certspotter/ct/x509/verify.go

477 lines
13 KiB
Go
Raw Blame History

This file contains ambiguous Unicode characters

This file contains Unicode characters that might be confused with other characters. If you think that this is intentional, you can safely ignore this warning. Use the Escape button to reveal them.

// 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 (e 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
DisableTimeChecks bool
// 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 {
if !opts.DisableTimeChecks {
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
// countrys 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.
//
// 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 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 {
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 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.
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
}
}
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
}