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  	"crypto"
    10  	"crypto/x509/pkix"
    11  	"errors"
    12  	"fmt"
    13  	"net"
    14  	"net/netip"
    15  	"net/url"
    16  	"reflect"
    17  	"runtime"
    18  	"strings"
    19  	"time"
    20  	"unicode/utf8"
    21  )
    22  
    23  type InvalidReason int
    24  
    25  const (
    26  	// NotAuthorizedToSign results when a certificate is signed by another
    27  	// which isn't marked as a CA certificate.
    28  	NotAuthorizedToSign InvalidReason = iota
    29  	// Expired results when a certificate has expired, based on the time
    30  	// given in the VerifyOptions.
    31  	Expired
    32  	// CANotAuthorizedForThisName results when an intermediate or root
    33  	// certificate has a name constraint which doesn't permit a DNS or
    34  	// other name (including IP address) in the leaf certificate.
    35  	CANotAuthorizedForThisName
    36  	// TooManyIntermediates results when a path length constraint is
    37  	// violated.
    38  	TooManyIntermediates
    39  	// IncompatibleUsage results when the certificate's key usage indicates
    40  	// that it may only be used for a different purpose.
    41  	IncompatibleUsage
    42  	// NameMismatch results when the subject name of a parent certificate
    43  	// does not match the issuer name in the child.
    44  	NameMismatch
    45  	// NameConstraintsWithoutSANs is a legacy error and is no longer returned.
    46  	NameConstraintsWithoutSANs
    47  	// UnconstrainedName results when a CA certificate contains permitted
    48  	// name constraints, but leaf certificate contains a name of an
    49  	// unsupported or unconstrained type.
    50  	UnconstrainedName
    51  	// TooManyConstraints results when the number of comparison operations
    52  	// needed to check a certificate exceeds the limit set by
    53  	// VerifyOptions.MaxConstraintComparisions. This limit exists to
    54  	// prevent pathological certificates can consuming excessive amounts of
    55  	// CPU time to verify.
    56  	TooManyConstraints
    57  	// CANotAuthorizedForExtKeyUsage results when an intermediate or root
    58  	// certificate does not permit a requested extended key usage.
    59  	CANotAuthorizedForExtKeyUsage
    60  )
    61  
    62  // CertificateInvalidError results when an odd error occurs. Users of this
    63  // library probably want to handle all these errors uniformly.
    64  type CertificateInvalidError struct {
    65  	Cert   *Certificate
    66  	Reason InvalidReason
    67  	Detail string
    68  }
    69  
    70  func (e CertificateInvalidError) Error() string {
    71  	switch e.Reason {
    72  	case NotAuthorizedToSign:
    73  		return "x509: certificate is not authorized to sign other certificates"
    74  	case Expired:
    75  		return "x509: certificate has expired or is not yet valid: " + e.Detail
    76  	case CANotAuthorizedForThisName:
    77  		return "x509: a root or intermediate certificate is not authorized to sign for this name: " + e.Detail
    78  	case CANotAuthorizedForExtKeyUsage:
    79  		return "x509: a root or intermediate certificate is not authorized for an extended key usage: " + e.Detail
    80  	case TooManyIntermediates:
    81  		return "x509: too many intermediates for path length constraint"
    82  	case IncompatibleUsage:
    83  		return "x509: certificate specifies an incompatible key usage"
    84  	case NameMismatch:
    85  		return "x509: issuer name does not match subject from issuing certificate"
    86  	case NameConstraintsWithoutSANs:
    87  		return "x509: issuer has name constraints but leaf doesn't have a SAN extension"
    88  	case UnconstrainedName:
    89  		return "x509: issuer has name constraints but leaf contains unknown or unconstrained name: " + e.Detail
    90  	}
    91  	return "x509: unknown error"
    92  }
    93  
    94  // HostnameError results when the set of authorized names doesn't match the
    95  // requested name.
    96  type HostnameError struct {
    97  	Certificate *Certificate
    98  	Host        string
    99  }
   100  
   101  func (h HostnameError) Error() string {
   102  	c := h.Certificate
   103  
   104  	if !c.hasSANExtension() && matchHostnames(c.Subject.CommonName, h.Host) {
   105  		return "x509: certificate relies on legacy Common Name field, use SANs instead"
   106  	}
   107  
   108  	var valid string
   109  	if ip := net.ParseIP(h.Host); ip != nil {
   110  		// Trying to validate an IP
   111  		if len(c.IPAddresses) == 0 {
   112  			return "x509: cannot validate certificate for " + h.Host + " because it doesn't contain any IP SANs"
   113  		}
   114  		for _, san := range c.IPAddresses {
   115  			if len(valid) > 0 {
   116  				valid += ", "
   117  			}
   118  			valid += san.String()
   119  		}
   120  	} else {
   121  		valid = strings.Join(c.DNSNames, ", ")
   122  	}
   123  
   124  	if len(valid) == 0 {
   125  		return "x509: certificate is not valid for any names, but wanted to match " + h.Host
   126  	}
   127  	return "x509: certificate is valid for " + valid + ", not " + h.Host
   128  }
   129  
   130  // UnknownAuthorityError results when the certificate issuer is unknown
   131  type UnknownAuthorityError struct {
   132  	Cert *Certificate
   133  	// hintErr contains an error that may be helpful in determining why an
   134  	// authority wasn't found.
   135  	hintErr error
   136  	// hintCert contains a possible authority certificate that was rejected
   137  	// because of the error in hintErr.
   138  	hintCert *Certificate
   139  }
   140  
   141  func (e UnknownAuthorityError) Error() string {
   142  	s := "x509: certificate signed by unknown authority"
   143  	if e.hintErr != nil {
   144  		certName := e.hintCert.Subject.CommonName
   145  		if len(certName) == 0 {
   146  			if len(e.hintCert.Subject.Organization) > 0 {
   147  				certName = e.hintCert.Subject.Organization[0]
   148  			} else {
   149  				certName = "serial:" + e.hintCert.SerialNumber.String()
   150  			}
   151  		}
   152  		s += fmt.Sprintf(" (possibly because of %q while trying to verify candidate authority certificate %q)", e.hintErr, certName)
   153  	}
   154  	return s
   155  }
   156  
   157  // SystemRootsError results when we fail to load the system root certificates.
   158  type SystemRootsError struct {
   159  	Err error
   160  }
   161  
   162  func (se SystemRootsError) Error() string {
   163  	msg := "x509: failed to load system roots and no roots provided"
   164  	if se.Err != nil {
   165  		return msg + "; " + se.Err.Error()
   166  	}
   167  	return msg
   168  }
   169  
   170  func (se SystemRootsError) Unwrap() error { return se.Err }
   171  
   172  // errNotParsed is returned when a certificate without ASN.1 contents is
   173  // verified. Platform-specific verification needs the ASN.1 contents.
   174  var errNotParsed = errors.New("x509: missing ASN.1 contents; use ParseCertificate")
   175  
   176  // VerifyOptions contains parameters for Certificate.Verify.
   177  type VerifyOptions struct {
   178  	// DNSName, if set, is checked against the leaf certificate with
   179  	// Certificate.VerifyHostname or the platform verifier.
   180  	DNSName string
   181  
   182  	// Intermediates is an optional pool of certificates that are not trust
   183  	// anchors, but can be used to form a chain from the leaf certificate to a
   184  	// root certificate.
   185  	Intermediates *CertPool
   186  	// Roots is the set of trusted root certificates the leaf certificate needs
   187  	// to chain up to. If nil, the system roots or the platform verifier are used.
   188  	Roots *CertPool
   189  
   190  	// CurrentTime is used to check the validity of all certificates in the
   191  	// chain. If zero, the current time is used.
   192  	CurrentTime time.Time
   193  
   194  	// KeyUsages specifies which Extended Key Usage values are acceptable. A
   195  	// chain is accepted if it allows any of the listed values. An empty list
   196  	// means ExtKeyUsageServerAuth. To accept any key usage, include ExtKeyUsageAny.
   197  	KeyUsages []ExtKeyUsage
   198  
   199  	// MaxConstraintComparisions is the maximum number of comparisons to
   200  	// perform when checking a given certificate's name constraints. If
   201  	// zero, a sensible default is used. This limit prevents pathological
   202  	// certificates from consuming excessive amounts of CPU time when
   203  	// validating. It does not apply to the platform verifier.
   204  	MaxConstraintComparisions int
   205  }
   206  
   207  const (
   208  	leafCertificate = iota
   209  	intermediateCertificate
   210  	rootCertificate
   211  )
   212  
   213  // rfc2821Mailbox represents a “mailbox” (which is an email address to most
   214  // people) by breaking it into the “local” (i.e. before the '@') and “domain”
   215  // parts.
   216  type rfc2821Mailbox struct {
   217  	local, domain string
   218  }
   219  
   220  // parseRFC2821Mailbox parses an email address into local and domain parts,
   221  // based on the ABNF for a “Mailbox” from RFC 2821. According to RFC 5280,
   222  // Section 4.2.1.6 that's correct for an rfc822Name from a certificate: “The
   223  // format of an rfc822Name is a "Mailbox" as defined in RFC 2821, Section 4.1.2”.
   224  func parseRFC2821Mailbox(in string) (mailbox rfc2821Mailbox, ok bool) {
   225  	if len(in) == 0 {
   226  		return mailbox, false
   227  	}
   228  
   229  	localPartBytes := make([]byte, 0, len(in)/2)
   230  
   231  	if in[0] == '"' {
   232  		// Quoted-string = DQUOTE *qcontent DQUOTE
   233  		// non-whitespace-control = %d1-8 / %d11 / %d12 / %d14-31 / %d127
   234  		// qcontent = qtext / quoted-pair
   235  		// qtext = non-whitespace-control /
   236  		//         %d33 / %d35-91 / %d93-126
   237  		// quoted-pair = ("\" text) / obs-qp
   238  		// text = %d1-9 / %d11 / %d12 / %d14-127 / obs-text
   239  		//
   240  		// (Names beginning with “obs-” are the obsolete syntax from RFC 2822,
   241  		// Section 4. Since it has been 16 years, we no longer accept that.)
   242  		in = in[1:]
   243  	QuotedString:
   244  		for {
   245  			if len(in) == 0 {
   246  				return mailbox, false
   247  			}
   248  			c := in[0]
   249  			in = in[1:]
   250  
   251  			switch {
   252  			case c == '"':
   253  				break QuotedString
   254  
   255  			case c == '\\':
   256  				// quoted-pair
   257  				if len(in) == 0 {
   258  					return mailbox, false
   259  				}
   260  				if in[0] == 11 ||
   261  					in[0] == 12 ||
   262  					(1 <= in[0] && in[0] <= 9) ||
   263  					(14 <= in[0] && in[0] <= 127) {
   264  					localPartBytes = append(localPartBytes, in[0])
   265  					in = in[1:]
   266  				} else {
   267  					return mailbox, false
   268  				}
   269  
   270  			case c == 11 ||
   271  				c == 12 ||
   272  				// Space (char 32) is not allowed based on the
   273  				// BNF, but RFC 3696 gives an example that
   274  				// assumes that it is. Several “verified”
   275  				// errata continue to argue about this point.
   276  				// We choose to accept it.
   277  				c == 32 ||
   278  				c == 33 ||
   279  				c == 127 ||
   280  				(1 <= c && c <= 8) ||
   281  				(14 <= c && c <= 31) ||
   282  				(35 <= c && c <= 91) ||
   283  				(93 <= c && c <= 126):
   284  				// qtext
   285  				localPartBytes = append(localPartBytes, c)
   286  
   287  			default:
   288  				return mailbox, false
   289  			}
   290  		}
   291  	} else {
   292  		// Atom ("." Atom)*
   293  	NextChar:
   294  		for len(in) > 0 {
   295  			// atext from RFC 2822, Section 3.2.4
   296  			c := in[0]
   297  
   298  			switch {
   299  			case c == '\\':
   300  				// Examples given in RFC 3696 suggest that
   301  				// escaped characters can appear outside of a
   302  				// quoted string. Several “verified” errata
   303  				// continue to argue the point. We choose to
   304  				// accept it.
   305  				in = in[1:]
   306  				if len(in) == 0 {
   307  					return mailbox, false
   308  				}
   309  				fallthrough
   310  
   311  			case ('0' <= c && c <= '9') ||
   312  				('a' <= c && c <= 'z') ||
   313  				('A' <= c && c <= 'Z') ||
   314  				c == '!' || c == '#' || c == '$' || c == '%' ||
   315  				c == '&' || c == '\'' || c == '*' || c == '+' ||
   316  				c == '-' || c == '/' || c == '=' || c == '?' ||
   317  				c == '^' || c == '_' || c == '`' || c == '{' ||
   318  				c == '|' || c == '}' || c == '~' || c == '.':
   319  				localPartBytes = append(localPartBytes, in[0])
   320  				in = in[1:]
   321  
   322  			default:
   323  				break NextChar
   324  			}
   325  		}
   326  
   327  		if len(localPartBytes) == 0 {
   328  			return mailbox, false
   329  		}
   330  
   331  		// From RFC 3696, Section 3:
   332  		// “period (".") may also appear, but may not be used to start
   333  		// or end the local part, nor may two or more consecutive
   334  		// periods appear.”
   335  		twoDots := []byte{'.', '.'}
   336  		if localPartBytes[0] == '.' ||
   337  			localPartBytes[len(localPartBytes)-1] == '.' ||
   338  			bytes.Contains(localPartBytes, twoDots) {
   339  			return mailbox, false
   340  		}
   341  	}
   342  
   343  	if len(in) == 0 || in[0] != '@' {
   344  		return mailbox, false
   345  	}
   346  	in = in[1:]
   347  
   348  	// The RFC species a format for domains, but that's known to be
   349  	// violated in practice so we accept that anything after an '@' is the
   350  	// domain part.
   351  	if _, ok := domainToReverseLabels(in); !ok {
   352  		return mailbox, false
   353  	}
   354  
   355  	mailbox.local = string(localPartBytes)
   356  	mailbox.domain = in
   357  	return mailbox, true
   358  }
   359  
   360  // domainToReverseLabels converts a textual domain name like foo.example.com to
   361  // the list of labels in reverse order, e.g. ["com", "example", "foo"].
   362  func domainToReverseLabels(domain string) (reverseLabels []string, ok bool) {
   363  	for len(domain) > 0 {
   364  		if i := strings.LastIndexByte(domain, '.'); i == -1 {
   365  			reverseLabels = append(reverseLabels, domain)
   366  			domain = ""
   367  		} else {
   368  			reverseLabels = append(reverseLabels, domain[i+1:])
   369  			domain = domain[:i]
   370  			if i == 0 { // domain == ""
   371  				// domain is prefixed with an empty label, append an empty
   372  				// string to reverseLabels to indicate this.
   373  				reverseLabels = append(reverseLabels, "")
   374  			}
   375  		}
   376  	}
   377  
   378  	if len(reverseLabels) > 0 && len(reverseLabels[0]) == 0 {
   379  		// An empty label at the end indicates an absolute value.
   380  		return nil, false
   381  	}
   382  
   383  	for _, label := range reverseLabels {
   384  		if len(label) == 0 {
   385  			// Empty labels are otherwise invalid.
   386  			return nil, false
   387  		}
   388  
   389  		for _, c := range label {
   390  			if c < 33 || c > 126 {
   391  				// Invalid character.
   392  				return nil, false
   393  			}
   394  		}
   395  	}
   396  
   397  	return reverseLabels, true
   398  }
   399  
   400  func matchEmailConstraint(mailbox rfc2821Mailbox, constraint string) (bool, error) {
   401  	// If the constraint contains an @, then it specifies an exact mailbox
   402  	// name.
   403  	if strings.Contains(constraint, "@") {
   404  		constraintMailbox, ok := parseRFC2821Mailbox(constraint)
   405  		if !ok {
   406  			return false, fmt.Errorf("x509: internal error: cannot parse constraint %q", constraint)
   407  		}
   408  		return mailbox.local == constraintMailbox.local && strings.EqualFold(mailbox.domain, constraintMailbox.domain), nil
   409  	}
   410  
   411  	// Otherwise the constraint is like a DNS constraint of the domain part
   412  	// of the mailbox.
   413  	return matchDomainConstraint(mailbox.domain, constraint)
   414  }
   415  
   416  func matchURIConstraint(uri *url.URL, constraint string) (bool, error) {
   417  	// From RFC 5280, Section 4.2.1.10:
   418  	// “a uniformResourceIdentifier that does not include an authority
   419  	// component with a host name specified as a fully qualified domain
   420  	// name (e.g., if the URI either does not include an authority
   421  	// component or includes an authority component in which the host name
   422  	// is specified as an IP address), then the application MUST reject the
   423  	// certificate.”
   424  
   425  	host := uri.Host
   426  	if len(host) == 0 {
   427  		return false, fmt.Errorf("URI with empty host (%q) cannot be matched against constraints", uri.String())
   428  	}
   429  
   430  	if strings.Contains(host, ":") && !strings.HasSuffix(host, "]") {
   431  		var err error
   432  		host, _, err = net.SplitHostPort(uri.Host)
   433  		if err != nil {
   434  			return false, err
   435  		}
   436  	}
   437  
   438  	// netip.ParseAddr will reject the URI IPv6 literal form "[...]", so we
   439  	// check if _either_ the string parses as an IP, or if it is enclosed in
   440  	// square brackets.
   441  	if _, err := netip.ParseAddr(host); err == nil || (strings.HasPrefix(host, "[") && strings.HasSuffix(host, "]")) {
   442  		return false, fmt.Errorf("URI with IP (%q) cannot be matched against constraints", uri.String())
   443  	}
   444  
   445  	return matchDomainConstraint(host, constraint)
   446  }
   447  
   448  func matchIPConstraint(ip net.IP, constraint *net.IPNet) (bool, error) {
   449  	if len(ip) != len(constraint.IP) {
   450  		return false, nil
   451  	}
   452  
   453  	for i := range ip {
   454  		if mask := constraint.Mask[i]; ip[i]&mask != constraint.IP[i]&mask {
   455  			return false, nil
   456  		}
   457  	}
   458  
   459  	return true, nil
   460  }
   461  
   462  func matchDomainConstraint(domain, constraint string) (bool, error) {
   463  	// The meaning of zero length constraints is not specified, but this
   464  	// code follows NSS and accepts them as matching everything.
   465  	if len(constraint) == 0 {
   466  		return true, nil
   467  	}
   468  
   469  	domainLabels, ok := domainToReverseLabels(domain)
   470  	if !ok {
   471  		return false, fmt.Errorf("x509: internal error: cannot parse domain %q", domain)
   472  	}
   473  
   474  	// RFC 5280 says that a leading period in a domain name means that at
   475  	// least one label must be prepended, but only for URI and email
   476  	// constraints, not DNS constraints. The code also supports that
   477  	// behaviour for DNS constraints.
   478  
   479  	mustHaveSubdomains := false
   480  	if constraint[0] == '.' {
   481  		mustHaveSubdomains = true
   482  		constraint = constraint[1:]
   483  	}
   484  
   485  	constraintLabels, ok := domainToReverseLabels(constraint)
   486  	if !ok {
   487  		return false, fmt.Errorf("x509: internal error: cannot parse domain %q", constraint)
   488  	}
   489  
   490  	if len(domainLabels) < len(constraintLabels) ||
   491  		(mustHaveSubdomains && len(domainLabels) == len(constraintLabels)) {
   492  		return false, nil
   493  	}
   494  
   495  	for i, constraintLabel := range constraintLabels {
   496  		if !strings.EqualFold(constraintLabel, domainLabels[i]) {
   497  			return false, nil
   498  		}
   499  	}
   500  
   501  	return true, nil
   502  }
   503  
   504  // checkNameConstraints checks that c permits a child certificate to claim the
   505  // given name, of type nameType. The argument parsedName contains the parsed
   506  // form of name, suitable for passing to the match function. The total number
   507  // of comparisons is tracked in the given count and should not exceed the given
   508  // limit.
   509  func (c *Certificate) checkNameConstraints(count *int,
   510  	maxConstraintComparisons int,
   511  	nameType string,
   512  	name string,
   513  	parsedName any,
   514  	match func(parsedName, constraint any) (match bool, err error),
   515  	permitted, excluded any) error {
   516  
   517  	excludedValue := reflect.ValueOf(excluded)
   518  
   519  	*count += excludedValue.Len()
   520  	if *count > maxConstraintComparisons {
   521  		return CertificateInvalidError{c, TooManyConstraints, ""}
   522  	}
   523  
   524  	for i := 0; i < excludedValue.Len(); i++ {
   525  		constraint := excludedValue.Index(i).Interface()
   526  		match, err := match(parsedName, constraint)
   527  		if err != nil {
   528  			return CertificateInvalidError{c, CANotAuthorizedForThisName, err.Error()}
   529  		}
   530  
   531  		if match {
   532  			return CertificateInvalidError{c, CANotAuthorizedForThisName, fmt.Sprintf("%s %q is excluded by constraint %q", nameType, name, constraint)}
   533  		}
   534  	}
   535  
   536  	permittedValue := reflect.ValueOf(permitted)
   537  
   538  	*count += permittedValue.Len()
   539  	if *count > maxConstraintComparisons {
   540  		return CertificateInvalidError{c, TooManyConstraints, ""}
   541  	}
   542  
   543  	ok := true
   544  	for i := 0; i < permittedValue.Len(); i++ {
   545  		constraint := permittedValue.Index(i).Interface()
   546  
   547  		var err error
   548  		if ok, err = match(parsedName, constraint); err != nil {
   549  			return CertificateInvalidError{c, CANotAuthorizedForThisName, err.Error()}
   550  		}
   551  
   552  		if ok {
   553  			break
   554  		}
   555  	}
   556  
   557  	if !ok {
   558  		return CertificateInvalidError{c, CANotAuthorizedForThisName, fmt.Sprintf("%s %q is not permitted by any constraint", nameType, name)}
   559  	}
   560  
   561  	return nil
   562  }
   563  
   564  // isValid performs validity checks on c given that it is a candidate to append
   565  // to the chain in currentChain.
   566  func (c *Certificate) isValid(certType int, currentChain []*Certificate, opts *VerifyOptions) error {
   567  	if len(c.UnhandledCriticalExtensions) > 0 {
   568  		return UnhandledCriticalExtension{}
   569  	}
   570  
   571  	if len(currentChain) > 0 {
   572  		child := currentChain[len(currentChain)-1]
   573  		if !bytes.Equal(child.RawIssuer, c.RawSubject) {
   574  			return CertificateInvalidError{c, NameMismatch, ""}
   575  		}
   576  	}
   577  
   578  	now := opts.CurrentTime
   579  	if now.IsZero() {
   580  		now = time.Now()
   581  	}
   582  	if now.Before(c.NotBefore) {
   583  		return CertificateInvalidError{
   584  			Cert:   c,
   585  			Reason: Expired,
   586  			Detail: fmt.Sprintf("current time %s is before %s", now.Format(time.RFC3339), c.NotBefore.Format(time.RFC3339)),
   587  		}
   588  	} else if now.After(c.NotAfter) {
   589  		return CertificateInvalidError{
   590  			Cert:   c,
   591  			Reason: Expired,
   592  			Detail: fmt.Sprintf("current time %s is after %s", now.Format(time.RFC3339), c.NotAfter.Format(time.RFC3339)),
   593  		}
   594  	}
   595  
   596  	maxConstraintComparisons := opts.MaxConstraintComparisions
   597  	if maxConstraintComparisons == 0 {
   598  		maxConstraintComparisons = 250000
   599  	}
   600  	comparisonCount := 0
   601  
   602  	if certType == intermediateCertificate || certType == rootCertificate {
   603  		if len(currentChain) == 0 {
   604  			return errors.New("x509: internal error: empty chain when appending CA cert")
   605  		}
   606  	}
   607  
   608  	if (certType == intermediateCertificate || certType == rootCertificate) &&
   609  		c.hasNameConstraints() {
   610  		toCheck := []*Certificate{}
   611  		for _, c := range currentChain {
   612  			if c.hasSANExtension() {
   613  				toCheck = append(toCheck, c)
   614  			}
   615  		}
   616  		for _, sanCert := range toCheck {
   617  			err := forEachSAN(sanCert.getSANExtension(), func(tag int, data []byte) error {
   618  				switch tag {
   619  				case nameTypeEmail:
   620  					name := string(data)
   621  					mailbox, ok := parseRFC2821Mailbox(name)
   622  					if !ok {
   623  						return fmt.Errorf("x509: cannot parse rfc822Name %q", mailbox)
   624  					}
   625  
   626  					if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "email address", name, mailbox,
   627  						func(parsedName, constraint any) (bool, error) {
   628  							return matchEmailConstraint(parsedName.(rfc2821Mailbox), constraint.(string))
   629  						}, c.PermittedEmailAddresses, c.ExcludedEmailAddresses); err != nil {
   630  						return err
   631  					}
   632  
   633  				case nameTypeDNS:
   634  					name := string(data)
   635  					if _, ok := domainToReverseLabels(name); !ok {
   636  						return fmt.Errorf("x509: cannot parse dnsName %q", name)
   637  					}
   638  
   639  					if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "DNS name", name, name,
   640  						func(parsedName, constraint any) (bool, error) {
   641  							return matchDomainConstraint(parsedName.(string), constraint.(string))
   642  						}, c.PermittedDNSDomains, c.ExcludedDNSDomains); err != nil {
   643  						return err
   644  					}
   645  
   646  				case nameTypeURI:
   647  					name := string(data)
   648  					uri, err := url.Parse(name)
   649  					if err != nil {
   650  						return fmt.Errorf("x509: internal error: URI SAN %q failed to parse", name)
   651  					}
   652  
   653  					if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "URI", name, uri,
   654  						func(parsedName, constraint any) (bool, error) {
   655  							return matchURIConstraint(parsedName.(*url.URL), constraint.(string))
   656  						}, c.PermittedURIDomains, c.ExcludedURIDomains); err != nil {
   657  						return err
   658  					}
   659  
   660  				case nameTypeIP:
   661  					ip := net.IP(data)
   662  					if l := len(ip); l != net.IPv4len && l != net.IPv6len {
   663  						return fmt.Errorf("x509: internal error: IP SAN %x failed to parse", data)
   664  					}
   665  
   666  					if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "IP address", ip.String(), ip,
   667  						func(parsedName, constraint any) (bool, error) {
   668  							return matchIPConstraint(parsedName.(net.IP), constraint.(*net.IPNet))
   669  						}, c.PermittedIPRanges, c.ExcludedIPRanges); err != nil {
   670  						return err
   671  					}
   672  
   673  				default:
   674  					// Unknown SAN types are ignored.
   675  				}
   676  
   677  				return nil
   678  			})
   679  
   680  			if err != nil {
   681  				return err
   682  			}
   683  		}
   684  	}
   685  
   686  	// KeyUsage status flags are ignored. From Engineering Security, Peter
   687  	// Gutmann: A European government CA marked its signing certificates as
   688  	// being valid for encryption only, but no-one noticed. Another
   689  	// European CA marked its signature keys as not being valid for
   690  	// signatures. A different CA marked its own trusted root certificate
   691  	// as being invalid for certificate signing. Another national CA
   692  	// distributed a certificate to be used to encrypt data for the
   693  	// country’s tax authority that was marked as only being usable for
   694  	// digital signatures but not for encryption. Yet another CA reversed
   695  	// the order of the bit flags in the keyUsage due to confusion over
   696  	// encoding endianness, essentially setting a random keyUsage in
   697  	// certificates that it issued. Another CA created a self-invalidating
   698  	// certificate by adding a certificate policy statement stipulating
   699  	// that the certificate had to be used strictly as specified in the
   700  	// keyUsage, and a keyUsage containing a flag indicating that the RSA
   701  	// encryption key could only be used for Diffie-Hellman key agreement.
   702  
   703  	if certType == intermediateCertificate && (!c.BasicConstraintsValid || !c.IsCA) {
   704  		return CertificateInvalidError{c, NotAuthorizedToSign, ""}
   705  	}
   706  
   707  	if c.BasicConstraintsValid && c.MaxPathLen >= 0 {
   708  		numIntermediates := len(currentChain) - 1
   709  		if numIntermediates > c.MaxPathLen {
   710  			return CertificateInvalidError{c, TooManyIntermediates, ""}
   711  		}
   712  	}
   713  
   714  	if !boringAllowCert(c) {
   715  		// IncompatibleUsage is not quite right here,
   716  		// but it's also the "no chains found" error
   717  		// and is close enough.
   718  		return CertificateInvalidError{c, IncompatibleUsage, ""}
   719  	}
   720  
   721  	return nil
   722  }
   723  
   724  // Verify attempts to verify c by building one or more chains from c to a
   725  // certificate in opts.Roots, using certificates in opts.Intermediates if
   726  // needed. If successful, it returns one or more chains where the first
   727  // element of the chain is c and the last element is from opts.Roots.
   728  //
   729  // If opts.Roots is nil, the platform verifier might be used, and
   730  // verification details might differ from what is described below. If system
   731  // roots are unavailable the returned error will be of type SystemRootsError.
   732  //
   733  // Name constraints in the intermediates will be applied to all names claimed
   734  // in the chain, not just opts.DNSName. Thus it is invalid for a leaf to claim
   735  // example.com if an intermediate doesn't permit it, even if example.com is not
   736  // the name being validated. Note that DirectoryName constraints are not
   737  // supported.
   738  //
   739  // Name constraint validation follows the rules from RFC 5280, with the
   740  // addition that DNS name constraints may use the leading period format
   741  // defined for emails and URIs. When a constraint has a leading period
   742  // it indicates that at least one additional label must be prepended to
   743  // the constrained name to be considered valid.
   744  //
   745  // Extended Key Usage values are enforced nested down a chain, so an intermediate
   746  // or root that enumerates EKUs prevents a leaf from asserting an EKU not in that
   747  // list. (While this is not specified, it is common practice in order to limit
   748  // the types of certificates a CA can issue.)
   749  //
   750  // Certificates that use SHA1WithRSA and ECDSAWithSHA1 signatures are not supported,
   751  // and will not be used to build chains.
   752  //
   753  // Certificates other than c in the returned chains should not be modified.
   754  //
   755  // WARNING: this function doesn't do any revocation checking.
   756  func (c *Certificate) Verify(opts VerifyOptions) (chains [][]*Certificate, err error) {
   757  	// Platform-specific verification needs the ASN.1 contents so
   758  	// this makes the behavior consistent across platforms.
   759  	if len(c.Raw) == 0 {
   760  		return nil, errNotParsed
   761  	}
   762  	for i := 0; i < opts.Intermediates.len(); i++ {
   763  		c, _, err := opts.Intermediates.cert(i)
   764  		if err != nil {
   765  			return nil, fmt.Errorf("crypto/x509: error fetching intermediate: %w", err)
   766  		}
   767  		if len(c.Raw) == 0 {
   768  			return nil, errNotParsed
   769  		}
   770  	}
   771  
   772  	// Use platform verifiers, where available, if Roots is from SystemCertPool.
   773  	if runtime.GOOS == "windows" || runtime.GOOS == "darwin" || runtime.GOOS == "ios" {
   774  		// Don't use the system verifier if the system pool was replaced with a non-system pool,
   775  		// i.e. if SetFallbackRoots was called with x509usefallbackroots=1.
   776  		systemPool := systemRootsPool()
   777  		if opts.Roots == nil && (systemPool == nil || systemPool.systemPool) {
   778  			return c.systemVerify(&opts)
   779  		}
   780  		if opts.Roots != nil && opts.Roots.systemPool {
   781  			platformChains, err := c.systemVerify(&opts)
   782  			// If the platform verifier succeeded, or there are no additional
   783  			// roots, return the platform verifier result. Otherwise, continue
   784  			// with the Go verifier.
   785  			if err == nil || opts.Roots.len() == 0 {
   786  				return platformChains, err
   787  			}
   788  		}
   789  	}
   790  
   791  	if opts.Roots == nil {
   792  		opts.Roots = systemRootsPool()
   793  		if opts.Roots == nil {
   794  			return nil, SystemRootsError{systemRootsErr}
   795  		}
   796  	}
   797  
   798  	err = c.isValid(leafCertificate, nil, &opts)
   799  	if err != nil {
   800  		return
   801  	}
   802  
   803  	if len(opts.DNSName) > 0 {
   804  		err = c.VerifyHostname(opts.DNSName)
   805  		if err != nil {
   806  			return
   807  		}
   808  	}
   809  
   810  	var candidateChains [][]*Certificate
   811  	if opts.Roots.contains(c) {
   812  		candidateChains = [][]*Certificate{{c}}
   813  	} else {
   814  		candidateChains, err = c.buildChains([]*Certificate{c}, nil, &opts)
   815  		if err != nil {
   816  			return nil, err
   817  		}
   818  	}
   819  
   820  	if len(opts.KeyUsages) == 0 {
   821  		opts.KeyUsages = []ExtKeyUsage{ExtKeyUsageServerAuth}
   822  	}
   823  
   824  	for _, eku := range opts.KeyUsages {
   825  		if eku == ExtKeyUsageAny {
   826  			// If any key usage is acceptable, no need to check the chain for
   827  			// key usages.
   828  			return candidateChains, nil
   829  		}
   830  	}
   831  
   832  	chains = make([][]*Certificate, 0, len(candidateChains))
   833  	for _, candidate := range candidateChains {
   834  		if checkChainForKeyUsage(candidate, opts.KeyUsages) {
   835  			chains = append(chains, candidate)
   836  		}
   837  	}
   838  
   839  	if len(chains) == 0 {
   840  		return nil, CertificateInvalidError{c, IncompatibleUsage, ""}
   841  	}
   842  
   843  	return chains, nil
   844  }
   845  
   846  func appendToFreshChain(chain []*Certificate, cert *Certificate) []*Certificate {
   847  	n := make([]*Certificate, len(chain)+1)
   848  	copy(n, chain)
   849  	n[len(chain)] = cert
   850  	return n
   851  }
   852  
   853  // alreadyInChain checks whether a candidate certificate is present in a chain.
   854  // Rather than doing a direct byte for byte equivalency check, we check if the
   855  // subject, public key, and SAN, if present, are equal. This prevents loops that
   856  // are created by mutual cross-signatures, or other cross-signature bridge
   857  // oddities.
   858  func alreadyInChain(candidate *Certificate, chain []*Certificate) bool {
   859  	type pubKeyEqual interface {
   860  		Equal(crypto.PublicKey) bool
   861  	}
   862  
   863  	var candidateSAN *pkix.Extension
   864  	for _, ext := range candidate.Extensions {
   865  		if ext.Id.Equal(oidExtensionSubjectAltName) {
   866  			candidateSAN = &ext
   867  			break
   868  		}
   869  	}
   870  
   871  	for _, cert := range chain {
   872  		if !bytes.Equal(candidate.RawSubject, cert.RawSubject) {
   873  			continue
   874  		}
   875  		if !candidate.PublicKey.(pubKeyEqual).Equal(cert.PublicKey) {
   876  			continue
   877  		}
   878  		var certSAN *pkix.Extension
   879  		for _, ext := range cert.Extensions {
   880  			if ext.Id.Equal(oidExtensionSubjectAltName) {
   881  				certSAN = &ext
   882  				break
   883  			}
   884  		}
   885  		if candidateSAN == nil && certSAN == nil {
   886  			return true
   887  		} else if candidateSAN == nil || certSAN == nil {
   888  			return false
   889  		}
   890  		if bytes.Equal(candidateSAN.Value, certSAN.Value) {
   891  			return true
   892  		}
   893  	}
   894  	return false
   895  }
   896  
   897  // maxChainSignatureChecks is the maximum number of CheckSignatureFrom calls
   898  // that an invocation of buildChains will (transitively) make. Most chains are
   899  // less than 15 certificates long, so this leaves space for multiple chains and
   900  // for failed checks due to different intermediates having the same Subject.
   901  const maxChainSignatureChecks = 100
   902  
   903  func (c *Certificate) buildChains(currentChain []*Certificate, sigChecks *int, opts *VerifyOptions) (chains [][]*Certificate, err error) {
   904  	var (
   905  		hintErr  error
   906  		hintCert *Certificate
   907  	)
   908  
   909  	considerCandidate := func(certType int, candidate potentialParent) {
   910  		if candidate.cert.PublicKey == nil || alreadyInChain(candidate.cert, currentChain) {
   911  			return
   912  		}
   913  
   914  		if sigChecks == nil {
   915  			sigChecks = new(int)
   916  		}
   917  		*sigChecks++
   918  		if *sigChecks > maxChainSignatureChecks {
   919  			err = errors.New("x509: signature check attempts limit reached while verifying certificate chain")
   920  			return
   921  		}
   922  
   923  		if err := c.CheckSignatureFrom(candidate.cert); err != nil {
   924  			if hintErr == nil {
   925  				hintErr = err
   926  				hintCert = candidate.cert
   927  			}
   928  			return
   929  		}
   930  
   931  		err = candidate.cert.isValid(certType, currentChain, opts)
   932  		if err != nil {
   933  			if hintErr == nil {
   934  				hintErr = err
   935  				hintCert = candidate.cert
   936  			}
   937  			return
   938  		}
   939  
   940  		if candidate.constraint != nil {
   941  			if err := candidate.constraint(currentChain); err != nil {
   942  				if hintErr == nil {
   943  					hintErr = err
   944  					hintCert = candidate.cert
   945  				}
   946  				return
   947  			}
   948  		}
   949  
   950  		switch certType {
   951  		case rootCertificate:
   952  			chains = append(chains, appendToFreshChain(currentChain, candidate.cert))
   953  		case intermediateCertificate:
   954  			var childChains [][]*Certificate
   955  			childChains, err = candidate.cert.buildChains(appendToFreshChain(currentChain, candidate.cert), sigChecks, opts)
   956  			chains = append(chains, childChains...)
   957  		}
   958  	}
   959  
   960  	for _, root := range opts.Roots.findPotentialParents(c) {
   961  		considerCandidate(rootCertificate, root)
   962  	}
   963  	for _, intermediate := range opts.Intermediates.findPotentialParents(c) {
   964  		considerCandidate(intermediateCertificate, intermediate)
   965  	}
   966  
   967  	if len(chains) > 0 {
   968  		err = nil
   969  	}
   970  	if len(chains) == 0 && err == nil {
   971  		err = UnknownAuthorityError{c, hintErr, hintCert}
   972  	}
   973  
   974  	return
   975  }
   976  
   977  func validHostnamePattern(host string) bool { return validHostname(host, true) }
   978  func validHostnameInput(host string) bool   { return validHostname(host, false) }
   979  
   980  // validHostname reports whether host is a valid hostname that can be matched or
   981  // matched against according to RFC 6125 2.2, with some leniency to accommodate
   982  // legacy values.
   983  func validHostname(host string, isPattern bool) bool {
   984  	if !isPattern {
   985  		host = strings.TrimSuffix(host, ".")
   986  	}
   987  	if len(host) == 0 {
   988  		return false
   989  	}
   990  	if host == "*" {
   991  		// Bare wildcards are not allowed, they are not valid DNS names,
   992  		// nor are they allowed per RFC 6125.
   993  		return false
   994  	}
   995  
   996  	for i, part := range strings.Split(host, ".") {
   997  		if part == "" {
   998  			// Empty label.
   999  			return false
  1000  		}
  1001  		if isPattern && i == 0 && part == "*" {
  1002  			// Only allow full left-most wildcards, as those are the only ones
  1003  			// we match, and matching literal '*' characters is probably never
  1004  			// the expected behavior.
  1005  			continue
  1006  		}
  1007  		for j, c := range part {
  1008  			if 'a' <= c && c <= 'z' {
  1009  				continue
  1010  			}
  1011  			if '0' <= c && c <= '9' {
  1012  				continue
  1013  			}
  1014  			if 'A' <= c && c <= 'Z' {
  1015  				continue
  1016  			}
  1017  			if c == '-' && j != 0 {
  1018  				continue
  1019  			}
  1020  			if c == '_' {
  1021  				// Not a valid character in hostnames, but commonly
  1022  				// found in deployments outside the WebPKI.
  1023  				continue
  1024  			}
  1025  			return false
  1026  		}
  1027  	}
  1028  
  1029  	return true
  1030  }
  1031  
  1032  func matchExactly(hostA, hostB string) bool {
  1033  	if hostA == "" || hostA == "." || hostB == "" || hostB == "." {
  1034  		return false
  1035  	}
  1036  	return toLowerCaseASCII(hostA) == toLowerCaseASCII(hostB)
  1037  }
  1038  
  1039  func matchHostnames(pattern, host string) bool {
  1040  	pattern = toLowerCaseASCII(pattern)
  1041  	host = toLowerCaseASCII(strings.TrimSuffix(host, "."))
  1042  
  1043  	if len(pattern) == 0 || len(host) == 0 {
  1044  		return false
  1045  	}
  1046  
  1047  	patternParts := strings.Split(pattern, ".")
  1048  	hostParts := strings.Split(host, ".")
  1049  
  1050  	if len(patternParts) != len(hostParts) {
  1051  		return false
  1052  	}
  1053  
  1054  	for i, patternPart := range patternParts {
  1055  		if i == 0 && patternPart == "*" {
  1056  			continue
  1057  		}
  1058  		if patternPart != hostParts[i] {
  1059  			return false
  1060  		}
  1061  	}
  1062  
  1063  	return true
  1064  }
  1065  
  1066  // toLowerCaseASCII returns a lower-case version of in. See RFC 6125 6.4.1. We use
  1067  // an explicitly ASCII function to avoid any sharp corners resulting from
  1068  // performing Unicode operations on DNS labels.
  1069  func toLowerCaseASCII(in string) string {
  1070  	// If the string is already lower-case then there's nothing to do.
  1071  	isAlreadyLowerCase := true
  1072  	for _, c := range in {
  1073  		if c == utf8.RuneError {
  1074  			// If we get a UTF-8 error then there might be
  1075  			// upper-case ASCII bytes in the invalid sequence.
  1076  			isAlreadyLowerCase = false
  1077  			break
  1078  		}
  1079  		if 'A' <= c && c <= 'Z' {
  1080  			isAlreadyLowerCase = false
  1081  			break
  1082  		}
  1083  	}
  1084  
  1085  	if isAlreadyLowerCase {
  1086  		return in
  1087  	}
  1088  
  1089  	out := []byte(in)
  1090  	for i, c := range out {
  1091  		if 'A' <= c && c <= 'Z' {
  1092  			out[i] += 'a' - 'A'
  1093  		}
  1094  	}
  1095  	return string(out)
  1096  }
  1097  
  1098  // VerifyHostname returns nil if c is a valid certificate for the named host.
  1099  // Otherwise it returns an error describing the mismatch.
  1100  //
  1101  // IP addresses can be optionally enclosed in square brackets and are checked
  1102  // against the IPAddresses field. Other names are checked case insensitively
  1103  // against the DNSNames field. If the names are valid hostnames, the certificate
  1104  // fields can have a wildcard as the complete left-most label (e.g. *.example.com).
  1105  //
  1106  // Note that the legacy Common Name field is ignored.
  1107  func (c *Certificate) VerifyHostname(h string) error {
  1108  	// IP addresses may be written in [ ].
  1109  	candidateIP := h
  1110  	if len(h) >= 3 && h[0] == '[' && h[len(h)-1] == ']' {
  1111  		candidateIP = h[1 : len(h)-1]
  1112  	}
  1113  	if ip := net.ParseIP(candidateIP); ip != nil {
  1114  		// We only match IP addresses against IP SANs.
  1115  		// See RFC 6125, Appendix B.2.
  1116  		for _, candidate := range c.IPAddresses {
  1117  			if ip.Equal(candidate) {
  1118  				return nil
  1119  			}
  1120  		}
  1121  		return HostnameError{c, candidateIP}
  1122  	}
  1123  
  1124  	candidateName := toLowerCaseASCII(h) // Save allocations inside the loop.
  1125  	validCandidateName := validHostnameInput(candidateName)
  1126  
  1127  	for _, match := range c.DNSNames {
  1128  		// Ideally, we'd only match valid hostnames according to RFC 6125 like
  1129  		// browsers (more or less) do, but in practice Go is used in a wider
  1130  		// array of contexts and can't even assume DNS resolution. Instead,
  1131  		// always allow perfect matches, and only apply wildcard and trailing
  1132  		// dot processing to valid hostnames.
  1133  		if validCandidateName && validHostnamePattern(match) {
  1134  			if matchHostnames(match, candidateName) {
  1135  				return nil
  1136  			}
  1137  		} else {
  1138  			if matchExactly(match, candidateName) {
  1139  				return nil
  1140  			}
  1141  		}
  1142  	}
  1143  
  1144  	return HostnameError{c, h}
  1145  }
  1146  
  1147  func checkChainForKeyUsage(chain []*Certificate, keyUsages []ExtKeyUsage) bool {
  1148  	usages := make([]ExtKeyUsage, len(keyUsages))
  1149  	copy(usages, keyUsages)
  1150  
  1151  	if len(chain) == 0 {
  1152  		return false
  1153  	}
  1154  
  1155  	usagesRemaining := len(usages)
  1156  
  1157  	// We walk down the list and cross out any usages that aren't supported
  1158  	// by each certificate. If we cross out all the usages, then the chain
  1159  	// is unacceptable.
  1160  
  1161  NextCert:
  1162  	for i := len(chain) - 1; i >= 0; i-- {
  1163  		cert := chain[i]
  1164  		if len(cert.ExtKeyUsage) == 0 && len(cert.UnknownExtKeyUsage) == 0 {
  1165  			// The certificate doesn't have any extended key usage specified.
  1166  			continue
  1167  		}
  1168  
  1169  		for _, usage := range cert.ExtKeyUsage {
  1170  			if usage == ExtKeyUsageAny {
  1171  				// The certificate is explicitly good for any usage.
  1172  				continue NextCert
  1173  			}
  1174  		}
  1175  
  1176  		const invalidUsage ExtKeyUsage = -1
  1177  
  1178  	NextRequestedUsage:
  1179  		for i, requestedUsage := range usages {
  1180  			if requestedUsage == invalidUsage {
  1181  				continue
  1182  			}
  1183  
  1184  			for _, usage := range cert.ExtKeyUsage {
  1185  				if requestedUsage == usage {
  1186  					continue NextRequestedUsage
  1187  				}
  1188  			}
  1189  
  1190  			usages[i] = invalidUsage
  1191  			usagesRemaining--
  1192  			if usagesRemaining == 0 {
  1193  				return false
  1194  			}
  1195  		}
  1196  	}
  1197  
  1198  	return true
  1199  }
  1200  

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