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
   226  // https://tools.ietf.org/html/rfc5280#section-4.2.1.6 that's correct for an
   227  // rfc822Name from a certificate: “The format of an rfc822Name is a "Mailbox"
   228  // as defined in https://tools.ietf.org/html/rfc2821#section-4.1.2”.
   229  func parseRFC2821Mailbox(in string) (mailbox rfc2821Mailbox, ok bool) {
   230  	if len(in) == 0 {
   231  		return mailbox, false
   232  	}
   233  
   234  	localPartBytes := make([]byte, 0, len(in)/2)
   235  
   236  	if in[0] == '"' {
   237  		// Quoted-string = DQUOTE *qcontent DQUOTE
   238  		// non-whitespace-control = %d1-8 / %d11 / %d12 / %d14-31 / %d127
   239  		// qcontent = qtext / quoted-pair
   240  		// qtext = non-whitespace-control /
   241  		//         %d33 / %d35-91 / %d93-126
   242  		// quoted-pair = ("\" text) / obs-qp
   243  		// text = %d1-9 / %d11 / %d12 / %d14-127 / obs-text
   244  		//
   245  		// (Names beginning with “obs-” are the obsolete syntax from
   246  		// https://tools.ietf.org/html/rfc2822#section-4. Since it has
   247  		// been 16 years, we no longer accept that.)
   248  		in = in[1:]
   249  	QuotedString:
   250  		for {
   251  			if len(in) == 0 {
   252  				return mailbox, false
   253  			}
   254  			c := in[0]
   255  			in = in[1:]
   256  
   257  			switch {
   258  			case c == '"':
   259  				break QuotedString
   260  
   261  			case c == '\\':
   262  				// quoted-pair
   263  				if len(in) == 0 {
   264  					return mailbox, false
   265  				}
   266  				if in[0] == 11 ||
   267  					in[0] == 12 ||
   268  					(1 <= in[0] && in[0] <= 9) ||
   269  					(14 <= in[0] && in[0] <= 127) {
   270  					localPartBytes = append(localPartBytes, in[0])
   271  					in = in[1:]
   272  				} else {
   273  					return mailbox, false
   274  				}
   275  
   276  			case c == 11 ||
   277  				c == 12 ||
   278  				// Space (char 32) is not allowed based on the
   279  				// BNF, but RFC 3696 gives an example that
   280  				// assumes that it is. Several “verified”
   281  				// errata continue to argue about this point.
   282  				// We choose to accept it.
   283  				c == 32 ||
   284  				c == 33 ||
   285  				c == 127 ||
   286  				(1 <= c && c <= 8) ||
   287  				(14 <= c && c <= 31) ||
   288  				(35 <= c && c <= 91) ||
   289  				(93 <= c && c <= 126):
   290  				// qtext
   291  				localPartBytes = append(localPartBytes, c)
   292  
   293  			default:
   294  				return mailbox, false
   295  			}
   296  		}
   297  	} else {
   298  		// Atom ("." Atom)*
   299  	NextChar:
   300  		for len(in) > 0 {
   301  			// atext from https://tools.ietf.org/html/rfc2822#section-3.2.4
   302  			c := in[0]
   303  
   304  			switch {
   305  			case c == '\\':
   306  				// Examples given in RFC 3696 suggest that
   307  				// escaped characters can appear outside of a
   308  				// quoted string. Several “verified” errata
   309  				// continue to argue the point. We choose to
   310  				// accept it.
   311  				in = in[1:]
   312  				if len(in) == 0 {
   313  					return mailbox, false
   314  				}
   315  				fallthrough
   316  
   317  			case ('0' <= c && c <= '9') ||
   318  				('a' <= c && c <= 'z') ||
   319  				('A' <= c && c <= 'Z') ||
   320  				c == '!' || c == '#' || c == '$' || c == '%' ||
   321  				c == '&' || c == '\'' || c == '*' || c == '+' ||
   322  				c == '-' || c == '/' || c == '=' || c == '?' ||
   323  				c == '^' || c == '_' || c == '`' || c == '{' ||
   324  				c == '|' || c == '}' || c == '~' || c == '.':
   325  				localPartBytes = append(localPartBytes, in[0])
   326  				in = in[1:]
   327  
   328  			default:
   329  				break NextChar
   330  			}
   331  		}
   332  
   333  		if len(localPartBytes) == 0 {
   334  			return mailbox, false
   335  		}
   336  
   337  		// https://tools.ietf.org/html/rfc3696#section-3
   338  		// “period (".") may also appear, but may not be used to start
   339  		// or end the local part, nor may two or more consecutive
   340  		// periods appear.”
   341  		twoDots := []byte{'.', '.'}
   342  		if localPartBytes[0] == '.' ||
   343  			localPartBytes[len(localPartBytes)-1] == '.' ||
   344  			bytes.Contains(localPartBytes, twoDots) {
   345  			return mailbox, false
   346  		}
   347  	}
   348  
   349  	if len(in) == 0 || in[0] != '@' {
   350  		return mailbox, false
   351  	}
   352  	in = in[1:]
   353  
   354  	// The RFC species a format for domains, but that's known to be
   355  	// violated in practice so we accept that anything after an '@' is the
   356  	// domain part.
   357  	if _, ok := domainToReverseLabels(in); !ok {
   358  		return mailbox, false
   359  	}
   360  
   361  	mailbox.local = string(localPartBytes)
   362  	mailbox.domain = in
   363  	return mailbox, true
   364  }
   365  
   366  // domainToReverseLabels converts a textual domain name like foo.example.com to
   367  // the list of labels in reverse order, e.g. ["com", "example", "foo"].
   368  func domainToReverseLabels(domain string) (reverseLabels []string, ok bool) {
   369  	for len(domain) > 0 {
   370  		if i := strings.LastIndexByte(domain, '.'); i == -1 {
   371  			reverseLabels = append(reverseLabels, domain)
   372  			domain = ""
   373  		} else {
   374  			reverseLabels = append(reverseLabels, domain[i+1:len(domain)])
   375  			domain = domain[:i]
   376  		}
   377  	}
   378  
   379  	if len(reverseLabels) > 0 && len(reverseLabels[0]) == 0 {
   380  		// An empty label at the end indicates an absolute value.
   381  		return nil, false
   382  	}
   383  
   384  	for _, label := range reverseLabels {
   385  		if len(label) == 0 {
   386  			// Empty labels are otherwise invalid.
   387  			return nil, false
   388  		}
   389  
   390  		for _, c := range label {
   391  			if c < 33 || c > 126 {
   392  				// Invalid character.
   393  				return nil, false
   394  			}
   395  		}
   396  	}
   397  
   398  	return reverseLabels, true
   399  }
   400  
   401  func matchEmailConstraint(mailbox rfc2821Mailbox, constraint string) (bool, error) {
   402  	// If the constraint contains an @, then it specifies an exact mailbox
   403  	// name.
   404  	if strings.Contains(constraint, "@") {
   405  		constraintMailbox, ok := parseRFC2821Mailbox(constraint)
   406  		if !ok {
   407  			return false, fmt.Errorf("x509: internal error: cannot parse constraint %q", constraint)
   408  		}
   409  		return mailbox.local == constraintMailbox.local && strings.EqualFold(mailbox.domain, constraintMailbox.domain), nil
   410  	}
   411  
   412  	// Otherwise the constraint is like a DNS constraint of the domain part
   413  	// of the mailbox.
   414  	return matchDomainConstraint(mailbox.domain, constraint)
   415  }
   416  
   417  func matchURIConstraint(uri *url.URL, constraint string) (bool, error) {
   418  	// https://tools.ietf.org/html/rfc5280#section-4.2.1.10
   419  	// “a uniformResourceIdentifier that does not include an authority
   420  	// component with a host name specified as a fully qualified domain
   421  	// name (e.g., if the URI either does not include an authority
   422  	// component or includes an authority component in which the host name
   423  	// is specified as an IP address), then the application MUST reject the
   424  	// certificate.”
   425  
   426  	host := uri.Host
   427  	if len(host) == 0 {
   428  		return false, fmt.Errorf("URI with empty host (%q) cannot be matched against constraints", uri.String())
   429  	}
   430  
   431  	if strings.Contains(host, ":") && !strings.HasSuffix(host, "]") {
   432  		var err error
   433  		host, _, err = net.SplitHostPort(uri.Host)
   434  		if err != nil {
   435  			return false, err
   436  		}
   437  	}
   438  
   439  	if strings.HasPrefix(host, "[") && strings.HasSuffix(host, "]") ||
   440  		net.ParseIP(host) != nil {
   441  		return false, fmt.Errorf("URI with IP (%q) cannot be matched against constraints", uri.String())
   442  	}
   443  
   444  	return matchDomainConstraint(host, constraint)
   445  }
   446  
   447  func matchIPConstraint(ip net.IP, constraint *net.IPNet) (bool, error) {
   448  	if len(ip) != len(constraint.IP) {
   449  		return false, nil
   450  	}
   451  
   452  	for i := range ip {
   453  		if mask := constraint.Mask[i]; ip[i]&mask != constraint.IP[i]&mask {
   454  			return false, nil
   455  		}
   456  	}
   457  
   458  	return true, nil
   459  }
   460  
   461  func matchDomainConstraint(domain, constraint string) (bool, error) {
   462  	// The meaning of zero length constraints is not specified, but this
   463  	// code follows NSS and accepts them as matching everything.
   464  	if len(constraint) == 0 {
   465  		return true, nil
   466  	}
   467  
   468  	domainLabels, ok := domainToReverseLabels(domain)
   469  	if !ok {
   470  		return false, fmt.Errorf("x509: internal error: cannot parse domain %q", domain)
   471  	}
   472  
   473  	// RFC 5280 says that a leading period in a domain name means that at
   474  	// least one label must be prepended, but only for URI and email
   475  	// constraints, not DNS constraints. The code also supports that
   476  	// behaviour for DNS constraints.
   477  
   478  	mustHaveSubdomains := false
   479  	if constraint[0] == '.' {
   480  		mustHaveSubdomains = true
   481  		constraint = constraint[1:]
   482  	}
   483  
   484  	constraintLabels, ok := domainToReverseLabels(constraint)
   485  	if !ok {
   486  		return false, fmt.Errorf("x509: internal error: cannot parse domain %q", constraint)
   487  	}
   488  
   489  	if len(domainLabels) < len(constraintLabels) ||
   490  		(mustHaveSubdomains && len(domainLabels) == len(constraintLabels)) {
   491  		return false, nil
   492  	}
   493  
   494  	for i, constraintLabel := range constraintLabels {
   495  		if !strings.EqualFold(constraintLabel, domainLabels[i]) {
   496  			return false, nil
   497  		}
   498  	}
   499  
   500  	return true, nil
   501  }
   502  
   503  // checkNameConstraints checks that c permits a child certificate to claim the
   504  // given name, of type nameType. The argument parsedName contains the parsed
   505  // form of name, suitable for passing to the match function. The total number
   506  // of comparisons is tracked in the given count and should not exceed the given
   507  // limit.
   508  func (c *Certificate) checkNameConstraints(count *int,
   509  	maxConstraintComparisons int,
   510  	nameType string,
   511  	name string,
   512  	parsedName interface{},
   513  	match func(parsedName, constraint interface{}) (match bool, err error),
   514  	permitted, excluded interface{}) error {
   515  
   516  	excludedValue := reflect.ValueOf(excluded)
   517  
   518  	*count += excludedValue.Len()
   519  	if *count > maxConstraintComparisons {
   520  		return CertificateInvalidError{c, TooManyConstraints, ""}
   521  	}
   522  
   523  	for i := 0; i < excludedValue.Len(); i++ {
   524  		constraint := excludedValue.Index(i).Interface()
   525  		match, err := match(parsedName, constraint)
   526  		if err != nil {
   527  			return CertificateInvalidError{c, CANotAuthorizedForThisName, err.Error()}
   528  		}
   529  
   530  		if match {
   531  			return CertificateInvalidError{c, CANotAuthorizedForThisName, fmt.Sprintf("%s %q is excluded by constraint %q", nameType, name, constraint)}
   532  		}
   533  	}
   534  
   535  	permittedValue := reflect.ValueOf(permitted)
   536  
   537  	*count += permittedValue.Len()
   538  	if *count > maxConstraintComparisons {
   539  		return CertificateInvalidError{c, TooManyConstraints, ""}
   540  	}
   541  
   542  	ok := true
   543  	for i := 0; i < permittedValue.Len(); i++ {
   544  		constraint := permittedValue.Index(i).Interface()
   545  
   546  		var err error
   547  		if ok, err = match(parsedName, constraint); err != nil {
   548  			return CertificateInvalidError{c, CANotAuthorizedForThisName, err.Error()}
   549  		}
   550  
   551  		if ok {
   552  			break
   553  		}
   554  	}
   555  
   556  	if !ok {
   557  		return CertificateInvalidError{c, CANotAuthorizedForThisName, fmt.Sprintf("%s %q is not permitted by any constraint", nameType, name)}
   558  	}
   559  
   560  	return nil
   561  }
   562  
   563  // isValid performs validity checks on c given that it is a candidate to append
   564  // to the chain in currentChain.
   565  func (c *Certificate) isValid(certType int, currentChain []*Certificate, opts *VerifyOptions) error {
   566  	if len(c.UnhandledCriticalExtensions) > 0 {
   567  		return UnhandledCriticalExtension{}
   568  	}
   569  
   570  	if len(currentChain) > 0 {
   571  		child := currentChain[len(currentChain)-1]
   572  		if !bytes.Equal(child.RawIssuer, c.RawSubject) {
   573  			return CertificateInvalidError{c, NameMismatch, ""}
   574  		}
   575  	}
   576  
   577  	now := opts.CurrentTime
   578  	if now.IsZero() {
   579  		now = time.Now()
   580  	}
   581  	if now.Before(c.NotBefore) || now.After(c.NotAfter) {
   582  		return CertificateInvalidError{c, Expired, ""}
   583  	}
   584  
   585  	maxConstraintComparisons := opts.MaxConstraintComparisions
   586  	if maxConstraintComparisons == 0 {
   587  		maxConstraintComparisons = 250000
   588  	}
   589  	comparisonCount := 0
   590  
   591  	var leaf *Certificate
   592  	if certType == intermediateCertificate || certType == rootCertificate {
   593  		if len(currentChain) == 0 {
   594  			return errors.New("x509: internal error: empty chain when appending CA cert")
   595  		}
   596  		leaf = currentChain[0]
   597  	}
   598  
   599  	checkNameConstraints := (certType == intermediateCertificate || certType == rootCertificate) && c.hasNameConstraints()
   600  	if checkNameConstraints && leaf.commonNameAsHostname() {
   601  		// This is the deprecated, legacy case of depending on the commonName as
   602  		// a hostname. We don't enforce name constraints against the CN, but
   603  		// VerifyHostname will look for hostnames in there if there are no SANs.
   604  		// In order to ensure VerifyHostname will not accept an unchecked name,
   605  		// return an error here.
   606  		return CertificateInvalidError{c, NameConstraintsWithoutSANs, ""}
   607  	} else if checkNameConstraints && leaf.hasSANExtension() {
   608  		err := forEachSAN(leaf.getSANExtension(), func(tag int, data []byte) error {
   609  			switch tag {
   610  			case nameTypeEmail:
   611  				name := string(data)
   612  				mailbox, ok := parseRFC2821Mailbox(name)
   613  				if !ok {
   614  					return fmt.Errorf("x509: cannot parse rfc822Name %q", mailbox)
   615  				}
   616  
   617  				if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "email address", name, mailbox,
   618  					func(parsedName, constraint interface{}) (bool, error) {
   619  						return matchEmailConstraint(parsedName.(rfc2821Mailbox), constraint.(string))
   620  					}, c.PermittedEmailAddresses, c.ExcludedEmailAddresses); err != nil {
   621  					return err
   622  				}
   623  
   624  			case nameTypeDNS:
   625  				name := string(data)
   626  				if _, ok := domainToReverseLabels(name); !ok {
   627  					return fmt.Errorf("x509: cannot parse dnsName %q", name)
   628  				}
   629  
   630  				if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "DNS name", name, name,
   631  					func(parsedName, constraint interface{}) (bool, error) {
   632  						return matchDomainConstraint(parsedName.(string), constraint.(string))
   633  					}, c.PermittedDNSDomains, c.ExcludedDNSDomains); err != nil {
   634  					return err
   635  				}
   636  
   637  			case nameTypeURI:
   638  				name := string(data)
   639  				uri, err := url.Parse(name)
   640  				if err != nil {
   641  					return fmt.Errorf("x509: internal error: URI SAN %q failed to parse", name)
   642  				}
   643  
   644  				if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "URI", name, uri,
   645  					func(parsedName, constraint interface{}) (bool, error) {
   646  						return matchURIConstraint(parsedName.(*url.URL), constraint.(string))
   647  					}, c.PermittedURIDomains, c.ExcludedURIDomains); err != nil {
   648  					return err
   649  				}
   650  
   651  			case nameTypeIP:
   652  				ip := net.IP(data)
   653  				if l := len(ip); l != net.IPv4len && l != net.IPv6len {
   654  					return fmt.Errorf("x509: internal error: IP SAN %x failed to parse", data)
   655  				}
   656  
   657  				if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "IP address", ip.String(), ip,
   658  					func(parsedName, constraint interface{}) (bool, error) {
   659  						return matchIPConstraint(parsedName.(net.IP), constraint.(*net.IPNet))
   660  					}, c.PermittedIPRanges, c.ExcludedIPRanges); err != nil {
   661  					return err
   662  				}
   663  
   664  			default:
   665  				// Unknown SAN types are ignored.
   666  			}
   667  
   668  			return nil
   669  		})
   670  
   671  		if err != nil {
   672  			return err
   673  		}
   674  	}
   675  
   676  	// KeyUsage status flags are ignored. From Engineering Security, Peter
   677  	// Gutmann: A European government CA marked its signing certificates as
   678  	// being valid for encryption only, but no-one noticed. Another
   679  	// European CA marked its signature keys as not being valid for
   680  	// signatures. A different CA marked its own trusted root certificate
   681  	// as being invalid for certificate signing. Another national CA
   682  	// distributed a certificate to be used to encrypt data for the
   683  	// country’s tax authority that was marked as only being usable for
   684  	// digital signatures but not for encryption. Yet another CA reversed
   685  	// the order of the bit flags in the keyUsage due to confusion over
   686  	// encoding endianness, essentially setting a random keyUsage in
   687  	// certificates that it issued. Another CA created a self-invalidating
   688  	// certificate by adding a certificate policy statement stipulating
   689  	// that the certificate had to be used strictly as specified in the
   690  	// keyUsage, and a keyUsage containing a flag indicating that the RSA
   691  	// encryption key could only be used for Diffie-Hellman key agreement.
   692  
   693  	if certType == intermediateCertificate && (!c.BasicConstraintsValid || !c.IsCA) {
   694  		return CertificateInvalidError{c, NotAuthorizedToSign, ""}
   695  	}
   696  
   697  	if c.BasicConstraintsValid && c.MaxPathLen >= 0 {
   698  		numIntermediates := len(currentChain) - 1
   699  		if numIntermediates > c.MaxPathLen {
   700  			return CertificateInvalidError{c, TooManyIntermediates, ""}
   701  		}
   702  	}
   703  
   704  	return nil
   705  }
   706  
   707  // Verify attempts to verify c by building one or more chains from c to a
   708  // certificate in opts.Roots, using certificates in opts.Intermediates if
   709  // needed. If successful, it returns one or more chains where the first
   710  // element of the chain is c and the last element is from opts.Roots.
   711  //
   712  // If opts.Roots is nil and system roots are unavailable the returned error
   713  // will be of type SystemRootsError.
   714  //
   715  // Name constraints in the intermediates will be applied to all names claimed
   716  // in the chain, not just opts.DNSName. Thus it is invalid for a leaf to claim
   717  // example.com if an intermediate doesn't permit it, even if example.com is not
   718  // the name being validated. Note that DirectoryName constraints are not
   719  // supported.
   720  //
   721  // Extended Key Usage values are enforced down a chain, so an intermediate or
   722  // root that enumerates EKUs prevents a leaf from asserting an EKU not in that
   723  // list.
   724  //
   725  // WARNING: this function doesn't do any revocation checking.
   726  func (c *Certificate) Verify(opts VerifyOptions) (chains [][]*Certificate, err error) {
   727  	// Platform-specific verification needs the ASN.1 contents so
   728  	// this makes the behavior consistent across platforms.
   729  	if len(c.Raw) == 0 {
   730  		return nil, errNotParsed
   731  	}
   732  	if opts.Intermediates != nil {
   733  		for _, intermediate := range opts.Intermediates.certs {
   734  			if len(intermediate.Raw) == 0 {
   735  				return nil, errNotParsed
   736  			}
   737  		}
   738  	}
   739  
   740  	// Use Windows's own verification and chain building.
   741  	if opts.Roots == nil && runtime.GOOS == "windows" {
   742  		return c.systemVerify(&opts)
   743  	}
   744  
   745  	if opts.Roots == nil {
   746  		opts.Roots = systemRootsPool()
   747  		if opts.Roots == nil {
   748  			return nil, SystemRootsError{systemRootsErr}
   749  		}
   750  	}
   751  
   752  	err = c.isValid(leafCertificate, nil, &opts)
   753  	if err != nil {
   754  		return
   755  	}
   756  
   757  	if len(opts.DNSName) > 0 {
   758  		err = c.VerifyHostname(opts.DNSName)
   759  		if err != nil {
   760  			return
   761  		}
   762  	}
   763  
   764  	var candidateChains [][]*Certificate
   765  	if opts.Roots.contains(c) {
   766  		candidateChains = append(candidateChains, []*Certificate{c})
   767  	} else {
   768  		if candidateChains, err = c.buildChains(nil, []*Certificate{c}, nil, &opts); err != nil {
   769  			return nil, err
   770  		}
   771  	}
   772  
   773  	keyUsages := opts.KeyUsages
   774  	if len(keyUsages) == 0 {
   775  		keyUsages = []ExtKeyUsage{ExtKeyUsageServerAuth}
   776  	}
   777  
   778  	// If any key usage is acceptable then we're done.
   779  	for _, usage := range keyUsages {
   780  		if usage == ExtKeyUsageAny {
   781  			return candidateChains, nil
   782  		}
   783  	}
   784  
   785  	for _, candidate := range candidateChains {
   786  		if checkChainForKeyUsage(candidate, keyUsages) {
   787  			chains = append(chains, candidate)
   788  		}
   789  	}
   790  
   791  	if len(chains) == 0 {
   792  		return nil, CertificateInvalidError{c, IncompatibleUsage, ""}
   793  	}
   794  
   795  	return chains, nil
   796  }
   797  
   798  func appendToFreshChain(chain []*Certificate, cert *Certificate) []*Certificate {
   799  	n := make([]*Certificate, len(chain)+1)
   800  	copy(n, chain)
   801  	n[len(chain)] = cert
   802  	return n
   803  }
   804  
   805  // maxChainSignatureChecks is the maximum number of CheckSignatureFrom calls
   806  // that an invocation of buildChains will (tranistively) make. Most chains are
   807  // less than 15 certificates long, so this leaves space for multiple chains and
   808  // for failed checks due to different intermediates having the same Subject.
   809  const maxChainSignatureChecks = 100
   810  
   811  func (c *Certificate) buildChains(cache map[*Certificate][][]*Certificate, currentChain []*Certificate, sigChecks *int, opts *VerifyOptions) (chains [][]*Certificate, err error) {
   812  	var (
   813  		hintErr  error
   814  		hintCert *Certificate
   815  	)
   816  
   817  	considerCandidate := func(certType int, candidate *Certificate) {
   818  		for _, cert := range currentChain {
   819  			if cert.Equal(candidate) {
   820  				return
   821  			}
   822  		}
   823  
   824  		if sigChecks == nil {
   825  			sigChecks = new(int)
   826  		}
   827  		*sigChecks++
   828  		if *sigChecks > maxChainSignatureChecks {
   829  			err = errors.New("x509: signature check attempts limit reached while verifying certificate chain")
   830  			return
   831  		}
   832  
   833  		if err := c.CheckSignatureFrom(candidate); err != nil {
   834  			if hintErr == nil {
   835  				hintErr = err
   836  				hintCert = candidate
   837  			}
   838  			return
   839  		}
   840  
   841  		err = candidate.isValid(certType, currentChain, opts)
   842  		if err != nil {
   843  			return
   844  		}
   845  
   846  		switch certType {
   847  		case rootCertificate:
   848  			chains = append(chains, appendToFreshChain(currentChain, candidate))
   849  		case intermediateCertificate:
   850  			if cache == nil {
   851  				cache = make(map[*Certificate][][]*Certificate)
   852  			}
   853  			childChains, ok := cache[candidate]
   854  			if !ok {
   855  				childChains, err = candidate.buildChains(cache, appendToFreshChain(currentChain, candidate), sigChecks, opts)
   856  				cache[candidate] = childChains
   857  			}
   858  			chains = append(chains, childChains...)
   859  		}
   860  	}
   861  
   862  	for _, rootNum := range opts.Roots.findPotentialParents(c) {
   863  		considerCandidate(rootCertificate, opts.Roots.certs[rootNum])
   864  	}
   865  	for _, intermediateNum := range opts.Intermediates.findPotentialParents(c) {
   866  		considerCandidate(intermediateCertificate, opts.Intermediates.certs[intermediateNum])
   867  	}
   868  
   869  	if len(chains) > 0 {
   870  		err = nil
   871  	}
   872  	if len(chains) == 0 && err == nil {
   873  		err = UnknownAuthorityError{c, hintErr, hintCert}
   874  	}
   875  
   876  	return
   877  }
   878  
   879  // validHostname returns whether host is a valid hostname that can be matched or
   880  // matched against according to RFC 6125 2.2, with some leniency to accomodate
   881  // legacy values.
   882  func validHostname(host string) bool {
   883  	host = strings.TrimSuffix(host, ".")
   884  
   885  	if len(host) == 0 {
   886  		return false
   887  	}
   888  
   889  	for i, part := range strings.Split(host, ".") {
   890  		if part == "" {
   891  			// Empty label.
   892  			return false
   893  		}
   894  		if i == 0 && part == "*" {
   895  			// Only allow full left-most wildcards, as those are the only ones
   896  			// we match, and matching literal '*' characters is probably never
   897  			// the expected behavior.
   898  			continue
   899  		}
   900  		for j, c := range part {
   901  			if 'a' <= c && c <= 'z' {
   902  				continue
   903  			}
   904  			if '0' <= c && c <= '9' {
   905  				continue
   906  			}
   907  			if 'A' <= c && c <= 'Z' {
   908  				continue
   909  			}
   910  			if c == '-' && j != 0 {
   911  				continue
   912  			}
   913  			if c == '_' || c == ':' {
   914  				// Not valid characters in hostnames, but commonly
   915  				// found in deployments outside the WebPKI.
   916  				continue
   917  			}
   918  			return false
   919  		}
   920  	}
   921  
   922  	return true
   923  }
   924  
   925  // commonNameAsHostname reports whether the Common Name field should be
   926  // considered the hostname that the certificate is valid for. This is a legacy
   927  // behavior, disabled if the Subject Alt Name extension is present.
   928  //
   929  // It applies the strict validHostname check to the Common Name field, so that
   930  // certificates without SANs can still be validated against CAs with name
   931  // constraints if there is no risk the CN would be matched as a hostname.
   932  // See NameConstraintsWithoutSANs and issue 24151.
   933  func (c *Certificate) commonNameAsHostname() bool {
   934  	return !ignoreCN && !c.hasSANExtension() && validHostname(c.Subject.CommonName)
   935  }
   936  
   937  func matchHostnames(pattern, host string) bool {
   938  	host = strings.TrimSuffix(host, ".")
   939  	pattern = strings.TrimSuffix(pattern, ".")
   940  
   941  	if len(pattern) == 0 || len(host) == 0 {
   942  		return false
   943  	}
   944  
   945  	patternParts := strings.Split(pattern, ".")
   946  	hostParts := strings.Split(host, ".")
   947  
   948  	if len(patternParts) != len(hostParts) {
   949  		return false
   950  	}
   951  
   952  	for i, patternPart := range patternParts {
   953  		if i == 0 && patternPart == "*" {
   954  			continue
   955  		}
   956  		if patternPart != hostParts[i] {
   957  			return false
   958  		}
   959  	}
   960  
   961  	return true
   962  }
   963  
   964  // toLowerCaseASCII returns a lower-case version of in. See RFC 6125 6.4.1. We use
   965  // an explicitly ASCII function to avoid any sharp corners resulting from
   966  // performing Unicode operations on DNS labels.
   967  func toLowerCaseASCII(in string) string {
   968  	// If the string is already lower-case then there's nothing to do.
   969  	isAlreadyLowerCase := true
   970  	for _, c := range in {
   971  		if c == utf8.RuneError {
   972  			// If we get a UTF-8 error then there might be
   973  			// upper-case ASCII bytes in the invalid sequence.
   974  			isAlreadyLowerCase = false
   975  			break
   976  		}
   977  		if 'A' <= c && c <= 'Z' {
   978  			isAlreadyLowerCase = false
   979  			break
   980  		}
   981  	}
   982  
   983  	if isAlreadyLowerCase {
   984  		return in
   985  	}
   986  
   987  	out := []byte(in)
   988  	for i, c := range out {
   989  		if 'A' <= c && c <= 'Z' {
   990  			out[i] += 'a' - 'A'
   991  		}
   992  	}
   993  	return string(out)
   994  }
   995  
   996  // VerifyHostname returns nil if c is a valid certificate for the named host.
   997  // Otherwise it returns an error describing the mismatch.
   998  func (c *Certificate) VerifyHostname(h string) error {
   999  	// IP addresses may be written in [ ].
  1000  	candidateIP := h
  1001  	if len(h) >= 3 && h[0] == '[' && h[len(h)-1] == ']' {
  1002  		candidateIP = h[1 : len(h)-1]
  1003  	}
  1004  	if ip := net.ParseIP(candidateIP); ip != nil {
  1005  		// We only match IP addresses against IP SANs.
  1006  		// https://tools.ietf.org/html/rfc6125#appendix-B.2
  1007  		for _, candidate := range c.IPAddresses {
  1008  			if ip.Equal(candidate) {
  1009  				return nil
  1010  			}
  1011  		}
  1012  		return HostnameError{c, candidateIP}
  1013  	}
  1014  
  1015  	lowered := toLowerCaseASCII(h)
  1016  
  1017  	if c.commonNameAsHostname() {
  1018  		if matchHostnames(toLowerCaseASCII(c.Subject.CommonName), lowered) {
  1019  			return nil
  1020  		}
  1021  	} else {
  1022  		for _, match := range c.DNSNames {
  1023  			if matchHostnames(toLowerCaseASCII(match), lowered) {
  1024  				return nil
  1025  			}
  1026  		}
  1027  	}
  1028  
  1029  	return HostnameError{c, h}
  1030  }
  1031  
  1032  func checkChainForKeyUsage(chain []*Certificate, keyUsages []ExtKeyUsage) bool {
  1033  	usages := make([]ExtKeyUsage, len(keyUsages))
  1034  	copy(usages, keyUsages)
  1035  
  1036  	if len(chain) == 0 {
  1037  		return false
  1038  	}
  1039  
  1040  	usagesRemaining := len(usages)
  1041  
  1042  	// We walk down the list and cross out any usages that aren't supported
  1043  	// by each certificate. If we cross out all the usages, then the chain
  1044  	// is unacceptable.
  1045  
  1046  NextCert:
  1047  	for i := len(chain) - 1; i >= 0; i-- {
  1048  		cert := chain[i]
  1049  		if len(cert.ExtKeyUsage) == 0 && len(cert.UnknownExtKeyUsage) == 0 {
  1050  			// The certificate doesn't have any extended key usage specified.
  1051  			continue
  1052  		}
  1053  
  1054  		for _, usage := range cert.ExtKeyUsage {
  1055  			if usage == ExtKeyUsageAny {
  1056  				// The certificate is explicitly good for any usage.
  1057  				continue NextCert
  1058  			}
  1059  		}
  1060  
  1061  		const invalidUsage ExtKeyUsage = -1
  1062  
  1063  	NextRequestedUsage:
  1064  		for i, requestedUsage := range usages {
  1065  			if requestedUsage == invalidUsage {
  1066  				continue
  1067  			}
  1068  
  1069  			for _, usage := range cert.ExtKeyUsage {
  1070  				if requestedUsage == usage {
  1071  					continue NextRequestedUsage
  1072  				} else if requestedUsage == ExtKeyUsageServerAuth &&
  1073  					(usage == ExtKeyUsageNetscapeServerGatedCrypto ||
  1074  						usage == ExtKeyUsageMicrosoftServerGatedCrypto) {
  1075  					// In order to support COMODO
  1076  					// certificate chains, we have to
  1077  					// accept Netscape or Microsoft SGC
  1078  					// usages as equal to ServerAuth.
  1079  					continue NextRequestedUsage
  1080  				}
  1081  			}
  1082  
  1083  			usages[i] = invalidUsage
  1084  			usagesRemaining--
  1085  			if usagesRemaining == 0 {
  1086  				return false
  1087  			}
  1088  		}
  1089  	}
  1090  
  1091  	return true
  1092  }
  1093  

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