certificates/scep/api/api.go

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package api
import (
"context"
"crypto/x509"
"encoding/base64"
"errors"
"fmt"
"io"
"net/http"
"net/url"
"strings"
"github.com/go-chi/chi"
microscep "github.com/micromdm/scep/v2/scep"
"go.mozilla.org/pkcs7"
"github.com/smallstep/certificates/api"
"github.com/smallstep/certificates/api/log"
"github.com/smallstep/certificates/authority/provisioner"
"github.com/smallstep/certificates/scep"
)
const (
opnGetCACert = "GetCACert"
opnGetCACaps = "GetCACaps"
opnPKIOperation = "PKIOperation"
// TODO: add other (more optional) operations and handling
)
const maxPayloadSize = 2 << 20
const (
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certChainHeader = "application/x-x509-ca-ra-cert"
leafHeader = "application/x-x509-ca-cert"
pkiOperationHeader = "application/x-pki-message"
)
// request is a SCEP server request.
type request struct {
Operation string
Message []byte
}
// response is a SCEP server response.
type response struct {
Operation string
CACertNum int
Data []byte
Certificate *x509.Certificate
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Error error
}
// handler is the SCEP request handler.
type handler struct {
Auth scep.Interface
}
// New returns a new SCEP API router.
func New(scepAuth scep.Interface) api.RouterHandler {
return &handler{scepAuth}
}
// Route traffic and implement the Router interface.
func (h *handler) Route(r api.Router) {
getLink := h.Auth.GetLinkExplicit
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r.MethodFunc(http.MethodGet, getLink("{provisionerName}/*", false, nil), h.lookupProvisioner(h.Get))
r.MethodFunc(http.MethodGet, getLink("{provisionerName}", false, nil), h.lookupProvisioner(h.Get))
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r.MethodFunc(http.MethodPost, getLink("{provisionerName}/*", false, nil), h.lookupProvisioner(h.Post))
r.MethodFunc(http.MethodPost, getLink("{provisionerName}", false, nil), h.lookupProvisioner(h.Post))
}
// Get handles all SCEP GET requests
func (h *handler) Get(w http.ResponseWriter, r *http.Request) {
req, err := decodeRequest(r)
if err != nil {
fail(w, fmt.Errorf("invalid scep get request: %w", err))
return
}
ctx := r.Context()
var res response
switch req.Operation {
case opnGetCACert:
res, err = h.GetCACert(ctx)
case opnGetCACaps:
res, err = h.GetCACaps(ctx)
case opnPKIOperation:
// TODO: implement the GET for PKI operation? Default CACAPS doesn't specify this is in use, though
default:
err = fmt.Errorf("unknown operation: %s", req.Operation)
}
if err != nil {
fail(w, fmt.Errorf("scep get request failed: %w", err))
return
}
writeResponse(w, res)
}
// Post handles all SCEP POST requests
func (h *handler) Post(w http.ResponseWriter, r *http.Request) {
req, err := decodeRequest(r)
if err != nil {
fail(w, fmt.Errorf("invalid scep post request: %w", err))
return
}
ctx := r.Context()
var res response
switch req.Operation {
case opnPKIOperation:
res, err = h.PKIOperation(ctx, req)
default:
err = fmt.Errorf("unknown operation: %s", req.Operation)
}
if err != nil {
fail(w, fmt.Errorf("scep post request failed: %w", err))
return
}
writeResponse(w, res)
}
func decodeRequest(r *http.Request) (request, error) {
defer r.Body.Close()
method := r.Method
query := r.URL.Query()
var operation string
if _, ok := query["operation"]; ok {
operation = query.Get("operation")
}
switch method {
case http.MethodGet:
switch operation {
case opnGetCACert, opnGetCACaps:
return request{
Operation: operation,
Message: []byte{},
}, nil
case opnPKIOperation:
var message string
if _, ok := query["message"]; ok {
message = query.Get("message")
}
// TODO: verify this; it seems like it should be StdEncoding instead of URLEncoding
decodedMessage, err := base64.URLEncoding.DecodeString(message)
if err != nil {
return request{}, err
}
return request{
Operation: operation,
Message: decodedMessage,
}, nil
default:
return request{}, fmt.Errorf("unsupported operation: %s", operation)
}
case http.MethodPost:
body, err := io.ReadAll(io.LimitReader(r.Body, maxPayloadSize))
if err != nil {
return request{}, err
}
return request{
Operation: operation,
Message: body,
}, nil
default:
return request{}, fmt.Errorf("unsupported method: %s", method)
}
}
// lookupProvisioner loads the provisioner associated with the request.
// Responds 404 if the provisioner does not exist.
func (h *handler) lookupProvisioner(next http.HandlerFunc) http.HandlerFunc {
return func(w http.ResponseWriter, r *http.Request) {
name := chi.URLParam(r, "provisionerName")
provisionerName, err := url.PathUnescape(name)
if err != nil {
fail(w, fmt.Errorf("error url unescaping provisioner name '%s'", name))
return
}
p, err := h.Auth.LoadProvisionerByName(provisionerName)
if err != nil {
fail(w, err)
return
}
prov, ok := p.(*provisioner.SCEP)
if !ok {
fail(w, errors.New("provisioner must be of type SCEP"))
return
}
ctx := r.Context()
ctx = context.WithValue(ctx, scep.ProvisionerContextKey, scep.Provisioner(prov))
next(w, r.WithContext(ctx))
}
}
// GetCACert returns the CA certificates in a SCEP response
func (h *handler) GetCACert(ctx context.Context) (response, error) {
certs, err := h.Auth.GetCACertificates(ctx)
if err != nil {
return response{}, err
}
if len(certs) == 0 {
return response{}, errors.New("missing CA cert")
}
res := response{
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Operation: opnGetCACert,
CACertNum: len(certs),
}
if len(certs) == 1 {
res.Data = certs[0].Raw
} else {
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// create degenerate pkcs7 certificate structure, according to
// https://tools.ietf.org/html/rfc8894#section-4.2.1.2, because
// not signed or encrypted data has to be returned.
data, err := microscep.DegenerateCertificates(certs)
if err != nil {
return response{}, err
}
res.Data = data
}
return res, nil
}
// GetCACaps returns the CA capabilities in a SCEP response
func (h *handler) GetCACaps(ctx context.Context) (response, error) {
caps := h.Auth.GetCACaps(ctx)
res := response{
Operation: opnGetCACaps,
Data: formatCapabilities(caps),
}
return res, nil
}
// PKIOperation performs PKI operations and returns a SCEP response
func (h *handler) PKIOperation(ctx context.Context, req request) (response, error) {
// parse the message using microscep implementation
microMsg, err := microscep.ParsePKIMessage(req.Message)
if err != nil {
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// return the error, because we can't use the msg for creating a CertRep
return response{}, err
}
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// this is essentially doing the same as microscep.ParsePKIMessage, but
// gives us access to the p7 itself in scep.PKIMessage. Essentially a small
// wrapper for the microscep implementation.
p7, err := pkcs7.Parse(microMsg.Raw)
if err != nil {
return response{}, err
}
// copy over properties to our internal PKIMessage
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msg := &scep.PKIMessage{
TransactionID: microMsg.TransactionID,
MessageType: microMsg.MessageType,
SenderNonce: microMsg.SenderNonce,
Raw: microMsg.Raw,
P7: p7,
}
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if err := h.Auth.DecryptPKIEnvelope(ctx, msg); err != nil {
return response{}, err
}
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// NOTE: at this point we have sufficient information for returning nicely signed CertReps
csr := msg.CSRReqMessage.CSR
// NOTE: we're blocking the RenewalReq if the challenge does not match, because otherwise we don't have any authentication.
// The macOS SCEP client performs renewals using PKCSreq. The CertNanny SCEP client will use PKCSreq with challenge too, it seems,
// even if using the renewal flow as described in the README.md. MicroMDM SCEP client also only does PKCSreq by default, unless
// a certificate exists; then it will use RenewalReq. Adding the challenge check here may be a small breaking change for clients.
// We'll have to see how it works out.
if msg.MessageType == microscep.PKCSReq || msg.MessageType == microscep.RenewalReq {
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challengeMatches, err := h.Auth.MatchChallengePassword(ctx, msg.CSRReqMessage.ChallengePassword)
if err != nil {
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return h.createFailureResponse(ctx, csr, msg, microscep.BadRequest, errors.New("error when checking password"))
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}
if !challengeMatches {
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// TODO: can this be returned safely to the client? In the end, if the password was correct, that gains a bit of info too.
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return h.createFailureResponse(ctx, csr, msg, microscep.BadRequest, errors.New("wrong password provided"))
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}
}
// TODO: authorize renewal: we can authorize renewals with the challenge password (if reusable secrets are used).
// Renewals OPTIONALLY include the challenge if the existing cert is used as authentication, but client SHOULD omit the challenge.
// This means that for renewal requests we should check the certificate provided to be signed before by the CA. We could
// enforce use of the challenge if we want too. That way we could be more flexible in terms of authentication scheme (i.e. reusing
// tokens from other provisioners, calling a webhook, storing multiple secrets, allowing them to be multi-use, etc).
// Authentication by the (self-signed) certificate with an optional challenge is required; supporting renewals incl. verification
// of the client cert is not.
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certRep, err := h.Auth.SignCSR(ctx, csr, msg)
if err != nil {
return h.createFailureResponse(ctx, csr, msg, microscep.BadRequest, fmt.Errorf("error when signing new certificate: %w", err))
}
res := response{
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Operation: opnPKIOperation,
Data: certRep.Raw,
Certificate: certRep.Certificate,
}
return res, nil
}
func formatCapabilities(caps []string) []byte {
return []byte(strings.Join(caps, "\r\n"))
}
// writeResponse writes a SCEP response back to the SCEP client.
func writeResponse(w http.ResponseWriter, res response) {
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if res.Error != nil {
log.Error(w, res.Error)
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}
if res.Certificate != nil {
api.LogCertificate(w, res.Certificate)
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}
w.Header().Set("Content-Type", contentHeader(res))
_, _ = w.Write(res.Data)
}
func fail(w http.ResponseWriter, err error) {
log.Error(w, err)
http.Error(w, err.Error(), http.StatusInternalServerError)
}
func (h *handler) createFailureResponse(ctx context.Context, csr *x509.CertificateRequest, msg *scep.PKIMessage, info microscep.FailInfo, failError error) (response, error) {
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certRepMsg, err := h.Auth.CreateFailureResponse(ctx, csr, msg, scep.FailInfoName(info), failError.Error())
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if err != nil {
return response{}, err
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}
return response{
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Operation: opnPKIOperation,
Data: certRepMsg.Raw,
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Error: failError,
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}, nil
}
func contentHeader(r response) string {
switch r.Operation {
case opnGetCACert:
if r.CACertNum > 1 {
return certChainHeader
}
return leafHeader
case opnPKIOperation:
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return pkiOperationHeader
default:
return "text/plain"
}
}