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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  	"reflect"
    14  	"runtime"
    15  	"strings"
    16  	"time"
    17  	"unicode/utf8"
    18  )
    19  
    20  type InvalidReason int
    21  
    22  const (
    23  	// NotAuthorizedToSign results when a certificate is signed by another
    24  	// which isn't marked as a CA certificate.
    25  	NotAuthorizedToSign InvalidReason = iota
    26  	// Expired results when a certificate has expired, based on the time
    27  	// given in the VerifyOptions.
    28  	Expired
    29  	// CANotAuthorizedForThisName results when an intermediate or root
    30  	// certificate has a name constraint which doesn't permit a DNS or
    31  	// other name (including IP address) in the leaf certificate.
    32  	CANotAuthorizedForThisName
    33  	// TooManyIntermediates results when a path length constraint is
    34  	// violated.
    35  	TooManyIntermediates
    36  	// IncompatibleUsage results when the certificate's key usage indicates
    37  	// that it may only be used for a different purpose.
    38  	IncompatibleUsage
    39  	// NameMismatch results when the subject name of a parent certificate
    40  	// does not match the issuer name in the child.
    41  	NameMismatch
    42  	// NameConstraintsWithoutSANs results when a leaf certificate doesn't
    43  	// contain a Subject Alternative Name extension, but a CA certificate
    44  	// contains name constraints.
    45  	NameConstraintsWithoutSANs
    46  	// UnconstrainedName results when a CA certificate contains permitted
    47  	// name constraints, but leaf certificate contains a name of an
    48  	// unsupported or unconstrained type.
    49  	UnconstrainedName
    50  	// TooManyConstraints results when the number of comparision operations
    51  	// needed to check a certificate exceeds the limit set by
    52  	// VerifyOptions.MaxConstraintComparisions. This limit exists to
    53  	// prevent pathological certificates can consuming excessive amounts of
    54  	// CPU time to verify.
    55  	TooManyConstraints
    56  	// CANotAuthorizedForExtKeyUsage results when an intermediate or root
    57  	// certificate does not permit an extended key usage that is claimed by
    58  	// the leaf certificate.
    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"
    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: " + e.Detail
    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  	var valid string
   105  	if ip := net.ParseIP(h.Host); ip != nil {
   106  		// Trying to validate an IP
   107  		if len(c.IPAddresses) == 0 {
   108  			return "x509: cannot validate certificate for " + h.Host + " because it doesn't contain any IP SANs"
   109  		}
   110  		for _, san := range c.IPAddresses {
   111  			if len(valid) > 0 {
   112  				valid += ", "
   113  			}
   114  			valid += san.String()
   115  		}
   116  	} else {
   117  		if c.hasSANExtension() {
   118  			valid = strings.Join(c.DNSNames, ", ")
   119  		} else {
   120  			valid = c.Subject.CommonName
   121  		}
   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  // errNotParsed is returned when a certificate without ASN.1 contents is
   171  // verified. Platform-specific verification needs the ASN.1 contents.
   172  var errNotParsed = errors.New("x509: missing ASN.1 contents; use ParseCertificate")
   173  
   174  // VerifyOptions contains parameters for Certificate.Verify. It's a structure
   175  // because other PKIX verification APIs have ended up needing many options.
   176  type VerifyOptions struct {
   177  	DNSName       string
   178  	Intermediates *CertPool
   179  	Roots         *CertPool // if nil, the system roots are used
   180  	CurrentTime   time.Time // if zero, the current time is used
   181  	// KeyUsage specifies which Extended Key Usage values are acceptable.
   182  	// An empty list means ExtKeyUsageServerAuth. Key usage is considered a
   183  	// constraint down the chain which mirrors Windows CryptoAPI behavior,
   184  	// but not the spec. To accept any key usage, include ExtKeyUsageAny.
   185  	KeyUsages []ExtKeyUsage
   186  	// MaxConstraintComparisions is the maximum number of comparisons to
   187  	// perform when checking a given certificate's name constraints. If
   188  	// zero, a sensible default is used. This limit prevents pathalogical
   189  	// certificates from consuming excessive amounts of CPU time when
   190  	// validating.
   191  	MaxConstraintComparisions int
   192  }
   193  
   194  const (
   195  	leafCertificate = iota
   196  	intermediateCertificate
   197  	rootCertificate
   198  )
   199  
   200  // rfc2821Mailbox represents a “mailbox” (which is an email address to most
   201  // people) by breaking it into the “local” (i.e. before the '@') and “domain”
   202  // parts.
   203  type rfc2821Mailbox struct {
   204  	local, domain string
   205  }
   206  
   207  // parseRFC2821Mailbox parses an email address into local and domain parts,
   208  // based on the ABNF for a “Mailbox” from RFC 2821. According to
   209  // https://tools.ietf.org/html/rfc5280#section-4.2.1.6 that's correct for an
   210  // rfc822Name from a certificate: “The format of an rfc822Name is a "Mailbox"
   211  // as defined in https://tools.ietf.org/html/rfc2821#section-4.1.2”.
   212  func parseRFC2821Mailbox(in string) (mailbox rfc2821Mailbox, ok bool) {
   213  	if len(in) == 0 {
   214  		return mailbox, false
   215  	}
   216  
   217  	localPartBytes := make([]byte, 0, len(in)/2)
   218  
   219  	if in[0] == '"' {
   220  		// Quoted-string = DQUOTE *qcontent DQUOTE
   221  		// non-whitespace-control = %d1-8 / %d11 / %d12 / %d14-31 / %d127
   222  		// qcontent = qtext / quoted-pair
   223  		// qtext = non-whitespace-control /
   224  		//         %d33 / %d35-91 / %d93-126
   225  		// quoted-pair = ("\" text) / obs-qp
   226  		// text = %d1-9 / %d11 / %d12 / %d14-127 / obs-text
   227  		//
   228  		// (Names beginning with “obs-” are the obsolete syntax from
   229  		// https://tools.ietf.org/html/rfc2822#section-4. Since it has
   230  		// been 16 years, we no longer accept that.)
   231  		in = in[1:]
   232  	QuotedString:
   233  		for {
   234  			if len(in) == 0 {
   235  				return mailbox, false
   236  			}
   237  			c := in[0]
   238  			in = in[1:]
   239  
   240  			switch {
   241  			case c == '"':
   242  				break QuotedString
   243  
   244  			case c == '\\':
   245  				// quoted-pair
   246  				if len(in) == 0 {
   247  					return mailbox, false
   248  				}
   249  				if in[0] == 11 ||
   250  					in[0] == 12 ||
   251  					(1 <= in[0] && in[0] <= 9) ||
   252  					(14 <= in[0] && in[0] <= 127) {
   253  					localPartBytes = append(localPartBytes, in[0])
   254  					in = in[1:]
   255  				} else {
   256  					return mailbox, false
   257  				}
   258  
   259  			case c == 11 ||
   260  				c == 12 ||
   261  				// Space (char 32) is not allowed based on the
   262  				// BNF, but RFC 3696 gives an example that
   263  				// assumes that it is. Several “verified”
   264  				// errata continue to argue about this point.
   265  				// We choose to accept it.
   266  				c == 32 ||
   267  				c == 33 ||
   268  				c == 127 ||
   269  				(1 <= c && c <= 8) ||
   270  				(14 <= c && c <= 31) ||
   271  				(35 <= c && c <= 91) ||
   272  				(93 <= c && c <= 126):
   273  				// qtext
   274  				localPartBytes = append(localPartBytes, c)
   275  
   276  			default:
   277  				return mailbox, false
   278  			}
   279  		}
   280  	} else {
   281  		// Atom ("." Atom)*
   282  	NextChar:
   283  		for len(in) > 0 {
   284  			// atext from https://tools.ietf.org/html/rfc2822#section-3.2.4
   285  			c := in[0]
   286  
   287  			switch {
   288  			case c == '\\':
   289  				// Examples given in RFC 3696 suggest that
   290  				// escaped characters can appear outside of a
   291  				// quoted string. Several “verified” errata
   292  				// continue to argue the point. We choose to
   293  				// accept it.
   294  				in = in[1:]
   295  				if len(in) == 0 {
   296  					return mailbox, false
   297  				}
   298  				fallthrough
   299  
   300  			case ('0' <= c && c <= '9') ||
   301  				('a' <= c && c <= 'z') ||
   302  				('A' <= c && c <= 'Z') ||
   303  				c == '!' || c == '#' || c == '$' || c == '%' ||
   304  				c == '&' || c == '\'' || c == '*' || c == '+' ||
   305  				c == '-' || c == '/' || c == '=' || c == '?' ||
   306  				c == '^' || c == '_' || c == '`' || c == '{' ||
   307  				c == '|' || c == '}' || c == '~' || c == '.':
   308  				localPartBytes = append(localPartBytes, in[0])
   309  				in = in[1:]
   310  
   311  			default:
   312  				break NextChar
   313  			}
   314  		}
   315  
   316  		if len(localPartBytes) == 0 {
   317  			return mailbox, false
   318  		}
   319  
   320  		// https://tools.ietf.org/html/rfc3696#section-3
   321  		// “period (".") may also appear, but may not be used to start
   322  		// or end the local part, nor may two or more consecutive
   323  		// periods appear.”
   324  		twoDots := []byte{'.', '.'}
   325  		if localPartBytes[0] == '.' ||
   326  			localPartBytes[len(localPartBytes)-1] == '.' ||
   327  			bytes.Contains(localPartBytes, twoDots) {
   328  			return mailbox, false
   329  		}
   330  	}
   331  
   332  	if len(in) == 0 || in[0] != '@' {
   333  		return mailbox, false
   334  	}
   335  	in = in[1:]
   336  
   337  	// The RFC species a format for domains, but that's known to be
   338  	// violated in practice so we accept that anything after an '@' is the
   339  	// domain part.
   340  	if _, ok := domainToReverseLabels(in); !ok {
   341  		return mailbox, false
   342  	}
   343  
   344  	mailbox.local = string(localPartBytes)
   345  	mailbox.domain = in
   346  	return mailbox, true
   347  }
   348  
   349  // domainToReverseLabels converts a textual domain name like foo.example.com to
   350  // the list of labels in reverse order, e.g. ["com", "example", "foo"].
   351  func domainToReverseLabels(domain string) (reverseLabels []string, ok bool) {
   352  	for len(domain) > 0 {
   353  		if i := strings.LastIndexByte(domain, '.'); i == -1 {
   354  			reverseLabels = append(reverseLabels, domain)
   355  			domain = ""
   356  		} else {
   357  			reverseLabels = append(reverseLabels, domain[i+1:len(domain)])
   358  			domain = domain[:i]
   359  		}
   360  	}
   361  
   362  	if len(reverseLabels) > 0 && len(reverseLabels[0]) == 0 {
   363  		// An empty label at the end indicates an absolute value.
   364  		return nil, false
   365  	}
   366  
   367  	for _, label := range reverseLabels {
   368  		if len(label) == 0 {
   369  			// Empty labels are otherwise invalid.
   370  			return nil, false
   371  		}
   372  
   373  		for _, c := range label {
   374  			if c < 33 || c > 126 {
   375  				// Invalid character.
   376  				return nil, false
   377  			}
   378  		}
   379  	}
   380  
   381  	return reverseLabels, true
   382  }
   383  
   384  func matchEmailConstraint(mailbox rfc2821Mailbox, constraint string) (bool, error) {
   385  	// If the constraint contains an @, then it specifies an exact mailbox
   386  	// name.
   387  	if strings.Contains(constraint, "@") {
   388  		constraintMailbox, ok := parseRFC2821Mailbox(constraint)
   389  		if !ok {
   390  			return false, fmt.Errorf("x509: internal error: cannot parse constraint %q", constraint)
   391  		}
   392  		return mailbox.local == constraintMailbox.local && strings.EqualFold(mailbox.domain, constraintMailbox.domain), nil
   393  	}
   394  
   395  	// Otherwise the constraint is like a DNS constraint of the domain part
   396  	// of the mailbox.
   397  	return matchDomainConstraint(mailbox.domain, constraint)
   398  }
   399  
   400  func matchURIConstraint(uri *url.URL, constraint string) (bool, error) {
   401  	// https://tools.ietf.org/html/rfc5280#section-4.2.1.10
   402  	// “a uniformResourceIdentifier that does not include an authority
   403  	// component with a host name specified as a fully qualified domain
   404  	// name (e.g., if the URI either does not include an authority
   405  	// component or includes an authority component in which the host name
   406  	// is specified as an IP address), then the application MUST reject the
   407  	// certificate.”
   408  
   409  	host := uri.Host
   410  	if len(host) == 0 {
   411  		return false, fmt.Errorf("URI with empty host (%q) cannot be matched against constraints", uri.String())
   412  	}
   413  
   414  	if strings.Contains(host, ":") && !strings.HasSuffix(host, "]") {
   415  		var err error
   416  		host, _, err = net.SplitHostPort(uri.Host)
   417  		if err != nil {
   418  			return false, err
   419  		}
   420  	}
   421  
   422  	if strings.HasPrefix(host, "[") && strings.HasSuffix(host, "]") ||
   423  		net.ParseIP(host) != nil {
   424  		return false, fmt.Errorf("URI with IP (%q) cannot be matched against constraints", uri.String())
   425  	}
   426  
   427  	return matchDomainConstraint(host, constraint)
   428  }
   429  
   430  func matchIPConstraint(ip net.IP, constraint *net.IPNet) (bool, error) {
   431  	if len(ip) != len(constraint.IP) {
   432  		return false, nil
   433  	}
   434  
   435  	for i := range ip {
   436  		if mask := constraint.Mask[i]; ip[i]&mask != constraint.IP[i]&mask {
   437  			return false, nil
   438  		}
   439  	}
   440  
   441  	return true, nil
   442  }
   443  
   444  func matchDomainConstraint(domain, constraint string) (bool, error) {
   445  	// The meaning of zero length constraints is not specified, but this
   446  	// code follows NSS and accepts them as matching everything.
   447  	if len(constraint) == 0 {
   448  		return true, nil
   449  	}
   450  
   451  	domainLabels, ok := domainToReverseLabels(domain)
   452  	if !ok {
   453  		return false, fmt.Errorf("x509: internal error: cannot parse domain %q", domain)
   454  	}
   455  
   456  	// RFC 5280 says that a leading period in a domain name means that at
   457  	// least one label must be prepended, but only for URI and email
   458  	// constraints, not DNS constraints. The code also supports that
   459  	// behaviour for DNS constraints.
   460  
   461  	mustHaveSubdomains := false
   462  	if constraint[0] == '.' {
   463  		mustHaveSubdomains = true
   464  		constraint = constraint[1:]
   465  	}
   466  
   467  	constraintLabels, ok := domainToReverseLabels(constraint)
   468  	if !ok {
   469  		return false, fmt.Errorf("x509: internal error: cannot parse domain %q", constraint)
   470  	}
   471  
   472  	if len(domainLabels) < len(constraintLabels) ||
   473  		(mustHaveSubdomains && len(domainLabels) == len(constraintLabels)) {
   474  		return false, nil
   475  	}
   476  
   477  	for i, constraintLabel := range constraintLabels {
   478  		if !strings.EqualFold(constraintLabel, domainLabels[i]) {
   479  			return false, nil
   480  		}
   481  	}
   482  
   483  	return true, nil
   484  }
   485  
   486  // checkNameConstraints checks that c permits a child certificate to claim the
   487  // given name, of type nameType. The argument parsedName contains the parsed
   488  // form of name, suitable for passing to the match function. The total number
   489  // of comparisons is tracked in the given count and should not exceed the given
   490  // limit.
   491  func (c *Certificate) checkNameConstraints(count *int,
   492  	maxConstraintComparisons int,
   493  	nameType string,
   494  	name string,
   495  	parsedName interface{},
   496  	match func(parsedName, constraint interface{}) (match bool, err error),
   497  	permitted, excluded interface{}) error {
   498  
   499  	excludedValue := reflect.ValueOf(excluded)
   500  
   501  	*count += excludedValue.Len()
   502  	if *count > maxConstraintComparisons {
   503  		return CertificateInvalidError{c, TooManyConstraints, ""}
   504  	}
   505  
   506  	for i := 0; i < excludedValue.Len(); i++ {
   507  		constraint := excludedValue.Index(i).Interface()
   508  		match, err := match(parsedName, constraint)
   509  		if err != nil {
   510  			return CertificateInvalidError{c, CANotAuthorizedForThisName, err.Error()}
   511  		}
   512  
   513  		if match {
   514  			return CertificateInvalidError{c, CANotAuthorizedForThisName, fmt.Sprintf("%s %q is excluded by constraint %q", nameType, name, constraint)}
   515  		}
   516  	}
   517  
   518  	permittedValue := reflect.ValueOf(permitted)
   519  
   520  	*count += permittedValue.Len()
   521  	if *count > maxConstraintComparisons {
   522  		return CertificateInvalidError{c, TooManyConstraints, ""}
   523  	}
   524  
   525  	ok := true
   526  	for i := 0; i < permittedValue.Len(); i++ {
   527  		constraint := permittedValue.Index(i).Interface()
   528  
   529  		var err error
   530  		if ok, err = match(parsedName, constraint); err != nil {
   531  			return CertificateInvalidError{c, CANotAuthorizedForThisName, err.Error()}
   532  		}
   533  
   534  		if ok {
   535  			break
   536  		}
   537  	}
   538  
   539  	if !ok {
   540  		return CertificateInvalidError{c, CANotAuthorizedForThisName, fmt.Sprintf("%s %q is not permitted by any constraint", nameType, name)}
   541  	}
   542  
   543  	return nil
   544  }
   545  
   546  // ekuPermittedBy returns true iff the given extended key usage is permitted by
   547  // the given EKU from a certificate. Normally, this would be a simple
   548  // comparison plus a special case for the “any” EKU. But, in order to support
   549  // existing certificates, some exceptions are made.
   550  func ekuPermittedBy(eku, certEKU ExtKeyUsage) bool {
   551  	if certEKU == ExtKeyUsageAny || eku == certEKU {
   552  		return true
   553  	}
   554  
   555  	// Some exceptions are made to support existing certificates. Firstly,
   556  	// the ServerAuth and SGC EKUs are treated as a group.
   557  	mapServerAuthEKUs := func(eku ExtKeyUsage) ExtKeyUsage {
   558  		if eku == ExtKeyUsageNetscapeServerGatedCrypto || eku == ExtKeyUsageMicrosoftServerGatedCrypto {
   559  			return ExtKeyUsageServerAuth
   560  		}
   561  		return eku
   562  	}
   563  
   564  	eku = mapServerAuthEKUs(eku)
   565  	certEKU = mapServerAuthEKUs(certEKU)
   566  
   567  	if eku == certEKU ||
   568  		// ServerAuth in a CA permits ClientAuth in the leaf.
   569  		(eku == ExtKeyUsageClientAuth && certEKU == ExtKeyUsageServerAuth) ||
   570  		// Any CA may issue an OCSP responder certificate.
   571  		eku == ExtKeyUsageOCSPSigning ||
   572  		// Code-signing CAs can use Microsoft's commercial and
   573  		// kernel-mode EKUs.
   574  		((eku == ExtKeyUsageMicrosoftCommercialCodeSigning || eku == ExtKeyUsageMicrosoftKernelCodeSigning) && certEKU == ExtKeyUsageCodeSigning) {
   575  		return true
   576  	}
   577  
   578  	return false
   579  }
   580  
   581  // isValid performs validity checks on c given that it is a candidate to append
   582  // to the chain in currentChain.
   583  func (c *Certificate) isValid(certType int, currentChain []*Certificate, opts *VerifyOptions) error {
   584  	if len(c.UnhandledCriticalExtensions) > 0 {
   585  		return UnhandledCriticalExtension{}
   586  	}
   587  
   588  	if len(currentChain) > 0 {
   589  		child := currentChain[len(currentChain)-1]
   590  		if !bytes.Equal(child.RawIssuer, c.RawSubject) {
   591  			return CertificateInvalidError{c, NameMismatch, ""}
   592  		}
   593  	}
   594  
   595  	now := opts.CurrentTime
   596  	if now.IsZero() {
   597  		now = time.Now()
   598  	}
   599  	if now.Before(c.NotBefore) || now.After(c.NotAfter) {
   600  		return CertificateInvalidError{c, Expired, ""}
   601  	}
   602  
   603  	maxConstraintComparisons := opts.MaxConstraintComparisions
   604  	if maxConstraintComparisons == 0 {
   605  		maxConstraintComparisons = 250000
   606  	}
   607  	comparisonCount := 0
   608  
   609  	var leaf *Certificate
   610  	if certType == intermediateCertificate || certType == rootCertificate {
   611  		if len(currentChain) == 0 {
   612  			return errors.New("x509: internal error: empty chain when appending CA cert")
   613  		}
   614  		leaf = currentChain[0]
   615  	}
   616  
   617  	if (certType == intermediateCertificate || certType == rootCertificate) && c.hasNameConstraints() {
   618  		sanExtension, ok := leaf.getSANExtension()
   619  		if !ok {
   620  			// This is the deprecated, legacy case of depending on
   621  			// the CN as a hostname. Chains modern enough to be
   622  			// using name constraints should not be depending on
   623  			// CNs.
   624  			return CertificateInvalidError{c, NameConstraintsWithoutSANs, ""}
   625  		}
   626  
   627  		err := forEachSAN(sanExtension, func(tag int, data []byte) error {
   628  			switch tag {
   629  			case nameTypeEmail:
   630  				name := string(data)
   631  				mailbox, ok := parseRFC2821Mailbox(name)
   632  				if !ok {
   633  					// This certificate should not have parsed.
   634  					return errors.New("x509: internal error: rfc822Name SAN failed to parse")
   635  				}
   636  
   637  				if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "email address", name, mailbox,
   638  					func(parsedName, constraint interface{}) (bool, error) {
   639  						return matchEmailConstraint(parsedName.(rfc2821Mailbox), constraint.(string))
   640  					}, c.PermittedEmailAddresses, c.ExcludedEmailAddresses); err != nil {
   641  					return err
   642  				}
   643  
   644  			case nameTypeDNS:
   645  				name := string(data)
   646  				if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "DNS name", name, name,
   647  					func(parsedName, constraint interface{}) (bool, error) {
   648  						return matchDomainConstraint(parsedName.(string), constraint.(string))
   649  					}, c.PermittedDNSDomains, c.ExcludedDNSDomains); err != nil {
   650  					return err
   651  				}
   652  
   653  			case nameTypeURI:
   654  				name := string(data)
   655  				uri, err := url.Parse(name)
   656  				if err != nil {
   657  					return fmt.Errorf("x509: internal error: URI SAN %q failed to parse", name)
   658  				}
   659  
   660  				if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "URI", name, uri,
   661  					func(parsedName, constraint interface{}) (bool, error) {
   662  						return matchURIConstraint(parsedName.(*url.URL), constraint.(string))
   663  					}, c.PermittedURIDomains, c.ExcludedURIDomains); err != nil {
   664  					return err
   665  				}
   666  
   667  			case nameTypeIP:
   668  				ip := net.IP(data)
   669  				if l := len(ip); l != net.IPv4len && l != net.IPv6len {
   670  					return fmt.Errorf("x509: internal error: IP SAN %x failed to parse", data)
   671  				}
   672  
   673  				if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "IP address", ip.String(), ip,
   674  					func(parsedName, constraint interface{}) (bool, error) {
   675  						return matchIPConstraint(parsedName.(net.IP), constraint.(*net.IPNet))
   676  					}, c.PermittedIPRanges, c.ExcludedIPRanges); err != nil {
   677  					return err
   678  				}
   679  
   680  			default:
   681  				// Unknown SAN types are ignored.
   682  			}
   683  
   684  			return nil
   685  		})
   686  
   687  		if err != nil {
   688  			return err
   689  		}
   690  	}
   691  
   692  	checkEKUs := certType == intermediateCertificate
   693  
   694  	// If no extended key usages are specified, then all are acceptable.
   695  	if checkEKUs && (len(c.ExtKeyUsage) == 0 && len(c.UnknownExtKeyUsage) == 0) {
   696  		checkEKUs = false
   697  	}
   698  
   699  	// If the “any” key usage is permitted, then no more checks are needed.
   700  	if checkEKUs {
   701  		for _, caEKU := range c.ExtKeyUsage {
   702  			comparisonCount++
   703  			if caEKU == ExtKeyUsageAny {
   704  				checkEKUs = false
   705  				break
   706  			}
   707  		}
   708  	}
   709  
   710  	if checkEKUs {
   711  	NextEKU:
   712  		for _, eku := range leaf.ExtKeyUsage {
   713  			if comparisonCount > maxConstraintComparisons {
   714  				return CertificateInvalidError{c, TooManyConstraints, ""}
   715  			}
   716  
   717  			for _, caEKU := range c.ExtKeyUsage {
   718  				comparisonCount++
   719  				if ekuPermittedBy(eku, caEKU) {
   720  					continue NextEKU
   721  				}
   722  			}
   723  
   724  			oid, _ := oidFromExtKeyUsage(eku)
   725  			return CertificateInvalidError{c, CANotAuthorizedForExtKeyUsage, fmt.Sprintf("EKU not permitted: %#v", oid)}
   726  		}
   727  
   728  	NextUnknownEKU:
   729  		for _, eku := range leaf.UnknownExtKeyUsage {
   730  			if comparisonCount > maxConstraintComparisons {
   731  				return CertificateInvalidError{c, TooManyConstraints, ""}
   732  			}
   733  
   734  			for _, caEKU := range c.UnknownExtKeyUsage {
   735  				comparisonCount++
   736  				if caEKU.Equal(eku) {
   737  					continue NextUnknownEKU
   738  				}
   739  			}
   740  
   741  			return CertificateInvalidError{c, CANotAuthorizedForExtKeyUsage, fmt.Sprintf("EKU not permitted: %#v", eku)}
   742  		}
   743  	}
   744  
   745  	// KeyUsage status flags are ignored. From Engineering Security, Peter
   746  	// Gutmann: A European government CA marked its signing certificates as
   747  	// being valid for encryption only, but no-one noticed. Another
   748  	// European CA marked its signature keys as not being valid for
   749  	// signatures. A different CA marked its own trusted root certificate
   750  	// as being invalid for certificate signing. Another national CA
   751  	// distributed a certificate to be used to encrypt data for the
   752  	// country’s tax authority that was marked as only being usable for
   753  	// digital signatures but not for encryption. Yet another CA reversed
   754  	// the order of the bit flags in the keyUsage due to confusion over
   755  	// encoding endianness, essentially setting a random keyUsage in
   756  	// certificates that it issued. Another CA created a self-invalidating
   757  	// certificate by adding a certificate policy statement stipulating
   758  	// that the certificate had to be used strictly as specified in the
   759  	// keyUsage, and a keyUsage containing a flag indicating that the RSA
   760  	// encryption key could only be used for Diffie-Hellman key agreement.
   761  
   762  	if certType == intermediateCertificate && (!c.BasicConstraintsValid || !c.IsCA) {
   763  		return CertificateInvalidError{c, NotAuthorizedToSign, ""}
   764  	}
   765  
   766  	if c.BasicConstraintsValid && c.MaxPathLen >= 0 {
   767  		numIntermediates := len(currentChain) - 1
   768  		if numIntermediates > c.MaxPathLen {
   769  			return CertificateInvalidError{c, TooManyIntermediates, ""}
   770  		}
   771  	}
   772  
   773  	return nil
   774  }
   775  
   776  // Verify attempts to verify c by building one or more chains from c to a
   777  // certificate in opts.Roots, using certificates in opts.Intermediates if
   778  // needed. If successful, it returns one or more chains where the first
   779  // element of the chain is c and the last element is from opts.Roots.
   780  //
   781  // If opts.Roots is nil and system roots are unavailable the returned error
   782  // will be of type SystemRootsError.
   783  //
   784  // Name constraints in the intermediates will be applied to all names claimed
   785  // in the chain, not just opts.DNSName. Thus it is invalid for a leaf to claim
   786  // example.com if an intermediate doesn't permit it, even if example.com is not
   787  // the name being validated. Note that DirectoryName constraints are not
   788  // supported.
   789  //
   790  // Extended Key Usage values are enforced down a chain, so an intermediate or
   791  // root that enumerates EKUs prevents a leaf from asserting an EKU not in that
   792  // list.
   793  //
   794  // WARNING: this function doesn't do any revocation checking.
   795  func (c *Certificate) Verify(opts VerifyOptions) (chains [][]*Certificate, err error) {
   796  	// Platform-specific verification needs the ASN.1 contents so
   797  	// this makes the behavior consistent across platforms.
   798  	if len(c.Raw) == 0 {
   799  		return nil, errNotParsed
   800  	}
   801  	if opts.Intermediates != nil {
   802  		for _, intermediate := range opts.Intermediates.certs {
   803  			if len(intermediate.Raw) == 0 {
   804  				return nil, errNotParsed
   805  			}
   806  		}
   807  	}
   808  
   809  	// Use Windows's own verification and chain building.
   810  	if opts.Roots == nil && runtime.GOOS == "windows" {
   811  		return c.systemVerify(&opts)
   812  	}
   813  
   814  	if opts.Roots == nil {
   815  		opts.Roots = systemRootsPool()
   816  		if opts.Roots == nil {
   817  			return nil, SystemRootsError{systemRootsErr}
   818  		}
   819  	}
   820  
   821  	err = c.isValid(leafCertificate, nil, &opts)
   822  	if err != nil {
   823  		return
   824  	}
   825  
   826  	if len(opts.DNSName) > 0 {
   827  		err = c.VerifyHostname(opts.DNSName)
   828  		if err != nil {
   829  			return
   830  		}
   831  	}
   832  
   833  	requestedKeyUsages := make([]ExtKeyUsage, len(opts.KeyUsages))
   834  	copy(requestedKeyUsages, opts.KeyUsages)
   835  	if len(requestedKeyUsages) == 0 {
   836  		requestedKeyUsages = append(requestedKeyUsages, ExtKeyUsageServerAuth)
   837  	}
   838  
   839  	// If no key usages are specified, then any are acceptable.
   840  	checkEKU := len(c.ExtKeyUsage) > 0
   841  
   842  	for _, eku := range requestedKeyUsages {
   843  		if eku == ExtKeyUsageAny {
   844  			checkEKU = false
   845  			break
   846  		}
   847  	}
   848  
   849  	if checkEKU {
   850  	NextUsage:
   851  		for _, eku := range requestedKeyUsages {
   852  			for _, leafEKU := range c.ExtKeyUsage {
   853  				if ekuPermittedBy(eku, leafEKU) {
   854  					continue NextUsage
   855  				}
   856  			}
   857  
   858  			oid, _ := oidFromExtKeyUsage(eku)
   859  			return nil, CertificateInvalidError{c, IncompatibleUsage, fmt.Sprintf("%#v", oid)}
   860  		}
   861  	}
   862  
   863  	var candidateChains [][]*Certificate
   864  	if opts.Roots.contains(c) {
   865  		candidateChains = append(candidateChains, []*Certificate{c})
   866  	} else {
   867  		if candidateChains, err = c.buildChains(make(map[int][][]*Certificate), []*Certificate{c}, &opts); err != nil {
   868  			return nil, err
   869  		}
   870  	}
   871  
   872  	return candidateChains, nil
   873  }
   874  
   875  func appendToFreshChain(chain []*Certificate, cert *Certificate) []*Certificate {
   876  	n := make([]*Certificate, len(chain)+1)
   877  	copy(n, chain)
   878  	n[len(chain)] = cert
   879  	return n
   880  }
   881  
   882  func (c *Certificate) buildChains(cache map[int][][]*Certificate, currentChain []*Certificate, opts *VerifyOptions) (chains [][]*Certificate, err error) {
   883  	possibleRoots, failedRoot, rootErr := opts.Roots.findVerifiedParents(c)
   884  nextRoot:
   885  	for _, rootNum := range possibleRoots {
   886  		root := opts.Roots.certs[rootNum]
   887  
   888  		for _, cert := range currentChain {
   889  			if cert.Equal(root) {
   890  				continue nextRoot
   891  			}
   892  		}
   893  
   894  		err = root.isValid(rootCertificate, currentChain, opts)
   895  		if err != nil {
   896  			continue
   897  		}
   898  		chains = append(chains, appendToFreshChain(currentChain, root))
   899  	}
   900  
   901  	possibleIntermediates, failedIntermediate, intermediateErr := opts.Intermediates.findVerifiedParents(c)
   902  nextIntermediate:
   903  	for _, intermediateNum := range possibleIntermediates {
   904  		intermediate := opts.Intermediates.certs[intermediateNum]
   905  		for _, cert := range currentChain {
   906  			if cert.Equal(intermediate) {
   907  				continue nextIntermediate
   908  			}
   909  		}
   910  		err = intermediate.isValid(intermediateCertificate, currentChain, opts)
   911  		if err != nil {
   912  			continue
   913  		}
   914  		var childChains [][]*Certificate
   915  		childChains, ok := cache[intermediateNum]
   916  		if !ok {
   917  			childChains, err = intermediate.buildChains(cache, appendToFreshChain(currentChain, intermediate), opts)
   918  			cache[intermediateNum] = childChains
   919  		}
   920  		chains = append(chains, childChains...)
   921  	}
   922  
   923  	if len(chains) > 0 {
   924  		err = nil
   925  	}
   926  
   927  	if len(chains) == 0 && err == nil {
   928  		hintErr := rootErr
   929  		hintCert := failedRoot
   930  		if hintErr == nil {
   931  			hintErr = intermediateErr
   932  			hintCert = failedIntermediate
   933  		}
   934  		err = UnknownAuthorityError{c, hintErr, hintCert}
   935  	}
   936  
   937  	return
   938  }
   939  
   940  func matchHostnames(pattern, host string) bool {
   941  	host = strings.TrimSuffix(host, ".")
   942  	pattern = strings.TrimSuffix(pattern, ".")
   943  
   944  	if len(pattern) == 0 || len(host) == 0 {
   945  		return false
   946  	}
   947  
   948  	patternParts := strings.Split(pattern, ".")
   949  	hostParts := strings.Split(host, ".")
   950  
   951  	if len(patternParts) != len(hostParts) {
   952  		return false
   953  	}
   954  
   955  	for i, patternPart := range patternParts {
   956  		if i == 0 && patternPart == "*" {
   957  			continue
   958  		}
   959  		if patternPart != hostParts[i] {
   960  			return false
   961  		}
   962  	}
   963  
   964  	return true
   965  }
   966  
   967  // toLowerCaseASCII returns a lower-case version of in. See RFC 6125 6.4.1. We use
   968  // an explicitly ASCII function to avoid any sharp corners resulting from
   969  // performing Unicode operations on DNS labels.
   970  func toLowerCaseASCII(in string) string {
   971  	// If the string is already lower-case then there's nothing to do.
   972  	isAlreadyLowerCase := true
   973  	for _, c := range in {
   974  		if c == utf8.RuneError {
   975  			// If we get a UTF-8 error then there might be
   976  			// upper-case ASCII bytes in the invalid sequence.
   977  			isAlreadyLowerCase = false
   978  			break
   979  		}
   980  		if 'A' <= c && c <= 'Z' {
   981  			isAlreadyLowerCase = false
   982  			break
   983  		}
   984  	}
   985  
   986  	if isAlreadyLowerCase {
   987  		return in
   988  	}
   989  
   990  	out := []byte(in)
   991  	for i, c := range out {
   992  		if 'A' <= c && c <= 'Z' {
   993  			out[i] += 'a' - 'A'
   994  		}
   995  	}
   996  	return string(out)
   997  }
   998  
   999  // VerifyHostname returns nil if c is a valid certificate for the named host.
  1000  // Otherwise it returns an error describing the mismatch.
  1001  func (c *Certificate) VerifyHostname(h string) error {
  1002  	// IP addresses may be written in [ ].
  1003  	candidateIP := h
  1004  	if len(h) >= 3 && h[0] == '[' && h[len(h)-1] == ']' {
  1005  		candidateIP = h[1 : len(h)-1]
  1006  	}
  1007  	if ip := net.ParseIP(candidateIP); ip != nil {
  1008  		// We only match IP addresses against IP SANs.
  1009  		// https://tools.ietf.org/html/rfc6125#appendix-B.2
  1010  		for _, candidate := range c.IPAddresses {
  1011  			if ip.Equal(candidate) {
  1012  				return nil
  1013  			}
  1014  		}
  1015  		return HostnameError{c, candidateIP}
  1016  	}
  1017  
  1018  	lowered := toLowerCaseASCII(h)
  1019  
  1020  	if c.hasSANExtension() {
  1021  		for _, match := range c.DNSNames {
  1022  			if matchHostnames(toLowerCaseASCII(match), lowered) {
  1023  				return nil
  1024  			}
  1025  		}
  1026  		// If Subject Alt Name is given, we ignore the common name.
  1027  	} else if matchHostnames(toLowerCaseASCII(c.Subject.CommonName), lowered) {
  1028  		return nil
  1029  	}
  1030  
  1031  	return HostnameError{c, h}
  1032  }
  1033  

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