certspotter/ct/x509/verify.go

// 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 (
	"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
)

// 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"
	}
	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 len(c.DNSNames) > 0 {
			valid = strings.Join(c.DNSNames, ", ")
		} else {
			valid = c.Subject.CommonName
		}
	}
	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]
			}
			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 {
}

func (e SystemRootsError) Error() string {
	return "x509: failed to load system roots and no roots provided"
}

// 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
	DisableTimeChecks bool
	// 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 behaviour,
	// but not the spec. To accept any key usage, include ExtKeyUsageAny.
	KeyUsages []ExtKeyUsage
}

const (
	leafCertificate = iota
	intermediateCertificate
	rootCertificate
)

// isValid performs validity checks on the c.
func (c *Certificate) isValid(certType int, currentChain []*Certificate, opts *VerifyOptions) error {
	if !opts.DisableTimeChecks {
		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 _, domain := range c.PermittedDNSDomains {
			if opts.DNSName == domain ||
				(strings.HasSuffix(opts.DNSName, domain) &&
					len(opts.DNSName) >= 1+len(domain) &&
					opts.DNSName[len(opts.DNSName)-len(domain)-1] == '.') {
				ok = true
				break
			}
		}

		if !ok {
			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.
//
// WARNING: this doesn't do any revocation checking.
func (c *Certificate) Verify(opts VerifyOptions) (chains [][]*Certificate, err error) {
	// 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{}
		}
	}

	err = c.isValid(leafCertificate, nil, &opts)
	if err != nil {
		return
	}

	if len(opts.DNSName) > 0 {
		err = c.VerifyHostname(opts.DNSName)
		if err != nil {
			return
		}
	}

	candidateChains, err := c.buildChains(make(map[int][][]*Certificate), []*Certificate{c}, &opts)
	if err != nil {
		return
	}

	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)
	for _, rootNum := range possibleRoots {
		root := opts.Roots.certs[rootNum]
		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 == 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 {
	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 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 len(c.DNSNames) > 0 {
		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.

	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
			}
		}

		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
}