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// Copyright 2022 Google LLC.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package cert contains certificate tools for Google API clients.
// This package is intended to be used with crypto/tls.Config.GetClientCertificate.
//
// The certificates can be used to satisfy Google's Endpoint Validation.
// See https://cloud.google.com/endpoint-verification/docs/overview
//
// This package is not intended for use by end developers. Use the
// google.golang.org/api/option package to configure API clients.
package cert
import (
"crypto/tls"
"crypto/x509"
"encoding/json"
"errors"
"fmt"
"io/ioutil"
"os"
"os/exec"
"os/user"
"path/filepath"
"sync"
"time"
)
const (
metadataPath = ".secureConnect"
metadataFile = "context_aware_metadata.json"
)
type secureConnectSource struct {
metadata secureConnectMetadata
// Cache the cert to avoid executing helper command repeatedly.
cachedCertMutex sync.Mutex
cachedCert *tls.Certificate
}
type secureConnectMetadata struct {
Cmd []string `json:"cert_provider_command"`
}
// NewSecureConnectSource creates a certificate source using
// the Secure Connect Helper and its associated metadata file.
//
// The configFilePath points to the location of the context aware metadata file.
// If configFilePath is empty, use the default context aware metadata location.
func NewSecureConnectSource(configFilePath string) (Source, error) {
if configFilePath == "" {
user, err := user.Current()
if err != nil {
// Error locating the default config means Secure Connect is not supported.
return nil, errSourceUnavailable
}
configFilePath = filepath.Join(user.HomeDir, metadataPath, metadataFile)
}
file, err := ioutil.ReadFile(configFilePath)
if err != nil {
if errors.Is(err, os.ErrNotExist) {
// Config file missing means Secure Connect is not supported.
return nil, errSourceUnavailable
}
return nil, err
}
var metadata secureConnectMetadata
if err := json.Unmarshal(file, &metadata); err != nil {
return nil, fmt.Errorf("cert: could not parse JSON in %q: %w", configFilePath, err)
}
if err := validateMetadata(metadata); err != nil {
return nil, fmt.Errorf("cert: invalid config in %q: %w", configFilePath, err)
}
return (&secureConnectSource{
metadata: metadata,
}).getClientCertificate, nil
}
func validateMetadata(metadata secureConnectMetadata) error {
if len(metadata.Cmd) == 0 {
return errors.New("empty cert_provider_command")
}
return nil
}
func (s *secureConnectSource) getClientCertificate(info *tls.CertificateRequestInfo) (*tls.Certificate, error) {
s.cachedCertMutex.Lock()
defer s.cachedCertMutex.Unlock()
if s.cachedCert != nil && !isCertificateExpired(s.cachedCert) {
return s.cachedCert, nil
}
// Expand OS environment variables in the cert provider command such as "$HOME".
for i := 0; i < len(s.metadata.Cmd); i++ {
s.metadata.Cmd[i] = os.ExpandEnv(s.metadata.Cmd[i])
}
command := s.metadata.Cmd
data, err := exec.Command(command[0], command[1:]...).Output()
if err != nil {
return nil, err
}
cert, err := tls.X509KeyPair(data, data)
if err != nil {
return nil, err
}
s.cachedCert = &cert
return &cert, nil
}
// isCertificateExpired returns true if the given cert is expired or invalid.
func isCertificateExpired(cert *tls.Certificate) bool {
if len(cert.Certificate) == 0 {
return true
}
parsed, err := x509.ParseCertificate(cert.Certificate[0])
if err != nil {
return true
}
return time.Now().After(parsed.NotAfter)
}
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