Source file src/crypto/x509/verify.go

Documentation: crypto/x509

  // 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 (
  	"bytes"
  	"errors"
  	"fmt"
  	"net"
  	"net/url"
  	"os"
  	"reflect"
  	"runtime"
  	"strings"
  	"time"
  	"unicode/utf8"
  )
  
  // ignoreCN disables interpreting Common Name as a hostname. See issue 24151.
  var ignoreCN = strings.Contains(os.Getenv("GODEBUG"), "x509ignoreCN=1")
  
  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.
  	//
  	// You can avoid this error by setting the experimental GODEBUG environment
  	// variable to "x509ignoreCN=1", disabling Common Name matching entirely.
  	// This behavior might become the default 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 (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 for this name: " + e.Detail
  	case CANotAuthorizedForExtKeyUsage:
  		return "x509: a root or intermediate certificate is not authorized for an extended key usage: " + e.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: " + e.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 (h HostnameError) Error() string {
  	c := h.Certificate
  
  	if !c.hasSANExtension() && !validHostname(c.Subject.CommonName) &&
  		matchHostnames(toLowerCaseASCII(c.Subject.CommonName), toLowerCaseASCII(h.Host)) {
  		// This would have validated, if it weren't for the validHostname check on Common Name.
  		return "x509: Common Name is not a valid hostname: " + c.Subject.CommonName
  	}
  
  	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 c.commonNameAsHostname() {
  			valid = c.Subject.CommonName
  		} else {
  			valid = strings.Join(c.DNSNames, ", ")
  		}
  	}
  
  	if len(valid) == 0 {
  		return "x509: certificate is not valid for any names, but wanted to match " + h.Host
  	}
  	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]
  			} else {
  				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 {
  	Err error
  }
  
  func (se SystemRootsError) Error() string {
  	msg := "x509: failed to load system roots and no roots provided"
  	if se.Err != nil {
  		return msg + "; " + se.Err.Error()
  	}
  	return msg
  }
  
  // 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. 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. A leaf
  	// certificate is accepted if it contains any of the listed values. An empty
  	// list means ExtKeyUsageServerAuth. To accept any key usage, include
  	// ExtKeyUsageAny.
  	//
  	// Certificate chains are required to nest these extended key usage values.
  	// (This matches the Windows CryptoAPI behavior, but not the spec.)
  	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.
  	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
  // https://tools.ietf.org/html/rfc5280#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 https://tools.ietf.org/html/rfc2821#section-4.1.2”.
  func parseRFC2821Mailbox(in string) (mailbox rfc2821Mailbox, ok bool) {
  	if len(in) == 0 {
  		return mailbox, false
  	}
  
  	localPartBytes := make([]byte, 0, len(in)/2)
  
  	if in[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
  		// https://tools.ietf.org/html/rfc2822#section-4. Since it has
  		// been 16 years, we no longer accept that.)
  		in = in[1:]
  	QuotedString:
  		for {
  			if len(in) == 0 {
  				return mailbox, false
  			}
  			c := in[0]
  			in = in[1:]
  
  			switch {
  			case c == '"':
  				break QuotedString
  
  			case c == '\\':
  				// quoted-pair
  				if len(in) == 0 {
  					return mailbox, false
  				}
  				if in[0] == 11 ||
  					in[0] == 12 ||
  					(1 <= in[0] && in[0] <= 9) ||
  					(14 <= in[0] && in[0] <= 127) {
  					localPartBytes = append(localPartBytes, in[0])
  					in = in[1:]
  				} else {
  					return mailbox, false
  				}
  
  			case c == 11 ||
  				c == 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.
  				c == 32 ||
  				c == 33 ||
  				c == 127 ||
  				(1 <= c && c <= 8) ||
  				(14 <= c && c <= 31) ||
  				(35 <= c && c <= 91) ||
  				(93 <= c && c <= 126):
  				// qtext
  				localPartBytes = append(localPartBytes, c)
  
  			default:
  				return mailbox, false
  			}
  		}
  	} else {
  		// Atom ("." Atom)*
  	NextChar:
  		for len(in) > 0 {
  			// atext from https://tools.ietf.org/html/rfc2822#section-3.2.4
  			c := in[0]
  
  			switch {
  			case c == '\\':
  				// 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.
  				in = in[1:]
  				if len(in) == 0 {
  					return mailbox, false
  				}
  				fallthrough
  
  			case ('0' <= c && c <= '9') ||
  				('a' <= c && c <= 'z') ||
  				('A' <= c && c <= 'Z') ||
  				c == '!' || c == '#' || c == '$' || c == '%' ||
  				c == '&' || c == '\'' || c == '*' || c == '+' ||
  				c == '-' || c == '/' || c == '=' || c == '?' ||
  				c == '^' || c == '_' || c == '`' || c == '{' ||
  				c == '|' || c == '}' || c == '~' || c == '.':
  				localPartBytes = append(localPartBytes, in[0])
  				in = in[1:]
  
  			default:
  				break NextChar
  			}
  		}
  
  		if len(localPartBytes) == 0 {
  			return mailbox, false
  		}
  
  		// https://tools.ietf.org/html/rfc3696#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.”
  		twoDots := []byte{'.', '.'}
  		if localPartBytes[0] == '.' ||
  			localPartBytes[len(localPartBytes)-1] == '.' ||
  			bytes.Contains(localPartBytes, twoDots) {
  			return mailbox, false
  		}
  	}
  
  	if len(in) == 0 || in[0] != '@' {
  		return mailbox, false
  	}
  	in = in[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 _, ok := domainToReverseLabels(in); !ok {
  		return mailbox, false
  	}
  
  	mailbox.local = string(localPartBytes)
  	mailbox.domain = in
  	return mailbox, 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(domain string) (reverseLabels []string, ok bool) {
  	for len(domain) > 0 {
  		if i := strings.LastIndexByte(domain, '.'); i == -1 {
  			reverseLabels = append(reverseLabels, domain)
  			domain = ""
  		} else {
  			reverseLabels = append(reverseLabels, domain[i+1:len(domain)])
  			domain = domain[:i]
  		}
  	}
  
  	if len(reverseLabels) > 0 && len(reverseLabels[0]) == 0 {
  		// An empty label at the end indicates an absolute value.
  		return nil, false
  	}
  
  	for _, label := range reverseLabels {
  		if len(label) == 0 {
  			// Empty labels are otherwise invalid.
  			return nil, false
  		}
  
  		for _, c := range label {
  			if c < 33 || c > 126 {
  				// Invalid character.
  				return nil, false
  			}
  		}
  	}
  
  	return reverseLabels, true
  }
  
  func matchEmailConstraint(mailbox rfc2821Mailbox, constraint string) (bool, error) {
  	// If the constraint contains an @, then it specifies an exact mailbox
  	// name.
  	if strings.Contains(constraint, "@") {
  		constraintMailbox, ok := parseRFC2821Mailbox(constraint)
  		if !ok {
  			return false, fmt.Errorf("x509: internal error: cannot parse constraint %q", constraint)
  		}
  		return mailbox.local == constraintMailbox.local && strings.EqualFold(mailbox.domain, constraintMailbox.domain), nil
  	}
  
  	// Otherwise the constraint is like a DNS constraint of the domain part
  	// of the mailbox.
  	return matchDomainConstraint(mailbox.domain, constraint)
  }
  
  func matchURIConstraint(uri *url.URL, constraint string) (bool, error) {
  	// https://tools.ietf.org/html/rfc5280#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 := uri.Host
  	if len(host) == 0 {
  		return false, fmt.Errorf("URI with empty host (%q) cannot be matched against constraints", uri.String())
  	}
  
  	if strings.Contains(host, ":") && !strings.HasSuffix(host, "]") {
  		var err error
  		host, _, err = net.SplitHostPort(uri.Host)
  		if err != nil {
  			return false, err
  		}
  	}
  
  	if strings.HasPrefix(host, "[") && strings.HasSuffix(host, "]") ||
  		net.ParseIP(host) != nil {
  		return false, fmt.Errorf("URI with IP (%q) cannot be matched against constraints", uri.String())
  	}
  
  	return matchDomainConstraint(host, constraint)
  }
  
  func matchIPConstraint(ip net.IP, constraint *net.IPNet) (bool, error) {
  	if len(ip) != len(constraint.IP) {
  		return false, nil
  	}
  
  	for i := range ip {
  		if mask := constraint.Mask[i]; ip[i]&mask != constraint.IP[i]&mask {
  			return false, nil
  		}
  	}
  
  	return true, nil
  }
  
  func matchDomainConstraint(domain, constraint 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(constraint) == 0 {
  		return true, nil
  	}
  
  	domainLabels, ok := domainToReverseLabels(domain)
  	if !ok {
  		return false, fmt.Errorf("x509: internal error: cannot parse domain %q", domain)
  	}
  
  	// 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.
  
  	mustHaveSubdomains := false
  	if constraint[0] == '.' {
  		mustHaveSubdomains = true
  		constraint = constraint[1:]
  	}
  
  	constraintLabels, ok := domainToReverseLabels(constraint)
  	if !ok {
  		return false, fmt.Errorf("x509: internal error: cannot parse domain %q", constraint)
  	}
  
  	if len(domainLabels) < len(constraintLabels) ||
  		(mustHaveSubdomains && len(domainLabels) == len(constraintLabels)) {
  		return false, nil
  	}
  
  	for i, constraintLabel := range constraintLabels {
  		if !strings.EqualFold(constraintLabel, domainLabels[i]) {
  			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 (c *Certificate) checkNameConstraints(count *int,
  	maxConstraintComparisons int,
  	nameType string,
  	name string,
  	parsedName interface{},
  	match func(parsedName, constraint interface{}) (match bool, err error),
  	permitted, excluded interface{}) error {
  
  	excludedValue := reflect.ValueOf(excluded)
  
  	*count += excludedValue.Len()
  	if *count > maxConstraintComparisons {
  		return CertificateInvalidError{c, TooManyConstraints, ""}
  	}
  
  	for i := 0; i < excludedValue.Len(); i++ {
  		constraint := excludedValue.Index(i).Interface()
  		match, err := match(parsedName, constraint)
  		if err != nil {
  			return CertificateInvalidError{c, CANotAuthorizedForThisName, err.Error()}
  		}
  
  		if match {
  			return CertificateInvalidError{c, CANotAuthorizedForThisName, fmt.Sprintf("%s %q is excluded by constraint %q", nameType, name, constraint)}
  		}
  	}
  
  	permittedValue := reflect.ValueOf(permitted)
  
  	*count += permittedValue.Len()
  	if *count > maxConstraintComparisons {
  		return CertificateInvalidError{c, TooManyConstraints, ""}
  	}
  
  	ok := true
  	for i := 0; i < permittedValue.Len(); i++ {
  		constraint := permittedValue.Index(i).Interface()
  
  		var err error
  		if ok, err = match(parsedName, constraint); err != nil {
  			return CertificateInvalidError{c, CANotAuthorizedForThisName, err.Error()}
  		}
  
  		if ok {
  			break
  		}
  	}
  
  	if !ok {
  		return CertificateInvalidError{c, CANotAuthorizedForThisName, fmt.Sprintf("%s %q is not permitted by any constraint", nameType, name)}
  	}
  
  	return nil
  }
  
  // isValid performs validity checks on c given that it is a candidate to append
  // to the chain in currentChain.
  func (c *Certificate) isValid(certType int, currentChain []*Certificate, opts *VerifyOptions) error {
  	if len(c.UnhandledCriticalExtensions) > 0 {
  		return UnhandledCriticalExtension{}
  	}
  
  	if len(currentChain) > 0 {
  		child := currentChain[len(currentChain)-1]
  		if !bytes.Equal(child.RawIssuer, c.RawSubject) {
  			return CertificateInvalidError{c, NameMismatch, ""}
  		}
  	}
  
  	now := opts.CurrentTime
  	if now.IsZero() {
  		now = time.Now()
  	}
  	if now.Before(c.NotBefore) || now.After(c.NotAfter) {
  		return CertificateInvalidError{c, Expired, ""}
  	}
  
  	maxConstraintComparisons := opts.MaxConstraintComparisions
  	if maxConstraintComparisons == 0 {
  		maxConstraintComparisons = 250000
  	}
  	comparisonCount := 0
  
  	var leaf *Certificate
  	if certType == intermediateCertificate || certType == rootCertificate {
  		if len(currentChain) == 0 {
  			return errors.New("x509: internal error: empty chain when appending CA cert")
  		}
  		leaf = currentChain[0]
  	}
  
  	checkNameConstraints := (certType == intermediateCertificate || certType == rootCertificate) && c.hasNameConstraints()
  	if checkNameConstraints && leaf.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{c, NameConstraintsWithoutSANs, ""}
  	} else if checkNameConstraints && leaf.hasSANExtension() {
  		err := forEachSAN(leaf.getSANExtension(), func(tag int, data []byte) error {
  			switch tag {
  			case nameTypeEmail:
  				name := string(data)
  				mailbox, ok := parseRFC2821Mailbox(name)
  				if !ok {
  					return fmt.Errorf("x509: cannot parse rfc822Name %q", mailbox)
  				}
  
  				if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "email address", name, mailbox,
  					func(parsedName, constraint interface{}) (bool, error) {
  						return matchEmailConstraint(parsedName.(rfc2821Mailbox), constraint.(string))
  					}, c.PermittedEmailAddresses, c.ExcludedEmailAddresses); err != nil {
  					return err
  				}
  
  			case nameTypeDNS:
  				name := string(data)
  				if _, ok := domainToReverseLabels(name); !ok {
  					return fmt.Errorf("x509: cannot parse dnsName %q", name)
  				}
  
  				if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "DNS name", name, name,
  					func(parsedName, constraint interface{}) (bool, error) {
  						return matchDomainConstraint(parsedName.(string), constraint.(string))
  					}, c.PermittedDNSDomains, c.ExcludedDNSDomains); err != nil {
  					return err
  				}
  
  			case nameTypeURI:
  				name := string(data)
  				uri, err := url.Parse(name)
  				if err != nil {
  					return fmt.Errorf("x509: internal error: URI SAN %q failed to parse", name)
  				}
  
  				if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "URI", name, uri,
  					func(parsedName, constraint interface{}) (bool, error) {
  						return matchURIConstraint(parsedName.(*url.URL), constraint.(string))
  					}, c.PermittedURIDomains, c.ExcludedURIDomains); err != nil {
  					return err
  				}
  
  			case nameTypeIP:
  				ip := net.IP(data)
  				if l := len(ip); l != net.IPv4len && l != net.IPv6len {
  					return fmt.Errorf("x509: internal error: IP SAN %x failed to parse", data)
  				}
  
  				if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "IP address", ip.String(), ip,
  					func(parsedName, constraint interface{}) (bool, error) {
  						return matchIPConstraint(parsedName.(net.IP), constraint.(*net.IPNet))
  					}, c.PermittedIPRanges, c.ExcludedIPRanges); err != nil {
  					return err
  				}
  
  			default:
  				// Unknown SAN types are ignored.
  			}
  
  			return nil
  		})
  
  		if err != nil {
  			return err
  		}
  	}
  
  	// 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.
  //
  // 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.
  //
  // Extended Key Usage values are enforced down a chain, so an intermediate or
  // root that enumerates EKUs prevents a leaf from asserting an EKU not in that
  // list.
  //
  // WARNING: this function doesn't do any revocation checking.
  func (c *Certificate) Verify(opts VerifyOptions) (chains [][]*Certificate, err error) {
  	// Platform-specific verification needs the ASN.1 contents so
  	// this makes the behavior consistent across platforms.
  	if len(c.Raw) == 0 {
  		return nil, errNotParsed
  	}
  	if opts.Intermediates != nil {
  		for _, intermediate := range opts.Intermediates.certs {
  			if len(intermediate.Raw) == 0 {
  				return nil, errNotParsed
  			}
  		}
  	}
  
  	// 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{systemRootsErr}
  		}
  	}
  
  	err = c.isValid(leafCertificate, nil, &opts)
  	if err != nil {
  		return
  	}
  
  	if len(opts.DNSName) > 0 {
  		err = c.VerifyHostname(opts.DNSName)
  		if err != nil {
  			return
  		}
  	}
  
  	var candidateChains [][]*Certificate
  	if opts.Roots.contains(c) {
  		candidateChains = append(candidateChains, []*Certificate{c})
  	} else {
  		if candidateChains, err = c.buildChains(make(map[int][][]*Certificate), []*Certificate{c}, &opts); err != nil {
  			return nil, err
  		}
  	}
  
  	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 {
  			return candidateChains, nil
  		}
  	}
  
  	for _, candidate := range candidateChains {
  		if checkChainForKeyUsage(candidate, keyUsages) {
  			chains = append(chains, candidate)
  		}
  	}
  
  	if len(chains) == 0 {
  		return nil, CertificateInvalidError{c, IncompatibleUsage, ""}
  	}
  
  	return chains, nil
  }
  
  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)
  nextRoot:
  	for _, rootNum := range possibleRoots {
  		root := opts.Roots.certs[rootNum]
  
  		for _, cert := range currentChain {
  			if cert.Equal(root) {
  				continue nextRoot
  			}
  		}
  
  		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.Equal(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
  }
  
  // validHostname returns whether host is a valid hostname that can be matched or
  // matched against according to RFC 6125 2.2, with some leniency to accomodate
  // legacy values.
  func validHostname(host string) bool {
  	host = strings.TrimSuffix(host, ".")
  
  	if len(host) == 0 {
  		return false
  	}
  
  	for i, part := range strings.Split(host, ".") {
  		if part == "" {
  			// Empty label.
  			return false
  		}
  		if i == 0 && part == "*" {
  			// 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 j, c := range part {
  			if 'a' <= c && c <= 'z' {
  				continue
  			}
  			if '0' <= c && c <= '9' {
  				continue
  			}
  			if 'A' <= c && c <= 'Z' {
  				continue
  			}
  			if c == '-' && j != 0 {
  				continue
  			}
  			if c == '_' || c == ':' {
  				// Not valid characters 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 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 (c *Certificate) commonNameAsHostname() bool {
  	return !ignoreCN && !c.hasSANExtension() && validHostname(c.Subject.CommonName)
  }
  
  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 c.commonNameAsHostname() {
  		if matchHostnames(toLowerCaseASCII(c.Subject.CommonName), lowered) {
  			return nil
  		}
  	} else {
  		for _, match := range c.DNSNames {
  			if matchHostnames(toLowerCaseASCII(match), 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
  }
  

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