Source file src/crypto/x509/verify.go

Documentation: crypto/x509

     1  // Copyright 2011 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package x509
     6  
     7  import (
     8  	"bytes"
     9  	"errors"
    10  	"fmt"
    11  	"net"
    12  	"net/url"
    13  	"os"
    14  	"reflect"
    15  	"runtime"
    16  	"strings"
    17  	"time"
    18  	"unicode/utf8"
    19  )
    20  
    21  // ignoreCN disables interpreting Common Name as a hostname. See issue 24151.
    22  var ignoreCN = !strings.Contains(os.Getenv("GODEBUG"), "x509ignoreCN=0")
    23  
    24  type InvalidReason int
    25  
    26  const (
    27  	// NotAuthorizedToSign results when a certificate is signed by another
    28  	// which isn't marked as a CA certificate.
    29  	NotAuthorizedToSign InvalidReason = iota
    30  	// Expired results when a certificate has expired, based on the time
    31  	// given in the VerifyOptions.
    32  	Expired
    33  	// CANotAuthorizedForThisName results when an intermediate or root
    34  	// certificate has a name constraint which doesn't permit a DNS or
    35  	// other name (including IP address) in the leaf certificate.
    36  	CANotAuthorizedForThisName
    37  	// TooManyIntermediates results when a path length constraint is
    38  	// violated.
    39  	TooManyIntermediates
    40  	// IncompatibleUsage results when the certificate's key usage indicates
    41  	// that it may only be used for a different purpose.
    42  	IncompatibleUsage
    43  	// NameMismatch results when the subject name of a parent certificate
    44  	// does not match the issuer name in the child.
    45  	NameMismatch
    46  	// NameConstraintsWithoutSANs results when a leaf certificate doesn't
    47  	// contain a Subject Alternative Name extension, but a CA certificate
    48  	// contains name constraints, and the Common Name can be interpreted as
    49  	// a hostname.
    50  	//
    51  	// This error is only returned when legacy Common Name matching is enabled
    52  	// by setting the GODEBUG environment variable to "x509ignoreCN=1". This
    53  	// setting might be removed in the future.
    54  	NameConstraintsWithoutSANs
    55  	// UnconstrainedName results when a CA certificate contains permitted
    56  	// name constraints, but leaf certificate contains a name of an
    57  	// unsupported or unconstrained type.
    58  	UnconstrainedName
    59  	// TooManyConstraints results when the number of comparison operations
    60  	// needed to check a certificate exceeds the limit set by
    61  	// VerifyOptions.MaxConstraintComparisions. This limit exists to
    62  	// prevent pathological certificates can consuming excessive amounts of
    63  	// CPU time to verify.
    64  	TooManyConstraints
    65  	// CANotAuthorizedForExtKeyUsage results when an intermediate or root
    66  	// certificate does not permit a requested extended key usage.
    67  	CANotAuthorizedForExtKeyUsage
    68  )
    69  
    70  // CertificateInvalidError results when an odd error occurs. Users of this
    71  // library probably want to handle all these errors uniformly.
    72  type CertificateInvalidError struct {
    73  	Cert   *Certificate
    74  	Reason InvalidReason
    75  	Detail string
    76  }
    77  
    78  func (e CertificateInvalidError) Error() string {
    79  	switch e.Reason {
    80  	case NotAuthorizedToSign:
    81  		return "x509: certificate is not authorized to sign other certificates"
    82  	case Expired:
    83  		return "x509: certificate has expired or is not yet valid: " + e.Detail
    84  	case CANotAuthorizedForThisName:
    85  		return "x509: a root or intermediate certificate is not authorized to sign for this name: " + e.Detail
    86  	case CANotAuthorizedForExtKeyUsage:
    87  		return "x509: a root or intermediate certificate is not authorized for an extended key usage: " + e.Detail
    88  	case TooManyIntermediates:
    89  		return "x509: too many intermediates for path length constraint"
    90  	case IncompatibleUsage:
    91  		return "x509: certificate specifies an incompatible key usage"
    92  	case NameMismatch:
    93  		return "x509: issuer name does not match subject from issuing certificate"
    94  	case NameConstraintsWithoutSANs:
    95  		return "x509: issuer has name constraints but leaf doesn't have a SAN extension"
    96  	case UnconstrainedName:
    97  		return "x509: issuer has name constraints but leaf contains unknown or unconstrained name: " + e.Detail
    98  	}
    99  	return "x509: unknown error"
   100  }
   101  
   102  // HostnameError results when the set of authorized names doesn't match the
   103  // requested name.
   104  type HostnameError struct {
   105  	Certificate *Certificate
   106  	Host        string
   107  }
   108  
   109  func (h HostnameError) Error() string {
   110  	c := h.Certificate
   111  
   112  	if !c.hasSANExtension() && matchHostnames(c.Subject.CommonName, h.Host) {
   113  		if !ignoreCN && !validHostnamePattern(c.Subject.CommonName) {
   114  			// This would have validated, if it weren't for the validHostname check on Common Name.
   115  			return "x509: Common Name is not a valid hostname: " + c.Subject.CommonName
   116  		}
   117  		if ignoreCN && validHostnamePattern(c.Subject.CommonName) {
   118  			// This would have validated if x509ignoreCN=0 were set.
   119  			return "x509: certificate relies on legacy Common Name field, " +
   120  				"use SANs or temporarily enable Common Name matching with GODEBUG=x509ignoreCN=0"
   121  		}
   122  	}
   123  
   124  	var valid string
   125  	if ip := net.ParseIP(h.Host); ip != nil {
   126  		// Trying to validate an IP
   127  		if len(c.IPAddresses) == 0 {
   128  			return "x509: cannot validate certificate for " + h.Host + " because it doesn't contain any IP SANs"
   129  		}
   130  		for _, san := range c.IPAddresses {
   131  			if len(valid) > 0 {
   132  				valid += ", "
   133  			}
   134  			valid += san.String()
   135  		}
   136  	} else {
   137  		if c.commonNameAsHostname() {
   138  			valid = c.Subject.CommonName
   139  		} else {
   140  			valid = strings.Join(c.DNSNames, ", ")
   141  		}
   142  	}
   143  
   144  	if len(valid) == 0 {
   145  		return "x509: certificate is not valid for any names, but wanted to match " + h.Host
   146  	}
   147  	return "x509: certificate is valid for " + valid + ", not " + h.Host
   148  }
   149  
   150  // UnknownAuthorityError results when the certificate issuer is unknown
   151  type UnknownAuthorityError struct {
   152  	Cert *Certificate
   153  	// hintErr contains an error that may be helpful in determining why an
   154  	// authority wasn't found.
   155  	hintErr error
   156  	// hintCert contains a possible authority certificate that was rejected
   157  	// because of the error in hintErr.
   158  	hintCert *Certificate
   159  }
   160  
   161  func (e UnknownAuthorityError) Error() string {
   162  	s := "x509: certificate signed by unknown authority"
   163  	if e.hintErr != nil {
   164  		certName := e.hintCert.Subject.CommonName
   165  		if len(certName) == 0 {
   166  			if len(e.hintCert.Subject.Organization) > 0 {
   167  				certName = e.hintCert.Subject.Organization[0]
   168  			} else {
   169  				certName = "serial:" + e.hintCert.SerialNumber.String()
   170  			}
   171  		}
   172  		s += fmt.Sprintf(" (possibly because of %q while trying to verify candidate authority certificate %q)", e.hintErr, certName)
   173  	}
   174  	return s
   175  }
   176  
   177  // SystemRootsError results when we fail to load the system root certificates.
   178  type SystemRootsError struct {
   179  	Err error
   180  }
   181  
   182  func (se SystemRootsError) Error() string {
   183  	msg := "x509: failed to load system roots and no roots provided"
   184  	if se.Err != nil {
   185  		return msg + "; " + se.Err.Error()
   186  	}
   187  	return msg
   188  }
   189  
   190  func (se SystemRootsError) Unwrap() error { return se.Err }
   191  
   192  // errNotParsed is returned when a certificate without ASN.1 contents is
   193  // verified. Platform-specific verification needs the ASN.1 contents.
   194  var errNotParsed = errors.New("x509: missing ASN.1 contents; use ParseCertificate")
   195  
   196  // VerifyOptions contains parameters for Certificate.Verify.
   197  type VerifyOptions struct {
   198  	// DNSName, if set, is checked against the leaf certificate with
   199  	// Certificate.VerifyHostname or the platform verifier.
   200  	DNSName string
   201  
   202  	// Intermediates is an optional pool of certificates that are not trust
   203  	// anchors, but can be used to form a chain from the leaf certificate to a
   204  	// root certificate.
   205  	Intermediates *CertPool
   206  	// Roots is the set of trusted root certificates the leaf certificate needs
   207  	// to chain up to. If nil, the system roots or the platform verifier are used.
   208  	Roots *CertPool
   209  
   210  	// CurrentTime is used to check the validity of all certificates in the
   211  	// chain. If zero, the current time is used.
   212  	CurrentTime time.Time
   213  
   214  	// KeyUsages specifies which Extended Key Usage values are acceptable. A
   215  	// chain is accepted if it allows any of the listed values. An empty list
   216  	// means ExtKeyUsageServerAuth. To accept any key usage, include ExtKeyUsageAny.
   217  	KeyUsages []ExtKeyUsage
   218  
   219  	// MaxConstraintComparisions is the maximum number of comparisons to
   220  	// perform when checking a given certificate's name constraints. If
   221  	// zero, a sensible default is used. This limit prevents pathological
   222  	// certificates from consuming excessive amounts of CPU time when
   223  	// validating. It does not apply to the platform verifier.
   224  	MaxConstraintComparisions int
   225  }
   226  
   227  const (
   228  	leafCertificate = iota
   229  	intermediateCertificate
   230  	rootCertificate
   231  )
   232  
   233  // rfc2821Mailbox represents a “mailbox” (which is an email address to most
   234  // people) by breaking it into the “local” (i.e. before the '@') and “domain”
   235  // parts.
   236  type rfc2821Mailbox struct {
   237  	local, domain string
   238  }
   239  
   240  // parseRFC2821Mailbox parses an email address into local and domain parts,
   241  // based on the ABNF for a “Mailbox” from RFC 2821. According to RFC 5280,
   242  // Section 4.2.1.6 that's correct for an rfc822Name from a certificate: “The
   243  // format of an rfc822Name is a "Mailbox" as defined in RFC 2821, Section 4.1.2”.
   244  func parseRFC2821Mailbox(in string) (mailbox rfc2821Mailbox, ok bool) {
   245  	if len(in) == 0 {
   246  		return mailbox, false
   247  	}
   248  
   249  	localPartBytes := make([]byte, 0, len(in)/2)
   250  
   251  	if in[0] == '"' {
   252  		// Quoted-string = DQUOTE *qcontent DQUOTE
   253  		// non-whitespace-control = %d1-8 / %d11 / %d12 / %d14-31 / %d127
   254  		// qcontent = qtext / quoted-pair
   255  		// qtext = non-whitespace-control /
   256  		//         %d33 / %d35-91 / %d93-126
   257  		// quoted-pair = ("\" text) / obs-qp
   258  		// text = %d1-9 / %d11 / %d12 / %d14-127 / obs-text
   259  		//
   260  		// (Names beginning with “obs-” are the obsolete syntax from RFC 2822,
   261  		// Section 4. Since it has been 16 years, we no longer accept that.)
   262  		in = in[1:]
   263  	QuotedString:
   264  		for {
   265  			if len(in) == 0 {
   266  				return mailbox, false
   267  			}
   268  			c := in[0]
   269  			in = in[1:]
   270  
   271  			switch {
   272  			case c == '"':
   273  				break QuotedString
   274  
   275  			case c == '\\':
   276  				// quoted-pair
   277  				if len(in) == 0 {
   278  					return mailbox, false
   279  				}
   280  				if in[0] == 11 ||
   281  					in[0] == 12 ||
   282  					(1 <= in[0] && in[0] <= 9) ||
   283  					(14 <= in[0] && in[0] <= 127) {
   284  					localPartBytes = append(localPartBytes, in[0])
   285  					in = in[1:]
   286  				} else {
   287  					return mailbox, false
   288  				}
   289  
   290  			case c == 11 ||
   291  				c == 12 ||
   292  				// Space (char 32) is not allowed based on the
   293  				// BNF, but RFC 3696 gives an example that
   294  				// assumes that it is. Several “verified”
   295  				// errata continue to argue about this point.
   296  				// We choose to accept it.
   297  				c == 32 ||
   298  				c == 33 ||
   299  				c == 127 ||
   300  				(1 <= c && c <= 8) ||
   301  				(14 <= c && c <= 31) ||
   302  				(35 <= c && c <= 91) ||
   303  				(93 <= c && c <= 126):
   304  				// qtext
   305  				localPartBytes = append(localPartBytes, c)
   306  
   307  			default:
   308  				return mailbox, false
   309  			}
   310  		}
   311  	} else {
   312  		// Atom ("." Atom)*
   313  	NextChar:
   314  		for len(in) > 0 {
   315  			// atext from RFC 2822, Section 3.2.4
   316  			c := in[0]
   317  
   318  			switch {
   319  			case c == '\\':
   320  				// Examples given in RFC 3696 suggest that
   321  				// escaped characters can appear outside of a
   322  				// quoted string. Several “verified” errata
   323  				// continue to argue the point. We choose to
   324  				// accept it.
   325  				in = in[1:]
   326  				if len(in) == 0 {
   327  					return mailbox, false
   328  				}
   329  				fallthrough
   330  
   331  			case ('0' <= c && c <= '9') ||
   332  				('a' <= c && c <= 'z') ||
   333  				('A' <= c && c <= 'Z') ||
   334  				c == '!' || c == '#' || c == '$' || c == '%' ||
   335  				c == '&' || c == '\'' || c == '*' || c == '+' ||
   336  				c == '-' || c == '/' || c == '=' || c == '?' ||
   337  				c == '^' || c == '_' || c == '`' || c == '{' ||
   338  				c == '|' || c == '}' || c == '~' || c == '.':
   339  				localPartBytes = append(localPartBytes, in[0])
   340  				in = in[1:]
   341  
   342  			default:
   343  				break NextChar
   344  			}
   345  		}
   346  
   347  		if len(localPartBytes) == 0 {
   348  			return mailbox, false
   349  		}
   350  
   351  		// From RFC 3696, Section 3:
   352  		// “period (".") may also appear, but may not be used to start
   353  		// or end the local part, nor may two or more consecutive
   354  		// periods appear.”
   355  		twoDots := []byte{'.', '.'}
   356  		if localPartBytes[0] == '.' ||
   357  			localPartBytes[len(localPartBytes)-1] == '.' ||
   358  			bytes.Contains(localPartBytes, twoDots) {
   359  			return mailbox, false
   360  		}
   361  	}
   362  
   363  	if len(in) == 0 || in[0] != '@' {
   364  		return mailbox, false
   365  	}
   366  	in = in[1:]
   367  
   368  	// The RFC species a format for domains, but that's known to be
   369  	// violated in practice so we accept that anything after an '@' is the
   370  	// domain part.
   371  	if _, ok := domainToReverseLabels(in); !ok {
   372  		return mailbox, false
   373  	}
   374  
   375  	mailbox.local = string(localPartBytes)
   376  	mailbox.domain = in
   377  	return mailbox, true
   378  }
   379  
   380  // domainToReverseLabels converts a textual domain name like foo.example.com to
   381  // the list of labels in reverse order, e.g. ["com", "example", "foo"].
   382  func domainToReverseLabels(domain string) (reverseLabels []string, ok bool) {
   383  	for len(domain) > 0 {
   384  		if i := strings.LastIndexByte(domain, '.'); i == -1 {
   385  			reverseLabels = append(reverseLabels, domain)
   386  			domain = ""
   387  		} else {
   388  			reverseLabels = append(reverseLabels, domain[i+1:])
   389  			domain = domain[:i]
   390  		}
   391  	}
   392  
   393  	if len(reverseLabels) > 0 && len(reverseLabels[0]) == 0 {
   394  		// An empty label at the end indicates an absolute value.
   395  		return nil, false
   396  	}
   397  
   398  	for _, label := range reverseLabels {
   399  		if len(label) == 0 {
   400  			// Empty labels are otherwise invalid.
   401  			return nil, false
   402  		}
   403  
   404  		for _, c := range label {
   405  			if c < 33 || c > 126 {
   406  				// Invalid character.
   407  				return nil, false
   408  			}
   409  		}
   410  	}
   411  
   412  	return reverseLabels, true
   413  }
   414  
   415  func matchEmailConstraint(mailbox rfc2821Mailbox, constraint string) (bool, error) {
   416  	// If the constraint contains an @, then it specifies an exact mailbox
   417  	// name.
   418  	if strings.Contains(constraint, "@") {
   419  		constraintMailbox, ok := parseRFC2821Mailbox(constraint)
   420  		if !ok {
   421  			return false, fmt.Errorf("x509: internal error: cannot parse constraint %q", constraint)
   422  		}
   423  		return mailbox.local == constraintMailbox.local && strings.EqualFold(mailbox.domain, constraintMailbox.domain), nil
   424  	}
   425  
   426  	// Otherwise the constraint is like a DNS constraint of the domain part
   427  	// of the mailbox.
   428  	return matchDomainConstraint(mailbox.domain, constraint)
   429  }
   430  
   431  func matchURIConstraint(uri *url.URL, constraint string) (bool, error) {
   432  	// From RFC 5280, Section 4.2.1.10:
   433  	// “a uniformResourceIdentifier that does not include an authority
   434  	// component with a host name specified as a fully qualified domain
   435  	// name (e.g., if the URI either does not include an authority
   436  	// component or includes an authority component in which the host name
   437  	// is specified as an IP address), then the application MUST reject the
   438  	// certificate.”
   439  
   440  	host := uri.Host
   441  	if len(host) == 0 {
   442  		return false, fmt.Errorf("URI with empty host (%q) cannot be matched against constraints", uri.String())
   443  	}
   444  
   445  	if strings.Contains(host, ":") && !strings.HasSuffix(host, "]") {
   446  		var err error
   447  		host, _, err = net.SplitHostPort(uri.Host)
   448  		if err != nil {
   449  			return false, err
   450  		}
   451  	}
   452  
   453  	if strings.HasPrefix(host, "[") && strings.HasSuffix(host, "]") ||
   454  		net.ParseIP(host) != nil {
   455  		return false, fmt.Errorf("URI with IP (%q) cannot be matched against constraints", uri.String())
   456  	}
   457  
   458  	return matchDomainConstraint(host, constraint)
   459  }
   460  
   461  func matchIPConstraint(ip net.IP, constraint *net.IPNet) (bool, error) {
   462  	if len(ip) != len(constraint.IP) {
   463  		return false, nil
   464  	}
   465  
   466  	for i := range ip {
   467  		if mask := constraint.Mask[i]; ip[i]&mask != constraint.IP[i]&mask {
   468  			return false, nil
   469  		}
   470  	}
   471  
   472  	return true, nil
   473  }
   474  
   475  func matchDomainConstraint(domain, constraint string) (bool, error) {
   476  	// The meaning of zero length constraints is not specified, but this
   477  	// code follows NSS and accepts them as matching everything.
   478  	if len(constraint) == 0 {
   479  		return true, nil
   480  	}
   481  
   482  	domainLabels, ok := domainToReverseLabels(domain)
   483  	if !ok {
   484  		return false, fmt.Errorf("x509: internal error: cannot parse domain %q", domain)
   485  	}
   486  
   487  	// RFC 5280 says that a leading period in a domain name means that at
   488  	// least one label must be prepended, but only for URI and email
   489  	// constraints, not DNS constraints. The code also supports that
   490  	// behaviour for DNS constraints.
   491  
   492  	mustHaveSubdomains := false
   493  	if constraint[0] == '.' {
   494  		mustHaveSubdomains = true
   495  		constraint = constraint[1:]
   496  	}
   497  
   498  	constraintLabels, ok := domainToReverseLabels(constraint)
   499  	if !ok {
   500  		return false, fmt.Errorf("x509: internal error: cannot parse domain %q", constraint)
   501  	}
   502  
   503  	if len(domainLabels) < len(constraintLabels) ||
   504  		(mustHaveSubdomains && len(domainLabels) == len(constraintLabels)) {
   505  		return false, nil
   506  	}
   507  
   508  	for i, constraintLabel := range constraintLabels {
   509  		if !strings.EqualFold(constraintLabel, domainLabels[i]) {
   510  			return false, nil
   511  		}
   512  	}
   513  
   514  	return true, nil
   515  }
   516  
   517  // checkNameConstraints checks that c permits a child certificate to claim the
   518  // given name, of type nameType. The argument parsedName contains the parsed
   519  // form of name, suitable for passing to the match function. The total number
   520  // of comparisons is tracked in the given count and should not exceed the given
   521  // limit.
   522  func (c *Certificate) checkNameConstraints(count *int,
   523  	maxConstraintComparisons int,
   524  	nameType string,
   525  	name string,
   526  	parsedName interface{},
   527  	match func(parsedName, constraint interface{}) (match bool, err error),
   528  	permitted, excluded interface{}) error {
   529  
   530  	excludedValue := reflect.ValueOf(excluded)
   531  
   532  	*count += excludedValue.Len()
   533  	if *count > maxConstraintComparisons {
   534  		return CertificateInvalidError{c, TooManyConstraints, ""}
   535  	}
   536  
   537  	for i := 0; i < excludedValue.Len(); i++ {
   538  		constraint := excludedValue.Index(i).Interface()
   539  		match, err := match(parsedName, constraint)
   540  		if err != nil {
   541  			return CertificateInvalidError{c, CANotAuthorizedForThisName, err.Error()}
   542  		}
   543  
   544  		if match {
   545  			return CertificateInvalidError{c, CANotAuthorizedForThisName, fmt.Sprintf("%s %q is excluded by constraint %q", nameType, name, constraint)}
   546  		}
   547  	}
   548  
   549  	permittedValue := reflect.ValueOf(permitted)
   550  
   551  	*count += permittedValue.Len()
   552  	if *count > maxConstraintComparisons {
   553  		return CertificateInvalidError{c, TooManyConstraints, ""}
   554  	}
   555  
   556  	ok := true
   557  	for i := 0; i < permittedValue.Len(); i++ {
   558  		constraint := permittedValue.Index(i).Interface()
   559  
   560  		var err error
   561  		if ok, err = match(parsedName, constraint); err != nil {
   562  			return CertificateInvalidError{c, CANotAuthorizedForThisName, err.Error()}
   563  		}
   564  
   565  		if ok {
   566  			break
   567  		}
   568  	}
   569  
   570  	if !ok {
   571  		return CertificateInvalidError{c, CANotAuthorizedForThisName, fmt.Sprintf("%s %q is not permitted by any constraint", nameType, name)}
   572  	}
   573  
   574  	return nil
   575  }
   576  
   577  // isValid performs validity checks on c given that it is a candidate to append
   578  // to the chain in currentChain.
   579  func (c *Certificate) isValid(certType int, currentChain []*Certificate, opts *VerifyOptions) error {
   580  	if len(c.UnhandledCriticalExtensions) > 0 {
   581  		return UnhandledCriticalExtension{}
   582  	}
   583  
   584  	if len(currentChain) > 0 {
   585  		child := currentChain[len(currentChain)-1]
   586  		if !bytes.Equal(child.RawIssuer, c.RawSubject) {
   587  			return CertificateInvalidError{c, NameMismatch, ""}
   588  		}
   589  	}
   590  
   591  	now := opts.CurrentTime
   592  	if now.IsZero() {
   593  		now = time.Now()
   594  	}
   595  	if now.Before(c.NotBefore) {
   596  		return CertificateInvalidError{
   597  			Cert:   c,
   598  			Reason: Expired,
   599  			Detail: fmt.Sprintf("current time %s is before %s", now.Format(time.RFC3339), c.NotBefore.Format(time.RFC3339)),
   600  		}
   601  	} else if now.After(c.NotAfter) {
   602  		return CertificateInvalidError{
   603  			Cert:   c,
   604  			Reason: Expired,
   605  			Detail: fmt.Sprintf("current time %s is after %s", now.Format(time.RFC3339), c.NotAfter.Format(time.RFC3339)),
   606  		}
   607  	}
   608  
   609  	maxConstraintComparisons := opts.MaxConstraintComparisions
   610  	if maxConstraintComparisons == 0 {
   611  		maxConstraintComparisons = 250000
   612  	}
   613  	comparisonCount := 0
   614  
   615  	var leaf *Certificate
   616  	if certType == intermediateCertificate || certType == rootCertificate {
   617  		if len(currentChain) == 0 {
   618  			return errors.New("x509: internal error: empty chain when appending CA cert")
   619  		}
   620  		leaf = currentChain[0]
   621  	}
   622  
   623  	checkNameConstraints := (certType == intermediateCertificate || certType == rootCertificate) && c.hasNameConstraints()
   624  	if checkNameConstraints && leaf.commonNameAsHostname() {
   625  		// This is the deprecated, legacy case of depending on the commonName as
   626  		// a hostname. We don't enforce name constraints against the CN, but
   627  		// VerifyHostname will look for hostnames in there if there are no SANs.
   628  		// In order to ensure VerifyHostname will not accept an unchecked name,
   629  		// return an error here.
   630  		return CertificateInvalidError{c, NameConstraintsWithoutSANs, ""}
   631  	} else if checkNameConstraints && leaf.hasSANExtension() {
   632  		err := forEachSAN(leaf.getSANExtension(), func(tag int, data []byte) error {
   633  			switch tag {
   634  			case nameTypeEmail:
   635  				name := string(data)
   636  				mailbox, ok := parseRFC2821Mailbox(name)
   637  				if !ok {
   638  					return fmt.Errorf("x509: cannot parse rfc822Name %q", mailbox)
   639  				}
   640  
   641  				if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "email address", name, mailbox,
   642  					func(parsedName, constraint interface{}) (bool, error) {
   643  						return matchEmailConstraint(parsedName.(rfc2821Mailbox), constraint.(string))
   644  					}, c.PermittedEmailAddresses, c.ExcludedEmailAddresses); err != nil {
   645  					return err
   646  				}
   647  
   648  			case nameTypeDNS:
   649  				name := string(data)
   650  				if _, ok := domainToReverseLabels(name); !ok {
   651  					return fmt.Errorf("x509: cannot parse dnsName %q", name)
   652  				}
   653  
   654  				if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "DNS name", name, name,
   655  					func(parsedName, constraint interface{}) (bool, error) {
   656  						return matchDomainConstraint(parsedName.(string), constraint.(string))
   657  					}, c.PermittedDNSDomains, c.ExcludedDNSDomains); err != nil {
   658  					return err
   659  				}
   660  
   661  			case nameTypeURI:
   662  				name := string(data)
   663  				uri, err := url.Parse(name)
   664  				if err != nil {
   665  					return fmt.Errorf("x509: internal error: URI SAN %q failed to parse", name)
   666  				}
   667  
   668  				if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "URI", name, uri,
   669  					func(parsedName, constraint interface{}) (bool, error) {
   670  						return matchURIConstraint(parsedName.(*url.URL), constraint.(string))
   671  					}, c.PermittedURIDomains, c.ExcludedURIDomains); err != nil {
   672  					return err
   673  				}
   674  
   675  			case nameTypeIP:
   676  				ip := net.IP(data)
   677  				if l := len(ip); l != net.IPv4len && l != net.IPv6len {
   678  					return fmt.Errorf("x509: internal error: IP SAN %x failed to parse", data)
   679  				}
   680  
   681  				if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "IP address", ip.String(), ip,
   682  					func(parsedName, constraint interface{}) (bool, error) {
   683  						return matchIPConstraint(parsedName.(net.IP), constraint.(*net.IPNet))
   684  					}, c.PermittedIPRanges, c.ExcludedIPRanges); err != nil {
   685  					return err
   686  				}
   687  
   688  			default:
   689  				// Unknown SAN types are ignored.
   690  			}
   691  
   692  			return nil
   693  		})
   694  
   695  		if err != nil {
   696  			return err
   697  		}
   698  	}
   699  
   700  	// KeyUsage status flags are ignored. From Engineering Security, Peter
   701  	// Gutmann: A European government CA marked its signing certificates as
   702  	// being valid for encryption only, but no-one noticed. Another
   703  	// European CA marked its signature keys as not being valid for
   704  	// signatures. A different CA marked its own trusted root certificate
   705  	// as being invalid for certificate signing. Another national CA
   706  	// distributed a certificate to be used to encrypt data for the
   707  	// country’s tax authority that was marked as only being usable for
   708  	// digital signatures but not for encryption. Yet another CA reversed
   709  	// the order of the bit flags in the keyUsage due to confusion over
   710  	// encoding endianness, essentially setting a random keyUsage in
   711  	// certificates that it issued. Another CA created a self-invalidating
   712  	// certificate by adding a certificate policy statement stipulating
   713  	// that the certificate had to be used strictly as specified in the
   714  	// keyUsage, and a keyUsage containing a flag indicating that the RSA
   715  	// encryption key could only be used for Diffie-Hellman key agreement.
   716  
   717  	if certType == intermediateCertificate && (!c.BasicConstraintsValid || !c.IsCA) {
   718  		return CertificateInvalidError{c, NotAuthorizedToSign, ""}
   719  	}
   720  
   721  	if c.BasicConstraintsValid && c.MaxPathLen >= 0 {
   722  		numIntermediates := len(currentChain) - 1
   723  		if numIntermediates > c.MaxPathLen {
   724  			return CertificateInvalidError{c, TooManyIntermediates, ""}
   725  		}
   726  	}
   727  
   728  	return nil
   729  }
   730  
   731  // Verify attempts to verify c by building one or more chains from c to a
   732  // certificate in opts.Roots, using certificates in opts.Intermediates if
   733  // needed. If successful, it returns one or more chains where the first
   734  // element of the chain is c and the last element is from opts.Roots.
   735  //
   736  // If opts.Roots is nil, the platform verifier might be used, and
   737  // verification details might differ from what is described below. If system
   738  // roots are unavailable the returned error will be of type SystemRootsError.
   739  //
   740  // Name constraints in the intermediates will be applied to all names claimed
   741  // in the chain, not just opts.DNSName. Thus it is invalid for a leaf to claim
   742  // example.com if an intermediate doesn't permit it, even if example.com is not
   743  // the name being validated. Note that DirectoryName constraints are not
   744  // supported.
   745  //
   746  // Name constraint validation follows the rules from RFC 5280, with the
   747  // addition that DNS name constraints may use the leading period format
   748  // defined for emails and URIs. When a constraint has a leading period
   749  // it indicates that at least one additional label must be prepended to
   750  // the constrained name to be considered valid.
   751  //
   752  // Extended Key Usage values are enforced nested down a chain, so an intermediate
   753  // or root that enumerates EKUs prevents a leaf from asserting an EKU not in that
   754  // list. (While this is not specified, it is common practice in order to limit
   755  // the types of certificates a CA can issue.)
   756  //
   757  // WARNING: this function doesn't do any revocation checking.
   758  func (c *Certificate) Verify(opts VerifyOptions) (chains [][]*Certificate, err error) {
   759  	// Platform-specific verification needs the ASN.1 contents so
   760  	// this makes the behavior consistent across platforms.
   761  	if len(c.Raw) == 0 {
   762  		return nil, errNotParsed
   763  	}
   764  	for i := 0; i < opts.Intermediates.len(); i++ {
   765  		c, err := opts.Intermediates.cert(i)
   766  		if err != nil {
   767  			return nil, fmt.Errorf("crypto/x509: error fetching intermediate: %w", err)
   768  		}
   769  		if len(c.Raw) == 0 {
   770  			return nil, errNotParsed
   771  		}
   772  	}
   773  
   774  	// Use Windows's own verification and chain building.
   775  	if opts.Roots == nil && runtime.GOOS == "windows" {
   776  		return c.systemVerify(&opts)
   777  	}
   778  
   779  	if opts.Roots == nil {
   780  		opts.Roots = systemRootsPool()
   781  		if opts.Roots == nil {
   782  			return nil, SystemRootsError{systemRootsErr}
   783  		}
   784  	}
   785  
   786  	err = c.isValid(leafCertificate, nil, &opts)
   787  	if err != nil {
   788  		return
   789  	}
   790  
   791  	if len(opts.DNSName) > 0 {
   792  		err = c.VerifyHostname(opts.DNSName)
   793  		if err != nil {
   794  			return
   795  		}
   796  	}
   797  
   798  	var candidateChains [][]*Certificate
   799  	if opts.Roots.contains(c) {
   800  		candidateChains = append(candidateChains, []*Certificate{c})
   801  	} else {
   802  		if candidateChains, err = c.buildChains(nil, []*Certificate{c}, nil, &opts); err != nil {
   803  			return nil, err
   804  		}
   805  	}
   806  
   807  	keyUsages := opts.KeyUsages
   808  	if len(keyUsages) == 0 {
   809  		keyUsages = []ExtKeyUsage{ExtKeyUsageServerAuth}
   810  	}
   811  
   812  	// If any key usage is acceptable then we're done.
   813  	for _, usage := range keyUsages {
   814  		if usage == ExtKeyUsageAny {
   815  			return candidateChains, nil
   816  		}
   817  	}
   818  
   819  	for _, candidate := range candidateChains {
   820  		if checkChainForKeyUsage(candidate, keyUsages) {
   821  			chains = append(chains, candidate)
   822  		}
   823  	}
   824  
   825  	if len(chains) == 0 {
   826  		return nil, CertificateInvalidError{c, IncompatibleUsage, ""}
   827  	}
   828  
   829  	return chains, nil
   830  }
   831  
   832  func appendToFreshChain(chain []*Certificate, cert *Certificate) []*Certificate {
   833  	n := make([]*Certificate, len(chain)+1)
   834  	copy(n, chain)
   835  	n[len(chain)] = cert
   836  	return n
   837  }
   838  
   839  // maxChainSignatureChecks is the maximum number of CheckSignatureFrom calls
   840  // that an invocation of buildChains will (tranistively) make. Most chains are
   841  // less than 15 certificates long, so this leaves space for multiple chains and
   842  // for failed checks due to different intermediates having the same Subject.
   843  const maxChainSignatureChecks = 100
   844  
   845  func (c *Certificate) buildChains(cache map[*Certificate][][]*Certificate, currentChain []*Certificate, sigChecks *int, opts *VerifyOptions) (chains [][]*Certificate, err error) {
   846  	var (
   847  		hintErr  error
   848  		hintCert *Certificate
   849  	)
   850  
   851  	considerCandidate := func(certType int, candidate *Certificate) {
   852  		for _, cert := range currentChain {
   853  			if cert.Equal(candidate) {
   854  				return
   855  			}
   856  		}
   857  
   858  		if sigChecks == nil {
   859  			sigChecks = new(int)
   860  		}
   861  		*sigChecks++
   862  		if *sigChecks > maxChainSignatureChecks {
   863  			err = errors.New("x509: signature check attempts limit reached while verifying certificate chain")
   864  			return
   865  		}
   866  
   867  		if err := c.CheckSignatureFrom(candidate); err != nil {
   868  			if hintErr == nil {
   869  				hintErr = err
   870  				hintCert = candidate
   871  			}
   872  			return
   873  		}
   874  
   875  		err = candidate.isValid(certType, currentChain, opts)
   876  		if err != nil {
   877  			return
   878  		}
   879  
   880  		switch certType {
   881  		case rootCertificate:
   882  			chains = append(chains, appendToFreshChain(currentChain, candidate))
   883  		case intermediateCertificate:
   884  			if cache == nil {
   885  				cache = make(map[*Certificate][][]*Certificate)
   886  			}
   887  			childChains, ok := cache[candidate]
   888  			if !ok {
   889  				childChains, err = candidate.buildChains(cache, appendToFreshChain(currentChain, candidate), sigChecks, opts)
   890  				cache[candidate] = childChains
   891  			}
   892  			chains = append(chains, childChains...)
   893  		}
   894  	}
   895  
   896  	for _, root := range opts.Roots.findPotentialParents(c) {
   897  		considerCandidate(rootCertificate, root)
   898  	}
   899  	for _, intermediate := range opts.Intermediates.findPotentialParents(c) {
   900  		considerCandidate(intermediateCertificate, intermediate)
   901  	}
   902  
   903  	if len(chains) > 0 {
   904  		err = nil
   905  	}
   906  	if len(chains) == 0 && err == nil {
   907  		err = UnknownAuthorityError{c, hintErr, hintCert}
   908  	}
   909  
   910  	return
   911  }
   912  
   913  func validHostnamePattern(host string) bool { return validHostname(host, true) }
   914  func validHostnameInput(host string) bool   { return validHostname(host, false) }
   915  
   916  // validHostname reports whether host is a valid hostname that can be matched or
   917  // matched against according to RFC 6125 2.2, with some leniency to accommodate
   918  // legacy values.
   919  func validHostname(host string, isPattern bool) bool {
   920  	if !isPattern {
   921  		host = strings.TrimSuffix(host, ".")
   922  	}
   923  	if len(host) == 0 {
   924  		return false
   925  	}
   926  
   927  	for i, part := range strings.Split(host, ".") {
   928  		if part == "" {
   929  			// Empty label.
   930  			return false
   931  		}
   932  		if isPattern && i == 0 && part == "*" {
   933  			// Only allow full left-most wildcards, as those are the only ones
   934  			// we match, and matching literal '*' characters is probably never
   935  			// the expected behavior.
   936  			continue
   937  		}
   938  		for j, c := range part {
   939  			if 'a' <= c && c <= 'z' {
   940  				continue
   941  			}
   942  			if '0' <= c && c <= '9' {
   943  				continue
   944  			}
   945  			if 'A' <= c && c <= 'Z' {
   946  				continue
   947  			}
   948  			if c == '-' && j != 0 {
   949  				continue
   950  			}
   951  			if c == '_' {
   952  				// Not a valid character in hostnames, but commonly
   953  				// found in deployments outside the WebPKI.
   954  				continue
   955  			}
   956  			return false
   957  		}
   958  	}
   959  
   960  	return true
   961  }
   962  
   963  // commonNameAsHostname reports whether the Common Name field should be
   964  // considered the hostname that the certificate is valid for. This is a legacy
   965  // behavior, disabled by default or if the Subject Alt Name extension is present.
   966  //
   967  // It applies the strict validHostname check to the Common Name field, so that
   968  // certificates without SANs can still be validated against CAs with name
   969  // constraints if there is no risk the CN would be matched as a hostname.
   970  // See NameConstraintsWithoutSANs and issue 24151.
   971  func (c *Certificate) commonNameAsHostname() bool {
   972  	return !ignoreCN && !c.hasSANExtension() && validHostnamePattern(c.Subject.CommonName)
   973  }
   974  
   975  func matchExactly(hostA, hostB string) bool {
   976  	if hostA == "" || hostA == "." || hostB == "" || hostB == "." {
   977  		return false
   978  	}
   979  	return toLowerCaseASCII(hostA) == toLowerCaseASCII(hostB)
   980  }
   981  
   982  func matchHostnames(pattern, host string) bool {
   983  	pattern = toLowerCaseASCII(pattern)
   984  	host = toLowerCaseASCII(strings.TrimSuffix(host, "."))
   985  
   986  	if len(pattern) == 0 || len(host) == 0 {
   987  		return false
   988  	}
   989  
   990  	patternParts := strings.Split(pattern, ".")
   991  	hostParts := strings.Split(host, ".")
   992  
   993  	if len(patternParts) != len(hostParts) {
   994  		return false
   995  	}
   996  
   997  	for i, patternPart := range patternParts {
   998  		if i == 0 && patternPart == "*" {
   999  			continue
  1000  		}
  1001  		if patternPart != hostParts[i] {
  1002  			return false
  1003  		}
  1004  	}
  1005  
  1006  	return true
  1007  }
  1008  
  1009  // toLowerCaseASCII returns a lower-case version of in. See RFC 6125 6.4.1. We use
  1010  // an explicitly ASCII function to avoid any sharp corners resulting from
  1011  // performing Unicode operations on DNS labels.
  1012  func toLowerCaseASCII(in string) string {
  1013  	// If the string is already lower-case then there's nothing to do.
  1014  	isAlreadyLowerCase := true
  1015  	for _, c := range in {
  1016  		if c == utf8.RuneError {
  1017  			// If we get a UTF-8 error then there might be
  1018  			// upper-case ASCII bytes in the invalid sequence.
  1019  			isAlreadyLowerCase = false
  1020  			break
  1021  		}
  1022  		if 'A' <= c && c <= 'Z' {
  1023  			isAlreadyLowerCase = false
  1024  			break
  1025  		}
  1026  	}
  1027  
  1028  	if isAlreadyLowerCase {
  1029  		return in
  1030  	}
  1031  
  1032  	out := []byte(in)
  1033  	for i, c := range out {
  1034  		if 'A' <= c && c <= 'Z' {
  1035  			out[i] += 'a' - 'A'
  1036  		}
  1037  	}
  1038  	return string(out)
  1039  }
  1040  
  1041  // VerifyHostname returns nil if c is a valid certificate for the named host.
  1042  // Otherwise it returns an error describing the mismatch.
  1043  //
  1044  // IP addresses can be optionally enclosed in square brackets and are checked
  1045  // against the IPAddresses field. Other names are checked case insensitively
  1046  // against the DNSNames field. If the names are valid hostnames, the certificate
  1047  // fields can have a wildcard as the left-most label.
  1048  //
  1049  // The legacy Common Name field is ignored unless it's a valid hostname, the
  1050  // certificate doesn't have any Subject Alternative Names, and the GODEBUG
  1051  // environment variable is set to "x509ignoreCN=0". Support for Common Name is
  1052  // deprecated will be entirely removed in the future.
  1053  func (c *Certificate) VerifyHostname(h string) error {
  1054  	// IP addresses may be written in [ ].
  1055  	candidateIP := h
  1056  	if len(h) >= 3 && h[0] == '[' && h[len(h)-1] == ']' {
  1057  		candidateIP = h[1 : len(h)-1]
  1058  	}
  1059  	if ip := net.ParseIP(candidateIP); ip != nil {
  1060  		// We only match IP addresses against IP SANs.
  1061  		// See RFC 6125, Appendix B.2.
  1062  		for _, candidate := range c.IPAddresses {
  1063  			if ip.Equal(candidate) {
  1064  				return nil
  1065  			}
  1066  		}
  1067  		return HostnameError{c, candidateIP}
  1068  	}
  1069  
  1070  	names := c.DNSNames
  1071  	if c.commonNameAsHostname() {
  1072  		names = []string{c.Subject.CommonName}
  1073  	}
  1074  
  1075  	candidateName := toLowerCaseASCII(h) // Save allocations inside the loop.
  1076  	validCandidateName := validHostnameInput(candidateName)
  1077  
  1078  	for _, match := range names {
  1079  		// Ideally, we'd only match valid hostnames according to RFC 6125 like
  1080  		// browsers (more or less) do, but in practice Go is used in a wider
  1081  		// array of contexts and can't even assume DNS resolution. Instead,
  1082  		// always allow perfect matches, and only apply wildcard and trailing
  1083  		// dot processing to valid hostnames.
  1084  		if validCandidateName && validHostnamePattern(match) {
  1085  			if matchHostnames(match, candidateName) {
  1086  				return nil
  1087  			}
  1088  		} else {
  1089  			if matchExactly(match, candidateName) {
  1090  				return nil
  1091  			}
  1092  		}
  1093  	}
  1094  
  1095  	return HostnameError{c, h}
  1096  }
  1097  
  1098  func checkChainForKeyUsage(chain []*Certificate, keyUsages []ExtKeyUsage) bool {
  1099  	usages := make([]ExtKeyUsage, len(keyUsages))
  1100  	copy(usages, keyUsages)
  1101  
  1102  	if len(chain) == 0 {
  1103  		return false
  1104  	}
  1105  
  1106  	usagesRemaining := len(usages)
  1107  
  1108  	// We walk down the list and cross out any usages that aren't supported
  1109  	// by each certificate. If we cross out all the usages, then the chain
  1110  	// is unacceptable.
  1111  
  1112  NextCert:
  1113  	for i := len(chain) - 1; i >= 0; i-- {
  1114  		cert := chain[i]
  1115  		if len(cert.ExtKeyUsage) == 0 && len(cert.UnknownExtKeyUsage) == 0 {
  1116  			// The certificate doesn't have any extended key usage specified.
  1117  			continue
  1118  		}
  1119  
  1120  		for _, usage := range cert.ExtKeyUsage {
  1121  			if usage == ExtKeyUsageAny {
  1122  				// The certificate is explicitly good for any usage.
  1123  				continue NextCert
  1124  			}
  1125  		}
  1126  
  1127  		const invalidUsage ExtKeyUsage = -1
  1128  
  1129  	NextRequestedUsage:
  1130  		for i, requestedUsage := range usages {
  1131  			if requestedUsage == invalidUsage {
  1132  				continue
  1133  			}
  1134  
  1135  			for _, usage := range cert.ExtKeyUsage {
  1136  				if requestedUsage == usage {
  1137  					continue NextRequestedUsage
  1138  				} else if requestedUsage == ExtKeyUsageServerAuth &&
  1139  					(usage == ExtKeyUsageNetscapeServerGatedCrypto ||
  1140  						usage == ExtKeyUsageMicrosoftServerGatedCrypto) {
  1141  					// In order to support COMODO
  1142  					// certificate chains, we have to
  1143  					// accept Netscape or Microsoft SGC
  1144  					// usages as equal to ServerAuth.
  1145  					continue NextRequestedUsage
  1146  				}
  1147  			}
  1148  
  1149  			usages[i] = invalidUsage
  1150  			usagesRemaining--
  1151  			if usagesRemaining == 0 {
  1152  				return false
  1153  			}
  1154  		}
  1155  	}
  1156  
  1157  	return true
  1158  }
  1159  

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