoft SGC
// usages as equal to ServerAuth.
continue NextRequestedUsage
}
}
usages[i] = invalidUsage
usagesRemaining--
if usagesRemaining == 0 {
return false
}
}
}
return true
}