certificates/authority/provisioner/scep.go
2023-05-09 23:47:28 -07:00

261 lines
8.3 KiB
Go

package provisioner
import (
"context"
"crypto/subtle"
"fmt"
"net/http"
"time"
"github.com/pkg/errors"
"go.step.sm/linkedca"
"github.com/smallstep/certificates/webhook"
)
// SCEP is the SCEP provisioner type, an entity that can authorize the
// SCEP provisioning flow
type SCEP struct {
*base
ID string `json:"-"`
Type string `json:"type"`
Name string `json:"name"`
ForceCN bool `json:"forceCN,omitempty"`
ChallengePassword string `json:"challenge,omitempty"`
Capabilities []string `json:"capabilities,omitempty"`
// IncludeRoot makes the provisioner return the CA root in addition to the
// intermediate in the GetCACerts response
IncludeRoot bool `json:"includeRoot,omitempty"`
// MinimumPublicKeyLength is the minimum length for public keys in CSRs
MinimumPublicKeyLength int `json:"minimumPublicKeyLength,omitempty"`
// Numerical identifier for the ContentEncryptionAlgorithm as defined in github.com/mozilla-services/pkcs7
// at https://github.com/mozilla-services/pkcs7/blob/33d05740a3526e382af6395d3513e73d4e66d1cb/encrypt.go#L63
// Defaults to 0, being DES-CBC
EncryptionAlgorithmIdentifier int `json:"encryptionAlgorithmIdentifier,omitempty"`
Options *Options `json:"options,omitempty"`
Claims *Claims `json:"claims,omitempty"`
ctl *Controller
encryptionAlgorithm int
challengeValidationController *challengeValidationController
}
// GetID returns the provisioner unique identifier.
func (s *SCEP) GetID() string {
if s.ID != "" {
return s.ID
}
return s.GetIDForToken()
}
// GetIDForToken returns an identifier that will be used to load the provisioner
// from a token.
func (s *SCEP) GetIDForToken() string {
return "scep/" + s.Name
}
// GetName returns the name of the provisioner.
func (s *SCEP) GetName() string {
return s.Name
}
// GetType returns the type of provisioner.
func (s *SCEP) GetType() Type {
return TypeSCEP
}
// GetEncryptedKey returns the base provisioner encrypted key if it's defined.
func (s *SCEP) GetEncryptedKey() (string, string, bool) {
return "", "", false
}
// GetTokenID returns the identifier of the token.
func (s *SCEP) GetTokenID(string) (string, error) {
return "", errors.New("scep provisioner does not implement GetTokenID")
}
// GetOptions returns the configured provisioner options.
func (s *SCEP) GetOptions() *Options {
return s.Options
}
// DefaultTLSCertDuration returns the default TLS cert duration enforced by
// the provisioner.
func (s *SCEP) DefaultTLSCertDuration() time.Duration {
return s.ctl.Claimer.DefaultTLSCertDuration()
}
type challengeValidationController struct {
client *http.Client
webhooks []*Webhook
}
// newChallengeValidationController creates a new challengeValidationController
// that performs challenge validation through webhooks.
func newChallengeValidationController(client *http.Client, webhooks []*Webhook) *challengeValidationController {
scepHooks := []*Webhook{}
for _, wh := range webhooks {
if wh.Kind != linkedca.Webhook_SCEPCHALLENGE.String() {
continue
}
if !isCertTypeOK(wh) {
continue
}
scepHooks = append(scepHooks, wh)
}
return &challengeValidationController{
client: client,
webhooks: scepHooks,
}
}
var (
ErrSCEPChallengeInvalid = errors.New("webhook server did not allow request")
)
// Validate executes zero or more configured webhooks to
// validate the SCEP challenge. If at least one of them indicates
// the challenge value is accepted, validation succeeds. In
// that case, the other webhooks will be skipped. If none of
// the webhooks indicates the value of the challenge was accepted,
// an error is returned.
func (c *challengeValidationController) Validate(ctx context.Context, challenge, transactionID string) error {
for _, wh := range c.webhooks {
req := &webhook.RequestBody{
SCEPChallenge: challenge,
SCEPTransactionID: transactionID,
}
resp, err := wh.DoWithContext(ctx, c.client, req, nil) // TODO(hs): support templated URL? Requires some refactoring
if err != nil {
return fmt.Errorf("failed executing webhook request: %w", err)
}
if resp.Allow {
return nil // return early when response is positive
}
}
return ErrSCEPChallengeInvalid
}
// isCertTypeOK returns whether or not the webhook can be used
// with the SCEP challenge validation webhook controller.
func isCertTypeOK(wh *Webhook) bool {
if wh.CertType == linkedca.Webhook_ALL.String() || wh.CertType == "" {
return true
}
return linkedca.Webhook_X509.String() == wh.CertType
}
// Init initializes and validates the fields of a SCEP type.
func (s *SCEP) Init(config Config) (err error) {
switch {
case s.Type == "":
return errors.New("provisioner type cannot be empty")
case s.Name == "":
return errors.New("provisioner name cannot be empty")
}
// Default to 2048 bits minimum public key length (for CSRs) if not set
if s.MinimumPublicKeyLength == 0 {
s.MinimumPublicKeyLength = 2048
}
if s.MinimumPublicKeyLength%8 != 0 {
return errors.Errorf("%d bits is not exactly divisible by 8", s.MinimumPublicKeyLength)
}
s.encryptionAlgorithm = s.EncryptionAlgorithmIdentifier // TODO(hs): we might want to upgrade the default security to AES-CBC?
if s.encryptionAlgorithm < 0 || s.encryptionAlgorithm > 4 {
return errors.New("only encryption algorithm identifiers from 0 to 4 are valid")
}
s.challengeValidationController = newChallengeValidationController(
config.WebhookClient,
s.GetOptions().GetWebhooks(),
)
// TODO: add other, SCEP specific, options?
s.ctl, err = NewController(s, s.Claims, config, s.Options)
return
}
// AuthorizeSign does not do any verification, because all verification is handled
// in the SCEP protocol. This method returns a list of modifiers / constraints
// on the resulting certificate.
func (s *SCEP) AuthorizeSign(context.Context, string) ([]SignOption, error) {
return []SignOption{
s,
// modifiers / withOptions
newProvisionerExtensionOption(TypeSCEP, s.Name, ""),
newForceCNOption(s.ForceCN),
profileDefaultDuration(s.ctl.Claimer.DefaultTLSCertDuration()),
// validators
newPublicKeyMinimumLengthValidator(s.MinimumPublicKeyLength),
newValidityValidator(s.ctl.Claimer.MinTLSCertDuration(), s.ctl.Claimer.MaxTLSCertDuration()),
newX509NamePolicyValidator(s.ctl.getPolicy().getX509()),
s.ctl.newWebhookController(nil, linkedca.Webhook_X509),
}, nil
}
// GetCapabilities returns the CA capabilities
func (s *SCEP) GetCapabilities() []string {
return s.Capabilities
}
// ShouldIncludeRootInChain indicates if the CA should
// return its intermediate, which is currently used for
// both signing and decryption, as well as the root in
// its chain.
func (s *SCEP) ShouldIncludeRootInChain() bool {
return s.IncludeRoot
}
// GetContentEncryptionAlgorithm returns the numeric identifier
// for the pkcs7 package encryption algorithm to use.
func (s *SCEP) GetContentEncryptionAlgorithm() int {
return s.encryptionAlgorithm
}
// ValidateChallenge validates the provided challenge. It starts by
// selecting the validation method to use, then performs validation
// according to that method.
func (s *SCEP) ValidateChallenge(ctx context.Context, challenge, transactionID string) error {
if s.challengeValidationController == nil {
return fmt.Errorf("provisioner %q wasn't initialized", s.Name)
}
switch s.selectValidationMethod() {
case validationMethodWebhook:
return s.challengeValidationController.Validate(ctx, challenge, transactionID)
default:
if subtle.ConstantTimeCompare([]byte(s.ChallengePassword), []byte(challenge)) == 0 {
return errors.New("invalid challenge password provided")
}
return nil
}
}
type validationMethod string
const (
validationMethodNone validationMethod = "none"
validationMethodStatic validationMethod = "static"
validationMethodWebhook validationMethod = "webhook"
)
// selectValidationMethod returns the method to validate SCEP
// challenges. If a webhook is configured with kind `SCEPCHALLENGE`,
// the webhook method will be used. If a challenge password is set,
// the static method is used. It will default to the `none` method.
func (s *SCEP) selectValidationMethod() validationMethod {
if len(s.challengeValidationController.webhooks) > 0 {
return validationMethodWebhook
}
if s.ChallengePassword != "" {
return validationMethodStatic
}
return validationMethodNone
}