Source file src/crypto/x509/verify.go

     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  	"crypto"
    10  	"crypto/x509/pkix"
    11  	"errors"
    12  	"fmt"
    13  	"net"
    14  	"net/url"
    15  	"reflect"
    16  	"runtime"
    17  	"strings"
    18  	"time"
    19  	"unicode/utf8"
    20  )
    21  
    22  type InvalidReason int
    23  
    24  const (
    25  	// NotAuthorizedToSign results when a certificate is signed by another
    26  	// which isn't marked as a CA certificate.
    27  	NotAuthorizedToSign InvalidReason = iota
    28  	// Expired results when a certificate has expired, based on the time
    29  	// given in the VerifyOptions.
    30  	Expired
    31  	// CANotAuthorizedForThisName results when an intermediate or root
    32  	// certificate has a name constraint which doesn't permit a DNS or
    33  	// other name (including IP address) in the leaf certificate.
    34  	CANotAuthorizedForThisName
    35  	// TooManyIntermediates results when a path length constraint is
    36  	// violated.
    37  	TooManyIntermediates
    38  	// IncompatibleUsage results when the certificate's key usage indicates
    39  	// that it may only be used for a different purpose.
    40  	IncompatibleUsage
    41  	// NameMismatch results when the subject name of a parent certificate
    42  	// does not match the issuer name in the child.
    43  	NameMismatch
    44  	// NameConstraintsWithoutSANs is a legacy error and is no longer returned.
    45  	NameConstraintsWithoutSANs
    46  	// UnconstrainedName results when a CA certificate contains permitted
    47  	// name constraints, but leaf certificate contains a name of an
    48  	// unsupported or unconstrained type.
    49  	UnconstrainedName
    50  	// TooManyConstraints results when the number of comparison operations
    51  	// needed to check a certificate exceeds the limit set by
    52  	// VerifyOptions.MaxConstraintComparisions. This limit exists to
    53  	// prevent pathological certificates can consuming excessive amounts of
    54  	// CPU time to verify.
    55  	TooManyConstraints
    56  	// CANotAuthorizedForExtKeyUsage results when an intermediate or root
    57  	// certificate does not permit a requested extended key usage.
    58  	CANotAuthorizedForExtKeyUsage
    59  )
    60  
    61  // CertificateInvalidError results when an odd error occurs. Users of this
    62  // library probably want to handle all these errors uniformly.
    63  type CertificateInvalidError struct {
    64  	Cert   *Certificate
    65  	Reason InvalidReason
    66  	Detail string
    67  }
    68  
    69  func (e CertificateInvalidError) Error() string {
    70  	switch e.Reason {
    71  	case NotAuthorizedToSign:
    72  		return "x509: certificate is not authorized to sign other certificates"
    73  	case Expired:
    74  		return "x509: certificate has expired or is not yet valid: " + e.Detail
    75  	case CANotAuthorizedForThisName:
    76  		return "x509: a root or intermediate certificate is not authorized to sign for this name: " + e.Detail
    77  	case CANotAuthorizedForExtKeyUsage:
    78  		return "x509: a root or intermediate certificate is not authorized for an extended key usage: " + e.Detail
    79  	case TooManyIntermediates:
    80  		return "x509: too many intermediates for path length constraint"
    81  	case IncompatibleUsage:
    82  		return "x509: certificate specifies an incompatible key usage"
    83  	case NameMismatch:
    84  		return "x509: issuer name does not match subject from issuing certificate"
    85  	case NameConstraintsWithoutSANs:
    86  		return "x509: issuer has name constraints but leaf doesn't have a SAN extension"
    87  	case UnconstrainedName:
    88  		return "x509: issuer has name constraints but leaf contains unknown or unconstrained name: " + e.Detail
    89  	}
    90  	return "x509: unknown error"
    91  }
    92  
    93  // HostnameError results when the set of authorized names doesn't match the
    94  // requested name.
    95  type HostnameError struct {
    96  	Certificate *Certificate
    97  	Host        string
    98  }
    99  
   100  func (h HostnameError) Error() string {
   101  	c := h.Certificate
   102  
   103  	if !c.hasSANExtension() && matchHostnames(c.Subject.CommonName, h.Host) {
   104  		return "x509: certificate relies on legacy Common Name field, use SANs instead"
   105  	}
   106  
   107  	var valid string
   108  	if ip := net.ParseIP(h.Host); ip != nil {
   109  		// Trying to validate an IP
   110  		if len(c.IPAddresses) == 0 {
   111  			return "x509: cannot validate certificate for " + h.Host + " because it doesn't contain any IP SANs"
   112  		}
   113  		for _, san := range c.IPAddresses {
   114  			if len(valid) > 0 {
   115  				valid += ", "
   116  			}
   117  			valid += san.String()
   118  		}
   119  	} else {
   120  		valid = strings.Join(c.DNSNames, ", ")
   121  	}
   122  
   123  	if len(valid) == 0 {
   124  		return "x509: certificate is not valid for any names, but wanted to match " + h.Host
   125  	}
   126  	return "x509: certificate is valid for " + valid + ", not " + h.Host
   127  }
   128  
   129  // UnknownAuthorityError results when the certificate issuer is unknown
   130  type UnknownAuthorityError struct {
   131  	Cert *Certificate
   132  	// hintErr contains an error that may be helpful in determining why an
   133  	// authority wasn't found.
   134  	hintErr error
   135  	// hintCert contains a possible authority certificate that was rejected
   136  	// because of the error in hintErr.
   137  	hintCert *Certificate
   138  }
   139  
   140  func (e UnknownAuthorityError) Error() string {
   141  	s := "x509: certificate signed by unknown authority"
   142  	if e.hintErr != nil {
   143  		certName := e.hintCert.Subject.CommonName
   144  		if len(certName) == 0 {
   145  			if len(e.hintCert.Subject.Organization) > 0 {
   146  				certName = e.hintCert.Subject.Organization[0]
   147  			} else {
   148  				certName = "serial:" + e.hintCert.SerialNumber.String()
   149  			}
   150  		}
   151  		s += fmt.Sprintf(" (possibly because of %q while trying to verify candidate authority certificate %q)", e.hintErr, certName)
   152  	}
   153  	return s
   154  }
   155  
   156  // SystemRootsError results when we fail to load the system root certificates.
   157  type SystemRootsError struct {
   158  	Err error
   159  }
   160  
   161  func (se SystemRootsError) Error() string {
   162  	msg := "x509: failed to load system roots and no roots provided"
   163  	if se.Err != nil {
   164  		return msg + "; " + se.Err.Error()
   165  	}
   166  	return msg
   167  }
   168  
   169  func (se SystemRootsError) Unwrap() error { return se.Err }
   170  
   171  // errNotParsed is returned when a certificate without ASN.1 contents is
   172  // verified. Platform-specific verification needs the ASN.1 contents.
   173  var errNotParsed = errors.New("x509: missing ASN.1 contents; use ParseCertificate")
   174  
   175  // VerifyOptions contains parameters for Certificate.Verify.
   176  type VerifyOptions struct {
   177  	// DNSName, if set, is checked against the leaf certificate with
   178  	// Certificate.VerifyHostname or the platform verifier.
   179  	DNSName string
   180  
   181  	// Intermediates is an optional pool of certificates that are not trust
   182  	// anchors, but can be used to form a chain from the leaf certificate to a
   183  	// root certificate.
   184  	Intermediates *CertPool
   185  	// Roots is the set of trusted root certificates the leaf certificate needs
   186  	// to chain up to. If nil, the system roots or the platform verifier are used.
   187  	Roots *CertPool
   188  
   189  	// CurrentTime is used to check the validity of all certificates in the
   190  	// chain. If zero, the current time is used.
   191  	CurrentTime time.Time
   192  
   193  	// KeyUsages specifies which Extended Key Usage values are acceptable. A
   194  	// chain is accepted if it allows any of the listed values. An empty list
   195  	// means ExtKeyUsageServerAuth. To accept any key usage, include ExtKeyUsageAny.
   196  	KeyUsages []ExtKeyUsage
   197  
   198  	// MaxConstraintComparisions is the maximum number of comparisons to
   199  	// perform when checking a given certificate's name constraints. If
   200  	// zero, a sensible default is used. This limit prevents pathological
   201  	// certificates from consuming excessive amounts of CPU time when
   202  	// validating. It does not apply to the platform verifier.
   203  	MaxConstraintComparisions int
   204  }
   205  
   206  const (
   207  	leafCertificate = iota
   208  	intermediateCertificate
   209  	rootCertificate
   210  )
   211  
   212  // rfc2821Mailbox represents a “mailbox” (which is an email address to most
   213  // people) by breaking it into the “local” (i.e. before the '@') and “domain”
   214  // parts.
   215  type rfc2821Mailbox struct {
   216  	local, domain string
   217  }
   218  
   219  // parseRFC2821Mailbox parses an email address into local and domain parts,
   220  // based on the ABNF for a “Mailbox” from RFC 2821. According to RFC 5280,
   221  // Section 4.2.1.6 that's correct for an rfc822Name from a certificate: “The
   222  // format of an rfc822Name is a "Mailbox" as defined in RFC 2821, Section 4.1.2”.
   223  func parseRFC2821Mailbox(in string) (mailbox rfc2821Mailbox, ok bool) {
   224  	if len(in) == 0 {
   225  		return mailbox, false
   226  	}
   227  
   228  	localPartBytes := make([]byte, 0, len(in)/2)
   229  
   230  	if in[0] == '"' {
   231  		// Quoted-string = DQUOTE *qcontent DQUOTE
   232  		// non-whitespace-control = %d1-8 / %d11 / %d12 / %d14-31 / %d127
   233  		// qcontent = qtext / quoted-pair
   234  		// qtext = non-whitespace-control /
   235  		//         %d33 / %d35-91 / %d93-126
   236  		// quoted-pair = ("\" text) / obs-qp
   237  		// text = %d1-9 / %d11 / %d12 / %d14-127 / obs-text
   238  		//
   239  		// (Names beginning with “obs-” are the obsolete syntax from RFC 2822,
   240  		// Section 4. Since it has been 16 years, we no longer accept that.)
   241  		in = in[1:]
   242  	QuotedString:
   243  		for {
   244  			if len(in) == 0 {
   245  				return mailbox, false
   246  			}
   247  			c := in[0]
   248  			in = in[1:]
   249  
   250  			switch {
   251  			case c == '"':
   252  				break QuotedString
   253  
   254  			case c == '\\':
   255  				// quoted-pair
   256  				if len(in) == 0 {
   257  					return mailbox, false
   258  				}
   259  				if in[0] == 11 ||
   260  					in[0] == 12 ||
   261  					(1 <= in[0] && in[0] <= 9) ||
   262  					(14 <= in[0] && in[0] <= 127) {
   263  					localPartBytes = append(localPartBytes, in[0])
   264  					in = in[1:]
   265  				} else {
   266  					return mailbox, false
   267  				}
   268  
   269  			case c == 11 ||
   270  				c == 12 ||
   271  				// Space (char 32) is not allowed based on the
   272  				// BNF, but RFC 3696 gives an example that
   273  				// assumes that it is. Several “verified”
   274  				// errata continue to argue about this point.
   275  				// We choose to accept it.
   276  				c == 32 ||
   277  				c == 33 ||
   278  				c == 127 ||
   279  				(1 <= c && c <= 8) ||
   280  				(14 <= c && c <= 31) ||
   281  				(35 <= c && c <= 91) ||
   282  				(93 <= c && c <= 126):
   283  				// qtext
   284  				localPartBytes = append(localPartBytes, c)
   285  
   286  			default:
   287  				return mailbox, false
   288  			}
   289  		}
   290  	} else {
   291  		// Atom ("." Atom)*
   292  	NextChar:
   293  		for len(in) > 0 {
   294  			// atext from RFC 2822, Section 3.2.4
   295  			c := in[0]
   296  
   297  			switch {
   298  			case c == '\\':
   299  				// Examples given in RFC 3696 suggest that
   300  				// escaped characters can appear outside of a
   301  				// quoted string. Several “verified” errata
   302  				// continue to argue the point. We choose to
   303  				// accept it.
   304  				in = in[1:]
   305  				if len(in) == 0 {
   306  					return mailbox, false
   307  				}
   308  				fallthrough
   309  
   310  			case ('0' <= c && c <= '9') ||
   311  				('a' <= c && c <= 'z') ||
   312  				('A' <= c && c <= 'Z') ||
   313  				c == '!' || c == '#' || c == '$' || c == '%' ||
   314  				c == '&' || c == '\'' || c == '*' || c == '+' ||
   315  				c == '-' || c == '/' || c == '=' || c == '?' ||
   316  				c == '^' || c == '_' || c == '`' || c == '{' ||
   317  				c == '|' || c == '}' || c == '~' || c == '.':
   318  				localPartBytes = append(localPartBytes, in[0])
   319  				in = in[1:]
   320  
   321  			default:
   322  				break NextChar
   323  			}
   324  		}
   325  
   326  		if len(localPartBytes) == 0 {
   327  			return mailbox, false
   328  		}
   329  
   330  		// From RFC 3696, Section 3:
   331  		// “period (".") may also appear, but may not be used to start
   332  		// or end the local part, nor may two or more consecutive
   333  		// periods appear.”
   334  		twoDots := []byte{'.', '.'}
   335  		if localPartBytes[0] == '.' ||
   336  			localPartBytes[len(localPartBytes)-1] == '.' ||
   337  			bytes.Contains(localPartBytes, twoDots) {
   338  			return mailbox, false
   339  		}
   340  	}
   341  
   342  	if len(in) == 0 || in[0] != '@' {
   343  		return mailbox, false
   344  	}
   345  	in = in[1:]
   346  
   347  	// The RFC species a format for domains, but that's known to be
   348  	// violated in practice so we accept that anything after an '@' is the
   349  	// domain part.
   350  	if _, ok := domainToReverseLabels(in); !ok {
   351  		return mailbox, false
   352  	}
   353  
   354  	mailbox.local = string(localPartBytes)
   355  	mailbox.domain = in
   356  	return mailbox, true
   357  }
   358  
   359  // domainToReverseLabels converts a textual domain name like foo.example.com to
   360  // the list of labels in reverse order, e.g. ["com", "example", "foo"].
   361  func domainToReverseLabels(domain string) (reverseLabels []string, ok bool) {
   362  	for len(domain) > 0 {
   363  		if i := strings.LastIndexByte(domain, '.'); i == -1 {
   364  			reverseLabels = append(reverseLabels, domain)
   365  			domain = ""
   366  		} else {
   367  			reverseLabels = append(reverseLabels, domain[i+1:])
   368  			domain = domain[:i]
   369  			if i == 0 { // domain == ""
   370  				// domain is prefixed with an empty label, append an empty
   371  				// string to reverseLabels to indicate this.
   372  				reverseLabels = append(reverseLabels, "")
   373  			}
   374  		}
   375  	}
   376  
   377  	if len(reverseLabels) > 0 && len(reverseLabels[0]) == 0 {
   378  		// An empty label at the end indicates an absolute value.
   379  		return nil, false
   380  	}
   381  
   382  	for _, label := range reverseLabels {
   383  		if len(label) == 0 {
   384  			// Empty labels are otherwise invalid.
   385  			return nil, false
   386  		}
   387  
   388  		for _, c := range label {
   389  			if c < 33 || c > 126 {
   390  				// Invalid character.
   391  				return nil, false
   392  			}
   393  		}
   394  	}
   395  
   396  	return reverseLabels, true
   397  }
   398  
   399  func matchEmailConstraint(mailbox rfc2821Mailbox, constraint string) (bool, error) {
   400  	// If the constraint contains an @, then it specifies an exact mailbox
   401  	// name.
   402  	if strings.Contains(constraint, "@") {
   403  		constraintMailbox, ok := parseRFC2821Mailbox(constraint)
   404  		if !ok {
   405  			return false, fmt.Errorf("x509: internal error: cannot parse constraint %q", constraint)
   406  		}
   407  		return mailbox.local == constraintMailbox.local && strings.EqualFold(mailbox.domain, constraintMailbox.domain), nil
   408  	}
   409  
   410  	// Otherwise the constraint is like a DNS constraint of the domain part
   411  	// of the mailbox.
   412  	return matchDomainConstraint(mailbox.domain, constraint)
   413  }
   414  
   415  func matchURIConstraint(uri *url.URL, constraint string) (bool, error) {
   416  	// From RFC 5280, Section 4.2.1.10:
   417  	// “a uniformResourceIdentifier that does not include an authority
   418  	// component with a host name specified as a fully qualified domain
   419  	// name (e.g., if the URI either does not include an authority
   420  	// component or includes an authority component in which the host name
   421  	// is specified as an IP address), then the application MUST reject the
   422  	// certificate.”
   423  
   424  	host := uri.Host
   425  	if len(host) == 0 {
   426  		return false, fmt.Errorf("URI with empty host (%q) cannot be matched against constraints", uri.String())
   427  	}
   428  
   429  	if strings.Contains(host, ":") && !strings.HasSuffix(host, "]") {
   430  		var err error
   431  		host, _, err = net.SplitHostPort(uri.Host)
   432  		if err != nil {
   433  			return false, err
   434  		}
   435  	}
   436  
   437  	if strings.HasPrefix(host, "[") && strings.HasSuffix(host, "]") ||
   438  		net.ParseIP(host) != nil {
   439  		return false, fmt.Errorf("URI with IP (%q) cannot be matched against constraints", uri.String())
   440  	}
   441  
   442  	return matchDomainConstraint(host, constraint)
   443  }
   444  
   445  func matchIPConstraint(ip net.IP, constraint *net.IPNet) (bool, error) {
   446  	if len(ip) != len(constraint.IP) {
   447  		return false, nil
   448  	}
   449  
   450  	for i := range ip {
   451  		if mask := constraint.Mask[i]; ip[i]&mask != constraint.IP[i]&mask {
   452  			return false, nil
   453  		}
   454  	}
   455  
   456  	return true, nil
   457  }
   458  
   459  func matchDomainConstraint(domain, constraint string) (bool, error) {
   460  	// The meaning of zero length constraints is not specified, but this
   461  	// code follows NSS and accepts them as matching everything.
   462  	if len(constraint) == 0 {
   463  		return true, nil
   464  	}
   465  
   466  	domainLabels, ok := domainToReverseLabels(domain)
   467  	if !ok {
   468  		return false, fmt.Errorf("x509: internal error: cannot parse domain %q", domain)
   469  	}
   470  
   471  	// RFC 5280 says that a leading period in a domain name means that at
   472  	// least one label must be prepended, but only for URI and email
   473  	// constraints, not DNS constraints. The code also supports that
   474  	// behaviour for DNS constraints.
   475  
   476  	mustHaveSubdomains := false
   477  	if constraint[0] == '.' {
   478  		mustHaveSubdomains = true
   479  		constraint = constraint[1:]
   480  	}
   481  
   482  	constraintLabels, ok := domainToReverseLabels(constraint)
   483  	if !ok {
   484  		return false, fmt.Errorf("x509: internal error: cannot parse domain %q", constraint)
   485  	}
   486  
   487  	if len(domainLabels) < len(constraintLabels) ||
   488  		(mustHaveSubdomains && len(domainLabels) == len(constraintLabels)) {
   489  		return false, nil
   490  	}
   491  
   492  	for i, constraintLabel := range constraintLabels {
   493  		if !strings.EqualFold(constraintLabel, domainLabels[i]) {
   494  			return false, nil
   495  		}
   496  	}
   497  
   498  	return true, nil
   499  }
   500  
   501  // checkNameConstraints checks that c permits a child certificate to claim the
   502  // given name, of type nameType. The argument parsedName contains the parsed
   503  // form of name, suitable for passing to the match function. The total number
   504  // of comparisons is tracked in the given count and should not exceed the given
   505  // limit.
   506  func (c *Certificate) checkNameConstraints(count *int,
   507  	maxConstraintComparisons int,
   508  	nameType string,
   509  	name string,
   510  	parsedName any,
   511  	match func(parsedName, constraint any) (match bool, err error),
   512  	permitted, excluded any) error {
   513  
   514  	excludedValue := reflect.ValueOf(excluded)
   515  
   516  	*count += excludedValue.Len()
   517  	if *count > maxConstraintComparisons {
   518  		return CertificateInvalidError{c, TooManyConstraints, ""}
   519  	}
   520  
   521  	for i := 0; i < excludedValue.Len(); i++ {
   522  		constraint := excludedValue.Index(i).Interface()
   523  		match, err := match(parsedName, constraint)
   524  		if err != nil {
   525  			return CertificateInvalidError{c, CANotAuthorizedForThisName, err.Error()}
   526  		}
   527  
   528  		if match {
   529  			return CertificateInvalidError{c, CANotAuthorizedForThisName, fmt.Sprintf("%s %q is excluded by constraint %q", nameType, name, constraint)}
   530  		}
   531  	}
   532  
   533  	permittedValue := reflect.ValueOf(permitted)
   534  
   535  	*count += permittedValue.Len()
   536  	if *count > maxConstraintComparisons {
   537  		return CertificateInvalidError{c, TooManyConstraints, ""}
   538  	}
   539  
   540  	ok := true
   541  	for i := 0; i < permittedValue.Len(); i++ {
   542  		constraint := permittedValue.Index(i).Interface()
   543  
   544  		var err error
   545  		if ok, err = match(parsedName, constraint); err != nil {
   546  			return CertificateInvalidError{c, CANotAuthorizedForThisName, err.Error()}
   547  		}
   548  
   549  		if ok {
   550  			break
   551  		}
   552  	}
   553  
   554  	if !ok {
   555  		return CertificateInvalidError{c, CANotAuthorizedForThisName, fmt.Sprintf("%s %q is not permitted by any constraint", nameType, name)}
   556  	}
   557  
   558  	return nil
   559  }
   560  
   561  // isValid performs validity checks on c given that it is a candidate to append
   562  // to the chain in currentChain.
   563  func (c *Certificate) isValid(certType int, currentChain []*Certificate, opts *VerifyOptions) error {
   564  	if len(c.UnhandledCriticalExtensions) > 0 {
   565  		return UnhandledCriticalExtension{}
   566  	}
   567  
   568  	if len(currentChain) > 0 {
   569  		child := currentChain[len(currentChain)-1]
   570  		if !bytes.Equal(child.RawIssuer, c.RawSubject) {
   571  			return CertificateInvalidError{c, NameMismatch, ""}
   572  		}
   573  	}
   574  
   575  	now := opts.CurrentTime
   576  	if now.IsZero() {
   577  		now = time.Now()
   578  	}
   579  	if now.Before(c.NotBefore) {
   580  		return CertificateInvalidError{
   581  			Cert:   c,
   582  			Reason: Expired,
   583  			Detail: fmt.Sprintf("current time %s is before %s", now.Format(time.RFC3339), c.NotBefore.Format(time.RFC3339)),
   584  		}
   585  	} else if now.After(c.NotAfter) {
   586  		return CertificateInvalidError{
   587  			Cert:   c,
   588  			Reason: Expired,
   589  			Detail: fmt.Sprintf("current time %s is after %s", now.Format(time.RFC3339), c.NotAfter.Format(time.RFC3339)),
   590  		}
   591  	}
   592  
   593  	maxConstraintComparisons := opts.MaxConstraintComparisions
   594  	if maxConstraintComparisons == 0 {
   595  		maxConstraintComparisons = 250000
   596  	}
   597  	comparisonCount := 0
   598  
   599  	if certType == intermediateCertificate || certType == rootCertificate {
   600  		if len(currentChain) == 0 {
   601  			return errors.New("x509: internal error: empty chain when appending CA cert")
   602  		}
   603  	}
   604  
   605  	if (certType == intermediateCertificate || certType == rootCertificate) &&
   606  		c.hasNameConstraints() {
   607  		toCheck := []*Certificate{}
   608  		for _, c := range currentChain {
   609  			if c.hasSANExtension() {
   610  				toCheck = append(toCheck, c)
   611  			}
   612  		}
   613  		for _, sanCert := range toCheck {
   614  			err := forEachSAN(sanCert.getSANExtension(), func(tag int, data []byte) error {
   615  				switch tag {
   616  				case nameTypeEmail:
   617  					name := string(data)
   618  					mailbox, ok := parseRFC2821Mailbox(name)
   619  					if !ok {
   620  						return fmt.Errorf("x509: cannot parse rfc822Name %q", mailbox)
   621  					}
   622  
   623  					if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "email address", name, mailbox,
   624  						func(parsedName, constraint any) (bool, error) {
   625  							return matchEmailConstraint(parsedName.(rfc2821Mailbox), constraint.(string))
   626  						}, c.PermittedEmailAddresses, c.ExcludedEmailAddresses); err != nil {
   627  						return err
   628  					}
   629  
   630  				case nameTypeDNS:
   631  					name := string(data)
   632  					if _, ok := domainToReverseLabels(name); !ok {
   633  						return fmt.Errorf("x509: cannot parse dnsName %q", name)
   634  					}
   635  
   636  					if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "DNS name", name, name,
   637  						func(parsedName, constraint any) (bool, error) {
   638  							return matchDomainConstraint(parsedName.(string), constraint.(string))
   639  						}, c.PermittedDNSDomains, c.ExcludedDNSDomains); err != nil {
   640  						return err
   641  					}
   642  
   643  				case nameTypeURI:
   644  					name := string(data)
   645  					uri, err := url.Parse(name)
   646  					if err != nil {
   647  						return fmt.Errorf("x509: internal error: URI SAN %q failed to parse", name)
   648  					}
   649  
   650  					if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "URI", name, uri,
   651  						func(parsedName, constraint any) (bool, error) {
   652  							return matchURIConstraint(parsedName.(*url.URL), constraint.(string))
   653  						}, c.PermittedURIDomains, c.ExcludedURIDomains); err != nil {
   654  						return err
   655  					}
   656  
   657  				case nameTypeIP:
   658  					ip := net.IP(data)
   659  					if l := len(ip); l != net.IPv4len && l != net.IPv6len {
   660  						return fmt.Errorf("x509: internal error: IP SAN %x failed to parse", data)
   661  					}
   662  
   663  					if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "IP address", ip.String(), ip,
   664  						func(parsedName, constraint any) (bool, error) {
   665  							return matchIPConstraint(parsedName.(net.IP), constraint.(*net.IPNet))
   666  						}, c.PermittedIPRanges, c.ExcludedIPRanges); err != nil {
   667  						return err
   668  					}
   669  
   670  				default:
   671  					// Unknown SAN types are ignored.
   672  				}
   673  
   674  				return nil
   675  			})
   676  
   677  			if err != nil {
   678  				return err
   679  			}
   680  		}
   681  	}
   682  
   683  	// KeyUsage status flags are ignored. From Engineering Security, Peter
   684  	// Gutmann: A European government CA marked its signing certificates as
   685  	// being valid for encryption only, but no-one noticed. Another
   686  	// European CA marked its signature keys as not being valid for
   687  	// signatures. A different CA marked its own trusted root certificate
   688  	// as being invalid for certificate signing. Another national CA
   689  	// distributed a certificate to be used to encrypt data for the
   690  	// country’s tax authority that was marked as only being usable for
   691  	// digital signatures but not for encryption. Yet another CA reversed
   692  	// the order of the bit flags in the keyUsage due to confusion over
   693  	// encoding endianness, essentially setting a random keyUsage in
   694  	// certificates that it issued. Another CA created a self-invalidating
   695  	// certificate by adding a certificate policy statement stipulating
   696  	// that the certificate had to be used strictly as specified in the
   697  	// keyUsage, and a keyUsage containing a flag indicating that the RSA
   698  	// encryption key could only be used for Diffie-Hellman key agreement.
   699  
   700  	if certType == intermediateCertificate && (!c.BasicConstraintsValid || !c.IsCA) {
   701  		return CertificateInvalidError{c, NotAuthorizedToSign, ""}
   702  	}
   703  
   704  	if c.BasicConstraintsValid && c.MaxPathLen >= 0 {
   705  		numIntermediates := len(currentChain) - 1
   706  		if numIntermediates > c.MaxPathLen {
   707  			return CertificateInvalidError{c, TooManyIntermediates, ""}
   708  		}
   709  	}
   710  
   711  	if !boringAllowCert(c) {
   712  		// IncompatibleUsage is not quite right here,
   713  		// but it's also the "no chains found" error
   714  		// and is close enough.
   715  		return CertificateInvalidError{c, IncompatibleUsage, ""}
   716  	}
   717  
   718  	return nil
   719  }
   720  
   721  // Verify attempts to verify c by building one or more chains from c to a
   722  // certificate in opts.Roots, using certificates in opts.Intermediates if
   723  // needed. If successful, it returns one or more chains where the first
   724  // element of the chain is c and the last element is from opts.Roots.
   725  //
   726  // If opts.Roots is nil, the platform verifier might be used, and
   727  // verification details might differ from what is described below. If system
   728  // roots are unavailable the returned error will be of type SystemRootsError.
   729  //
   730  // Name constraints in the intermediates will be applied to all names claimed
   731  // in the chain, not just opts.DNSName. Thus it is invalid for a leaf to claim
   732  // example.com if an intermediate doesn't permit it, even if example.com is not
   733  // the name being validated. Note that DirectoryName constraints are not
   734  // supported.
   735  //
   736  // Name constraint validation follows the rules from RFC 5280, with the
   737  // addition that DNS name constraints may use the leading period format
   738  // defined for emails and URIs. When a constraint has a leading period
   739  // it indicates that at least one additional label must be prepended to
   740  // the constrained name to be considered valid.
   741  //
   742  // Extended Key Usage values are enforced nested down a chain, so an intermediate
   743  // or root that enumerates EKUs prevents a leaf from asserting an EKU not in that
   744  // list. (While this is not specified, it is common practice in order to limit
   745  // the types of certificates a CA can issue.)
   746  //
   747  // Certificates that use SHA1WithRSA and ECDSAWithSHA1 signatures are not supported,
   748  // and will not be used to build chains.
   749  //
   750  // Certificates other than c in the returned chains should not be modified.
   751  //
   752  // WARNING: this function doesn't do any revocation checking.
   753  func (c *Certificate) Verify(opts VerifyOptions) (chains [][]*Certificate, err error) {
   754  	// Platform-specific verification needs the ASN.1 contents so
   755  	// this makes the behavior consistent across platforms.
   756  	if len(c.Raw) == 0 {
   757  		return nil, errNotParsed
   758  	}
   759  	for i := 0; i < opts.Intermediates.len(); i++ {
   760  		c, _, err := opts.Intermediates.cert(i)
   761  		if err != nil {
   762  			return nil, fmt.Errorf("crypto/x509: error fetching intermediate: %w", err)
   763  		}
   764  		if len(c.Raw) == 0 {
   765  			return nil, errNotParsed
   766  		}
   767  	}
   768  
   769  	// Use platform verifiers, where available, if Roots is from SystemCertPool.
   770  	if runtime.GOOS == "windows" || runtime.GOOS == "darwin" || runtime.GOOS == "ios" {
   771  		// Don't use the system verifier if the system pool was replaced with a non-system pool,
   772  		// i.e. if SetFallbackRoots was called with x509usefallbackroots=1.
   773  		systemPool := systemRootsPool()
   774  		if opts.Roots == nil && (systemPool == nil || systemPool.systemPool) {
   775  			return c.systemVerify(&opts)
   776  		}
   777  		if opts.Roots != nil && opts.Roots.systemPool {
   778  			platformChains, err := c.systemVerify(&opts)
   779  			// If the platform verifier succeeded, or there are no additional
   780  			// roots, return the platform verifier result. Otherwise, continue
   781  			// with the Go verifier.
   782  			if err == nil || opts.Roots.len() == 0 {
   783  				return platformChains, err
   784  			}
   785  		}
   786  	}
   787  
   788  	if opts.Roots == nil {
   789  		opts.Roots = systemRootsPool()
   790  		if opts.Roots == nil {
   791  			return nil, SystemRootsError{systemRootsErr}
   792  		}
   793  	}
   794  
   795  	err = c.isValid(leafCertificate, nil, &opts)
   796  	if err != nil {
   797  		return
   798  	}
   799  
   800  	if len(opts.DNSName) > 0 {
   801  		err = c.VerifyHostname(opts.DNSName)
   802  		if err != nil {
   803  			return
   804  		}
   805  	}
   806  
   807  	var candidateChains [][]*Certificate
   808  	if opts.Roots.contains(c) {
   809  		candidateChains = [][]*Certificate{{c}}
   810  	} else {
   811  		candidateChains, err = c.buildChains([]*Certificate{c}, nil, &opts)
   812  		if err != nil {
   813  			return nil, err
   814  		}
   815  	}
   816  
   817  	if len(opts.KeyUsages) == 0 {
   818  		opts.KeyUsages = []ExtKeyUsage{ExtKeyUsageServerAuth}
   819  	}
   820  
   821  	for _, eku := range opts.KeyUsages {
   822  		if eku == ExtKeyUsageAny {
   823  			// If any key usage is acceptable, no need to check the chain for
   824  			// key usages.
   825  			return candidateChains, nil
   826  		}
   827  	}
   828  
   829  	chains = make([][]*Certificate, 0, len(candidateChains))
   830  	for _, candidate := range candidateChains {
   831  		if checkChainForKeyUsage(candidate, opts.KeyUsages) {
   832  			chains = append(chains, candidate)
   833  		}
   834  	}
   835  
   836  	if len(chains) == 0 {
   837  		return nil, CertificateInvalidError{c, IncompatibleUsage, ""}
   838  	}
   839  
   840  	return chains, nil
   841  }
   842  
   843  func appendToFreshChain(chain []*Certificate, cert *Certificate) []*Certificate {
   844  	n := make([]*Certificate, len(chain)+1)
   845  	copy(n, chain)
   846  	n[len(chain)] = cert
   847  	return n
   848  }
   849  
   850  // alreadyInChain checks whether a candidate certificate is present in a chain.
   851  // Rather than doing a direct byte for byte equivalency check, we check if the
   852  // subject, public key, and SAN, if present, are equal. This prevents loops that
   853  // are created by mutual cross-signatures, or other cross-signature bridge
   854  // oddities.
   855  func alreadyInChain(candidate *Certificate, chain []*Certificate) bool {
   856  	type pubKeyEqual interface {
   857  		Equal(crypto.PublicKey) bool
   858  	}
   859  
   860  	var candidateSAN *pkix.Extension
   861  	for _, ext := range candidate.Extensions {
   862  		if ext.Id.Equal(oidExtensionSubjectAltName) {
   863  			candidateSAN = &ext
   864  			break
   865  		}
   866  	}
   867  
   868  	for _, cert := range chain {
   869  		if !bytes.Equal(candidate.RawSubject, cert.RawSubject) {
   870  			continue
   871  		}
   872  		if !candidate.PublicKey.(pubKeyEqual).Equal(cert.PublicKey) {
   873  			continue
   874  		}
   875  		var certSAN *pkix.Extension
   876  		for _, ext := range cert.Extensions {
   877  			if ext.Id.Equal(oidExtensionSubjectAltName) {
   878  				certSAN = &ext
   879  				break
   880  			}
   881  		}
   882  		if candidateSAN == nil && certSAN == nil {
   883  			return true
   884  		} else if candidateSAN == nil || certSAN == nil {
   885  			return false
   886  		}
   887  		if bytes.Equal(candidateSAN.Value, certSAN.Value) {
   888  			return true
   889  		}
   890  	}
   891  	return false
   892  }
   893  
   894  // maxChainSignatureChecks is the maximum number of CheckSignatureFrom calls
   895  // that an invocation of buildChains will (transitively) make. Most chains are
   896  // less than 15 certificates long, so this leaves space for multiple chains and
   897  // for failed checks due to different intermediates having the same Subject.
   898  const maxChainSignatureChecks = 100
   899  
   900  func (c *Certificate) buildChains(currentChain []*Certificate, sigChecks *int, opts *VerifyOptions) (chains [][]*Certificate, err error) {
   901  	var (
   902  		hintErr  error
   903  		hintCert *Certificate
   904  	)
   905  
   906  	considerCandidate := func(certType int, candidate potentialParent) {
   907  		if candidate.cert.PublicKey == nil || alreadyInChain(candidate.cert, currentChain) {
   908  			return
   909  		}
   910  
   911  		if sigChecks == nil {
   912  			sigChecks = new(int)
   913  		}
   914  		*sigChecks++
   915  		if *sigChecks > maxChainSignatureChecks {
   916  			err = errors.New("x509: signature check attempts limit reached while verifying certificate chain")
   917  			return
   918  		}
   919  
   920  		if err := c.CheckSignatureFrom(candidate.cert); err != nil {
   921  			if hintErr == nil {
   922  				hintErr = err
   923  				hintCert = candidate.cert
   924  			}
   925  			return
   926  		}
   927  
   928  		err = candidate.cert.isValid(certType, currentChain, opts)
   929  		if err != nil {
   930  			if hintErr == nil {
   931  				hintErr = err
   932  				hintCert = candidate.cert
   933  			}
   934  			return
   935  		}
   936  
   937  		if candidate.constraint != nil {
   938  			if err := candidate.constraint(currentChain); err != nil {
   939  				if hintErr == nil {
   940  					hintErr = err
   941  					hintCert = candidate.cert
   942  				}
   943  				return
   944  			}
   945  		}
   946  
   947  		switch certType {
   948  		case rootCertificate:
   949  			chains = append(chains, appendToFreshChain(currentChain, candidate.cert))
   950  		case intermediateCertificate:
   951  			var childChains [][]*Certificate
   952  			childChains, err = candidate.cert.buildChains(appendToFreshChain(currentChain, candidate.cert), sigChecks, opts)
   953  			chains = append(chains, childChains...)
   954  		}
   955  	}
   956  
   957  	for _, root := range opts.Roots.findPotentialParents(c) {
   958  		considerCandidate(rootCertificate, root)
   959  	}
   960  	for _, intermediate := range opts.Intermediates.findPotentialParents(c) {
   961  		considerCandidate(intermediateCertificate, intermediate)
   962  	}
   963  
   964  	if len(chains) > 0 {
   965  		err = nil
   966  	}
   967  	if len(chains) == 0 && err == nil {
   968  		err = UnknownAuthorityError{c, hintErr, hintCert}
   969  	}
   970  
   971  	return
   972  }
   973  
   974  func validHostnamePattern(host string) bool { return validHostname(host, true) }
   975  func validHostnameInput(host string) bool   { return validHostname(host, false) }
   976  
   977  // validHostname reports whether host is a valid hostname that can be matched or
   978  // matched against according to RFC 6125 2.2, with some leniency to accommodate
   979  // legacy values.
   980  func validHostname(host string, isPattern bool) bool {
   981  	if !isPattern {
   982  		host = strings.TrimSuffix(host, ".")
   983  	}
   984  	if len(host) == 0 {
   985  		return false
   986  	}
   987  	if host == "*" {
   988  		// Bare wildcards are not allowed, they are not valid DNS names,
   989  		// nor are they allowed per RFC 6125.
   990  		return false
   991  	}
   992  
   993  	for i, part := range strings.Split(host, ".") {
   994  		if part == "" {
   995  			// Empty label.
   996  			return false
   997  		}
   998  		if isPattern && i == 0 && part == "*" {
   999  			// Only allow full left-most wildcards, as those are the only ones
  1000  			// we match, and matching literal '*' characters is probably never
  1001  			// the expected behavior.
  1002  			continue
  1003  		}
  1004  		for j, c := range part {
  1005  			if 'a' <= c && c <= 'z' {
  1006  				continue
  1007  			}
  1008  			if '0' <= c && c <= '9' {
  1009  				continue
  1010  			}
  1011  			if 'A' <= c && c <= 'Z' {
  1012  				continue
  1013  			}
  1014  			if c == '-' && j != 0 {
  1015  				continue
  1016  			}
  1017  			if c == '_' {
  1018  				// Not a valid character in hostnames, but commonly
  1019  				// found in deployments outside the WebPKI.
  1020  				continue
  1021  			}
  1022  			return false
  1023  		}
  1024  	}
  1025  
  1026  	return true
  1027  }
  1028  
  1029  func matchExactly(hostA, hostB string) bool {
  1030  	if hostA == "" || hostA == "." || hostB == "" || hostB == "." {
  1031  		return false
  1032  	}
  1033  	return toLowerCaseASCII(hostA) == toLowerCaseASCII(hostB)
  1034  }
  1035  
  1036  func matchHostnames(pattern, host string) bool {
  1037  	pattern = toLowerCaseASCII(pattern)
  1038  	host = toLowerCaseASCII(strings.TrimSuffix(host, "."))
  1039  
  1040  	if len(pattern) == 0 || len(host) == 0 {
  1041  		return false
  1042  	}
  1043  
  1044  	patternParts := strings.Split(pattern, ".")
  1045  	hostParts := strings.Split(host, ".")
  1046  
  1047  	if len(patternParts) != len(hostParts) {
  1048  		return false
  1049  	}
  1050  
  1051  	for i, patternPart := range patternParts {
  1052  		if i == 0 && patternPart == "*" {
  1053  			continue
  1054  		}
  1055  		if patternPart != hostParts[i] {
  1056  			return false
  1057  		}
  1058  	}
  1059  
  1060  	return true
  1061  }
  1062  
  1063  // toLowerCaseASCII returns a lower-case version of in. See RFC 6125 6.4.1. We use
  1064  // an explicitly ASCII function to avoid any sharp corners resulting from
  1065  // performing Unicode operations on DNS labels.
  1066  func toLowerCaseASCII(in string) string {
  1067  	// If the string is already lower-case then there's nothing to do.
  1068  	isAlreadyLowerCase := true
  1069  	for _, c := range in {
  1070  		if c == utf8.RuneError {
  1071  			// If we get a UTF-8 error then there might be
  1072  			// upper-case ASCII bytes in the invalid sequence.
  1073  			isAlreadyLowerCase = false
  1074  			break
  1075  		}
  1076  		if 'A' <= c && c <= 'Z' {
  1077  			isAlreadyLowerCase = false
  1078  			break
  1079  		}
  1080  	}
  1081  
  1082  	if isAlreadyLowerCase {
  1083  		return in
  1084  	}
  1085  
  1086  	out := []byte(in)
  1087  	for i, c := range out {
  1088  		if 'A' <= c && c <= 'Z' {
  1089  			out[i] += 'a' - 'A'
  1090  		}
  1091  	}
  1092  	return string(out)
  1093  }
  1094  
  1095  // VerifyHostname returns nil if c is a valid certificate for the named host.
  1096  // Otherwise it returns an error describing the mismatch.
  1097  //
  1098  // IP addresses can be optionally enclosed in square brackets and are checked
  1099  // against the IPAddresses field. Other names are checked case insensitively
  1100  // against the DNSNames field. If the names are valid hostnames, the certificate
  1101  // fields can have a wildcard as the complete left-most label (e.g. *.example.com).
  1102  //
  1103  // Note that the legacy Common Name field is ignored.
  1104  func (c *Certificate) VerifyHostname(h string) error {
  1105  	// IP addresses may be written in [ ].
  1106  	candidateIP := h
  1107  	if len(h) >= 3 && h[0] == '[' && h[len(h)-1] == ']' {
  1108  		candidateIP = h[1 : len(h)-1]
  1109  	}
  1110  	if ip := net.ParseIP(candidateIP); ip != nil {
  1111  		// We only match IP addresses against IP SANs.
  1112  		// See RFC 6125, Appendix B.2.
  1113  		for _, candidate := range c.IPAddresses {
  1114  			if ip.Equal(candidate) {
  1115  				return nil
  1116  			}
  1117  		}
  1118  		return HostnameError{c, candidateIP}
  1119  	}
  1120  
  1121  	candidateName := toLowerCaseASCII(h) // Save allocations inside the loop.
  1122  	validCandidateName := validHostnameInput(candidateName)
  1123  
  1124  	for _, match := range c.DNSNames {
  1125  		// Ideally, we'd only match valid hostnames according to RFC 6125 like
  1126  		// browsers (more or less) do, but in practice Go is used in a wider
  1127  		// array of contexts and can't even assume DNS resolution. Instead,
  1128  		// always allow perfect matches, and only apply wildcard and trailing
  1129  		// dot processing to valid hostnames.
  1130  		if validCandidateName && validHostnamePattern(match) {
  1131  			if matchHostnames(match, candidateName) {
  1132  				return nil
  1133  			}
  1134  		} else {
  1135  			if matchExactly(match, candidateName) {
  1136  				return nil
  1137  			}
  1138  		}
  1139  	}
  1140  
  1141  	return HostnameError{c, h}
  1142  }
  1143  
  1144  func checkChainForKeyUsage(chain []*Certificate, keyUsages []ExtKeyUsage) bool {
  1145  	usages := make([]ExtKeyUsage, len(keyUsages))
  1146  	copy(usages, keyUsages)
  1147  
  1148  	if len(chain) == 0 {
  1149  		return false
  1150  	}
  1151  
  1152  	usagesRemaining := len(usages)
  1153  
  1154  	// We walk down the list and cross out any usages that aren't supported
  1155  	// by each certificate. If we cross out all the usages, then the chain
  1156  	// is unacceptable.
  1157  
  1158  NextCert:
  1159  	for i := len(chain) - 1; i >= 0; i-- {
  1160  		cert := chain[i]
  1161  		if len(cert.ExtKeyUsage) == 0 && len(cert.UnknownExtKeyUsage) == 0 {
  1162  			// The certificate doesn't have any extended key usage specified.
  1163  			continue
  1164  		}
  1165  
  1166  		for _, usage := range cert.ExtKeyUsage {
  1167  			if usage == ExtKeyUsageAny {
  1168  				// The certificate is explicitly good for any usage.
  1169  				continue NextCert
  1170  			}
  1171  		}
  1172  
  1173  		const invalidUsage ExtKeyUsage = -1
  1174  
  1175  	NextRequestedUsage:
  1176  		for i, requestedUsage := range usages {
  1177  			if requestedUsage == invalidUsage {
  1178  				continue
  1179  			}
  1180  
  1181  			for _, usage := range cert.ExtKeyUsage {
  1182  				if requestedUsage == usage {
  1183  					continue NextRequestedUsage
  1184  				}
  1185  			}
  1186  
  1187  			usages[i] = invalidUsage
  1188  			usagesRemaining--
  1189  			if usagesRemaining == 0 {
  1190  				return false
  1191  			}
  1192  		}
  1193  	}
  1194  
  1195  	return true
  1196  }
  1197  

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