Source file src/crypto/x509/verify.go

Documentation: crypto/x509

     1  // Copyright 2011 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package x509
     6  
     7  import (
     8  	"bytes"
     9  	"errors"
    10  	"fmt"
    11  	"net"
    12  	"net/url"
    13  	"os"
    14  	"reflect"
    15  	"runtime"
    16  	"strings"
    17  	"time"
    18  	"unicode/utf8"
    19  )
    20  
    21  // ignoreCN disables interpreting Common Name as a hostname. See issue 24151.
    22  var ignoreCN = strings.Contains(os.Getenv("GODEBUG"), "x509ignoreCN=1")
    23  
    24  type InvalidReason int
    25  
    26  const (
    27  	// NotAuthorizedToSign results when a certificate is signed by another
    28  	// which isn't marked as a CA certificate.
    29  	NotAuthorizedToSign InvalidReason = iota
    30  	// Expired results when a certificate has expired, based on the time
    31  	// given in the VerifyOptions.
    32  	Expired
    33  	// CANotAuthorizedForThisName results when an intermediate or root
    34  	// certificate has a name constraint which doesn't permit a DNS or
    35  	// other name (including IP address) in the leaf certificate.
    36  	CANotAuthorizedForThisName
    37  	// TooManyIntermediates results when a path length constraint is
    38  	// violated.
    39  	TooManyIntermediates
    40  	// IncompatibleUsage results when the certificate's key usage indicates
    41  	// that it may only be used for a different purpose.
    42  	IncompatibleUsage
    43  	// NameMismatch results when the subject name of a parent certificate
    44  	// does not match the issuer name in the child.
    45  	NameMismatch
    46  	// NameConstraintsWithoutSANs results when a leaf certificate doesn't
    47  	// contain a Subject Alternative Name extension, but a CA certificate
    48  	// contains name constraints, and the Common Name can be interpreted as
    49  	// a hostname.
    50  	//
    51  	// You can avoid this error by setting the experimental GODEBUG environment
    52  	// variable to "x509ignoreCN=1", disabling Common Name matching entirely.
    53  	// This behavior might become the default in the future.
    54  	NameConstraintsWithoutSANs
    55  	// UnconstrainedName results when a CA certificate contains permitted
    56  	// name constraints, but leaf certificate contains a name of an
    57  	// unsupported or unconstrained type.
    58  	UnconstrainedName
    59  	// TooManyConstraints results when the number of comparison operations
    60  	// needed to check a certificate exceeds the limit set by
    61  	// VerifyOptions.MaxConstraintComparisions. This limit exists to
    62  	// prevent pathological certificates can consuming excessive amounts of
    63  	// CPU time to verify.
    64  	TooManyConstraints
    65  	// CANotAuthorizedForExtKeyUsage results when an intermediate or root
    66  	// certificate does not permit a requested extended key usage.
    67  	CANotAuthorizedForExtKeyUsage
    68  )
    69  
    70  // CertificateInvalidError results when an odd error occurs. Users of this
    71  // library probably want to handle all these errors uniformly.
    72  type CertificateInvalidError struct {
    73  	Cert   *Certificate
    74  	Reason InvalidReason
    75  	Detail string
    76  }
    77  
    78  func (e CertificateInvalidError) Error() string {
    79  	switch e.Reason {
    80  	case NotAuthorizedToSign:
    81  		return "x509: certificate is not authorized to sign other certificates"
    82  	case Expired:
    83  		return "x509: certificate has expired or is not yet valid"
    84  	case CANotAuthorizedForThisName:
    85  		return "x509: a root or intermediate certificate is not authorized to sign for this name: " + e.Detail
    86  	case CANotAuthorizedForExtKeyUsage:
    87  		return "x509: a root or intermediate certificate is not authorized for an extended key usage: " + e.Detail
    88  	case TooManyIntermediates:
    89  		return "x509: too many intermediates for path length constraint"
    90  	case IncompatibleUsage:
    91  		return "x509: certificate specifies an incompatible key usage"
    92  	case NameMismatch:
    93  		return "x509: issuer name does not match subject from issuing certificate"
    94  	case NameConstraintsWithoutSANs:
    95  		return "x509: issuer has name constraints but leaf doesn't have a SAN extension"
    96  	case UnconstrainedName:
    97  		return "x509: issuer has name constraints but leaf contains unknown or unconstrained name: " + e.Detail
    98  	}
    99  	return "x509: unknown error"
   100  }
   101  
   102  // HostnameError results when the set of authorized names doesn't match the
   103  // requested name.
   104  type HostnameError struct {
   105  	Certificate *Certificate
   106  	Host        string
   107  }
   108  
   109  func (h HostnameError) Error() string {
   110  	c := h.Certificate
   111  
   112  	if !c.hasSANExtension() && !validHostname(c.Subject.CommonName) &&
   113  		matchHostnames(toLowerCaseASCII(c.Subject.CommonName), toLowerCaseASCII(h.Host)) {
   114  		// This would have validated, if it weren't for the validHostname check on Common Name.
   115  		return "x509: Common Name is not a valid hostname: " + c.Subject.CommonName
   116  	}
   117  
   118  	var valid string
   119  	if ip := net.ParseIP(h.Host); ip != nil {
   120  		// Trying to validate an IP
   121  		if len(c.IPAddresses) == 0 {
   122  			return "x509: cannot validate certificate for " + h.Host + " because it doesn't contain any IP SANs"
   123  		}
   124  		for _, san := range c.IPAddresses {
   125  			if len(valid) > 0 {
   126  				valid += ", "
   127  			}
   128  			valid += san.String()
   129  		}
   130  	} else {
   131  		if c.commonNameAsHostname() {
   132  			valid = c.Subject.CommonName
   133  		} else {
   134  			valid = strings.Join(c.DNSNames, ", ")
   135  		}
   136  	}
   137  
   138  	if len(valid) == 0 {
   139  		return "x509: certificate is not valid for any names, but wanted to match " + h.Host
   140  	}
   141  	return "x509: certificate is valid for " + valid + ", not " + h.Host
   142  }
   143  
   144  // UnknownAuthorityError results when the certificate issuer is unknown
   145  type UnknownAuthorityError struct {
   146  	Cert *Certificate
   147  	// hintErr contains an error that may be helpful in determining why an
   148  	// authority wasn't found.
   149  	hintErr error
   150  	// hintCert contains a possible authority certificate that was rejected
   151  	// because of the error in hintErr.
   152  	hintCert *Certificate
   153  }
   154  
   155  func (e UnknownAuthorityError) Error() string {
   156  	s := "x509: certificate signed by unknown authority"
   157  	if e.hintErr != nil {
   158  		certName := e.hintCert.Subject.CommonName
   159  		if len(certName) == 0 {
   160  			if len(e.hintCert.Subject.Organization) > 0 {
   161  				certName = e.hintCert.Subject.Organization[0]
   162  			} else {
   163  				certName = "serial:" + e.hintCert.SerialNumber.String()
   164  			}
   165  		}
   166  		s += fmt.Sprintf(" (possibly because of %q while trying to verify candidate authority certificate %q)", e.hintErr, certName)
   167  	}
   168  	return s
   169  }
   170  
   171  // SystemRootsError results when we fail to load the system root certificates.
   172  type SystemRootsError struct {
   173  	Err error
   174  }
   175  
   176  func (se SystemRootsError) Error() string {
   177  	msg := "x509: failed to load system roots and no roots provided"
   178  	if se.Err != nil {
   179  		return msg + "; " + se.Err.Error()
   180  	}
   181  	return msg
   182  }
   183  
   184  // errNotParsed is returned when a certificate without ASN.1 contents is
   185  // verified. Platform-specific verification needs the ASN.1 contents.
   186  var errNotParsed = errors.New("x509: missing ASN.1 contents; use ParseCertificate")
   187  
   188  // VerifyOptions contains parameters for Certificate.Verify. It's a structure
   189  // because other PKIX verification APIs have ended up needing many options.
   190  type VerifyOptions struct {
   191  	DNSName       string
   192  	Intermediates *CertPool
   193  	Roots         *CertPool // if nil, the system roots are used
   194  	CurrentTime   time.Time // if zero, the current time is used
   195  	// KeyUsage specifies which Extended Key Usage values are acceptable. A leaf
   196  	// certificate is accepted if it contains any of the listed values. An empty
   197  	// list means ExtKeyUsageServerAuth. To accept any key usage, include
   198  	// ExtKeyUsageAny.
   199  	//
   200  	// Certificate chains are required to nest these extended key usage values.
   201  	// (This matches the Windows CryptoAPI behavior, but not the spec.)
   202  	KeyUsages []ExtKeyUsage
   203  	// MaxConstraintComparisions is the maximum number of comparisons to
   204  	// perform when checking a given certificate's name constraints. If
   205  	// zero, a sensible default is used. This limit prevents pathological
   206  	// certificates from consuming excessive amounts of CPU time when
   207  	// validating.
   208  	MaxConstraintComparisions int
   209  }
   210  
   211  const (
   212  	leafCertificate = iota
   213  	intermediateCertificate
   214  	rootCertificate
   215  )
   216  
   217  // rfc2821Mailbox represents a “mailbox” (which is an email address to most
   218  // people) by breaking it into the “local” (i.e. before the '@') and “domain”
   219  // parts.
   220  type rfc2821Mailbox struct {
   221  	local, domain string
   222  }
   223  
   224  // parseRFC2821Mailbox parses an email address into local and domain parts,
   225  // based on the ABNF for a “Mailbox” from RFC 2821. According to RFC 5280,
   226  // Section 4.2.1.6 that's correct for an rfc822Name from a certificate: “The
   227  // format of an rfc822Name is a "Mailbox" as defined in RFC 2821, Section 4.1.2”.
   228  func parseRFC2821Mailbox(in string) (mailbox rfc2821Mailbox, ok bool) {
   229  	if len(in) == 0 {
   230  		return mailbox, false
   231  	}
   232  
   233  	localPartBytes := make([]byte, 0, len(in)/2)
   234  
   235  	if in[0] == '"' {
   236  		// Quoted-string = DQUOTE *qcontent DQUOTE
   237  		// non-whitespace-control = %d1-8 / %d11 / %d12 / %d14-31 / %d127
   238  		// qcontent = qtext / quoted-pair
   239  		// qtext = non-whitespace-control /
   240  		//         %d33 / %d35-91 / %d93-126
   241  		// quoted-pair = ("\" text) / obs-qp
   242  		// text = %d1-9 / %d11 / %d12 / %d14-127 / obs-text
   243  		//
   244  		// (Names beginning with “obs-” are the obsolete syntax from RFC 2822,
   245  		// Section 4. Since it has been 16 years, we no longer accept that.)
   246  		in = in[1:]
   247  	QuotedString:
   248  		for {
   249  			if len(in) == 0 {
   250  				return mailbox, false
   251  			}
   252  			c := in[0]
   253  			in = in[1:]
   254  
   255  			switch {
   256  			case c == '"':
   257  				break QuotedString
   258  
   259  			case c == '\\':
   260  				// quoted-pair
   261  				if len(in) == 0 {
   262  					return mailbox, false
   263  				}
   264  				if in[0] == 11 ||
   265  					in[0] == 12 ||
   266  					(1 <= in[0] && in[0] <= 9) ||
   267  					(14 <= in[0] && in[0] <= 127) {
   268  					localPartBytes = append(localPartBytes, in[0])
   269  					in = in[1:]
   270  				} else {
   271  					return mailbox, false
   272  				}
   273  
   274  			case c == 11 ||
   275  				c == 12 ||
   276  				// Space (char 32) is not allowed based on the
   277  				// BNF, but RFC 3696 gives an example that
   278  				// assumes that it is. Several “verified”
   279  				// errata continue to argue about this point.
   280  				// We choose to accept it.
   281  				c == 32 ||
   282  				c == 33 ||
   283  				c == 127 ||
   284  				(1 <= c && c <= 8) ||
   285  				(14 <= c && c <= 31) ||
   286  				(35 <= c && c <= 91) ||
   287  				(93 <= c && c <= 126):
   288  				// qtext
   289  				localPartBytes = append(localPartBytes, c)
   290  
   291  			default:
   292  				return mailbox, false
   293  			}
   294  		}
   295  	} else {
   296  		// Atom ("." Atom)*
   297  	NextChar:
   298  		for len(in) > 0 {
   299  			// atext from RFC 2822, Section 3.2.4
   300  			c := in[0]
   301  
   302  			switch {
   303  			case c == '\\':
   304  				// Examples given in RFC 3696 suggest that
   305  				// escaped characters can appear outside of a
   306  				// quoted string. Several “verified” errata
   307  				// continue to argue the point. We choose to
   308  				// accept it.
   309  				in = in[1:]
   310  				if len(in) == 0 {
   311  					return mailbox, false
   312  				}
   313  				fallthrough
   314  
   315  			case ('0' <= c && c <= '9') ||
   316  				('a' <= c && c <= 'z') ||
   317  				('A' <= c && c <= 'Z') ||
   318  				c == '!' || c == '#' || c == '$' || c == '%' ||
   319  				c == '&' || c == '\'' || c == '*' || c == '+' ||
   320  				c == '-' || c == '/' || c == '=' || c == '?' ||
   321  				c == '^' || c == '_' || c == '`' || c == '{' ||
   322  				c == '|' || c == '}' || c == '~' || c == '.':
   323  				localPartBytes = append(localPartBytes, in[0])
   324  				in = in[1:]
   325  
   326  			default:
   327  				break NextChar
   328  			}
   329  		}
   330  
   331  		if len(localPartBytes) == 0 {
   332  			return mailbox, false
   333  		}
   334  
   335  		// From RFC 3696, Section 3:
   336  		// “period (".") may also appear, but may not be used to start
   337  		// or end the local part, nor may two or more consecutive
   338  		// periods appear.”
   339  		twoDots := []byte{'.', '.'}
   340  		if localPartBytes[0] == '.' ||
   341  			localPartBytes[len(localPartBytes)-1] == '.' ||
   342  			bytes.Contains(localPartBytes, twoDots) {
   343  			return mailbox, false
   344  		}
   345  	}
   346  
   347  	if len(in) == 0 || in[0] != '@' {
   348  		return mailbox, false
   349  	}
   350  	in = in[1:]
   351  
   352  	// The RFC species a format for domains, but that's known to be
   353  	// violated in practice so we accept that anything after an '@' is the
   354  	// domain part.
   355  	if _, ok := domainToReverseLabels(in); !ok {
   356  		return mailbox, false
   357  	}
   358  
   359  	mailbox.local = string(localPartBytes)
   360  	mailbox.domain = in
   361  	return mailbox, true
   362  }
   363  
   364  // domainToReverseLabels converts a textual domain name like foo.example.com to
   365  // the list of labels in reverse order, e.g. ["com", "example", "foo"].
   366  func domainToReverseLabels(domain string) (reverseLabels []string, ok bool) {
   367  	for len(domain) > 0 {
   368  		if i := strings.LastIndexByte(domain, '.'); i == -1 {
   369  			reverseLabels = append(reverseLabels, domain)
   370  			domain = ""
   371  		} else {
   372  			reverseLabels = append(reverseLabels, domain[i+1:])
   373  			domain = domain[:i]
   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 interface{},
   511  	match func(parsedName, constraint interface{}) (match bool, err error),
   512  	permitted, excluded interface{}) 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) || now.After(c.NotAfter) {
   580  		return CertificateInvalidError{c, Expired, ""}
   581  	}
   582  
   583  	maxConstraintComparisons := opts.MaxConstraintComparisions
   584  	if maxConstraintComparisons == 0 {
   585  		maxConstraintComparisons = 250000
   586  	}
   587  	comparisonCount := 0
   588  
   589  	var leaf *Certificate
   590  	if certType == intermediateCertificate || certType == rootCertificate {
   591  		if len(currentChain) == 0 {
   592  			return errors.New("x509: internal error: empty chain when appending CA cert")
   593  		}
   594  		leaf = currentChain[0]
   595  	}
   596  
   597  	checkNameConstraints := (certType == intermediateCertificate || certType == rootCertificate) && c.hasNameConstraints()
   598  	if checkNameConstraints && leaf.commonNameAsHostname() {
   599  		// This is the deprecated, legacy case of depending on the commonName as
   600  		// a hostname. We don't enforce name constraints against the CN, but
   601  		// VerifyHostname will look for hostnames in there if there are no SANs.
   602  		// In order to ensure VerifyHostname will not accept an unchecked name,
   603  		// return an error here.
   604  		return CertificateInvalidError{c, NameConstraintsWithoutSANs, ""}
   605  	} else if checkNameConstraints && leaf.hasSANExtension() {
   606  		err := forEachSAN(leaf.getSANExtension(), func(tag int, data []byte) error {
   607  			switch tag {
   608  			case nameTypeEmail:
   609  				name := string(data)
   610  				mailbox, ok := parseRFC2821Mailbox(name)
   611  				if !ok {
   612  					return fmt.Errorf("x509: cannot parse rfc822Name %q", mailbox)
   613  				}
   614  
   615  				if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "email address", name, mailbox,
   616  					func(parsedName, constraint interface{}) (bool, error) {
   617  						return matchEmailConstraint(parsedName.(rfc2821Mailbox), constraint.(string))
   618  					}, c.PermittedEmailAddresses, c.ExcludedEmailAddresses); err != nil {
   619  					return err
   620  				}
   621  
   622  			case nameTypeDNS:
   623  				name := string(data)
   624  				if _, ok := domainToReverseLabels(name); !ok {
   625  					return fmt.Errorf("x509: cannot parse dnsName %q", name)
   626  				}
   627  
   628  				if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "DNS name", name, name,
   629  					func(parsedName, constraint interface{}) (bool, error) {
   630  						return matchDomainConstraint(parsedName.(string), constraint.(string))
   631  					}, c.PermittedDNSDomains, c.ExcludedDNSDomains); err != nil {
   632  					return err
   633  				}
   634  
   635  			case nameTypeURI:
   636  				name := string(data)
   637  				uri, err := url.Parse(name)
   638  				if err != nil {
   639  					return fmt.Errorf("x509: internal error: URI SAN %q failed to parse", name)
   640  				}
   641  
   642  				if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "URI", name, uri,
   643  					func(parsedName, constraint interface{}) (bool, error) {
   644  						return matchURIConstraint(parsedName.(*url.URL), constraint.(string))
   645  					}, c.PermittedURIDomains, c.ExcludedURIDomains); err != nil {
   646  					return err
   647  				}
   648  
   649  			case nameTypeIP:
   650  				ip := net.IP(data)
   651  				if l := len(ip); l != net.IPv4len && l != net.IPv6len {
   652  					return fmt.Errorf("x509: internal error: IP SAN %x failed to parse", data)
   653  				}
   654  
   655  				if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "IP address", ip.String(), ip,
   656  					func(parsedName, constraint interface{}) (bool, error) {
   657  						return matchIPConstraint(parsedName.(net.IP), constraint.(*net.IPNet))
   658  					}, c.PermittedIPRanges, c.ExcludedIPRanges); err != nil {
   659  					return err
   660  				}
   661  
   662  			default:
   663  				// Unknown SAN types are ignored.
   664  			}
   665  
   666  			return nil
   667  		})
   668  
   669  		if err != nil {
   670  			return err
   671  		}
   672  	}
   673  
   674  	// KeyUsage status flags are ignored. From Engineering Security, Peter
   675  	// Gutmann: A European government CA marked its signing certificates as
   676  	// being valid for encryption only, but no-one noticed. Another
   677  	// European CA marked its signature keys as not being valid for
   678  	// signatures. A different CA marked its own trusted root certificate
   679  	// as being invalid for certificate signing. Another national CA
   680  	// distributed a certificate to be used to encrypt data for the
   681  	// country’s tax authority that was marked as only being usable for
   682  	// digital signatures but not for encryption. Yet another CA reversed
   683  	// the order of the bit flags in the keyUsage due to confusion over
   684  	// encoding endianness, essentially setting a random keyUsage in
   685  	// certificates that it issued. Another CA created a self-invalidating
   686  	// certificate by adding a certificate policy statement stipulating
   687  	// that the certificate had to be used strictly as specified in the
   688  	// keyUsage, and a keyUsage containing a flag indicating that the RSA
   689  	// encryption key could only be used for Diffie-Hellman key agreement.
   690  
   691  	if certType == intermediateCertificate && (!c.BasicConstraintsValid || !c.IsCA) {
   692  		return CertificateInvalidError{c, NotAuthorizedToSign, ""}
   693  	}
   694  
   695  	if c.BasicConstraintsValid && c.MaxPathLen >= 0 {
   696  		numIntermediates := len(currentChain) - 1
   697  		if numIntermediates > c.MaxPathLen {
   698  			return CertificateInvalidError{c, TooManyIntermediates, ""}
   699  		}
   700  	}
   701  
   702  	return nil
   703  }
   704  
   705  // Verify attempts to verify c by building one or more chains from c to a
   706  // certificate in opts.Roots, using certificates in opts.Intermediates if
   707  // needed. If successful, it returns one or more chains where the first
   708  // element of the chain is c and the last element is from opts.Roots.
   709  //
   710  // If opts.Roots is nil and system roots are unavailable the returned error
   711  // will be of type SystemRootsError.
   712  //
   713  // Name constraints in the intermediates will be applied to all names claimed
   714  // in the chain, not just opts.DNSName. Thus it is invalid for a leaf to claim
   715  // example.com if an intermediate doesn't permit it, even if example.com is not
   716  // the name being validated. Note that DirectoryName constraints are not
   717  // supported.
   718  //
   719  // Extended Key Usage values are enforced down a chain, so an intermediate or
   720  // root that enumerates EKUs prevents a leaf from asserting an EKU not in that
   721  // list.
   722  //
   723  // WARNING: this function doesn't do any revocation checking.
   724  func (c *Certificate) Verify(opts VerifyOptions) (chains [][]*Certificate, err error) {
   725  	// Platform-specific verification needs the ASN.1 contents so
   726  	// this makes the behavior consistent across platforms.
   727  	if len(c.Raw) == 0 {
   728  		return nil, errNotParsed
   729  	}
   730  	if opts.Intermediates != nil {
   731  		for _, intermediate := range opts.Intermediates.certs {
   732  			if len(intermediate.Raw) == 0 {
   733  				return nil, errNotParsed
   734  			}
   735  		}
   736  	}
   737  
   738  	// Use Windows's own verification and chain building.
   739  	if opts.Roots == nil && runtime.GOOS == "windows" {
   740  		return c.systemVerify(&opts)
   741  	}
   742  
   743  	if opts.Roots == nil {
   744  		opts.Roots = systemRootsPool()
   745  		if opts.Roots == nil {
   746  			return nil, SystemRootsError{systemRootsErr}
   747  		}
   748  	}
   749  
   750  	err = c.isValid(leafCertificate, nil, &opts)
   751  	if err != nil {
   752  		return
   753  	}
   754  
   755  	if len(opts.DNSName) > 0 {
   756  		err = c.VerifyHostname(opts.DNSName)
   757  		if err != nil {
   758  			return
   759  		}
   760  	}
   761  
   762  	var candidateChains [][]*Certificate
   763  	if opts.Roots.contains(c) {
   764  		candidateChains = append(candidateChains, []*Certificate{c})
   765  	} else {
   766  		if candidateChains, err = c.buildChains(nil, []*Certificate{c}, nil, &opts); err != nil {
   767  			return nil, err
   768  		}
   769  	}
   770  
   771  	keyUsages := opts.KeyUsages
   772  	if len(keyUsages) == 0 {
   773  		keyUsages = []ExtKeyUsage{ExtKeyUsageServerAuth}
   774  	}
   775  
   776  	// If any key usage is acceptable then we're done.
   777  	for _, usage := range keyUsages {
   778  		if usage == ExtKeyUsageAny {
   779  			return candidateChains, nil
   780  		}
   781  	}
   782  
   783  	for _, candidate := range candidateChains {
   784  		if checkChainForKeyUsage(candidate, keyUsages) {
   785  			chains = append(chains, candidate)
   786  		}
   787  	}
   788  
   789  	if len(chains) == 0 {
   790  		return nil, CertificateInvalidError{c, IncompatibleUsage, ""}
   791  	}
   792  
   793  	return chains, nil
   794  }
   795  
   796  func appendToFreshChain(chain []*Certificate, cert *Certificate) []*Certificate {
   797  	n := make([]*Certificate, len(chain)+1)
   798  	copy(n, chain)
   799  	n[len(chain)] = cert
   800  	return n
   801  }
   802  
   803  // maxChainSignatureChecks is the maximum number of CheckSignatureFrom calls
   804  // that an invocation of buildChains will (tranistively) make. Most chains are
   805  // less than 15 certificates long, so this leaves space for multiple chains and
   806  // for failed checks due to different intermediates having the same Subject.
   807  const maxChainSignatureChecks = 100
   808  
   809  func (c *Certificate) buildChains(cache map[*Certificate][][]*Certificate, currentChain []*Certificate, sigChecks *int, opts *VerifyOptions) (chains [][]*Certificate, err error) {
   810  	var (
   811  		hintErr  error
   812  		hintCert *Certificate
   813  	)
   814  
   815  	considerCandidate := func(certType int, candidate *Certificate) {
   816  		for _, cert := range currentChain {
   817  			if cert.Equal(candidate) {
   818  				return
   819  			}
   820  		}
   821  
   822  		if sigChecks == nil {
   823  			sigChecks = new(int)
   824  		}
   825  		*sigChecks++
   826  		if *sigChecks > maxChainSignatureChecks {
   827  			err = errors.New("x509: signature check attempts limit reached while verifying certificate chain")
   828  			return
   829  		}
   830  
   831  		if err := c.CheckSignatureFrom(candidate); err != nil {
   832  			if hintErr == nil {
   833  				hintErr = err
   834  				hintCert = candidate
   835  			}
   836  			return
   837  		}
   838  
   839  		err = candidate.isValid(certType, currentChain, opts)
   840  		if err != nil {
   841  			return
   842  		}
   843  
   844  		switch certType {
   845  		case rootCertificate:
   846  			chains = append(chains, appendToFreshChain(currentChain, candidate))
   847  		case intermediateCertificate:
   848  			if cache == nil {
   849  				cache = make(map[*Certificate][][]*Certificate)
   850  			}
   851  			childChains, ok := cache[candidate]
   852  			if !ok {
   853  				childChains, err = candidate.buildChains(cache, appendToFreshChain(currentChain, candidate), sigChecks, opts)
   854  				cache[candidate] = childChains
   855  			}
   856  			chains = append(chains, childChains...)
   857  		}
   858  	}
   859  
   860  	for _, rootNum := range opts.Roots.findPotentialParents(c) {
   861  		considerCandidate(rootCertificate, opts.Roots.certs[rootNum])
   862  	}
   863  	for _, intermediateNum := range opts.Intermediates.findPotentialParents(c) {
   864  		considerCandidate(intermediateCertificate, opts.Intermediates.certs[intermediateNum])
   865  	}
   866  
   867  	if len(chains) > 0 {
   868  		err = nil
   869  	}
   870  	if len(chains) == 0 && err == nil {
   871  		err = UnknownAuthorityError{c, hintErr, hintCert}
   872  	}
   873  
   874  	return
   875  }
   876  
   877  // validHostname reports whether host is a valid hostname that can be matched or
   878  // matched against according to RFC 6125 2.2, with some leniency to accommodate
   879  // legacy values.
   880  func validHostname(host string) bool {
   881  	host = strings.TrimSuffix(host, ".")
   882  
   883  	if len(host) == 0 {
   884  		return false
   885  	}
   886  
   887  	for i, part := range strings.Split(host, ".") {
   888  		if part == "" {
   889  			// Empty label.
   890  			return false
   891  		}
   892  		if i == 0 && part == "*" {
   893  			// Only allow full left-most wildcards, as those are the only ones
   894  			// we match, and matching literal '*' characters is probably never
   895  			// the expected behavior.
   896  			continue
   897  		}
   898  		for j, c := range part {
   899  			if 'a' <= c && c <= 'z' {
   900  				continue
   901  			}
   902  			if '0' <= c && c <= '9' {
   903  				continue
   904  			}
   905  			if 'A' <= c && c <= 'Z' {
   906  				continue
   907  			}
   908  			if c == '-' && j != 0 {
   909  				continue
   910  			}
   911  			if c == '_' || c == ':' {
   912  				// Not valid characters in hostnames, but commonly
   913  				// found in deployments outside the WebPKI.
   914  				continue
   915  			}
   916  			return false
   917  		}
   918  	}
   919  
   920  	return true
   921  }
   922  
   923  // commonNameAsHostname reports whether the Common Name field should be
   924  // considered the hostname that the certificate is valid for. This is a legacy
   925  // behavior, disabled if the Subject Alt Name extension is present.
   926  //
   927  // It applies the strict validHostname check to the Common Name field, so that
   928  // certificates without SANs can still be validated against CAs with name
   929  // constraints if there is no risk the CN would be matched as a hostname.
   930  // See NameConstraintsWithoutSANs and issue 24151.
   931  func (c *Certificate) commonNameAsHostname() bool {
   932  	return !ignoreCN && !c.hasSANExtension() && validHostname(c.Subject.CommonName)
   933  }
   934  
   935  func matchHostnames(pattern, host string) bool {
   936  	host = strings.TrimSuffix(host, ".")
   937  	pattern = strings.TrimSuffix(pattern, ".")
   938  
   939  	if len(pattern) == 0 || len(host) == 0 {
   940  		return false
   941  	}
   942  
   943  	patternParts := strings.Split(pattern, ".")
   944  	hostParts := strings.Split(host, ".")
   945  
   946  	if len(patternParts) != len(hostParts) {
   947  		return false
   948  	}
   949  
   950  	for i, patternPart := range patternParts {
   951  		if i == 0 && patternPart == "*" {
   952  			continue
   953  		}
   954  		if patternPart != hostParts[i] {
   955  			return false
   956  		}
   957  	}
   958  
   959  	return true
   960  }
   961  
   962  // toLowerCaseASCII returns a lower-case version of in. See RFC 6125 6.4.1. We use
   963  // an explicitly ASCII function to avoid any sharp corners resulting from
   964  // performing Unicode operations on DNS labels.
   965  func toLowerCaseASCII(in string) string {
   966  	// If the string is already lower-case then there's nothing to do.
   967  	isAlreadyLowerCase := true
   968  	for _, c := range in {
   969  		if c == utf8.RuneError {
   970  			// If we get a UTF-8 error then there might be
   971  			// upper-case ASCII bytes in the invalid sequence.
   972  			isAlreadyLowerCase = false
   973  			break
   974  		}
   975  		if 'A' <= c && c <= 'Z' {
   976  			isAlreadyLowerCase = false
   977  			break
   978  		}
   979  	}
   980  
   981  	if isAlreadyLowerCase {
   982  		return in
   983  	}
   984  
   985  	out := []byte(in)
   986  	for i, c := range out {
   987  		if 'A' <= c && c <= 'Z' {
   988  			out[i] += 'a' - 'A'
   989  		}
   990  	}
   991  	return string(out)
   992  }
   993  
   994  // VerifyHostname returns nil if c is a valid certificate for the named host.
   995  // Otherwise it returns an error describing the mismatch.
   996  func (c *Certificate) VerifyHostname(h string) error {
   997  	// IP addresses may be written in [ ].
   998  	candidateIP := h
   999  	if len(h) >= 3 && h[0] == '[' && h[len(h)-1] == ']' {
  1000  		candidateIP = h[1 : len(h)-1]
  1001  	}
  1002  	if ip := net.ParseIP(candidateIP); ip != nil {
  1003  		// We only match IP addresses against IP SANs.
  1004  		// See RFC 6125, Appendix B.2.
  1005  		for _, candidate := range c.IPAddresses {
  1006  			if ip.Equal(candidate) {
  1007  				return nil
  1008  			}
  1009  		}
  1010  		return HostnameError{c, candidateIP}
  1011  	}
  1012  
  1013  	lowered := toLowerCaseASCII(h)
  1014  
  1015  	if c.commonNameAsHostname() {
  1016  		if matchHostnames(toLowerCaseASCII(c.Subject.CommonName), lowered) {
  1017  			return nil
  1018  		}
  1019  	} else {
  1020  		for _, match := range c.DNSNames {
  1021  			if matchHostnames(toLowerCaseASCII(match), lowered) {
  1022  				return nil
  1023  			}
  1024  		}
  1025  	}
  1026  
  1027  	return HostnameError{c, h}
  1028  }
  1029  
  1030  func checkChainForKeyUsage(chain []*Certificate, keyUsages []ExtKeyUsage) bool {
  1031  	usages := make([]ExtKeyUsage, len(keyUsages))
  1032  	copy(usages, keyUsages)
  1033  
  1034  	if len(chain) == 0 {
  1035  		return false
  1036  	}
  1037  
  1038  	usagesRemaining := len(usages)
  1039  
  1040  	// We walk down the list and cross out any usages that aren't supported
  1041  	// by each certificate. If we cross out all the usages, then the chain
  1042  	// is unacceptable.
  1043  
  1044  NextCert:
  1045  	for i := len(chain) - 1; i >= 0; i-- {
  1046  		cert := chain[i]
  1047  		if len(cert.ExtKeyUsage) == 0 && len(cert.UnknownExtKeyUsage) == 0 {
  1048  			// The certificate doesn't have any extended key usage specified.
  1049  			continue
  1050  		}
  1051  
  1052  		for _, usage := range cert.ExtKeyUsage {
  1053  			if usage == ExtKeyUsageAny {
  1054  				// The certificate is explicitly good for any usage.
  1055  				continue NextCert
  1056  			}
  1057  		}
  1058  
  1059  		const invalidUsage ExtKeyUsage = -1
  1060  
  1061  	NextRequestedUsage:
  1062  		for i, requestedUsage := range usages {
  1063  			if requestedUsage == invalidUsage {
  1064  				continue
  1065  			}
  1066  
  1067  			for _, usage := range cert.ExtKeyUsage {
  1068  				if requestedUsage == usage {
  1069  					continue NextRequestedUsage
  1070  				} else if requestedUsage == ExtKeyUsageServerAuth &&
  1071  					(usage == ExtKeyUsageNetscapeServerGatedCrypto ||
  1072  						usage == ExtKeyUsageMicrosoftServerGatedCrypto) {
  1073  					// In order to support COMODO
  1074  					// certificate chains, we have to
  1075  					// accept Netscape or Microsoft SGC
  1076  					// usages as equal to ServerAuth.
  1077  					continue NextRequestedUsage
  1078  				}
  1079  			}
  1080  
  1081  			usages[i] = invalidUsage
  1082  			usagesRemaining--
  1083  			if usagesRemaining == 0 {
  1084  				return false
  1085  			}
  1086  		}
  1087  	}
  1088  
  1089  	return true
  1090  }
  1091  

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