Adding upstream version 1.20.14.upstream/1.20.14upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1179 insertions, 0 deletions

diff --git a/src/crypto/x509/verify.go b/src/crypto/x509/verify.go
new file mode 100644
index 0000000..0b01f8b
--- /dev/null
+++ b/src/crypto/x509/verify.go
@@ -0,0 +1,1179 @@
+// 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"
+	"crypto"
+	"crypto/x509/pkix"
+	"errors"
+	"fmt"
+	"net"
+	"net/url"
+	"reflect"
+	"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 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 is a legacy error and is no longer returned.
+	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: " + e.Detail
+	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
+
+	if !c.hasSANExtension() && matchHostnames(c.Subject.CommonName, h.Host) {
+		return "x509: certificate relies on legacy Common Name field, use SANs instead"
+	}
+
+	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 {
+		valid = strings.Join(c.DNSNames, ", ")
+	}
+
+	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
+}
+
+func (se SystemRootsError) Unwrap() error { return se.Err }
+
+// 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.
+type VerifyOptions struct {
+	// DNSName, if set, is checked against the leaf certificate with
+	// Certificate.VerifyHostname or the platform verifier.
+	DNSName string
+
+	// Intermediates is an optional pool of certificates that are not trust
+	// anchors, but can be used to form a chain from the leaf certificate to a
+	// root certificate.
+	Intermediates *CertPool
+	// Roots is the set of trusted root certificates the leaf certificate needs
+	// to chain up to. If nil, the system roots or the platform verifier are used.
+	Roots *CertPool
+
+	// CurrentTime is used to check the validity of all certificates in the
+	// chain. If zero, the current time is used.
+	CurrentTime time.Time
+
+	// KeyUsages specifies which Extended Key Usage values are acceptable. A
+	// chain is accepted if it allows any of the listed values. An empty list
+	// means ExtKeyUsageServerAuth. To accept any key usage, include ExtKeyUsageAny.
+	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. It does not apply to the platform verifier.
+	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 RFC 5280,
+// 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 RFC 2821, 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 RFC 2822,
+		// 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 RFC 2822, 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
+		}
+
+		// From RFC 3696, 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:])
+			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) {
+	// From RFC 5280, 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 any,
+	match func(parsedName, constraint any) (match bool, err error),
+	permitted, excluded any) 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) {
+		return CertificateInvalidError{
+			Cert:   c,
+			Reason: Expired,
+			Detail: fmt.Sprintf("current time %s is before %s", now.Format(time.RFC3339), c.NotBefore.Format(time.RFC3339)),
+		}
+	} else if now.After(c.NotAfter) {
+		return CertificateInvalidError{
+			Cert:   c,
+			Reason: Expired,
+			Detail: fmt.Sprintf("current time %s is after %s", now.Format(time.RFC3339), c.NotAfter.Format(time.RFC3339)),
+		}
+	}
+
+	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() {
+		toCheck := []*Certificate{}
+		if leaf.hasSANExtension() {
+			toCheck = append(toCheck, leaf)
+		}
+		if c.hasSANExtension() {
+			toCheck = append(toCheck, c)
+		}
+		for _, sanCert := range toCheck {
+			err := forEachSAN(sanCert.getSANExtension(), 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 any) (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 any) (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 any) (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 any) (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, ""}
+		}
+	}
+
+	if !boringAllowCert(c) {
+		// IncompatibleUsage is not quite right here,
+		// but it's also the "no chains found" error
+		// and is close enough.
+		return CertificateInvalidError{c, IncompatibleUsage, ""}
+	}
+
+	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, the platform verifier might be used, and
+// verification details might differ from what is described below. If 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.
+//
+// Name constraint validation follows the rules from RFC 5280, with the
+// addition that DNS name constraints may use the leading period format
+// defined for emails and URIs. When a constraint has a leading period
+// it indicates that at least one additional label must be prepended to
+// the constrained name to be considered valid.
+//
+// Extended Key Usage values are enforced nested down a chain, so an intermediate
+// or root that enumerates EKUs prevents a leaf from asserting an EKU not in that
+// list. (While this is not specified, it is common practice in order to limit
+// the types of certificates a CA can issue.)
+//
+// Certificates that use SHA1WithRSA and ECDSAWithSHA1 signatures are not supported,
+// and will not be used to build chains.
+//
+// Certificates other than c in the returned chains should not be modified.
+//
+// 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
+	}
+	for i := 0; i < opts.Intermediates.len(); i++ {
+		c, err := opts.Intermediates.cert(i)
+		if err != nil {
+			return nil, fmt.Errorf("crypto/x509: error fetching intermediate: %w", err)
+		}
+		if len(c.Raw) == 0 {
+			return nil, errNotParsed
+		}
+	}
+
+	// Use platform verifiers, where available, if Roots is from SystemCertPool.
+	if runtime.GOOS == "windows" || runtime.GOOS == "darwin" || runtime.GOOS == "ios" {
+		// Don't use the system verifier if the system pool was replaced with a non-system pool,
+		// i.e. if SetFallbackRoots was called with x509usefallbackroots=1.
+		systemPool := systemRootsPool()
+		if opts.Roots == nil && (systemPool == nil || systemPool.systemPool) {
+			return c.systemVerify(&opts)
+		}
+		if opts.Roots != nil && opts.Roots.systemPool {
+			platformChains, err := c.systemVerify(&opts)
+			// If the platform verifier succeeded, or there are no additional
+			// roots, return the platform verifier result. Otherwise, continue
+			// with the Go verifier.
+			if err == nil || opts.Roots.len() == 0 {
+				return platformChains, err
+			}
+		}
+	}
+
+	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 = [][]*Certificate{{c}}
+	} else {
+		candidateChains, err = c.buildChains([]*Certificate{c}, nil, &opts)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	if len(opts.KeyUsages) == 0 {
+		opts.KeyUsages = []ExtKeyUsage{ExtKeyUsageServerAuth}
+	}
+
+	for _, eku := range opts.KeyUsages {
+		if eku == ExtKeyUsageAny {
+			// If any key usage is acceptable, no need to check the chain for
+			// key usages.
+			return candidateChains, nil
+		}
+	}
+
+	chains = make([][]*Certificate, 0, len(candidateChains))
+	for _, candidate := range candidateChains {
+		if checkChainForKeyUsage(candidate, opts.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
+}
+
+// alreadyInChain checks whether a candidate certificate is present in a chain.
+// Rather than doing a direct byte for byte equivalency check, we check if the
+// subject, public key, and SAN, if present, are equal. This prevents loops that
+// are created by mutual cross-signatures, or other cross-signature bridge
+// oddities.
+func alreadyInChain(candidate *Certificate, chain []*Certificate) bool {
+	type pubKeyEqual interface {
+		Equal(crypto.PublicKey) bool
+	}
+
+	var candidateSAN *pkix.Extension
+	for _, ext := range candidate.Extensions {
+		if ext.Id.Equal(oidExtensionSubjectAltName) {
+			candidateSAN = &ext
+			break
+		}
+	}
+
+	for _, cert := range chain {
+		if !bytes.Equal(candidate.RawSubject, cert.RawSubject) {
+			continue
+		}
+		if !candidate.PublicKey.(pubKeyEqual).Equal(cert.PublicKey) {
+			continue
+		}
+		var certSAN *pkix.Extension
+		for _, ext := range cert.Extensions {
+			if ext.Id.Equal(oidExtensionSubjectAltName) {
+				certSAN = &ext
+				break
+			}
+		}
+		if candidateSAN == nil && certSAN == nil {
+			return true
+		} else if candidateSAN == nil || certSAN == nil {
+			return false
+		}
+		if bytes.Equal(candidateSAN.Value, certSAN.Value) {
+			return true
+		}
+	}
+	return false
+}
+
+// maxChainSignatureChecks is the maximum number of CheckSignatureFrom calls
+// that an invocation of buildChains will (transitively) make. Most chains are
+// less than 15 certificates long, so this leaves space for multiple chains and
+// for failed checks due to different intermediates having the same Subject.
+const maxChainSignatureChecks = 100
+
+func (c *Certificate) buildChains(currentChain []*Certificate, sigChecks *int, opts *VerifyOptions) (chains [][]*Certificate, err error) {
+	var (
+		hintErr  error
+		hintCert *Certificate
+	)
+
+	considerCandidate := func(certType int, candidate *Certificate) {
+		if alreadyInChain(candidate, currentChain) {
+			return
+		}
+
+		if sigChecks == nil {
+			sigChecks = new(int)
+		}
+		*sigChecks++
+		if *sigChecks > maxChainSignatureChecks {
+			err = errors.New("x509: signature check attempts limit reached while verifying certificate chain")
+			return
+		}
+
+		if err := c.CheckSignatureFrom(candidate); err != nil {
+			if hintErr == nil {
+				hintErr = err
+				hintCert = candidate
+			}
+			return
+		}
+
+		err = candidate.isValid(certType, currentChain, opts)
+		if err != nil {
+			if hintErr == nil {
+				hintErr = err
+				hintCert = candidate
+			}
+			return
+		}
+
+		switch certType {
+		case rootCertificate:
+			chains = append(chains, appendToFreshChain(currentChain, candidate))
+		case intermediateCertificate:
+			var childChains [][]*Certificate
+			childChains, err = candidate.buildChains(appendToFreshChain(currentChain, candidate), sigChecks, opts)
+			chains = append(chains, childChains...)
+		}
+	}
+
+	for _, root := range opts.Roots.findPotentialParents(c) {
+		considerCandidate(rootCertificate, root)
+	}
+	for _, intermediate := range opts.Intermediates.findPotentialParents(c) {
+		considerCandidate(intermediateCertificate, intermediate)
+	}
+
+	if len(chains) > 0 {
+		err = nil
+	}
+	if len(chains) == 0 && err == nil {
+		err = UnknownAuthorityError{c, hintErr, hintCert}
+	}
+
+	return
+}
+
+func validHostnamePattern(host string) bool { return validHostname(host, true) }
+func validHostnameInput(host string) bool   { return validHostname(host, false) }
+
+// validHostname reports whether host is a valid hostname that can be matched or
+// matched against according to RFC 6125 2.2, with some leniency to accommodate
+// legacy values.
+func validHostname(host string, isPattern bool) bool {
+	if !isPattern {
+		host = strings.TrimSuffix(host, ".")
+	}
+	if len(host) == 0 {
+		return false
+	}
+
+	for i, part := range strings.Split(host, ".") {
+		if part == "" {
+			// Empty label.
+			return false
+		}
+		if isPattern && i == 0 && part == "*" {
+			// Only allow full left-most wildcards, as those are the only ones
+			// we match, and matching literal '*' characters is probably never
+			// the expected behavior.
+			continue
+		}
+		for j, c := range part {
+			if 'a' <= c && c <= 'z' {
+				continue
+			}
+			if '0' <= c && c <= '9' {
+				continue
+			}
+			if 'A' <= c && c <= 'Z' {
+				continue
+			}
+			if c == '-' && j != 0 {
+				continue
+			}
+			if c == '_' {
+				// Not a valid character in hostnames, but commonly
+				// found in deployments outside the WebPKI.
+				continue
+			}
+			return false
+		}
+	}
+
+	return true
+}
+
+func matchExactly(hostA, hostB string) bool {
+	if hostA == "" || hostA == "." || hostB == "" || hostB == "." {
+		return false
+	}
+	return toLowerCaseASCII(hostA) == toLowerCaseASCII(hostB)
+}
+
+func matchHostnames(pattern, host string) bool {
+	pattern = toLowerCaseASCII(pattern)
+	host = toLowerCaseASCII(strings.TrimSuffix(host, "."))
+
+	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.
+//
+// IP addresses can be optionally enclosed in square brackets and are checked
+// against the IPAddresses field. Other names are checked case insensitively
+// against the DNSNames field. If the names are valid hostnames, the certificate
+// fields can have a wildcard as the left-most label.
+//
+// Note that the legacy Common Name field is ignored.
+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.
+		// See RFC 6125, Appendix B.2.
+		for _, candidate := range c.IPAddresses {
+			if ip.Equal(candidate) {
+				return nil
+			}
+		}
+		return HostnameError{c, candidateIP}
+	}
+
+	candidateName := toLowerCaseASCII(h) // Save allocations inside the loop.
+	validCandidateName := validHostnameInput(candidateName)
+
+	for _, match := range c.DNSNames {
+		// Ideally, we'd only match valid hostnames according to RFC 6125 like
+		// browsers (more or less) do, but in practice Go is used in a wider
+		// array of contexts and can't even assume DNS resolution. Instead,
+		// always allow perfect matches, and only apply wildcard and trailing
+		// dot processing to valid hostnames.
+		if validCandidateName && validHostnamePattern(match) {
+			if matchHostnames(match, candidateName) {
+				return nil
+			}
+		} else {
+			if matchExactly(match, candidateName) {
+				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
+				}
+			}
+
+			usages[i] = invalidUsage
+			usagesRemaining--
+			if usagesRemaining == 0 {
+				return false
+			}
+		}
+	}
+
+	return true
+}