From 5b09e7df3d09c10f0e22f87f71ebb4f997e5e199 Mon Sep 17 00:00:00 2001
From: Mohammed Al Sahaf <msaa1990@gmail.com>
Date: Fri, 25 Aug 2023 22:00:28 +0300
Subject: [PATCH] pki: rough draft for generating CSR through API

---
 modules/caddypki/adminapi.go | 74 ++++++++++++++++++++++++++++++++++--
 modules/caddypki/ca.go       | 40 +++++++++++++++++++
 2 files changed, 111 insertions(+), 3 deletions(-)

diff --git a/modules/caddypki/adminapi.go b/modules/caddypki/adminapi.go
index 435af349a..720a5dee9 100644
--- a/modules/caddypki/adminapi.go
+++ b/modules/caddypki/adminapi.go
@@ -18,6 +18,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"net/http"
+	"regexp"
 	"strings"
 
 	"go.uber.org/zap"
@@ -29,6 +30,12 @@ func init() {
 	caddy.RegisterModule(adminAPI{})
 }
 
+var (
+	caInfoPathPattern     = regexp.MustCompile(`^ca/[^/]+$`)
+	getCertPathPattern    = regexp.MustCompile(`^ca/[^/]+/certificates$`)
+	produceCSRPathPattern = regexp.MustCompile(`^ca/[^/]+/csr$`)
+)
+
 // adminAPI is a module that serves PKI endpoints to retrieve
 // information about the CAs being managed by Caddy.
 type adminAPI struct {
@@ -71,12 +78,13 @@ func (a *adminAPI) Routes() []caddy.AdminRoute {
 // handleAPIEndpoints routes API requests within adminPKIEndpointBase.
 func (a *adminAPI) handleAPIEndpoints(w http.ResponseWriter, r *http.Request) error {
 	uri := strings.TrimPrefix(r.URL.Path, "/pki/")
-	parts := strings.Split(uri, "/")
 	switch {
-	case len(parts) == 2 && parts[0] == "ca" && parts[1] != "":
+	case caInfoPathPattern.MatchString(uri):
 		return a.handleCAInfo(w, r)
-	case len(parts) == 3 && parts[0] == "ca" && parts[1] != "" && parts[2] == "certificates":
+	case getCertPathPattern.MatchString(uri):
 		return a.handleCACerts(w, r)
+	case produceCSRPathPattern.MatchString(uri):
+		return a.handleCSRGeneration(w, r)
 	}
 	return caddy.APIError{
 		HTTPStatus: http.StatusNotFound,
@@ -168,6 +176,66 @@ func (a *adminAPI) handleCACerts(w http.ResponseWriter, r *http.Request) error {
 	return nil
 }
 
+type csrRequest struct {
+	// SANs is a list of subject alternative names for the certificate.
+	SANs []string `json:"sans"`
+}
+
+func (a *adminAPI) handleCSRGeneration(w http.ResponseWriter, r *http.Request) error {
+	if r.Method != http.MethodPost {
+		return caddy.APIError{
+			HTTPStatus: http.StatusMethodNotAllowed,
+			Err:        fmt.Errorf("method not allowed: %v", r.Method),
+		}
+	}
+
+	ca, err := a.getCAFromAPIRequestPath(r)
+	if err != nil {
+		return caddy.APIError{
+			HTTPStatus: http.StatusBadRequest,
+			Err:        err,
+		}
+	}
+
+	// Decode the CSR request from the request body
+	var csrReq csrRequest
+	if err := json.NewDecoder(r.Body).Decode(&csrReq); err != nil {
+		return caddy.APIError{
+			HTTPStatus: http.StatusBadRequest,
+			Err:        fmt.Errorf("failed to decode CSR request: %v", err),
+		}
+	}
+	// Generate the CSR
+	csr, err := ca.generateCSR(csrReq.SANs)
+	if err != nil {
+		return caddy.APIError{
+			HTTPStatus: http.StatusInternalServerError,
+			Err:        fmt.Errorf("failed to generate CSR: %v", err),
+		}
+	}
+	if r.Header.Get("Accept") != "application/pkcs10" {
+		return caddy.APIError{
+			HTTPStatus: http.StatusNotAcceptable,
+			Err:        fmt.Errorf("only accept application/pkcs10"),
+		}
+	}
+	bs, err := pemEncode("CERTIFICATE REQUEST", csr.Raw)
+	if err != nil {
+		return caddy.APIError{
+			HTTPStatus: http.StatusInternalServerError,
+			Err:        fmt.Errorf("failed to encode CSR to PEM: %v", err),
+		}
+	}
+	w.Header().Set("Content-Type", "application/pkcs10")
+	if _, err := w.Write(bs); err != nil {
+		return caddy.APIError{
+			HTTPStatus: http.StatusInternalServerError,
+			Err:        fmt.Errorf("failed to write CSR response: %v", err),
+		}
+	}
+	return nil
+}
+
 func (a *adminAPI) getCAFromAPIRequestPath(r *http.Request) (*CA, error) {
 	// Grab the CA ID from the request path, it should be the 4th segment (/pki/ca/<ca>)
 	id := strings.Split(r.URL.Path, "/")[3]
diff --git a/modules/caddypki/ca.go b/modules/caddypki/ca.go
index 6c48da6f9..15339cedb 100644
--- a/modules/caddypki/ca.go
+++ b/modules/caddypki/ca.go
@@ -29,6 +29,8 @@ import (
 	"github.com/smallstep/certificates/authority"
 	"github.com/smallstep/certificates/db"
 	"github.com/smallstep/truststore"
+	"go.step.sm/crypto/keyutil"
+	"go.step.sm/crypto/x509util"
 	"go.uber.org/zap"
 
 	"github.com/caddyserver/caddy/v2"
@@ -394,6 +396,10 @@ func (ca CA) storageKeyIntermediateKey() string {
 	return path.Join(ca.storageKeyCAPrefix(), "intermediate.key")
 }
 
+func (ca CA) storageKeyCSRKey() string {
+	return path.Join(ca.storageKeyCAPrefix(), "csr.key")
+}
+
 func (ca CA) newReplacer() *caddy.Replacer {
 	repl := caddy.NewReplacer()
 	repl.Set("pki.ca.name", ca.Name)
@@ -421,6 +427,40 @@ func (ca CA) installRoot() error {
 	)
 }
 
+func (ca CA) generateCSR(sans []string) (csr *x509.CertificateRequest, err error) {
+	var signer crypto.Signer
+	csrKeyPEM, err := ca.storage.Load(ca.ctx, ca.storageKeyCSRKey())
+	if err != nil {
+		if !errors.Is(err, fs.ErrNotExist) {
+			return nil, fmt.Errorf("loading csr key: %v", err)
+		}
+
+		signer, err = keyutil.GenerateDefaultSigner()
+		if err != nil {
+			return nil, err
+		}
+		csrKeyPEM, err = certmagic.PEMEncodePrivateKey(signer)
+		if err != nil {
+			return nil, fmt.Errorf("encoding csr key: %v", err)
+		}
+		if err := ca.storage.Store(ca.ctx, ca.storageKeyCSRKey(), csrKeyPEM); err != nil {
+			return nil, fmt.Errorf("saving csr key: %v", err)
+		}
+	}
+	if signer == nil {
+		signer, err = certmagic.PEMDecodePrivateKey(csrKeyPEM)
+		if err != nil {
+			return nil, fmt.Errorf("decoding root key: %v", err)
+		}
+	}
+
+	csr, err = x509util.CreateCertificateRequest("", sans, signer)
+	if err != nil {
+		return nil, err
+	}
+	return csr, nil
+}
+
 // AuthorityConfig is used to help a CA configure
 // the underlying signing authority.
 type AuthorityConfig struct {