libgo/go/crypto/x509/verify.go - gcc - Git at Google

 // 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 (
 	"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
 }

 func (e UnknownAuthorityError) Error() string {
 	return "x509: certificate signed by unknown authority"
 }

 // 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
 	// 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 {
 		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] == '.') {
 				continue
 			}

 			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.
 //
 // 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) {
 	for _, rootNum := range opts.Roots.findVerifiedParents(c) {
 		root := opts.Roots.certs[rootNum]
 		err = root.isValid(rootCertificate, currentChain, opts)
 		if err != nil {
 			continue
 		}
 		chains = append(chains, appendToFreshChain(currentChain, root))
 	}

 nextIntermediate:
 	for _, intermediateNum := range opts.Intermediates.findVerifiedParents(c) {
 		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 {
 		err = UnknownAuthorityError{c}
 	}

 	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
 }
	// 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 (
	"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
	}

	func (e UnknownAuthorityError) Error() string {
	return "x509: certificate signed by unknown authority"
	}

	// 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
	// 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 {
	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] == '.') {
	continue
	}

	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.
	//
	// 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) {
	for _, rootNum := range opts.Roots.findVerifiedParents(c) {
	root := opts.Roots.certs[rootNum]
	err = root.isValid(rootCertificate, currentChain, opts)
	if err != nil {
	continue
	}
	chains = append(chains, appendToFreshChain(currentChain, root))
	}

	nextIntermediate:
	for _, intermediateNum := range opts.Intermediates.findVerifiedParents(c) {
	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 {
	err = UnknownAuthorityError{c}
	}

	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
	}