feat(M51): add SCEP server (RFC 8894) for MDM and network device enrollment

Implements Simple Certificate Enrollment Protocol with single-endpoint operation-based dispatch (GetCACaps, GetCACert, PKIOperation), PKCS#7 SignedData CSR extraction with fallback for raw/base64 CSR, challenge password authentication via CSR attributes, and shared internal/pkcs7 package extracted from EST handler to eliminate code duplication. 24 new tests (11 service + 13 handler) plus 5 shared pkcs7 package tests. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
2026-06-10 14:48:56 +00:00 · 2026-04-15 16:47:18 -04:00
parent 75cf8475f5
commit bcefb11e65
16 changed files with 1390 additions and 173 deletions
@@ -0,0 +1,353 @@
+package handler
+
+import (
+	"context"
+	"crypto/x509"
+	"encoding/asn1"
+	"encoding/base64"
+	"encoding/pem"
+	"fmt"
+	"io"
+	"net/http"
+	"strings"
+
+	"github.com/shankar0123/certctl/internal/api/middleware"
+	"github.com/shankar0123/certctl/internal/domain"
+	"github.com/shankar0123/certctl/internal/pkcs7"
+)
+
+// SCEPService defines the service interface for SCEP enrollment operations.
+// SCEP (RFC 8894) is a protocol for certificate enrollment used by MDM platforms
+// and network devices.
+type SCEPService interface {
+	// GetCACaps returns the SCEP server capabilities as a newline-separated string.
+	GetCACaps(ctx context.Context) string
+
+	// GetCACert returns the PEM-encoded CA certificate chain.
+	GetCACert(ctx context.Context) (string, error)
+
+	// PKCSReq processes a PKCS#10 CSR and returns a signed certificate.
+	PKCSReq(ctx context.Context, csrPEM string, challengePassword string, transactionID string) (*domain.SCEPEnrollResult, error)
+}
+
+// SCEPHandler handles HTTP requests for the SCEP protocol (RFC 8894).
+//
+// SCEP uses a single endpoint with operation-based dispatch via query parameters.
+// All operations use GET or POST to the same path.
+//
+// Supported operations:
+//   - GET  ?operation=GetCACaps    — server capabilities
+//   - GET  ?operation=GetCACert    — CA certificate distribution
+//   - POST ?operation=PKIOperation — certificate enrollment (PKCSReq)
+type SCEPHandler struct {
+	svc SCEPService
+}
+
+// NewSCEPHandler creates a new SCEPHandler.
+func NewSCEPHandler(svc SCEPService) SCEPHandler {
+	return SCEPHandler{svc: svc}
+}
+
+// HandleSCEP is the single entry point for all SCEP operations.
+// It dispatches based on the "operation" query parameter.
+func (h SCEPHandler) HandleSCEP(w http.ResponseWriter, r *http.Request) {
+	operation := r.URL.Query().Get("operation")
+
+	switch operation {
+	case "GetCACaps":
+		h.getCACaps(w, r)
+	case "GetCACert":
+		h.getCACert(w, r)
+	case "PKIOperation":
+		h.pkiOperation(w, r)
+	default:
+		http.Error(w, fmt.Sprintf("Unknown SCEP operation: %s", operation), http.StatusBadRequest)
+	}
+}
+
+// getCACaps handles GET ?operation=GetCACaps
+// Returns the SCEP server capabilities as plaintext, one per line.
+func (h SCEPHandler) getCACaps(w http.ResponseWriter, r *http.Request) {
+	if r.Method != http.MethodGet {
+		http.Error(w, "Method not allowed", http.StatusMethodNotAllowed)
+		return
+	}
+
+	caps := h.svc.GetCACaps(r.Context())
+	w.Header().Set("Content-Type", "text/plain")
+	w.WriteHeader(http.StatusOK)
+	w.Write([]byte(caps))
+}
+
+// getCACert handles GET ?operation=GetCACert
+// Returns the CA certificate(s). Single cert as DER, chain as PKCS#7.
+func (h SCEPHandler) getCACert(w http.ResponseWriter, r *http.Request) {
+	if r.Method != http.MethodGet {
+		http.Error(w, "Method not allowed", http.StatusMethodNotAllowed)
+		return
+	}
+
+	caCertPEM, err := h.svc.GetCACert(r.Context())
+	if err != nil {
+		requestID := middleware.GetRequestID(r.Context())
+		ErrorWithRequestID(w, http.StatusInternalServerError, fmt.Sprintf("Failed to get CA certificate: %v", err), requestID)
+		return
+	}
+
+	// Parse PEM to DER chain
+	derCerts, err := pkcs7.PEMToDERChain(caCertPEM)
+	if err != nil {
+		requestID := middleware.GetRequestID(r.Context())
+		ErrorWithRequestID(w, http.StatusInternalServerError, "Failed to parse CA certificates", requestID)
+		return
+	}
+
+	if len(derCerts) == 1 {
+		// Single CA cert — return as raw DER
+		w.Header().Set("Content-Type", "application/x-x509-ca-cert")
+		w.WriteHeader(http.StatusOK)
+		w.Write(derCerts[0])
+		return
+	}
+
+	// Multiple certs (CA + RA or chain) — return as PKCS#7
+	pkcs7Data, err := pkcs7.BuildCertsOnlyPKCS7(derCerts)
+	if err != nil {
+		requestID := middleware.GetRequestID(r.Context())
+		ErrorWithRequestID(w, http.StatusInternalServerError, "Failed to build PKCS#7 response", requestID)
+		return
+	}
+
+	w.Header().Set("Content-Type", "application/x-x509-ca-ra-cert")
+	w.WriteHeader(http.StatusOK)
+	w.Write(pkcs7Data)
+}
+
+// pkiOperation handles POST ?operation=PKIOperation
+// Processes a SCEP enrollment request containing a PKCS#7-wrapped CSR.
+func (h SCEPHandler) pkiOperation(w http.ResponseWriter, r *http.Request) {
+	if r.Method != http.MethodPost {
+		http.Error(w, "Method not allowed", http.StatusMethodNotAllowed)
+		return
+	}
+
+	requestID := middleware.GetRequestID(r.Context())
+
+	body, err := io.ReadAll(io.LimitReader(r.Body, 1<<20)) // 1MB limit
+	if err != nil {
+		ErrorWithRequestID(w, http.StatusBadRequest, "Failed to read request body", requestID)
+		return
+	}
+	defer r.Body.Close()
+
+	if len(body) == 0 {
+		ErrorWithRequestID(w, http.StatusBadRequest, "Empty request body", requestID)
+		return
+	}
+
+	// Extract the PKCS#10 CSR from the PKCS#7 SignedData envelope
+	csrDER, challengePassword, transactionID, err := extractCSRFromPKCS7(body)
+	if err != nil {
+		ErrorWithRequestID(w, http.StatusBadRequest, fmt.Sprintf("Invalid SCEP message: %v", err), requestID)
+		return
+	}
+
+	// Validate the CSR
+	csr, err := x509.ParseCertificateRequest(csrDER)
+	if err != nil {
+		ErrorWithRequestID(w, http.StatusBadRequest, fmt.Sprintf("Invalid CSR: %v", err), requestID)
+		return
+	}
+	if err := csr.CheckSignature(); err != nil {
+		ErrorWithRequestID(w, http.StatusBadRequest, fmt.Sprintf("CSR signature invalid: %v", err), requestID)
+		return
+	}
+
+	// Convert DER CSR to PEM for the service layer
+	csrPEM := string(pem.EncodeToMemory(&pem.Block{
+		Type:  "CERTIFICATE REQUEST",
+		Bytes: csrDER,
+	}))
+
+	result, err := h.svc.PKCSReq(r.Context(), csrPEM, challengePassword, transactionID)
+	if err != nil {
+		if strings.Contains(err.Error(), "challenge password") {
+			ErrorWithRequestID(w, http.StatusForbidden, "Invalid challenge password", requestID)
+			return
+		}
+		ErrorWithRequestID(w, http.StatusInternalServerError, fmt.Sprintf("Enrollment failed: %v", err), requestID)
+		return
+	}
+
+	// Build response: issued cert wrapped in PKCS#7 certs-only
+	h.writeSCEPResponse(w, result)
+}
+
+// writeSCEPResponse writes a SCEP enrollment response as PKCS#7 certs-only (DER).
+func (h SCEPHandler) writeSCEPResponse(w http.ResponseWriter, result *domain.SCEPEnrollResult) {
+	var derCerts [][]byte
+
+	certDER, err := pkcs7.PEMToDERChain(result.CertPEM)
+	if err != nil || len(certDER) == 0 {
+		http.Error(w, "Failed to encode certificate", http.StatusInternalServerError)
+		return
+	}
+	derCerts = append(derCerts, certDER...)
+
+	if result.ChainPEM != "" {
+		chainDER, err := pkcs7.PEMToDERChain(result.ChainPEM)
+		if err == nil {
+			derCerts = append(derCerts, chainDER...)
+		}
+	}
+
+	pkcs7Data, err := pkcs7.BuildCertsOnlyPKCS7(derCerts)
+	if err != nil {
+		http.Error(w, "Failed to build PKCS#7 response", http.StatusInternalServerError)
+		return
+	}
+
+	w.Header().Set("Content-Type", "application/x-pki-message")
+	w.WriteHeader(http.StatusOK)
+	w.Write(pkcs7Data)
+}
+
+// extractCSRFromPKCS7 extracts a PKCS#10 CSR from a SCEP PKCS#7 SignedData envelope.
+//
+// SCEP clients wrap the CSR in a PKCS#7 SignedData structure. For the MVP, we parse
+// the outer ASN.1 structure to find the encapsulated content (the CSR bytes), and
+// extract the challenge password from the CSR attributes.
+//
+// Returns: csrDER, challengePassword, transactionID, error
+func extractCSRFromPKCS7(data []byte) ([]byte, string, string, error) {
+	// Try to decode as PKCS#7 SignedData
+	csrDER, err := parseSignedDataForCSR(data)
+	if err != nil {
+		// Fallback: some clients send the CSR directly (not wrapped in PKCS#7)
+		// or send base64-encoded data
+		decoded, decErr := base64.StdEncoding.DecodeString(strings.TrimSpace(string(data)))
+		if decErr == nil {
+			// Try the decoded data as PKCS#7
+			csrDER2, err2 := parseSignedDataForCSR(decoded)
+			if err2 == nil {
+				return extractCSRFields(csrDER2)
+			}
+			// Maybe the decoded data IS the CSR directly
+			if _, parseErr := x509.ParseCertificateRequest(decoded); parseErr == nil {
+				return extractCSRFields(decoded)
+			}
+		}
+		// Maybe the raw data IS the CSR directly (no PKCS#7 wrapping)
+		if _, parseErr := x509.ParseCertificateRequest(data); parseErr == nil {
+			return extractCSRFields(data)
+		}
+		return nil, "", "", fmt.Errorf("failed to extract CSR from PKCS#7: %w", err)
+	}
+	return extractCSRFields(csrDER)
+}
+
+// extractCSRFields extracts the challenge password and transaction ID from CSR attributes.
+func extractCSRFields(csrDER []byte) ([]byte, string, string, error) {
+	csr, err := x509.ParseCertificateRequest(csrDER)
+	if err != nil {
+		return nil, "", "", fmt.Errorf("invalid CSR: %w", err)
+	}
+
+	challengePassword := ""
+	transactionID := ""
+
+	// OID for challengePassword: 1.2.840.113549.1.9.7
+	oidChallengePassword := asn1.ObjectIdentifier{1, 2, 840, 113549, 1, 9, 7}
+
+	// Extract challenge password from parsed CSR attributes.
+	// Attributes is []pkix.AttributeTypeAndValueSET where each has Type (OID)
+	// and Value ([][]pkix.AttributeTypeAndValue). The challenge password value
+	// is stored as a string in the inner AttributeTypeAndValue.Value field.
+	for _, attr := range csr.Attributes {
+		if attr.Type.Equal(oidChallengePassword) {
+			if len(attr.Value) > 0 && len(attr.Value[0]) > 0 {
+				if pwd, ok := attr.Value[0][0].Value.(string); ok {
+					challengePassword = pwd
+				}
+			}
+		}
+	}
+
+	// Use CN as fallback transaction ID if not found in attributes
+	if transactionID == "" && csr.Subject.CommonName != "" {
+		transactionID = csr.Subject.CommonName
+	}
+
+	return csrDER, challengePassword, transactionID, nil
+}
+
+// pkcs7ContentInfo represents the outer ContentInfo structure.
+type pkcs7ContentInfo struct {
+	ContentType asn1.ObjectIdentifier
+	Content     asn1.RawValue `asn1:"explicit,tag:0"`
+}
+
+// pkcs7SignedData represents a simplified SignedData structure for CSR extraction.
+type pkcs7SignedData struct {
+	Version          int
+	DigestAlgorithms asn1.RawValue
+	EncapContentInfo asn1.RawValue
+}
+
+// pkcs7EncapContent represents the EncapsulatedContentInfo.
+type pkcs7EncapContent struct {
+	ContentType asn1.ObjectIdentifier
+	Content     asn1.RawValue `asn1:"explicit,optional,tag:0"`
+}
+
+// parseSignedDataForCSR extracts the encapsulated content (CSR) from PKCS#7 SignedData.
+func parseSignedDataForCSR(data []byte) ([]byte, error) {
+	var contentInfo pkcs7ContentInfo
+	rest, err := asn1.Unmarshal(data, &contentInfo)
+	if err != nil {
+		return nil, fmt.Errorf("failed to parse ContentInfo: %w", err)
+	}
+	if len(rest) > 0 {
+		// Trailing data is OK for some implementations
+	}
+
+	// OID for signedData: 1.2.840.113549.1.7.2
+	oidSignedData := asn1.ObjectIdentifier{1, 2, 840, 113549, 1, 7, 2}
+	if !contentInfo.ContentType.Equal(oidSignedData) {
+		return nil, fmt.Errorf("not SignedData: got OID %v", contentInfo.ContentType)
+	}
+
+	// Parse the SignedData
+	var signedData pkcs7SignedData
+	_, err = asn1.Unmarshal(contentInfo.Content.Bytes, &signedData)
+	if err != nil {
+		return nil, fmt.Errorf("failed to parse SignedData: %w", err)
+	}
+
+	// Parse the EncapsulatedContentInfo to get the CSR
+	var encapContent pkcs7EncapContent
+	_, err = asn1.Unmarshal(signedData.EncapContentInfo.FullBytes, &encapContent)
+	if err != nil {
+		return nil, fmt.Errorf("failed to parse EncapsulatedContentInfo: %w", err)
+	}
+
+	if len(encapContent.Content.Bytes) == 0 {
+		return nil, fmt.Errorf("empty encapsulated content")
+	}
+
+	// The content may be wrapped in an OCTET STRING
+	var csrBytes []byte
+	var octetString asn1.RawValue
+	if _, err := asn1.Unmarshal(encapContent.Content.Bytes, &octetString); err == nil && octetString.Tag == asn1.TagOctetString {
+		csrBytes = octetString.Bytes
+	} else {
+		csrBytes = encapContent.Content.Bytes
+	}
+
+	// Validate it's a parseable CSR
+	if _, err := x509.ParseCertificateRequest(csrBytes); err != nil {
+		return nil, fmt.Errorf("extracted content is not a valid CSR: %w", err)
+	}
+
+	return csrBytes, nil
+}