// 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 (
	
	
	
	
	
	
	
	
	
	
	
)

// ignoreCN disables interpreting Common Name as a hostname. See issue 24151.
var ignoreCN = !strings.Contains(os.Getenv("GODEBUG"), "x509ignoreCN=0")

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 permit a DNS or
	// other name (including IP address) in the leaf certificate.
	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
	// NameMismatch results when the subject name of a parent certificate
	// does not match the issuer name in the child.
	NameMismatch
	// NameConstraintsWithoutSANs results when a leaf certificate doesn't
	// contain a Subject Alternative Name extension, but a CA certificate
	// contains name constraints, and the Common Name can be interpreted as
	// a hostname.
	//
	// This error is only returned when legacy Common Name matching is enabled
	// by setting the GODEBUG environment variable to "x509ignoreCN=1". This
	// setting might be removed in the future.
	NameConstraintsWithoutSANs
	// UnconstrainedName results when a CA certificate contains permitted
	// name constraints, but leaf certificate contains a name of an
	// unsupported or unconstrained type.
	UnconstrainedName
	// TooManyConstraints results when the number of comparison operations
	// needed to check a certificate exceeds the limit set by
	// VerifyOptions.MaxConstraintComparisions. This limit exists to
	// prevent pathological certificates can consuming excessive amounts of
	// CPU time to verify.
	TooManyConstraints
	// CANotAuthorizedForExtKeyUsage results when an intermediate or root
	// certificate does not permit a requested extended key usage.
	CANotAuthorizedForExtKeyUsage
)

// 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
	Detail string
}

func ( CertificateInvalidError) () string {
	switch .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: " + .Detail
	case CANotAuthorizedForThisName:
		return "x509: a root or intermediate certificate is not authorized to sign for this name: " + .Detail
	case CANotAuthorizedForExtKeyUsage:
		return "x509: a root or intermediate certificate is not authorized for an extended key usage: " + .Detail
	case TooManyIntermediates:
		return "x509: too many intermediates for path length constraint"
	case IncompatibleUsage:
		return "x509: certificate specifies an incompatible key usage"
	case NameMismatch:
		return "x509: issuer name does not match subject from issuing certificate"
	case NameConstraintsWithoutSANs:
		return "x509: issuer has name constraints but leaf doesn't have a SAN extension"
	case UnconstrainedName:
		return "x509: issuer has name constraints but leaf contains unknown or unconstrained name: " + .Detail
	}
	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 ( HostnameError) () string {
	 := .Certificate

	if !.hasSANExtension() && matchHostnames(.Subject.CommonName, .Host) {
		if !ignoreCN && !validHostnamePattern(.Subject.CommonName) {
			// This would have validated, if it weren't for the validHostname check on Common Name.
			return "x509: Common Name is not a valid hostname: " + .Subject.CommonName
		}
		if ignoreCN && validHostnamePattern(.Subject.CommonName) {
			// This would have validated if x509ignoreCN=0 were set.
			return "x509: certificate relies on legacy Common Name field, " +
				"use SANs or temporarily enable Common Name matching with GODEBUG=x509ignoreCN=0"
		}
	}

	var  string
	if  := net.ParseIP(.Host);  != nil {
		// Trying to validate an IP
		if len(.IPAddresses) == 0 {
			return "x509: cannot validate certificate for " + .Host + " because it doesn't contain any IP SANs"
		}
		for ,  := range .IPAddresses {
			if len() > 0 {
				 += ", "
			}
			 += .String()
		}
	} else {
		if .commonNameAsHostname() {
			 = .Subject.CommonName
		} else {
			 = strings.Join(.DNSNames, ", ")
		}
	}

	if len() == 0 {
		return "x509: certificate is not valid for any names, but wanted to match " + .Host
	}
	return "x509: certificate is valid for " +  + ", not " + .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 ( UnknownAuthorityError) () string {
	 := "x509: certificate signed by unknown authority"
	if .hintErr != nil {
		 := .hintCert.Subject.CommonName
		if len() == 0 {
			if len(.hintCert.Subject.Organization) > 0 {
				 = .hintCert.Subject.Organization[0]
			} else {
				 = "serial:" + .hintCert.SerialNumber.String()
			}
		}
		 += fmt.Sprintf(" (possibly because of %q while trying to verify candidate authority certificate %q)", .hintErr, )
	}
	return 
}

// SystemRootsError results when we fail to load the system root certificates.
type SystemRootsError struct {
	Err error
}

func ( SystemRootsError) () string {
	 := "x509: failed to load system roots and no roots provided"
	if .Err != nil {
		return  + "; " + .Err.Error()
	}
	return 
}

func ( SystemRootsError) () error { return .Err }

// errNotParsed is returned when a certificate without ASN.1 contents is
// verified. Platform-specific verification needs the ASN.1 contents.
var errNotParsed = errors.New("x509: missing ASN.1 contents; use ParseCertificate")

// VerifyOptions contains parameters for Certificate.Verify.
type VerifyOptions struct {
	// DNSName, if set, is checked against the leaf certificate with
	// Certificate.VerifyHostname or the platform verifier.
	DNSName string

	// Intermediates is an optional pool of certificates that are not trust
	// anchors, but can be used to form a chain from the leaf certificate to a
	// root certificate.
	Intermediates *CertPool
	// Roots is the set of trusted root certificates the leaf certificate needs
	// to chain up to. If nil, the system roots or the platform verifier are used.
	Roots *CertPool

	// CurrentTime is used to check the validity of all certificates in the
	// chain. If zero, the current time is used.
	CurrentTime time.Time

	// KeyUsages specifies which Extended Key Usage values are acceptable. A
	// chain is accepted if it allows any of the listed values. An empty list
	// means ExtKeyUsageServerAuth. To accept any key usage, include ExtKeyUsageAny.
	KeyUsages []ExtKeyUsage

	// MaxConstraintComparisions is the maximum number of comparisons to
	// perform when checking a given certificate's name constraints. If
	// zero, a sensible default is used. This limit prevents pathological
	// certificates from consuming excessive amounts of CPU time when
	// validating. It does not apply to the platform verifier.
	MaxConstraintComparisions int
}

const (
	leafCertificate = iota
	intermediateCertificate
	rootCertificate
)

// rfc2821Mailbox represents a “mailbox” (which is an email address to most
// people) by breaking it into the “local” (i.e. before the '@') and “domain”
// parts.
type rfc2821Mailbox struct {
	local, domain string
}

// parseRFC2821Mailbox parses an email address into local and domain parts,
// based on the ABNF for a “Mailbox” from RFC 2821. According to RFC 5280,
// Section 4.2.1.6 that's correct for an rfc822Name from a certificate: “The
// format of an rfc822Name is a "Mailbox" as defined in RFC 2821, Section 4.1.2”.
func parseRFC2821Mailbox( string) ( rfc2821Mailbox,  bool) {
	if len() == 0 {
		return , false
	}

	 := make([]byte, 0, len()/2)

	if [0] == '"' {
		// Quoted-string = DQUOTE *qcontent DQUOTE
		// non-whitespace-control = %d1-8 / %d11 / %d12 / %d14-31 / %d127
		// qcontent = qtext / quoted-pair
		// qtext = non-whitespace-control /
		//         %d33 / %d35-91 / %d93-126
		// quoted-pair = ("\" text) / obs-qp
		// text = %d1-9 / %d11 / %d12 / %d14-127 / obs-text
		//
		// (Names beginning with “obs-” are the obsolete syntax from RFC 2822,
		// Section 4. Since it has been 16 years, we no longer accept that.)
		 = [1:]
	:
		for {
			if len() == 0 {
				return , false
			}
			 := [0]
			 = [1:]

			switch {
			case  == '"':
				break 

			case  == '\\':
				// quoted-pair
				if len() == 0 {
					return , false
				}
				if [0] == 11 ||
					[0] == 12 ||
					(1 <= [0] && [0] <= 9) ||
					(14 <= [0] && [0] <= 127) {
					 = append(, [0])
					 = [1:]
				} else {
					return , false
				}

			case  == 11 ||
				 == 12 ||
				// Space (char 32) is not allowed based on the
				// BNF, but RFC 3696 gives an example that
				// assumes that it is. Several “verified”
				// errata continue to argue about this point.
				// We choose to accept it.
				 == 32 ||
				 == 33 ||
				 == 127 ||
				(1 <=  &&  <= 8) ||
				(14 <=  &&  <= 31) ||
				(35 <=  &&  <= 91) ||
				(93 <=  &&  <= 126):
				// qtext
				 = append(, )

			default:
				return , false
			}
		}
	} else {
		// Atom ("." Atom)*
	:
		for len() > 0 {
			// atext from RFC 2822, Section 3.2.4
			 := [0]

			switch {
			case  == '\\':
				// Examples given in RFC 3696 suggest that
				// escaped characters can appear outside of a
				// quoted string. Several “verified” errata
				// continue to argue the point. We choose to
				// accept it.
				 = [1:]
				if len() == 0 {
					return , false
				}
				fallthrough

			case ('0' <=  &&  <= '9') ||
				('a' <=  &&  <= 'z') ||
				('A' <=  &&  <= 'Z') ||
				 == '!' ||  == '#' ||  == '$' ||  == '%' ||
				 == '&' ||  == '\'' ||  == '*' ||  == '+' ||
				 == '-' ||  == '/' ||  == '=' ||  == '?' ||
				 == '^' ||  == '_' ||  == '`' ||  == '{' ||
				 == '|' ||  == '}' ||  == '~' ||  == '.':
				 = append(, [0])
				 = [1:]

			default:
				break 
			}
		}

		if len() == 0 {
			return , false
		}

		// From RFC 3696, Section 3:
		// “period (".") may also appear, but may not be used to start
		// or end the local part, nor may two or more consecutive
		// periods appear.”
		 := []byte{'.', '.'}
		if [0] == '.' ||
			[len()-1] == '.' ||
			bytes.Contains(, ) {
			return , false
		}
	}

	if len() == 0 || [0] != '@' {
		return , false
	}
	 = [1:]

	// The RFC species a format for domains, but that's known to be
	// violated in practice so we accept that anything after an '@' is the
	// domain part.
	if ,  := domainToReverseLabels(); ! {
		return , false
	}

	.local = string()
	.domain = 
	return , true
}

// domainToReverseLabels converts a textual domain name like foo.example.com to
// the list of labels in reverse order, e.g. ["com", "example", "foo"].
func domainToReverseLabels( string) ( []string,  bool) {
	for len() > 0 {
		if  := strings.LastIndexByte(, '.');  == -1 {
			 = append(, )
			 = ""
		} else {
			 = append(, [+1:])
			 = [:]
		}
	}

	if len() > 0 && len([0]) == 0 {
		// An empty label at the end indicates an absolute value.
		return nil, false
	}

	for ,  := range  {
		if len() == 0 {
			// Empty labels are otherwise invalid.
			return nil, false
		}

		for ,  := range  {
			if  < 33 ||  > 126 {
				// Invalid character.
				return nil, false
			}
		}
	}

	return , true
}

func matchEmailConstraint( rfc2821Mailbox,  string) (bool, error) {
	// If the constraint contains an @, then it specifies an exact mailbox
	// name.
	if strings.Contains(, "@") {
		,  := parseRFC2821Mailbox()
		if ! {
			return false, fmt.Errorf("x509: internal error: cannot parse constraint %q", )
		}
		return .local == .local && strings.EqualFold(.domain, .domain), nil
	}

	// Otherwise the constraint is like a DNS constraint of the domain part
	// of the mailbox.
	return matchDomainConstraint(.domain, )
}

func matchURIConstraint( *url.URL,  string) (bool, error) {
	// From RFC 5280, Section 4.2.1.10:
	// “a uniformResourceIdentifier that does not include an authority
	// component with a host name specified as a fully qualified domain
	// name (e.g., if the URI either does not include an authority
	// component or includes an authority component in which the host name
	// is specified as an IP address), then the application MUST reject the
	// certificate.”

	 := .Host
	if len() == 0 {
		return false, fmt.Errorf("URI with empty host (%q) cannot be matched against constraints", .String())
	}

	if strings.Contains(, ":") && !strings.HasSuffix(, "]") {
		var  error
		, _,  = net.SplitHostPort(.Host)
		if  != nil {
			return false, 
		}
	}

	if strings.HasPrefix(, "[") && strings.HasSuffix(, "]") ||
		net.ParseIP() != nil {
		return false, fmt.Errorf("URI with IP (%q) cannot be matched against constraints", .String())
	}

	return matchDomainConstraint(, )
}

func matchIPConstraint( net.IP,  *net.IPNet) (bool, error) {
	if len() != len(.IP) {
		return false, nil
	}

	for  := range  {
		if  := .Mask[]; []& != .IP[]& {
			return false, nil
		}
	}

	return true, nil
}

func matchDomainConstraint(,  string) (bool, error) {
	// The meaning of zero length constraints is not specified, but this
	// code follows NSS and accepts them as matching everything.
	if len() == 0 {
		return true, nil
	}

	,  := domainToReverseLabels()
	if ! {
		return false, fmt.Errorf("x509: internal error: cannot parse domain %q", )
	}

	// RFC 5280 says that a leading period in a domain name means that at
	// least one label must be prepended, but only for URI and email
	// constraints, not DNS constraints. The code also supports that
	// behaviour for DNS constraints.

	 := false
	if [0] == '.' {
		 = true
		 = [1:]
	}

	,  := domainToReverseLabels()
	if ! {
		return false, fmt.Errorf("x509: internal error: cannot parse domain %q", )
	}

	if len() < len() ||
		( && len() == len()) {
		return false, nil
	}

	for ,  := range  {
		if !strings.EqualFold(, []) {
			return false, nil
		}
	}

	return true, nil
}

// checkNameConstraints checks that c permits a child certificate to claim the
// given name, of type nameType. The argument parsedName contains the parsed
// form of name, suitable for passing to the match function. The total number
// of comparisons is tracked in the given count and should not exceed the given
// limit.
func ( *Certificate) ( *int,
	 int,
	 string,
	 string,
	 interface{},
	 func(,  interface{}) ( bool,  error),
	,  interface{}) error {

	 := reflect.ValueOf()

	* += .Len()
	if * >  {
		return CertificateInvalidError{, TooManyConstraints, ""}
	}

	for  := 0;  < .Len(); ++ {
		 := .Index().Interface()
		,  := (, )
		if  != nil {
			return CertificateInvalidError{, CANotAuthorizedForThisName, .Error()}
		}

		if  {
			return CertificateInvalidError{, CANotAuthorizedForThisName, fmt.Sprintf("%s %q is excluded by constraint %q", , , )}
		}
	}

	 := reflect.ValueOf()

	* += .Len()
	if * >  {
		return CertificateInvalidError{, TooManyConstraints, ""}
	}

	 := true
	for  := 0;  < .Len(); ++ {
		 := .Index().Interface()

		var  error
		if ,  = (, );  != nil {
			return CertificateInvalidError{, CANotAuthorizedForThisName, .Error()}
		}

		if  {
			break
		}
	}

	if ! {
		return CertificateInvalidError{, CANotAuthorizedForThisName, fmt.Sprintf("%s %q is not permitted by any constraint", , )}
	}

	return nil
}

// isValid performs validity checks on c given that it is a candidate to append
// to the chain in currentChain.
func ( *Certificate) ( int,  []*Certificate,  *VerifyOptions) error {
	if len(.UnhandledCriticalExtensions) > 0 {
		return UnhandledCriticalExtension{}
	}

	if len() > 0 {
		 := [len()-1]
		if !bytes.Equal(.RawIssuer, .RawSubject) {
			return CertificateInvalidError{, NameMismatch, ""}
		}
	}

	 := .CurrentTime
	if .IsZero() {
		 = time.Now()
	}
	if .Before(.NotBefore) {
		return CertificateInvalidError{
			Cert:   ,
			Reason: Expired,
			Detail: fmt.Sprintf("current time %s is before %s", .Format(time.RFC3339), .NotBefore.Format(time.RFC3339)),
		}
	} else if .After(.NotAfter) {
		return CertificateInvalidError{
			Cert:   ,
			Reason: Expired,
			Detail: fmt.Sprintf("current time %s is after %s", .Format(time.RFC3339), .NotAfter.Format(time.RFC3339)),
		}
	}

	 := .MaxConstraintComparisions
	if  == 0 {
		 = 250000
	}
	 := 0

	var  *Certificate
	if  == intermediateCertificate ||  == rootCertificate {
		if len() == 0 {
			return errors.New("x509: internal error: empty chain when appending CA cert")
		}
		 = [0]
	}

	 := ( == intermediateCertificate ||  == rootCertificate) && .hasNameConstraints()
	if  && .commonNameAsHostname() {
		// This is the deprecated, legacy case of depending on the commonName as
		// a hostname. We don't enforce name constraints against the CN, but
		// VerifyHostname will look for hostnames in there if there are no SANs.
		// In order to ensure VerifyHostname will not accept an unchecked name,
		// return an error here.
		return CertificateInvalidError{, NameConstraintsWithoutSANs, ""}
	} else if  && .hasSANExtension() {
		 := forEachSAN(.getSANExtension(), func( int,  []byte) error {
			switch  {
			case nameTypeEmail:
				 := string()
				,  := parseRFC2821Mailbox()
				if ! {
					return fmt.Errorf("x509: cannot parse rfc822Name %q", )
				}

				if  := .checkNameConstraints(&, , "email address", , ,
					func(,  interface{}) (bool, error) {
						return matchEmailConstraint(.(rfc2821Mailbox), .(string))
					}, .PermittedEmailAddresses, .ExcludedEmailAddresses);  != nil {
					return 
				}

			case nameTypeDNS:
				 := string()
				if ,  := domainToReverseLabels(); ! {
					return fmt.Errorf("x509: cannot parse dnsName %q", )
				}

				if  := .checkNameConstraints(&, , "DNS name", , ,
					func(,  interface{}) (bool, error) {
						return matchDomainConstraint(.(string), .(string))
					}, .PermittedDNSDomains, .ExcludedDNSDomains);  != nil {
					return 
				}

			case nameTypeURI:
				 := string()
				,  := url.Parse()
				if  != nil {
					return fmt.Errorf("x509: internal error: URI SAN %q failed to parse", )
				}

				if  := .checkNameConstraints(&, , "URI", , ,
					func(,  interface{}) (bool, error) {
						return matchURIConstraint(.(*url.URL), .(string))
					}, .PermittedURIDomains, .ExcludedURIDomains);  != nil {
					return 
				}

			case nameTypeIP:
				 := net.IP()
				if  := len();  != net.IPv4len &&  != net.IPv6len {
					return fmt.Errorf("x509: internal error: IP SAN %x failed to parse", )
				}

				if  := .checkNameConstraints(&, , "IP address", .String(), ,
					func(,  interface{}) (bool, error) {
						return matchIPConstraint(.(net.IP), .(*net.IPNet))
					}, .PermittedIPRanges, .ExcludedIPRanges);  != nil {
					return 
				}

			default:
				// Unknown SAN types are ignored.
			}

			return nil
		})

		if  != nil {
			return 
		}
	}

	// 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  == intermediateCertificate && (!.BasicConstraintsValid || !.IsCA) {
		return CertificateInvalidError{, NotAuthorizedToSign, ""}
	}

	if .BasicConstraintsValid && .MaxPathLen >= 0 {
		 := len() - 1
		if  > .MaxPathLen {
			return CertificateInvalidError{, 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, the platform verifier might be used, and
// verification details might differ from what is described below. If system
// roots are unavailable the returned error will be of type SystemRootsError.
//
// Name constraints in the intermediates will be applied to all names claimed
// in the chain, not just opts.DNSName. Thus it is invalid for a leaf to claim
// example.com if an intermediate doesn't permit it, even if example.com is not
// the name being validated. Note that DirectoryName constraints are not
// supported.
//
// Name constraint validation follows the rules from RFC 5280, with the
// addition that DNS name constraints may use the leading period format
// defined for emails and URIs. When a constraint has a leading period
// it indicates that at least one additional label must be prepended to
// the constrained name to be considered valid.
//
// Extended Key Usage values are enforced nested down a chain, so an intermediate
// or root that enumerates EKUs prevents a leaf from asserting an EKU not in that
// list. (While this is not specified, it is common practice in order to limit
// the types of certificates a CA can issue.)
//
// WARNING: this function doesn't do any revocation checking.
func ( *Certificate) ( VerifyOptions) ( [][]*Certificate,  error) {
	// Platform-specific verification needs the ASN.1 contents so
	// this makes the behavior consistent across platforms.
	if len(.Raw) == 0 {
		return nil, errNotParsed
	}
	for  := 0;  < .Intermediates.len(); ++ {
		,  := .Intermediates.cert()
		if  != nil {
			return nil, fmt.Errorf("crypto/x509: error fetching intermediate: %w", )
		}
		if len(.Raw) == 0 {
			return nil, errNotParsed
		}
	}

	// Use Windows's own verification and chain building.
	if .Roots == nil && runtime.GOOS == "windows" {
		return .systemVerify(&)
	}

	if .Roots == nil {
		.Roots = systemRootsPool()
		if .Roots == nil {
			return nil, SystemRootsError{systemRootsErr}
		}
	}

	 = .isValid(leafCertificate, nil, &)
	if  != nil {
		return
	}

	if len(.DNSName) > 0 {
		 = .VerifyHostname(.DNSName)
		if  != nil {
			return
		}
	}

	var  [][]*Certificate
	if .Roots.contains() {
		 = append(, []*Certificate{})
	} else {
		if ,  = .buildChains(nil, []*Certificate{}, nil, &);  != nil {
			return nil, 
		}
	}

	 := .KeyUsages
	if len() == 0 {
		 = []ExtKeyUsage{ExtKeyUsageServerAuth}
	}

	// If any key usage is acceptable then we're done.
	for ,  := range  {
		if  == ExtKeyUsageAny {
			return , nil
		}
	}

	for ,  := range  {
		if checkChainForKeyUsage(, ) {
			 = append(, )
		}
	}

	if len() == 0 {
		return nil, CertificateInvalidError{, IncompatibleUsage, ""}
	}

	return , nil
}

func appendToFreshChain( []*Certificate,  *Certificate) []*Certificate {
	 := make([]*Certificate, len()+1)
	copy(, )
	[len()] = 
	return 
}

// maxChainSignatureChecks is the maximum number of CheckSignatureFrom calls
// that an invocation of buildChains will (tranistively) make. Most chains are
// less than 15 certificates long, so this leaves space for multiple chains and
// for failed checks due to different intermediates having the same Subject.
const maxChainSignatureChecks = 100

func ( *Certificate) ( map[*Certificate][][]*Certificate,  []*Certificate,  *int,  *VerifyOptions) ( [][]*Certificate,  error) {
	var (
		  error
		 *Certificate
	)

	 := func( int,  *Certificate) {
		for ,  := range  {
			if .Equal() {
				return
			}
		}

		if  == nil {
			 = new(int)
		}
		*++
		if * > maxChainSignatureChecks {
			 = errors.New("x509: signature check attempts limit reached while verifying certificate chain")
			return
		}

		if  := .CheckSignatureFrom();  != nil {
			if  == nil {
				 = 
				 = 
			}
			return
		}

		 = .isValid(, , )
		if  != nil {
			return
		}

		switch  {
		case rootCertificate:
			 = append(, appendToFreshChain(, ))
		case intermediateCertificate:
			if  == nil {
				 = make(map[*Certificate][][]*Certificate)
			}
			,  := []
			if ! {
				,  = .(, appendToFreshChain(, ), , )
				[] = 
			}
			 = append(, ...)
		}
	}

	for ,  := range .Roots.findPotentialParents() {
		(rootCertificate, )
	}
	for ,  := range .Intermediates.findPotentialParents() {
		(intermediateCertificate, )
	}

	if len() > 0 {
		 = nil
	}
	if len() == 0 &&  == nil {
		 = UnknownAuthorityError{, , }
	}

	return
}

func validHostnamePattern( string) bool { return validHostname(, true) }
func validHostnameInput( string) bool   { return validHostname(, false) }

// validHostname reports whether host is a valid hostname that can be matched or
// matched against according to RFC 6125 2.2, with some leniency to accommodate
// legacy values.
func validHostname( string,  bool) bool {
	if ! {
		 = strings.TrimSuffix(, ".")
	}
	if len() == 0 {
		return false
	}

	for ,  := range strings.Split(, ".") {
		if  == "" {
			// Empty label.
			return false
		}
		if  &&  == 0 &&  == "*" {
			// Only allow full left-most wildcards, as those are the only ones
			// we match, and matching literal '*' characters is probably never
			// the expected behavior.
			continue
		}
		for ,  := range  {
			if 'a' <=  &&  <= 'z' {
				continue
			}
			if '0' <=  &&  <= '9' {
				continue
			}
			if 'A' <=  &&  <= 'Z' {
				continue
			}
			if  == '-' &&  != 0 {
				continue
			}
			if  == '_' {
				// Not a valid character in hostnames, but commonly
				// found in deployments outside the WebPKI.
				continue
			}
			return false
		}
	}

	return true
}

// commonNameAsHostname reports whether the Common Name field should be
// considered the hostname that the certificate is valid for. This is a legacy
// behavior, disabled by default or if the Subject Alt Name extension is present.
//
// It applies the strict validHostname check to the Common Name field, so that
// certificates without SANs can still be validated against CAs with name
// constraints if there is no risk the CN would be matched as a hostname.
// See NameConstraintsWithoutSANs and issue 24151.
func ( *Certificate) () bool {
	return !ignoreCN && !.hasSANExtension() && validHostnamePattern(.Subject.CommonName)
}

func matchExactly(,  string) bool {
	if  == "" ||  == "." ||  == "" ||  == "." {
		return false
	}
	return toLowerCaseASCII() == toLowerCaseASCII()
}

func matchHostnames(,  string) bool {
	 = toLowerCaseASCII()
	 = toLowerCaseASCII(strings.TrimSuffix(, "."))

	if len() == 0 || len() == 0 {
		return false
	}

	 := strings.Split(, ".")
	 := strings.Split(, ".")

	if len() != len() {
		return false
	}

	for ,  := range  {
		if  == 0 &&  == "*" {
			continue
		}
		if  != [] {
			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( string) string {
	// If the string is already lower-case then there's nothing to do.
	 := true
	for ,  := range  {
		if  == utf8.RuneError {
			// If we get a UTF-8 error then there might be
			// upper-case ASCII bytes in the invalid sequence.
			 = false
			break
		}
		if 'A' <=  &&  <= 'Z' {
			 = false
			break
		}
	}

	if  {
		return 
	}

	 := []byte()
	for ,  := range  {
		if 'A' <=  &&  <= 'Z' {
			[] += 'a' - 'A'
		}
	}
	return string()
}

// VerifyHostname returns nil if c is a valid certificate for the named host.
// Otherwise it returns an error describing the mismatch.
//
// IP addresses can be optionally enclosed in square brackets and are checked
// against the IPAddresses field. Other names are checked case insensitively
// against the DNSNames field. If the names are valid hostnames, the certificate
// fields can have a wildcard as the left-most label.
//
// The legacy Common Name field is ignored unless it's a valid hostname, the
// certificate doesn't have any Subject Alternative Names, and the GODEBUG
// environment variable is set to "x509ignoreCN=0". Support for Common Name is
// deprecated will be entirely removed in the future.
func ( *Certificate) ( string) error {
	// IP addresses may be written in [ ].
	 := 
	if len() >= 3 && [0] == '[' && [len()-1] == ']' {
		 = [1 : len()-1]
	}
	if  := net.ParseIP();  != nil {
		// We only match IP addresses against IP SANs.
		// See RFC 6125, Appendix B.2.
		for ,  := range .IPAddresses {
			if .Equal() {
				return nil
			}
		}
		return HostnameError{, }
	}

	 := .DNSNames
	if .commonNameAsHostname() {
		 = []string{.Subject.CommonName}
	}

	 := toLowerCaseASCII() // Save allocations inside the loop.
	 := validHostnameInput()

	for ,  := range  {
		// Ideally, we'd only match valid hostnames according to RFC 6125 like
		// browsers (more or less) do, but in practice Go is used in a wider
		// array of contexts and can't even assume DNS resolution. Instead,
		// always allow perfect matches, and only apply wildcard and trailing
		// dot processing to valid hostnames.
		if  && validHostnamePattern() {
			if matchHostnames(, ) {
				return nil
			}
		} else {
			if matchExactly(, ) {
				return nil
			}
		}
	}

	return HostnameError{, }
}

func checkChainForKeyUsage( []*Certificate,  []ExtKeyUsage) bool {
	 := make([]ExtKeyUsage, len())
	copy(, )

	if len() == 0 {
		return false
	}

	 := len()

	// 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  := len() - 1;  >= 0; -- {
		 := []
		if len(.ExtKeyUsage) == 0 && len(.UnknownExtKeyUsage) == 0 {
			// The certificate doesn't have any extended key usage specified.
			continue
		}

		for ,  := range .ExtKeyUsage {
			if  == ExtKeyUsageAny {
				// The certificate is explicitly good for any usage.
				continue 
			}
		}

		const  ExtKeyUsage = -1

	:
		for ,  := range  {
			if  ==  {
				continue
			}

			for ,  := range .ExtKeyUsage {
				if  ==  {
					continue 
				} else if  == ExtKeyUsageServerAuth &&
					( == ExtKeyUsageNetscapeServerGatedCrypto ||
						 == ExtKeyUsageMicrosoftServerGatedCrypto) {
					// In order to support COMODO
					// certificate chains, we have to
					// accept Netscape or Microsoft SGC
					// usages as equal to ServerAuth.
					continue 
				}
			}

			[] = 
			--
			if  == 0 {
				return false
			}
		}
	}

	return true
}