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Source file src/crypto/x509/verify.go

Documentation: crypto/x509

  // Copyright 2011 The Go Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style
  // license that can be found in the LICENSE file.
  
  package x509
  
  import (
  	"bytes"
  	"errors"
  	"fmt"
  	"net"
  	"runtime"
  	"strings"
  	"time"
  	"unicode/utf8"
  )
  
  type InvalidReason int
  
  const (
  	// NotAuthorizedToSign results when a certificate is signed by another
  	// which isn't marked as a CA certificate.
  	NotAuthorizedToSign InvalidReason = iota
  	// Expired results when a certificate has expired, based on the time
  	// given in the VerifyOptions.
  	Expired
  	// CANotAuthorizedForThisName results when an intermediate or root
  	// certificate has a name constraint which doesn't include the name
  	// being checked.
  	CANotAuthorizedForThisName
  	// TooManyIntermediates results when a path length constraint is
  	// violated.
  	TooManyIntermediates
  	// IncompatibleUsage results when the certificate's key usage indicates
  	// that it may only be used for a different purpose.
  	IncompatibleUsage
  	// NameMismatch results when the subject name of a parent certificate
  	// does not match the issuer name in the child.
  	NameMismatch
  )
  
  // CertificateInvalidError results when an odd error occurs. Users of this
  // library probably want to handle all these errors uniformly.
  type CertificateInvalidError struct {
  	Cert   *Certificate
  	Reason InvalidReason
  }
  
  func (e CertificateInvalidError) Error() string {
  	switch e.Reason {
  	case NotAuthorizedToSign:
  		return "x509: certificate is not authorized to sign other certificates"
  	case Expired:
  		return "x509: certificate has expired or is not yet valid"
  	case CANotAuthorizedForThisName:
  		return "x509: a root or intermediate certificate is not authorized to sign in this domain"
  	case TooManyIntermediates:
  		return "x509: too many intermediates for path length constraint"
  	case IncompatibleUsage:
  		return "x509: certificate specifies an incompatible key usage"
  	case NameMismatch:
  		return "x509: issuer name does not match subject from issuing certificate"
  	}
  	return "x509: unknown error"
  }
  
  // HostnameError results when the set of authorized names doesn't match the
  // requested name.
  type HostnameError struct {
  	Certificate *Certificate
  	Host        string
  }
  
  func (h HostnameError) Error() string {
  	c := h.Certificate
  
  	var valid string
  	if ip := net.ParseIP(h.Host); ip != nil {
  		// Trying to validate an IP
  		if len(c.IPAddresses) == 0 {
  			return "x509: cannot validate certificate for " + h.Host + " because it doesn't contain any IP SANs"
  		}
  		for _, san := range c.IPAddresses {
  			if len(valid) > 0 {
  				valid += ", "
  			}
  			valid += san.String()
  		}
  	} else {
  		if c.hasSANExtension() {
  			valid = strings.Join(c.DNSNames, ", ")
  		} else {
  			valid = c.Subject.CommonName
  		}
  	}
  
  	if len(valid) == 0 {
  		return "x509: certificate is not valid for any names, but wanted to match " + h.Host
  	}
  	return "x509: certificate is valid for " + valid + ", not " + h.Host
  }
  
  // UnknownAuthorityError results when the certificate issuer is unknown
  type UnknownAuthorityError struct {
  	Cert *Certificate
  	// hintErr contains an error that may be helpful in determining why an
  	// authority wasn't found.
  	hintErr error
  	// hintCert contains a possible authority certificate that was rejected
  	// because of the error in hintErr.
  	hintCert *Certificate
  }
  
  func (e UnknownAuthorityError) Error() string {
  	s := "x509: certificate signed by unknown authority"
  	if e.hintErr != nil {
  		certName := e.hintCert.Subject.CommonName
  		if len(certName) == 0 {
  			if len(e.hintCert.Subject.Organization) > 0 {
  				certName = e.hintCert.Subject.Organization[0]
  			} else {
  				certName = "serial:" + e.hintCert.SerialNumber.String()
  			}
  		}
  		s += fmt.Sprintf(" (possibly because of %q while trying to verify candidate authority certificate %q)", e.hintErr, certName)
  	}
  	return s
  }
  
  // SystemRootsError results when we fail to load the system root certificates.
  type SystemRootsError struct {
  	Err error
  }
  
  func (se SystemRootsError) Error() string {
  	msg := "x509: failed to load system roots and no roots provided"
  	if se.Err != nil {
  		return msg + "; " + se.Err.Error()
  	}
  	return msg
  }
  
  // errNotParsed is returned when a certificate without ASN.1 contents is
  // verified. Platform-specific verification needs the ASN.1 contents.
  var errNotParsed = errors.New("x509: missing ASN.1 contents; use ParseCertificate")
  
  // VerifyOptions contains parameters for Certificate.Verify. It's a structure
  // because other PKIX verification APIs have ended up needing many options.
  type VerifyOptions struct {
  	DNSName       string
  	Intermediates *CertPool
  	Roots         *CertPool // if nil, the system roots are used
  	CurrentTime   time.Time // if zero, the current time is used
  	// KeyUsage specifies which Extended Key Usage values are acceptable.
  	// An empty list means ExtKeyUsageServerAuth. Key usage is considered a
  	// constraint down the chain which mirrors Windows CryptoAPI behavior,
  	// but not the spec. To accept any key usage, include ExtKeyUsageAny.
  	KeyUsages []ExtKeyUsage
  }
  
  const (
  	leafCertificate = iota
  	intermediateCertificate
  	rootCertificate
  )
  
  func matchNameConstraint(domain, constraint string) bool {
  	// The meaning of zero length constraints is not specified, but this
  	// code follows NSS and accepts them as matching everything.
  	if len(constraint) == 0 {
  		return true
  	}
  
  	if len(domain) < len(constraint) {
  		return false
  	}
  
  	prefixLen := len(domain) - len(constraint)
  	if !strings.EqualFold(domain[prefixLen:], constraint) {
  		return false
  	}
  
  	if prefixLen == 0 {
  		return true
  	}
  
  	isSubdomain := domain[prefixLen-1] == '.'
  	constraintHasLeadingDot := constraint[0] == '.'
  	return isSubdomain != constraintHasLeadingDot
  }
  
  // isValid performs validity checks on the c.
  func (c *Certificate) isValid(certType int, currentChain []*Certificate, opts *VerifyOptions) error {
  	if len(c.UnhandledCriticalExtensions) > 0 {
  		return UnhandledCriticalExtension{}
  	}
  
  	if len(currentChain) > 0 {
  		child := currentChain[len(currentChain)-1]
  		if !bytes.Equal(child.RawIssuer, c.RawSubject) {
  			return CertificateInvalidError{c, NameMismatch}
  		}
  	}
  
  	now := opts.CurrentTime
  	if now.IsZero() {
  		now = time.Now()
  	}
  	if now.Before(c.NotBefore) || now.After(c.NotAfter) {
  		return CertificateInvalidError{c, Expired}
  	}
  
  	if len(c.PermittedDNSDomains) > 0 {
  		ok := false
  		for _, constraint := range c.PermittedDNSDomains {
  			ok = matchNameConstraint(opts.DNSName, constraint)
  			if ok {
  				break
  			}
  		}
  
  		if !ok {
  			return CertificateInvalidError{c, CANotAuthorizedForThisName}
  		}
  	}
  
  	for _, constraint := range c.ExcludedDNSDomains {
  		if matchNameConstraint(opts.DNSName, constraint) {
  			return CertificateInvalidError{c, CANotAuthorizedForThisName}
  		}
  	}
  
  	// KeyUsage status flags are ignored. From Engineering Security, Peter
  	// Gutmann: A European government CA marked its signing certificates as
  	// being valid for encryption only, but no-one noticed. Another
  	// European CA marked its signature keys as not being valid for
  	// signatures. A different CA marked its own trusted root certificate
  	// as being invalid for certificate signing. Another national CA
  	// distributed a certificate to be used to encrypt data for the
  	// country’s tax authority that was marked as only being usable for
  	// digital signatures but not for encryption. Yet another CA reversed
  	// the order of the bit flags in the keyUsage due to confusion over
  	// encoding endianness, essentially setting a random keyUsage in
  	// certificates that it issued. Another CA created a self-invalidating
  	// certificate by adding a certificate policy statement stipulating
  	// that the certificate had to be used strictly as specified in the
  	// keyUsage, and a keyUsage containing a flag indicating that the RSA
  	// encryption key could only be used for Diffie-Hellman key agreement.
  
  	if certType == intermediateCertificate && (!c.BasicConstraintsValid || !c.IsCA) {
  		return CertificateInvalidError{c, NotAuthorizedToSign}
  	}
  
  	if c.BasicConstraintsValid && c.MaxPathLen >= 0 {
  		numIntermediates := len(currentChain) - 1
  		if numIntermediates > c.MaxPathLen {
  			return CertificateInvalidError{c, TooManyIntermediates}
  		}
  	}
  
  	return nil
  }
  
  // Verify attempts to verify c by building one or more chains from c to a
  // certificate in opts.Roots, using certificates in opts.Intermediates if
  // needed. If successful, it returns one or more chains where the first
  // element of the chain is c and the last element is from opts.Roots.
  //
  // If opts.Roots is nil and system roots are unavailable the returned error
  // will be of type SystemRootsError.
  //
  // WARNING: this doesn't do any revocation checking.
  func (c *Certificate) Verify(opts VerifyOptions) (chains [][]*Certificate, err error) {
  	// Platform-specific verification needs the ASN.1 contents so
  	// this makes the behavior consistent across platforms.
  	if len(c.Raw) == 0 {
  		return nil, errNotParsed
  	}
  	if opts.Intermediates != nil {
  		for _, intermediate := range opts.Intermediates.certs {
  			if len(intermediate.Raw) == 0 {
  				return nil, errNotParsed
  			}
  		}
  	}
  
  	// Use Windows's own verification and chain building.
  	if opts.Roots == nil && runtime.GOOS == "windows" {
  		return c.systemVerify(&opts)
  	}
  
  	if opts.Roots == nil {
  		opts.Roots = systemRootsPool()
  		if opts.Roots == nil {
  			return nil, SystemRootsError{systemRootsErr}
  		}
  	}
  
  	err = c.isValid(leafCertificate, nil, &opts)
  	if err != nil {
  		return
  	}
  
  	if len(opts.DNSName) > 0 {
  		err = c.VerifyHostname(opts.DNSName)
  		if err != nil {
  			return
  		}
  	}
  
  	var candidateChains [][]*Certificate
  	if opts.Roots.contains(c) {
  		candidateChains = append(candidateChains, []*Certificate{c})
  	} else {
  		if candidateChains, err = c.buildChains(make(map[int][][]*Certificate), []*Certificate{c}, &opts); err != nil {
  			return nil, err
  		}
  	}
  
  	keyUsages := opts.KeyUsages
  	if len(keyUsages) == 0 {
  		keyUsages = []ExtKeyUsage{ExtKeyUsageServerAuth}
  	}
  
  	// If any key usage is acceptable then we're done.
  	for _, usage := range keyUsages {
  		if usage == ExtKeyUsageAny {
  			chains = candidateChains
  			return
  		}
  	}
  
  	for _, candidate := range candidateChains {
  		if checkChainForKeyUsage(candidate, keyUsages) {
  			chains = append(chains, candidate)
  		}
  	}
  
  	if len(chains) == 0 {
  		err = CertificateInvalidError{c, IncompatibleUsage}
  	}
  
  	return
  }
  
  func appendToFreshChain(chain []*Certificate, cert *Certificate) []*Certificate {
  	n := make([]*Certificate, len(chain)+1)
  	copy(n, chain)
  	n[len(chain)] = cert
  	return n
  }
  
  func (c *Certificate) buildChains(cache map[int][][]*Certificate, currentChain []*Certificate, opts *VerifyOptions) (chains [][]*Certificate, err error) {
  	possibleRoots, failedRoot, rootErr := opts.Roots.findVerifiedParents(c)
  nextRoot:
  	for _, rootNum := range possibleRoots {
  		root := opts.Roots.certs[rootNum]
  
  		for _, cert := range currentChain {
  			if cert.Equal(root) {
  				continue nextRoot
  			}
  		}
  
  		err = root.isValid(rootCertificate, currentChain, opts)
  		if err != nil {
  			continue
  		}
  		chains = append(chains, appendToFreshChain(currentChain, root))
  	}
  
  	possibleIntermediates, failedIntermediate, intermediateErr := opts.Intermediates.findVerifiedParents(c)
  nextIntermediate:
  	for _, intermediateNum := range possibleIntermediates {
  		intermediate := opts.Intermediates.certs[intermediateNum]
  		for _, cert := range currentChain {
  			if cert.Equal(intermediate) {
  				continue nextIntermediate
  			}
  		}
  		err = intermediate.isValid(intermediateCertificate, currentChain, opts)
  		if err != nil {
  			continue
  		}
  		var childChains [][]*Certificate
  		childChains, ok := cache[intermediateNum]
  		if !ok {
  			childChains, err = intermediate.buildChains(cache, appendToFreshChain(currentChain, intermediate), opts)
  			cache[intermediateNum] = childChains
  		}
  		chains = append(chains, childChains...)
  	}
  
  	if len(chains) > 0 {
  		err = nil
  	}
  
  	if len(chains) == 0 && err == nil {
  		hintErr := rootErr
  		hintCert := failedRoot
  		if hintErr == nil {
  			hintErr = intermediateErr
  			hintCert = failedIntermediate
  		}
  		err = UnknownAuthorityError{c, hintErr, hintCert}
  	}
  
  	return
  }
  
  func matchHostnames(pattern, host string) bool {
  	host = strings.TrimSuffix(host, ".")
  	pattern = strings.TrimSuffix(pattern, ".")
  
  	if len(pattern) == 0 || len(host) == 0 {
  		return false
  	}
  
  	patternParts := strings.Split(pattern, ".")
  	hostParts := strings.Split(host, ".")
  
  	if len(patternParts) != len(hostParts) {
  		return false
  	}
  
  	for i, patternPart := range patternParts {
  		if i == 0 && patternPart == "*" {
  			continue
  		}
  		if patternPart != hostParts[i] {
  			return false
  		}
  	}
  
  	return true
  }
  
  // toLowerCaseASCII returns a lower-case version of in. See RFC 6125 6.4.1. We use
  // an explicitly ASCII function to avoid any sharp corners resulting from
  // performing Unicode operations on DNS labels.
  func toLowerCaseASCII(in string) string {
  	// If the string is already lower-case then there's nothing to do.
  	isAlreadyLowerCase := true
  	for _, c := range in {
  		if c == utf8.RuneError {
  			// If we get a UTF-8 error then there might be
  			// upper-case ASCII bytes in the invalid sequence.
  			isAlreadyLowerCase = false
  			break
  		}
  		if 'A' <= c && c <= 'Z' {
  			isAlreadyLowerCase = false
  			break
  		}
  	}
  
  	if isAlreadyLowerCase {
  		return in
  	}
  
  	out := []byte(in)
  	for i, c := range out {
  		if 'A' <= c && c <= 'Z' {
  			out[i] += 'a' - 'A'
  		}
  	}
  	return string(out)
  }
  
  // VerifyHostname returns nil if c is a valid certificate for the named host.
  // Otherwise it returns an error describing the mismatch.
  func (c *Certificate) VerifyHostname(h string) error {
  	// IP addresses may be written in [ ].
  	candidateIP := h
  	if len(h) >= 3 && h[0] == '[' && h[len(h)-1] == ']' {
  		candidateIP = h[1 : len(h)-1]
  	}
  	if ip := net.ParseIP(candidateIP); ip != nil {
  		// We only match IP addresses against IP SANs.
  		// https://tools.ietf.org/html/rfc6125#appendix-B.2
  		for _, candidate := range c.IPAddresses {
  			if ip.Equal(candidate) {
  				return nil
  			}
  		}
  		return HostnameError{c, candidateIP}
  	}
  
  	lowered := toLowerCaseASCII(h)
  
  	if c.hasSANExtension() {
  		for _, match := range c.DNSNames {
  			if matchHostnames(toLowerCaseASCII(match), lowered) {
  				return nil
  			}
  		}
  		// If Subject Alt Name is given, we ignore the common name.
  	} else if matchHostnames(toLowerCaseASCII(c.Subject.CommonName), lowered) {
  		return nil
  	}
  
  	return HostnameError{c, h}
  }
  
  func checkChainForKeyUsage(chain []*Certificate, keyUsages []ExtKeyUsage) bool {
  	usages := make([]ExtKeyUsage, len(keyUsages))
  	copy(usages, keyUsages)
  
  	if len(chain) == 0 {
  		return false
  	}
  
  	usagesRemaining := len(usages)
  
  	// We walk down the list and cross out any usages that aren't supported
  	// by each certificate. If we cross out all the usages, then the chain
  	// is unacceptable.
  
  NextCert:
  	for i := len(chain) - 1; i >= 0; i-- {
  		cert := chain[i]
  		if len(cert.ExtKeyUsage) == 0 && len(cert.UnknownExtKeyUsage) == 0 {
  			// The certificate doesn't have any extended key usage specified.
  			continue
  		}
  
  		for _, usage := range cert.ExtKeyUsage {
  			if usage == ExtKeyUsageAny {
  				// The certificate is explicitly good for any usage.
  				continue NextCert
  			}
  		}
  
  		const invalidUsage ExtKeyUsage = -1
  
  	NextRequestedUsage:
  		for i, requestedUsage := range usages {
  			if requestedUsage == invalidUsage {
  				continue
  			}
  
  			for _, usage := range cert.ExtKeyUsage {
  				if requestedUsage == usage {
  					continue NextRequestedUsage
  				} else if requestedUsage == ExtKeyUsageServerAuth &&
  					(usage == ExtKeyUsageNetscapeServerGatedCrypto ||
  						usage == ExtKeyUsageMicrosoftServerGatedCrypto) {
  					// In order to support COMODO
  					// certificate chains, we have to
  					// accept Netscape or Microsoft SGC
  					// usages as equal to ServerAuth.
  					continue NextRequestedUsage
  				}
  			}
  
  			usages[i] = invalidUsage
  			usagesRemaining--
  			if usagesRemaining == 0 {
  				return false
  			}
  		}
  	}
  
  	return true
  }
  

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