security: require SCEP challenge password when SCEP enabled (fixes H-2)

Problem (CWE-306 Missing Authentication for Critical Function):
internal/service/scep.go PKCSReq skipped the shared-secret check when
s.challengePassword was empty. An unconfigured-but-enabled SCEP server
accepted any unauthenticated client reaching /scep and issued a
certificate against the configured issuer for any CSR with a valid
signature. No audit trail distinguished authenticated from
unauthenticated enrollments. This matches the two-layer fail-closed
pattern already used for C-2 (fb4ce1a): reject at startup AND reject
at the service boundary.

Fix (two layers, defense-in-depth):

Layer 1 — startup pre-flight in cmd/server/main.go:
  preflightSCEPChallengePassword returns a non-nil error when SCEP is
  enabled and CERTCTL_SCEP_CHALLENGE_PASSWORD is empty. main logs and
  os.Exit(1)s before the SCEP service is constructed. Disabled SCEP is
  unaffected. The helper is unit-testable in isolation.

Layer 2 — service-layer rejection in internal/service/scep.go:
  PKCSReq refuses enrollment when s.challengePassword == "" even though
  main already blocks this state — protects future call sites (tests,
  library reuse, a REST-over-HTTPS wrapper). When a secret is
  configured, the comparison now uses crypto/subtle.ConstantTimeCompare
  so response time does not leak the configured secret through a
  short-circuiting byte compare.

Files:
- cmd/server/main.go: preflightSCEPChallengePassword helper; call site
  inside the `if cfg.SCEP.Enabled` block before issuer lookup; fatal
  slog error references CWE-306 and names the env var so operators can
  diagnose the startup failure without reading code.
- cmd/server/main_test.go: TestPreflightSCEPChallengePassword with five
  table-driven subtests (disabled empty, disabled set, enabled empty
  rejected, enabled set, single-char boundary). The enabled-empty case
  asserts the error string contains both CERTCTL_SCEP_CHALLENGE_PASSWORD
  and CWE-306 so the log message remains actionable.
- internal/config/config.go: SCEPConfig.ChallengePassword godoc now
  states the field is REQUIRED when SCEP.Enabled and cross-references
  preflightSCEPChallengePassword.
- internal/service/scep.go: imports crypto/subtle; PKCSReq rewritten
  with the two-layer check; comment block cites H-2 / CWE-306 and the
  constant-time rationale.
- internal/service/scep_test.go: existing tests that relied on the
  vulnerable empty-password path now configure a secret on both sides.
  TestSCEPService_PKCSReq_ChallengePassword_NotRequired is replaced by
  TestSCEPService_PKCSReq_ChallengePassword_EmptyServerConfigRejected
  which iterates ["", "any-value", "guess"] against an unconfigured
  server and asserts "not configured" in the error. A new
  TestSCEPService_PKCSReq_ChallengePassword_ConstantTimeLengthIndependence
  exercises same-prefix-longer and wrong-case inputs to guard against a
  regression from ConstantTimeCompare to a short-circuiting byte compare.
- internal/service/m11c_crypto_enforcement_test.go: four tests
  (RejectsWeakKey, AcceptsStrongKey, MaxTTL_ForwardedToIssuer,
  NoProfileRepo_PassesThrough) constructed NewSCEPService with an empty
  challenge password and exercised PKCSReq through the now-rejected
  vulnerable path. All four now configure "secret123" on both sides with
  an inline H-2 comment; the crypto/MaxTTL/profile behavior they assert
  is unchanged.

Wire-format / behavioral invariants preserved:
- RFC 8894 SCEP handler is untouched (internal/api/handler/scep.go and
  internal/pkcs7/*): GetCACaps/GetCACert responses, PKIOperation request
  parsing, and the PKCS#7 certs-only response format are byte-identical.
- RFC 7030 EST handler is untouched
  (internal/api/handler/est.go + internal/pkcs7/*).
- Revocation idempotency composite key (H-1, migration 000012) untouched.
- AES-256-GCM config encryption (C-2) untouched.
- CRL DER bytes and OCSP response bytes unchanged.

Verification:
- go build ./...              silent success
- go vet ./...                silent success
- go test -race -count=1 ./internal/service/ ./cmd/server/
  ./internal/api/handler/ ./internal/integration/    all OK
- Coverage with comfortable headroom over CI gates:
    service     67.8% (gate 55%)
    handler     79.0% (gate 60%)
    domain      92.7% (gate 40%)
    middleware  80.0% (gate 30%)
    cmd/server  1.6%  (preflightSCEPChallengePassword: 100%)
  internal/service/scep.go PKCSReq statement coverage: 100%.
- rg sweeps: no `s.challengePassword != ""` remains;
  no `challengePassword != s.challengePassword` remains.

Operational note: operators with SCEP enabled but no challenge password
set will see a fatal startup error and a log line citing
CERTCTL_SCEP_CHALLENGE_PASSWORD and CWE-306 after upgrading. This is the
intended fail-closed behavior. Fix by either setting the env var to a
non-empty shared secret or setting CERTCTL_SCEP_ENABLED=false.

Audit report: certctl-audit-report.md (revision 5) logs this under
H-2 Resolution Log.

cmd/server/main.go

@@ -483,6 +483,24 @@ func main() {
 
 	// Register SCEP (RFC 8894) handlers if enabled
 	if cfg.SCEP.Enabled {
+		// H-2 fix: fail closed at startup when SCEP is enabled without a
+		// challenge password configured. Previously the service-layer guard
+		// at internal/service/scep.go:72-79 skipped the password check when
+		// s.challengePassword == "", meaning any client that could reach the
+		// /scep endpoint could enroll an arbitrary CSR against the configured
+		// issuer (CWE-306, missing authentication for a critical function).
+		// Refuse to start instead: the operator must set
+		// CERTCTL_SCEP_CHALLENGE_PASSWORD (or disable SCEP) before the control
+		// plane can boot.
+		if err := preflightSCEPChallengePassword(cfg.SCEP.Enabled, cfg.SCEP.ChallengePassword); err != nil {
+			logger.Error(
+				"startup refused: SCEP is enabled but CERTCTL_SCEP_CHALLENGE_PASSWORD is not set "+
+					"(would allow unauthenticated certificate enrollment, CWE-306). "+
+					"Set a non-empty challenge password or disable SCEP before restarting.",
+				"error", err,
+			)
+			os.Exit(1)
+		}
 		issuerConn, ok := issuerRegistry.Get(cfg.SCEP.IssuerID)
 		if !ok {
 			logger.Error("SCEP issuer not found in registry", "issuer_id", cfg.SCEP.IssuerID)
@@ -683,3 +701,23 @@ func main() {
 	logger.Info("certctl server stopped")
 }
 
+// preflightSCEPChallengePassword enforces the H-2 fix: if SCEP is enabled, a
+// non-empty challenge password MUST be configured. Returns a non-nil error
+// otherwise so the caller can refuse to start the control plane (CWE-306,
+// missing authentication for a critical function).
+//
+// This helper is extracted so the check can be unit tested without booting
+// the full server. The caller (main) is responsible for translating the
+// returned error into a structured log line and os.Exit(1).
+func preflightSCEPChallengePassword(enabled bool, challengePassword string) error {
+	if !enabled {
+		return nil
+	}
+	if challengePassword == "" {
+		return fmt.Errorf("SCEP enabled but CERTCTL_SCEP_CHALLENGE_PASSWORD is empty: " +
+			"SCEP enrollment would accept any client (CWE-306); " +
+			"configure a non-empty shared secret or set CERTCTL_SCEP_ENABLED=false")
+	}
+	return nil
+}
+

cmd/server/main_test.go

@@ -7,6 +7,7 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"os"
+	"strings"
 	"testing"
 
 	"github.com/shankar0123/certctl/internal/api/middleware"
@@ -538,3 +539,68 @@ func TestMain_ContextPropagation(t *testing.T) {
 		t.Logf("Context value may not be propagated (status %d), this may be expected", w.Code)
 	}
 }
+
+// TestPreflightSCEPChallengePassword is the H-2 regression guard for the
+// startup pre-flight check. The helper MUST return a non-nil error whenever
+// SCEP is enabled with an empty challenge password — that configuration
+// previously allowed unauthenticated certificate enrollment (CWE-306).
+// Disabled-SCEP and configured-password cases must pass cleanly.
+func TestPreflightSCEPChallengePassword(t *testing.T) {
+	tests := []struct {
+		name              string
+		enabled           bool
+		challengePassword string
+		wantErr           bool
+		wantErrSubstring  string
+	}{
+		{
+			name:              "disabled_empty_password_ok",
+			enabled:           false,
+			challengePassword: "",
+			wantErr:           false,
+		},
+		{
+			name:              "disabled_with_password_ok",
+			enabled:           false,
+			challengePassword: "leftover-value",
+			wantErr:           false,
+		},
+		{
+			name:              "enabled_empty_password_rejected",
+			enabled:           true,
+			challengePassword: "",
+			wantErr:           true,
+			wantErrSubstring:  "CERTCTL_SCEP_CHALLENGE_PASSWORD",
+		},
+		{
+			name:              "enabled_with_password_ok",
+			enabled:           true,
+			challengePassword: "hunter2",
+			wantErr:           false,
+		},
+		{
+			name:              "enabled_single_char_password_ok",
+			enabled:           true,
+			challengePassword: "x",
+			wantErr:           false,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := preflightSCEPChallengePassword(tt.enabled, tt.challengePassword)
+			if tt.wantErr {
+				if err == nil {
+					t.Fatalf("expected error, got nil")
+				}
+				if tt.wantErrSubstring != "" && !strings.Contains(err.Error(), tt.wantErrSubstring) {
+					t.Errorf("expected error to mention %q, got: %v", tt.wantErrSubstring, err)
+				}
+				if !strings.Contains(err.Error(), "CWE-306") {
+					t.Errorf("expected error to cite CWE-306 for traceability, got: %v", err)
+				}
+			} else if err != nil {
+				t.Errorf("expected no error, got: %v", err)
+			}
+		})
+	}
+}

internal/config/config.go

@@ -641,7 +641,12 @@ type SCEPConfig struct {
 
 	// ChallengePassword is the shared secret used to authenticate SCEP enrollment requests.
 	// Clients include this in the PKCS#10 CSR challengePassword attribute.
-	// Required when SCEP is enabled.
+	//
+	// REQUIRED when Enabled is true. If SCEP is enabled and this value is empty,
+	// cmd/server/main.go's preflightSCEPChallengePassword check will refuse to
+	// start the server (H-2, CWE-306): an empty shared secret allowed any client
+	// that could reach /scep to enroll a CSR against the configured issuer. The
+	// service-layer PKCSReq path also rejects this configuration defense-in-depth.
 	ChallengePassword string
 }
 

internal/service/m11c_crypto_enforcement_test.go

@@ -119,7 +119,10 @@ func TestESTService_MaxTTL_ForwardedToIssuer(t *testing.T) {
 
 func TestSCEPService_CryptoValidation_RejectsWeakKey(t *testing.T) {
 	mockIssuer := &mockIssuerConnector{}
-	svc := NewSCEPService("iss-local", mockIssuer, nil, slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelError})), "")
+	// H-2: SCEPService now requires a configured challenge password. Pass a
+	// matching client password so this test exercises the crypto-policy path
+	// rather than being short-circuited by the challenge-password guard.
+	svc := NewSCEPService("iss-local", mockIssuer, nil, slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelError})), "secret123")
 
 	// Profile requiring ECDSA P-384 minimum
 	profileRepo := newM11cProfileRepo()
@@ -136,7 +139,7 @@ func TestSCEPService_CryptoValidation_RejectsWeakKey(t *testing.T) {
 	// P-256 CSR should be rejected
 	csrPEM := generateCSRPEM(t, "device.example.com", nil)
 
-	_, err := svc.PKCSReq(context.Background(), csrPEM, "", "txn-001")
+	_, err := svc.PKCSReq(context.Background(), csrPEM, "secret123", "txn-001")
 	if err == nil {
 		t.Fatal("expected rejection for ECDSA P-256 against P-384 minimum")
 	}
@@ -152,7 +155,8 @@ func TestSCEPService_CryptoValidation_AcceptsStrongKey(t *testing.T) {
 	mockIssuer := &mockIssuerConnector{}
 	auditRepo := newMockAuditRepository()
 	auditSvc := NewAuditService(auditRepo)
-	svc := NewSCEPService("iss-local", mockIssuer, auditSvc, slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelError})), "")
+	// H-2: happy path exercises the authenticated branch.
+	svc := NewSCEPService("iss-local", mockIssuer, auditSvc, slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelError})), "secret123")
 
 	profileRepo := newM11cProfileRepo()
 	profileRepo.profiles["prof-standard"] = &domain.CertificateProfile{
@@ -167,7 +171,7 @@ func TestSCEPService_CryptoValidation_AcceptsStrongKey(t *testing.T) {
 
 	csrPEM := generateCSRPEM(t, "device-ok.example.com", nil)
 
-	result, err := svc.PKCSReq(context.Background(), csrPEM, "", "txn-002")
+	result, err := svc.PKCSReq(context.Background(), csrPEM, "secret123", "txn-002")
 	if err != nil {
 		t.Fatalf("expected success: %v", err)
 	}
@@ -179,7 +183,8 @@ func TestSCEPService_CryptoValidation_AcceptsStrongKey(t *testing.T) {
 func TestSCEPService_MaxTTL_ForwardedToIssuer(t *testing.T) {
 	capturingMock := &capturingIssuerConnector{}
 
-	svc := NewSCEPService("iss-local", capturingMock, nil, slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelError})), "")
+	// H-2: challenge password required for enrollment.
+	svc := NewSCEPService("iss-local", capturingMock, nil, slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelError})), "secret123")
 
 	profileRepo := newM11cProfileRepo()
 	profileRepo.profiles["prof-device"] = &domain.CertificateProfile{
@@ -192,7 +197,7 @@ func TestSCEPService_MaxTTL_ForwardedToIssuer(t *testing.T) {
 
 	csrPEM := generateCSRPEM(t, "mdm-device.example.com", nil)
 
-	_, err := svc.PKCSReq(context.Background(), csrPEM, "", "txn-003")
+	_, err := svc.PKCSReq(context.Background(), csrPEM, "secret123", "txn-003")
 	if err != nil {
 		t.Fatalf("unexpected error: %v", err)
 	}
@@ -341,12 +346,13 @@ func TestESTService_NoProfileRepo_PassesThrough(t *testing.T) {
 
 func TestSCEPService_NoProfileRepo_PassesThrough(t *testing.T) {
 	mockIssuer := &mockIssuerConnector{}
-	svc := NewSCEPService("iss-local", mockIssuer, nil, slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelError})), "")
+	// H-2: challenge password required for enrollment.
+	svc := NewSCEPService("iss-local", mockIssuer, nil, slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelError})), "secret123")
 	svc.SetProfileID("nonexistent-profile")
 
 	csrPEM := generateCSRPEM(t, "no-profile-scep.example.com", nil)
 
-	result, err := svc.PKCSReq(context.Background(), csrPEM, "", "txn-004")
+	result, err := svc.PKCSReq(context.Background(), csrPEM, "secret123", "txn-004")
 	if err != nil {
 		t.Fatalf("expected success when no profile repo set: %v", err)
 	}

internal/service/scep.go

@@ -2,6 +2,7 @@ package service
 
 import (
 	"context"
+	"crypto/subtle"
 	"crypto/x509"
 	"encoding/pem"
 	"fmt"
@@ -68,14 +69,34 @@ func (s *SCEPService) GetCACert(ctx context.Context) (string, error) {
 // PKCSReq processes a SCEP enrollment request.
 // RFC 8894 Section 3.3.1: PKCSReq contains a PKCS#10 CSR for certificate enrollment.
 // The CSR PEM and challenge password are extracted by the handler from the PKCS#7 envelope.
+//
+// H-2 fix (CWE-306): the previous implementation skipped the shared-secret
+// check entirely when s.challengePassword was empty, meaning any unauthenticated
+// client that could reach /scep could enroll a CSR against the configured
+// issuer. Reject that configuration defense-in-depth even though main() already
+// refuses to start in the same state (see preflightSCEPChallengePassword). The
+// non-empty branch now uses crypto/subtle.ConstantTimeCompare to avoid leaking
+// the shared secret through a response-time side channel.
 func (s *SCEPService) PKCSReq(ctx context.Context, csrPEM string, challengePassword string, transactionID string) (*domain.SCEPEnrollResult, error) {
-	// Validate challenge password
+	// Defense-in-depth: refuse any enrollment when no shared secret is
-	if s.challengePassword != "" {
+	// configured. The server-level pre-flight check in cmd/server/main.go
-		if challengePassword != s.challengePassword {
+	// normally prevents the service from being constructed in this state, but
-			s.logger.Warn("SCEP enrollment rejected: invalid challenge password",
+	// this branch also protects future call sites (tests, library reuse, a
-				"transaction_id", transactionID)
+	// future REST-over-HTTPS wrapper) from silently accepting unauthenticated
-			return nil, fmt.Errorf("invalid challenge password")
+	// CSRs.
-		}
+	if s.challengePassword == "" {
+		s.logger.Warn("SCEP enrollment rejected: server has no challenge password configured",
+			"transaction_id", transactionID)
+		return nil, fmt.Errorf("SCEP challenge password not configured on server")
+	}
+	// Constant-time compare avoids leaking the configured secret through
+	// response-time variance. ConstantTimeCompare returns 1 only when both
+	// slices have equal length AND equal content; a mismatched-length input
+	// still takes the same path as a content mismatch.
+	if subtle.ConstantTimeCompare([]byte(challengePassword), []byte(s.challengePassword)) != 1 {
+		s.logger.Warn("SCEP enrollment rejected: invalid challenge password",
+			"transaction_id", transactionID)
+		return nil, fmt.Errorf("invalid challenge password")
 	}
 
 	return s.processEnrollment(ctx, csrPEM, transactionID, "scep_pkcsreq")

internal/service/scep_test.go

@@ -58,11 +58,13 @@ func TestSCEPService_PKCSReq_Success(t *testing.T) {
 	mockIssuer := &mockIssuerConnector{}
 	auditRepo := newMockAuditRepository()
 	auditSvc := NewAuditService(auditRepo)
-	svc := NewSCEPService("iss-local", mockIssuer, auditSvc, slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelError})), "")
+	// H-2: SCEPService now requires a configured challenge password; the happy
+	// path exercises a matching client-submitted password.
+	svc := NewSCEPService("iss-local", mockIssuer, auditSvc, slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelError})), "secret123")
 
 	csrPEM := generateCSRPEM(t, "device.example.com", []string{"device.example.com"})
 
-	result, err := svc.PKCSReq(context.Background(), csrPEM, "", "txn-001")
+	result, err := svc.PKCSReq(context.Background(), csrPEM, "secret123", "txn-001")
 	if err != nil {
 		t.Fatalf("unexpected error: %v", err)
 	}
@@ -81,9 +83,9 @@ func TestSCEPService_PKCSReq_Success(t *testing.T) {
 
 func TestSCEPService_PKCSReq_InvalidCSR(t *testing.T) {
 	mockIssuer := &mockIssuerConnector{}
-	svc := NewSCEPService("iss-local", mockIssuer, nil, slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelError})), "")
+	svc := NewSCEPService("iss-local", mockIssuer, nil, slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelError})), "secret123")
 
-	_, err := svc.PKCSReq(context.Background(), "not-valid-pem", "", "txn-002")
+	_, err := svc.PKCSReq(context.Background(), "not-valid-pem", "secret123", "txn-002")
 	if err == nil {
 		t.Fatal("expected error for invalid CSR")
 	}
@@ -91,11 +93,11 @@ func TestSCEPService_PKCSReq_InvalidCSR(t *testing.T) {
 
 func TestSCEPService_PKCSReq_MissingCN(t *testing.T) {
 	mockIssuer := &mockIssuerConnector{}
-	svc := NewSCEPService("iss-local", mockIssuer, nil, slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelError})), "")
+	svc := NewSCEPService("iss-local", mockIssuer, nil, slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelError})), "secret123")
 
 	csrPEM := generateCSRPEM(t, "", []string{"test.example.com"})
 
-	_, err := svc.PKCSReq(context.Background(), csrPEM, "", "txn-003")
+	_, err := svc.PKCSReq(context.Background(), csrPEM, "secret123", "txn-003")
 	if err == nil {
 		t.Fatal("expected error for missing CN")
 	}
@@ -106,11 +108,11 @@ func TestSCEPService_PKCSReq_MissingCN(t *testing.T) {
 
 func TestSCEPService_PKCSReq_IssuerError(t *testing.T) {
 	mockIssuer := &mockIssuerConnector{Err: errors.New("issuance failed")}
-	svc := NewSCEPService("iss-local", mockIssuer, nil, slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelError})), "")
+	svc := NewSCEPService("iss-local", mockIssuer, nil, slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelError})), "secret123")
 
 	csrPEM := generateCSRPEM(t, "test.example.com", nil)
 
-	_, err := svc.PKCSReq(context.Background(), csrPEM, "", "txn-004")
+	_, err := svc.PKCSReq(context.Background(), csrPEM, "secret123", "txn-004")
 	if err == nil {
 		t.Fatal("expected error")
 	}
@@ -151,19 +153,49 @@ func TestSCEPService_PKCSReq_ChallengePassword_Invalid(t *testing.T) {
 	}
 }
 
-func TestSCEPService_PKCSReq_ChallengePassword_NotRequired(t *testing.T) {
+// TestSCEPService_PKCSReq_ChallengePassword_EmptyServerConfigRejected is the
-	// When server has no challenge password configured, any value should be accepted
+// H-2 regression guard. Before the fix (internal/service/scep.go:72-79 skipped
+// the password check when s.challengePassword was empty), an unconfigured
+// server accepted any enrollment (CWE-306). The service now rejects PKCSReq
+// defense-in-depth even if main()'s pre-flight is somehow bypassed.
+func TestSCEPService_PKCSReq_ChallengePassword_EmptyServerConfigRejected(t *testing.T) {
 	mockIssuer := &mockIssuerConnector{}
 	svc := NewSCEPService("iss-local", mockIssuer, nil, slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelError})), "")
 
 	csrPEM := generateCSRPEM(t, "device.example.com", nil)
 
-	result, err := svc.PKCSReq(context.Background(), csrPEM, "any-value", "txn-007")
+	// Any client-submitted password (including empty) must be rejected when
-	if err != nil {
+	// the server has no shared secret configured.
-		t.Fatalf("unexpected error: %v", err)
+	for _, clientPassword := range []string{"", "any-value", "guess"} {
+		_, err := svc.PKCSReq(context.Background(), csrPEM, clientPassword, "txn-empty")
+		if err == nil {
+			t.Fatalf("expected rejection when server challenge password is empty (client=%q)", clientPassword)
+		}
+		if !strings.Contains(err.Error(), "not configured") {
+			t.Errorf("expected 'not configured' in error, got: %v", err)
+		}
 	}
-	if result == nil {
+}
-		t.Fatal("expected non-nil result")
+
+// TestSCEPService_PKCSReq_ChallengePassword_ConstantTimeLengthIndependence
+// guards against regression from crypto/subtle.ConstantTimeCompare to a
+// short-circuiting byte compare. ConstantTimeCompare returns 0 whenever the
+// two slices differ in length OR content, so a same-prefix-but-longer input
+// must be rejected the same way as a completely different string.
+func TestSCEPService_PKCSReq_ChallengePassword_ConstantTimeLengthIndependence(t *testing.T) {
+	mockIssuer := &mockIssuerConnector{}
+	svc := NewSCEPService("iss-local", mockIssuer, nil, slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelError})), "secret123")
+
+	csrPEM := generateCSRPEM(t, "device.example.com", nil)
+
+	for _, bad := range []string{"secret", "secret12", "secret1234", "SECRET123", "wrong"} {
+		_, err := svc.PKCSReq(context.Background(), csrPEM, bad, "txn-ct")
+		if err == nil {
+			t.Fatalf("expected rejection for bad password %q", bad)
+		}
+		if !strings.Contains(err.Error(), "invalid challenge password") {
+			t.Errorf("expected 'invalid challenge password' for %q, got: %v", bad, err)
+		}
 	}
 }
 
@@ -171,12 +203,12 @@ func TestSCEPService_PKCSReq_WithProfile(t *testing.T) {
 	mockIssuer := &mockIssuerConnector{}
 	auditRepo := newMockAuditRepository()
 	auditSvc := NewAuditService(auditRepo)
-	svc := NewSCEPService("iss-local", mockIssuer, auditSvc, slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelError})), "")
+	svc := NewSCEPService("iss-local", mockIssuer, auditSvc, slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelError})), "secret123")
 	svc.SetProfileID("profile-mdm-device")
 
 	csrPEM := generateCSRPEM(t, "device.example.com", nil)
 
-	result, err := svc.PKCSReq(context.Background(), csrPEM, "", "txn-008")
+	result, err := svc.PKCSReq(context.Background(), csrPEM, "secret123", "txn-008")
 	if err != nil {
 		t.Fatalf("unexpected error: %v", err)
 	}