crypto/x509: check EKUs like 1.9.

[gostls13.git] / src / crypto / 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 (
        "bytes"
        "encoding/asn1"
        "errors"
        "fmt"
        "net"
        "net/url"
        "reflect"
        "runtime"
        "strconv"
        "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 permit a DNS or
        // other name (including IP address) in the leaf certificate.
        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
        // NameConstraintsWithoutSANs results when a leaf certificate doesn't
        // contain a Subject Alternative Name extension, but a CA certificate
        // contains name constraints.
        NameConstraintsWithoutSANs
        // UnconstrainedName results when a CA certificate contains permitted
        // name constraints, but leaf certificate contains a name of an
        // unsupported or unconstrained type.
        UnconstrainedName
        // TooManyConstraints results when the number of comparison operations
        // needed to check a certificate exceeds the limit set by
        // VerifyOptions.MaxConstraintComparisions. This limit exists to
        // prevent pathological certificates can consuming excessive amounts of
        // CPU time to verify.
        TooManyConstraints
        // CANotAuthorizedForExtKeyUsage results when an intermediate or root
        // certificate does not permit a requested extended key usage.
        CANotAuthorizedForExtKeyUsage
)

// 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
        Detail string
}

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 for this name: " + e.Detail
        case CANotAuthorizedForExtKeyUsage:
                return "x509: a root or intermediate certificate is not authorized for an extended key usage: " + e.Detail
        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"
        case NameConstraintsWithoutSANs:
                return "x509: issuer has name constraints but leaf doesn't have a SAN extension"
        case UnconstrainedName:
                return "x509: issuer has name constraints but leaf contains unknown or unconstrained name: " + e.Detail
        }
        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. A leaf
        // certificate is accepted if it contains any of the listed values. An empty
        // list means ExtKeyUsageServerAuth. To accept any key usage, include
        // ExtKeyUsageAny.
        //
        // Certificate chains are required to nest these extended key usage values.
        // (This matches the Windows CryptoAPI behavior, but not the spec.)
        KeyUsages []ExtKeyUsage
        // MaxConstraintComparisions is the maximum number of comparisons to
        // perform when checking a given certificate's name constraints. If
        // zero, a sensible default is used. This limit prevents pathological
        // certificates from consuming excessive amounts of CPU time when
        // validating.
        MaxConstraintComparisions int
}

const (
        leafCertificate = iota
        intermediateCertificate
        rootCertificate
)

// rfc2821Mailbox represents a “mailbox” (which is an email address to most
// people) by breaking it into the “local” (i.e. before the '@') and “domain”
// parts.
type rfc2821Mailbox struct {
        local, domain string
}

// parseRFC2821Mailbox parses an email address into local and domain parts,
// based on the ABNF for a “Mailbox” from RFC 2821. According to
// https://tools.ietf.org/html/rfc5280#section-4.2.1.6 that's correct for an
// rfc822Name from a certificate: “The format of an rfc822Name is a "Mailbox"
// as defined in https://tools.ietf.org/html/rfc2821#section-4.1.2”.
func parseRFC2821Mailbox(in string) (mailbox rfc2821Mailbox, ok bool) {
        if len(in) == 0 {
                return mailbox, false
        }

        localPartBytes := make([]byte, 0, len(in)/2)

        if in[0] == '"' {
                // Quoted-string = DQUOTE *qcontent DQUOTE
                // non-whitespace-control = %d1-8 / %d11 / %d12 / %d14-31 / %d127
                // qcontent = qtext / quoted-pair
                // qtext = non-whitespace-control /
                //         %d33 / %d35-91 / %d93-126
                // quoted-pair = ("\" text) / obs-qp
                // text = %d1-9 / %d11 / %d12 / %d14-127 / obs-text
                //
                // (Names beginning with “obs-” are the obsolete syntax from
                // https://tools.ietf.org/html/rfc2822#section-4. Since it has
                // been 16 years, we no longer accept that.)
                in = in[1:]
        QuotedString:
                for {
                        if len(in) == 0 {
                                return mailbox, false
                        }
                        c := in[0]
                        in = in[1:]

                        switch {
                        case c == '"':
                                break QuotedString

                        case c == '\\':
                                // quoted-pair
                                if len(in) == 0 {
                                        return mailbox, false
                                }
                                if in[0] == 11 ||
                                        in[0] == 12 ||
                                        (1 <= in[0] && in[0] <= 9) ||
                                        (14 <= in[0] && in[0] <= 127) {
                                        localPartBytes = append(localPartBytes, in[0])
                                        in = in[1:]
                                } else {
                                        return mailbox, false
                                }

                        case c == 11 ||
                                c == 12 ||
                                // Space (char 32) is not allowed based on the
                                // BNF, but RFC 3696 gives an example that
                                // assumes that it is. Several “verified”
                                // errata continue to argue about this point.
                                // We choose to accept it.
                                c == 32 ||
                                c == 33 ||
                                c == 127 ||
                                (1 <= c && c <= 8) ||
                                (14 <= c && c <= 31) ||
                                (35 <= c && c <= 91) ||
                                (93 <= c && c <= 126):
                                // qtext
                                localPartBytes = append(localPartBytes, c)

                        default:
                                return mailbox, false
                        }
                }
        } else {
                // Atom ("." Atom)*
        NextChar:
                for len(in) > 0 {
                        // atext from https://tools.ietf.org/html/rfc2822#section-3.2.4
                        c := in[0]

                        switch {
                        case c == '\\':
                                // Examples given in RFC 3696 suggest that
                                // escaped characters can appear outside of a
                                // quoted string. Several “verified” errata
                                // continue to argue the point. We choose to
                                // accept it.
                                in = in[1:]
                                if len(in) == 0 {
                                        return mailbox, false
                                }
                                fallthrough

                        case ('0' <= c && c <= '9') ||
                                ('a' <= c && c <= 'z') ||
                                ('A' <= c && c <= 'Z') ||
                                c == '!' || c == '#' || c == '$' || c == '%' ||
                                c == '&' || c == '\'' || c == '*' || c == '+' ||
                                c == '-' || c == '/' || c == '=' || c == '?' ||
                                c == '^' || c == '_' || c == '`' || c == '{' ||
                                c == '|' || c == '}' || c == '~' || c == '.':
                                localPartBytes = append(localPartBytes, in[0])
                                in = in[1:]

                        default:
                                break NextChar
                        }
                }

                if len(localPartBytes) == 0 {
                        return mailbox, false
                }

                // https://tools.ietf.org/html/rfc3696#section-3
                // “period (".") may also appear, but may not be used to start
                // or end the local part, nor may two or more consecutive
                // periods appear.”
                twoDots := []byte{'.', '.'}
                if localPartBytes[0] == '.' ||
                        localPartBytes[len(localPartBytes)-1] == '.' ||
                        bytes.Contains(localPartBytes, twoDots) {
                        return mailbox, false
                }
        }

        if len(in) == 0 || in[0] != '@' {
                return mailbox, false
        }
        in = in[1:]

        // The RFC species a format for domains, but that's known to be
        // violated in practice so we accept that anything after an '@' is the
        // domain part.
        if _, ok := domainToReverseLabels(in); !ok {
                return mailbox, false
        }

        mailbox.local = string(localPartBytes)
        mailbox.domain = in
        return mailbox, true
}

// domainToReverseLabels converts a textual domain name like foo.example.com to
// the list of labels in reverse order, e.g. ["com", "example", "foo"].
func domainToReverseLabels(domain string) (reverseLabels []string, ok bool) {
        for len(domain) > 0 {
                if i := strings.LastIndexByte(domain, '.'); i == -1 {
                        reverseLabels = append(reverseLabels, domain)
                        domain = ""
                } else {
                        reverseLabels = append(reverseLabels, domain[i+1:len(domain)])
                        domain = domain[:i]
                }
        }

        if len(reverseLabels) > 0 && len(reverseLabels[0]) == 0 {
                // An empty label at the end indicates an absolute value.
                return nil, false
        }

        for _, label := range reverseLabels {
                if len(label) == 0 {
                        // Empty labels are otherwise invalid.
                        return nil, false
                }

                for _, c := range label {
                        if c < 33 || c > 126 {
                                // Invalid character.
                                return nil, false
                        }
                }
        }

        return reverseLabels, true
}

func matchEmailConstraint(mailbox rfc2821Mailbox, constraint string) (bool, error) {
        // If the constraint contains an @, then it specifies an exact mailbox
        // name.
        if strings.Contains(constraint, "@") {
                constraintMailbox, ok := parseRFC2821Mailbox(constraint)
                if !ok {
                        return false, fmt.Errorf("x509: internal error: cannot parse constraint %q", constraint)
                }
                return mailbox.local == constraintMailbox.local && strings.EqualFold(mailbox.domain, constraintMailbox.domain), nil
        }

        // Otherwise the constraint is like a DNS constraint of the domain part
        // of the mailbox.
        return matchDomainConstraint(mailbox.domain, constraint)
}

func matchURIConstraint(uri *url.URL, constraint string) (bool, error) {
        // https://tools.ietf.org/html/rfc5280#section-4.2.1.10
        // “a uniformResourceIdentifier that does not include an authority
        // component with a host name specified as a fully qualified domain
        // name (e.g., if the URI either does not include an authority
        // component or includes an authority component in which the host name
        // is specified as an IP address), then the application MUST reject the
        // certificate.”

        host := uri.Host
        if len(host) == 0 {
                return false, fmt.Errorf("URI with empty host (%q) cannot be matched against constraints", uri.String())
        }

        if strings.Contains(host, ":") && !strings.HasSuffix(host, "]") {
                var err error
                host, _, err = net.SplitHostPort(uri.Host)
                if err != nil {
                        return false, err
                }
        }

        if strings.HasPrefix(host, "[") && strings.HasSuffix(host, "]") ||
                net.ParseIP(host) != nil {
                return false, fmt.Errorf("URI with IP (%q) cannot be matched against constraints", uri.String())
        }

        return matchDomainConstraint(host, constraint)
}

func matchIPConstraint(ip net.IP, constraint *net.IPNet) (bool, error) {
        if len(ip) != len(constraint.IP) {
                return false, nil
        }

        for i := range ip {
                if mask := constraint.Mask[i]; ip[i]&mask != constraint.IP[i]&mask {
                        return false, nil
                }
        }

        return true, nil
}

func matchDomainConstraint(domain, constraint string) (bool, error) {
        // 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, nil
        }

        domainLabels, ok := domainToReverseLabels(domain)
        if !ok {
                return false, fmt.Errorf("x509: internal error: cannot parse domain %q", domain)
        }

        // RFC 5280 says that a leading period in a domain name means that at
        // least one label must be prepended, but only for URI and email
        // constraints, not DNS constraints. The code also supports that
        // behaviour for DNS constraints.

        mustHaveSubdomains := false
        if constraint[0] == '.' {
                mustHaveSubdomains = true
                constraint = constraint[1:]
        }

        constraintLabels, ok := domainToReverseLabels(constraint)
        if !ok {
                return false, fmt.Errorf("x509: internal error: cannot parse domain %q", constraint)
        }

        if len(domainLabels) < len(constraintLabels) ||
                (mustHaveSubdomains && len(domainLabels) == len(constraintLabels)) {
                return false, nil
        }

        for i, constraintLabel := range constraintLabels {
                if !strings.EqualFold(constraintLabel, domainLabels[i]) {
                        return false, nil
                }
        }

        return true, nil
}

// checkNameConstraints checks that c permits a child certificate to claim the
// given name, of type nameType. The argument parsedName contains the parsed
// form of name, suitable for passing to the match function. The total number
// of comparisons is tracked in the given count and should not exceed the given
// limit.
func (c *Certificate) checkNameConstraints(count *int,
        maxConstraintComparisons int,
        nameType string,
        name string,
        parsedName interface{},
        match func(parsedName, constraint interface{}) (match bool, err error),
        permitted, excluded interface{}) error {

        excludedValue := reflect.ValueOf(excluded)

        *count += excludedValue.Len()
        if *count > maxConstraintComparisons {
                return CertificateInvalidError{c, TooManyConstraints, ""}
        }

        for i := 0; i < excludedValue.Len(); i++ {
                constraint := excludedValue.Index(i).Interface()
                match, err := match(parsedName, constraint)
                if err != nil {
                        return CertificateInvalidError{c, CANotAuthorizedForThisName, err.Error()}
                }

                if match {
                        return CertificateInvalidError{c, CANotAuthorizedForThisName, fmt.Sprintf("%s %q is excluded by constraint %q", nameType, name, constraint)}
                }
        }

        permittedValue := reflect.ValueOf(permitted)

        *count += permittedValue.Len()
        if *count > maxConstraintComparisons {
                return CertificateInvalidError{c, TooManyConstraints, ""}
        }

        ok := true
        for i := 0; i < permittedValue.Len(); i++ {
                constraint := permittedValue.Index(i).Interface()

                var err error
                if ok, err = match(parsedName, constraint); err != nil {
                        return CertificateInvalidError{c, CANotAuthorizedForThisName, err.Error()}
                }

                if ok {
                        break
                }
        }

        if !ok {
                return CertificateInvalidError{c, CANotAuthorizedForThisName, fmt.Sprintf("%s %q is not permitted by any constraint", nameType, name)}
        }

        return nil
}

// isValid performs validity checks on c given that it is a candidate to append
// to the chain in currentChain.
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, ""}
        }

        maxConstraintComparisons := opts.MaxConstraintComparisions
        if maxConstraintComparisons == 0 {
                maxConstraintComparisons = 250000
        }
        comparisonCount := 0

        var leaf *Certificate
        if certType == intermediateCertificate || certType == rootCertificate {
                if len(currentChain) == 0 {
                        return errors.New("x509: internal error: empty chain when appending CA cert")
                }
                leaf = currentChain[0]
        }

        if (certType == intermediateCertificate || certType == rootCertificate) && c.hasNameConstraints() {
                sanExtension, ok := leaf.getSANExtension()
                if !ok {
                        // This is the deprecated, legacy case of depending on
                        // the CN as a hostname. Chains modern enough to be
                        // using name constraints should not be depending on
                        // CNs.
                        return CertificateInvalidError{c, NameConstraintsWithoutSANs, ""}
                }

                err := forEachSAN(sanExtension, func(tag int, data []byte) error {
                        switch tag {
                        case nameTypeEmail:
                                name := string(data)
                                mailbox, ok := parseRFC2821Mailbox(name)
                                if !ok {
                                        return fmt.Errorf("x509: cannot parse rfc822Name %q", mailbox)
                                }

                                if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "email address", name, mailbox,
                                        func(parsedName, constraint interface{}) (bool, error) {
                                                return matchEmailConstraint(parsedName.(rfc2821Mailbox), constraint.(string))
                                        }, c.PermittedEmailAddresses, c.ExcludedEmailAddresses); err != nil {
                                        return err
                                }

                        case nameTypeDNS:
                                name := string(data)
                                if _, ok := domainToReverseLabels(name); !ok {
                                        return fmt.Errorf("x509: cannot parse dnsName %q", name)
                                }

                                if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "DNS name", name, name,
                                        func(parsedName, constraint interface{}) (bool, error) {
                                                return matchDomainConstraint(parsedName.(string), constraint.(string))
                                        }, c.PermittedDNSDomains, c.ExcludedDNSDomains); err != nil {
                                        return err
                                }

                        case nameTypeURI:
                                name := string(data)
                                uri, err := url.Parse(name)
                                if err != nil {
                                        return fmt.Errorf("x509: internal error: URI SAN %q failed to parse", name)
                                }

                                if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "URI", name, uri,
                                        func(parsedName, constraint interface{}) (bool, error) {
                                                return matchURIConstraint(parsedName.(*url.URL), constraint.(string))
                                        }, c.PermittedURIDomains, c.ExcludedURIDomains); err != nil {
                                        return err
                                }

                        case nameTypeIP:
                                ip := net.IP(data)
                                if l := len(ip); l != net.IPv4len && l != net.IPv6len {
                                        return fmt.Errorf("x509: internal error: IP SAN %x failed to parse", data)
                                }

                                if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "IP address", ip.String(), ip,
                                        func(parsedName, constraint interface{}) (bool, error) {
                                                return matchIPConstraint(parsedName.(net.IP), constraint.(*net.IPNet))
                                        }, c.PermittedIPRanges, c.ExcludedIPRanges); err != nil {
                                        return err
                                }

                        default:
                                // Unknown SAN types are ignored.
                        }

                        return nil
                })

                if err != nil {
                        return err
                }
        }

        // 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
}

// formatOID formats an ASN.1 OBJECT IDENTIFER in the common, dotted style.
func formatOID(oid asn1.ObjectIdentifier) string {
        ret := ""
        for i, v := range oid {
                if i > 0 {
                        ret += "."
                }
                ret += strconv.Itoa(v)
        }
        return ret
}

// 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.
//
// Name constraints in the intermediates will be applied to all names claimed
// in the chain, not just opts.DNSName. Thus it is invalid for a leaf to claim
// example.com if an intermediate doesn't permit it, even if example.com is not
// the name being validated. Note that DirectoryName constraints are not
// supported.
//
// Extended Key Usage values are enforced down a chain, so an intermediate or
// root that enumerates EKUs prevents a leaf from asserting an EKU not in that
// list.
//
// WARNING: this function 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 {
                        return candidateChains, nil
                }
        }

        for _, candidate := range candidateChains {
                if checkChainForKeyUsage(candidate, keyUsages) {
                        chains = append(chains, candidate)
                }
        }

        if len(chains) == 0 {
                return nil, CertificateInvalidError{c, IncompatibleUsage, ""}
        }

        return chains, nil
}

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
}