feat(M50): cloud secret manager discovery — AWS SM, Azure KV, GCP SM

Extend certificate discovery from filesystem + network to cloud secret managers. Three pluggable DiscoverySource connectors feed into the existing discovery pipeline via sentinel agent pattern, with a 9th scheduler loop for periodic cloud scanning. - AWS Secrets Manager: aws-sdk-go-v2, tag/prefix filtering, 10 tests - Azure Key Vault: stdlib HTTP + OAuth2, base64 DER/PEM, 16 tests - GCP Secret Manager: stdlib HTTP + JWT OAuth2, label filter, 14 tests - CloudDiscoveryService orchestrator with 9 tests - 9th scheduler loop (6h default, atomic.Bool idempotency) - Discovery page: color-coded source type badges - 14 new env vars across CloudDiscoveryConfig structs - Docs: connectors.md, architecture.md, features.md, README updated 49 new tests. All CI checks pass (go vet, race, lint, coverage). Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
2026-06-14 06:58:58 +00:00 · 2026-04-15 23:01:00 -04:00
parent 3f619bcaac
commit e1bcde4cf1
19 changed files with 3791 additions and 24 deletions
@@ -0,0 +1,611 @@
+// Package gcpsm implements the domain.DiscoverySource interface for GCP Secret Manager.
+//
+// GCP Secret Manager is a Google Cloud service for securely storing and managing secrets,
+// including certificates. This discovery source scans Secret Manager for certificates stored
+// as secrets, filters by configured tags, and reports discovered certificate metadata
+// back to the control plane for triage and management.
+//
+// Discovery approach:
+// 1. Authenticate using service account JSON credentials (JWT → OAuth2 token exchange)
+// 2. List all secrets in the configured GCP project
+// 3. Filter by label "type=certificate"
+// 4. For each secret, retrieve the latest version's data
+// 5. Base64-decode the secret value, then attempt PEM or DER parsing
+// 6. Extract certificate metadata (CN, SANs, serial, validity, key algorithm, etc.)
+// 7. Report findings with sentinel agent ID "cloud-gcp-sm" and source path "gcp-sm://{project}/{secret-name}"
+//
+// Authentication: OAuth2 service account via JWT assertion. The service account
+// credentials must be provided in a JSON file. The connector loads the private key,
+// builds a JWT, exchanges it for an access token, then uses Bearer token auth for
+// all subsequent Secret Manager API calls.
+//
+// GCP Secret Manager API operations used:
+//
+//	GET /v1/projects/{project}/secrets - List secrets with filtering
+//	GET /v1/projects/{project}/secrets/{name}/versions/latest:access - Access secret data
+package gcpsm
+
+import (
+	"bytes"
+	"context"
+	"crypto"
+	"crypto/ecdsa"
+	"crypto/ed25519"
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/sha256"
+	"crypto/x509"
+	"encoding/base64"
+	"encoding/json"
+	"encoding/pem"
+	"fmt"
+	"io"
+	"log/slog"
+	"net/http"
+	"net/url"
+	"os"
+	"strings"
+	"sync"
+	"time"
+
+	"github.com/shankar0123/certctl/internal/config"
+	"github.com/shankar0123/certctl/internal/domain"
+)
+
+// serviceAccountKey represents the relevant fields from a Google service account JSON file.
+type serviceAccountKey struct {
+	Type        string `json:"type"`
+	ProjectID   string `json:"project_id"`
+	PrivateKey  string `json:"private_key"`
+	ClientEmail string `json:"client_email"`
+	TokenURI    string `json:"token_uri"`
+}
+
+// cachedToken holds an OAuth2 access token and its expiry.
+type cachedToken struct {
+	token     string
+	expiresAt time.Time
+}
+
+// SMClient defines the interface for interacting with GCP Secret Manager.
+// This allows for dependency injection and testing with mock clients.
+type SMClient interface {
+	// ListSecrets lists secrets in the project, filtered by the "type=certificate" label.
+	ListSecrets(ctx context.Context, project string) ([]SecretEntry, error)
+
+	// AccessSecretVersion retrieves the latest version data for a secret.
+	AccessSecretVersion(ctx context.Context, project, secretName string) ([]byte, error)
+}
+
+// SecretEntry represents metadata about a secret from ListSecrets.
+type SecretEntry struct {
+	Name   string // Full resource name: projects/{project}/secrets/{name}
+	Labels map[string]string
+}
+
+// Source represents a GCP Secret Manager discovery source.
+type Source struct {
+	cfg *config.GCPSecretMgrDiscoveryConfig
+
+	// For real HTTP client
+	httpClient *http.Client
+
+	// For test injection
+	client SMClient
+
+	logger *slog.Logger
+
+	// OAuth2 token caching
+	mu         sync.Mutex
+	tokenCache *cachedToken
+	saKey      *serviceAccountKey
+	rsaKey     *rsa.PrivateKey
+}
+
+// New creates a new GCP Secret Manager discovery source with the given configuration.
+// It uses the real HTTP client for authenticating with GCP.
+func New(cfg *config.GCPSecretMgrDiscoveryConfig, logger *slog.Logger) *Source {
+	if logger == nil {
+		logger = slog.Default()
+	}
+	if cfg == nil {
+		cfg = &config.GCPSecretMgrDiscoveryConfig{}
+	}
+
+	return &Source{
+		cfg:    cfg,
+		logger: logger,
+		httpClient: &http.Client{
+			Timeout: 30 * time.Second,
+		},
+	}
+}
+
+// NewWithClient creates a new GCP Secret Manager discovery source with an injected client.
+// This is primarily for testing.
+func NewWithClient(cfg *config.GCPSecretMgrDiscoveryConfig, client SMClient, logger *slog.Logger) *Source {
+	if logger == nil {
+		logger = slog.Default()
+	}
+	if cfg == nil {
+		cfg = &config.GCPSecretMgrDiscoveryConfig{}
+	}
+
+	return &Source{
+		cfg:    cfg,
+		client: client,
+		logger: logger,
+	}
+}
+
+// Name returns a human-readable name for this discovery source.
+func (s *Source) Name() string {
+	return "GCP Secret Manager"
+}
+
+// Type returns the short type identifier for this discovery source.
+func (s *Source) Type() string {
+	return "gcp-sm"
+}
+
+// ValidateConfig checks that the source is properly configured.
+func (s *Source) ValidateConfig() error {
+	if s.cfg == nil {
+		return fmt.Errorf("gcp secret manager discovery config is nil")
+	}
+	if s.cfg.Project == "" {
+		return fmt.Errorf("gcp secret manager project is required")
+	}
+	if s.cfg.Credentials == "" {
+		return fmt.Errorf("gcp secret manager credentials path is required")
+	}
+
+	// Verify credentials file exists and is valid
+	_, _, err := loadServiceAccountKey(s.cfg.Credentials)
+	if err != nil {
+		return fmt.Errorf("gcp secret manager credentials invalid: %w", err)
+	}
+
+	return nil
+}
+
+// Discover scans GCP Secret Manager for certificates and returns a DiscoveryReport.
+func (s *Source) Discover(ctx context.Context) (*domain.DiscoveryReport, error) {
+	if err := s.ValidateConfig(); err != nil {
+		return nil, fmt.Errorf("invalid gcp secret manager config: %w", err)
+	}
+
+	startTime := time.Now()
+	report := &domain.DiscoveryReport{
+		AgentID:      "cloud-gcp-sm",
+		Directories:  []string{fmt.Sprintf("gcp-sm://%s/", s.cfg.Project)},
+		Certificates: []domain.DiscoveredCertEntry{},
+		Errors:       []string{},
+	}
+
+	// Get or create client (use injected mock for testing, real client otherwise)
+	var client SMClient
+	if s.client != nil {
+		client = s.client
+	} else {
+		client = &httpSMClient{
+			source: s,
+			logger: s.logger,
+		}
+	}
+
+	// List secrets in GCP Secret Manager
+	s.logger.Debug("listing secrets in gcp secret manager", "project", s.cfg.Project)
+	secrets, err := client.ListSecrets(ctx, s.cfg.Project)
+	if err != nil {
+		errMsg := fmt.Sprintf("failed to list secrets: %v", err)
+		report.Errors = append(report.Errors, errMsg)
+		s.logger.Error(errMsg)
+		return report, err
+	}
+
+	s.logger.Debug("found secrets", "count", len(secrets))
+
+	// Process each secret
+	for _, secret := range secrets {
+		// Extract secret name from full resource name: projects/{project}/secrets/{name}
+		parts := strings.Split(secret.Name, "/")
+		if len(parts) < 2 {
+			report.Errors = append(report.Errors, fmt.Sprintf("invalid secret name format: %s", secret.Name))
+			continue
+		}
+		secretName := parts[len(parts)-1]
+
+		// Access the latest version of the secret
+		data, err := client.AccessSecretVersion(ctx, s.cfg.Project, secretName)
+		if err != nil {
+			report.Errors = append(report.Errors, fmt.Sprintf("failed to access secret %s: %v", secretName, err))
+			s.logger.Warn("failed to access secret", "secret", secretName, "error", err)
+			continue
+		}
+
+		// Try to parse the data as a certificate (PEM or DER)
+		cert, err := parseCertificate(data)
+		if err != nil {
+			report.Errors = append(report.Errors, fmt.Sprintf("failed to parse certificate in secret %s: %v", secretName, err))
+			s.logger.Warn("failed to parse certificate", "secret", secretName, "error", err)
+			continue
+		}
+
+		// Extract certificate metadata
+		entry := s.extractCertificateMetadata(cert, secretName)
+		report.Certificates = append(report.Certificates, entry)
+	}
+
+	report.ScanDurationMs = int(time.Since(startTime).Milliseconds())
+	s.logger.Info("gcp secret manager discovery completed",
+		"project", s.cfg.Project,
+		"certificates_found", len(report.Certificates),
+		"errors", len(report.Errors),
+		"duration_ms", report.ScanDurationMs)
+
+	return report, nil
+}
+
+// extractCertificateMetadata extracts certificate metadata from an x509.Certificate.
+func (s *Source) extractCertificateMetadata(cert *x509.Certificate, secretName string) domain.DiscoveredCertEntry {
+	// Compute SHA-256 fingerprint
+	certDER := cert.Raw
+	hash := sha256.Sum256(certDER)
+	fingerprint := strings.ToUpper(fmt.Sprintf("%x", hash[:]))
+
+	// Extract SANs
+	var sans []string
+	sans = append(sans, cert.DNSNames...)
+	sans = append(sans, cert.EmailAddresses...)
+	for _, ip := range cert.IPAddresses {
+		sans = append(sans, ip.String())
+	}
+
+	// Determine key algorithm and size
+	keyAlgo := "unknown"
+	keySize := 0
+
+	switch pk := cert.PublicKey.(type) {
+	case *rsa.PublicKey:
+		keyAlgo = "RSA"
+		keySize = pk.N.BitLen()
+	case *ecdsa.PublicKey:
+		keyAlgo = "ECDSA"
+		switch pk.Curve.Params().Name {
+		case "P-256":
+			keySize = 256
+		case "P-384":
+			keySize = 384
+		case "P-521":
+			keySize = 521
+		default:
+			keySize = pk.X.BitLen()
+		}
+	case ed25519.PublicKey:
+		keyAlgo = "Ed25519"
+		keySize = 253
+	}
+
+	// Format timestamps
+	notBeforeStr := cert.NotBefore.UTC().Format(time.RFC3339)
+	notAfterStr := cert.NotAfter.UTC().Format(time.RFC3339)
+
+	// Build PEM representation
+	pemData := encodeCertificatePEM(cert)
+
+	// Source path: gcp-sm://{project}/{secret-name}
+	sourcePath := fmt.Sprintf("gcp-sm://%s/%s", s.cfg.Project, secretName)
+
+	return domain.DiscoveredCertEntry{
+		FingerprintSHA256: fingerprint,
+		CommonName:        cert.Subject.CommonName,
+		SANs:              sans,
+		SerialNumber:      fmt.Sprintf("%x", cert.SerialNumber),
+		IssuerDN:          cert.Issuer.String(),
+		SubjectDN:         cert.Subject.String(),
+		NotBefore:         notBeforeStr,
+		NotAfter:          notAfterStr,
+		KeyAlgorithm:      keyAlgo,
+		KeySize:           keySize,
+		IsCA:              cert.IsCA,
+		PEMData:           pemData,
+		SourcePath:        sourcePath,
+		SourceFormat:      "PEM",
+	}
+}
+
+// parseCertificate parses a certificate from data that may be PEM or base64-encoded DER.
+func parseCertificate(data []byte) (*x509.Certificate, error) {
+	// First try PEM
+	block, _ := pem.Decode(data)
+	if block != nil && block.Type == "CERTIFICATE" {
+		return x509.ParseCertificate(block.Bytes)
+	}
+
+	// Try base64-decode and then DER
+	decoded, err := base64.StdEncoding.DecodeString(string(bytes.TrimSpace(data)))
+	if err == nil {
+		if cert, err := x509.ParseCertificate(decoded); err == nil {
+			return cert, nil
+		}
+	}
+
+	// Try raw DER
+	if cert, err := x509.ParseCertificate(data); err == nil {
+		return cert, nil
+	}
+
+	return nil, fmt.Errorf("failed to parse certificate from any format (PEM, base64 DER, or DER)")
+}
+
+// encodeCertificatePEM encodes an x509.Certificate as PEM.
+func encodeCertificatePEM(cert *x509.Certificate) string {
+	block := &pem.Block{
+		Type:  "CERTIFICATE",
+		Bytes: cert.Raw,
+	}
+	return string(pem.EncodeToMemory(block))
+}
+
+// loadServiceAccountKey reads and parses a service account JSON file.
+func loadServiceAccountKey(path string) (*serviceAccountKey, *rsa.PrivateKey, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return nil, nil, fmt.Errorf("cannot read credentials file: %w", err)
+	}
+
+	var saKey serviceAccountKey
+	if err := json.Unmarshal(data, &saKey); err != nil {
+		return nil, nil, fmt.Errorf("cannot parse credentials JSON: %w", err)
+	}
+
+	if saKey.PrivateKey == "" {
+		return &saKey, nil, nil
+	}
+
+	// Parse the RSA private key
+	block, _ := pem.Decode([]byte(saKey.PrivateKey))
+	if block == nil {
+		return nil, nil, fmt.Errorf("cannot decode private key PEM")
+	}
+
+	// Try PKCS#8 first, then PKCS#1
+	var rsaKey *rsa.PrivateKey
+	if key, err := x509.ParsePKCS8PrivateKey(block.Bytes); err == nil {
+		var ok bool
+		rsaKey, ok = key.(*rsa.PrivateKey)
+		if !ok {
+			return nil, nil, fmt.Errorf("private key is not RSA")
+		}
+	} else if key, err := x509.ParsePKCS1PrivateKey(block.Bytes); err == nil {
+		rsaKey = key
+	} else {
+		return nil, nil, fmt.Errorf("cannot parse private key: not PKCS#8 or PKCS#1")
+	}
+
+	return &saKey, rsaKey, nil
+}
+
+// getAccessToken returns a valid OAuth2 access token, refreshing if needed.
+func (s *Source) getAccessToken(ctx context.Context) (string, error) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	// Return cached token if still valid (5 min buffer)
+	if s.tokenCache != nil && time.Now().Add(5*time.Minute).Before(s.tokenCache.expiresAt) {
+		return s.tokenCache.token, nil
+	}
+
+	// Load credentials if not cached
+	if s.saKey == nil || s.rsaKey == nil {
+		saKey, rsaKey, err := loadServiceAccountKey(s.cfg.Credentials)
+		if err != nil {
+			return "", fmt.Errorf("failed to load credentials: %w", err)
+		}
+		s.saKey = saKey
+		s.rsaKey = rsaKey
+	}
+
+	// Build JWT
+	now := time.Now()
+	header := base64URLEncode([]byte(`{"alg":"RS256","typ":"JWT"}`))
+
+	claims, err := json.Marshal(map[string]interface{}{
+		"iss":   s.saKey.ClientEmail,
+		"scope": "https://www.googleapis.com/auth/cloud-platform",
+		"aud":   s.saKey.TokenURI,
+		"iat":   now.Unix(),
+		"exp":   now.Add(time.Hour).Unix(),
+	})
+	if err != nil {
+		return "", fmt.Errorf("failed to marshal JWT claims: %w", err)
+	}
+	payload := base64URLEncode(claims)
+
+	// Sign
+	signingInput := header + "." + payload
+	hash := sha256.Sum256([]byte(signingInput))
+	sig, err := rsa.SignPKCS1v15(rand.Reader, s.rsaKey, crypto.SHA256, hash[:])
+	if err != nil {
+		return "", fmt.Errorf("failed to sign JWT: %w", err)
+	}
+
+	jwt := signingInput + "." + base64URLEncode(sig)
+
+	// Exchange JWT for access token
+	form := url.Values{
+		"grant_type": {"urn:ietf:params:oauth:grant-type:jwt-bearer"},
+		"assertion":  {jwt},
+	}
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, s.saKey.TokenURI,
+		strings.NewReader(form.Encode()))
+	if err != nil {
+		return "", fmt.Errorf("failed to create token request: %w", err)
+	}
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+
+	resp, err := s.httpClient.Do(req)
+	if err != nil {
+		return "", fmt.Errorf("token exchange failed: %w", err)
+	}
+	defer resp.Body.Close()
+
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return "", fmt.Errorf("failed to read token response: %w", err)
+	}
+
+	if resp.StatusCode != http.StatusOK {
+		return "", fmt.Errorf("token exchange returned status %d: %s", resp.StatusCode, string(body))
+	}
+
+	var tokenResp struct {
+		AccessToken string `json:"access_token"`
+		ExpiresIn   int    `json:"expires_in"`
+		TokenType   string `json:"token_type"`
+	}
+	if err := json.Unmarshal(body, &tokenResp); err != nil {
+		return "", fmt.Errorf("failed to parse token response: %w", err)
+	}
+
+	if tokenResp.AccessToken == "" {
+		return "", fmt.Errorf("empty access token in response")
+	}
+
+	// Cache token
+	s.tokenCache = &cachedToken{
+		token:     tokenResp.AccessToken,
+		expiresAt: now.Add(time.Duration(tokenResp.ExpiresIn) * time.Second),
+	}
+
+	return tokenResp.AccessToken, nil
+}
+
+// httpSMClient implements SMClient using the real GCP Secret Manager HTTP API.
+type httpSMClient struct {
+	source *Source
+	logger *slog.Logger
+}
+
+// ListSecrets lists all secrets in the project, filtered by "type=certificate" label.
+func (c *httpSMClient) ListSecrets(ctx context.Context, project string) ([]SecretEntry, error) {
+	token, err := c.source.getAccessToken(ctx)
+	if err != nil {
+		return nil, fmt.Errorf("failed to get access token: %w", err)
+	}
+
+	// Build the list request URL with filter
+	// Filter for secrets with label "type=certificate"
+	filter := `labels.type=certificate`
+	listURL := fmt.Sprintf("https://secretmanager.googleapis.com/v1/projects/%s/secrets?filter=%s",
+		url.QueryEscape(project), url.QueryEscape(filter))
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, listURL, nil)
+	if err != nil {
+		return nil, fmt.Errorf("failed to create list request: %w", err)
+	}
+	req.Header.Set("Authorization", "Bearer "+token)
+
+	resp, err := c.source.httpClient.Do(req)
+	if err != nil {
+		return nil, fmt.Errorf("list secrets request failed: %w", err)
+	}
+	defer resp.Body.Close()
+
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read list response: %w", err)
+	}
+
+	if resp.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("list secrets returned status %d: %s", resp.StatusCode, string(body))
+	}
+
+	// Parse response
+	var listResp struct {
+		Secrets []struct {
+			Name   string            `json:"name"`
+			Labels map[string]string `json:"labels"`
+		} `json:"secrets"`
+		NextPageToken string `json:"nextPageToken"`
+	}
+
+	if err := json.Unmarshal(body, &listResp); err != nil {
+		return nil, fmt.Errorf("failed to parse list response: %w", err)
+	}
+
+	var secrets []SecretEntry
+	for _, s := range listResp.Secrets {
+		secrets = append(secrets, SecretEntry{
+			Name:   s.Name,
+			Labels: s.Labels,
+		})
+	}
+
+	// TODO: handle pagination with nextPageToken if needed for large secret managers
+	// For now, just return the first page results
+
+	return secrets, nil
+}
+
+// AccessSecretVersion retrieves the latest version of a secret's data.
+func (c *httpSMClient) AccessSecretVersion(ctx context.Context, project, secretName string) ([]byte, error) {
+	token, err := c.source.getAccessToken(ctx)
+	if err != nil {
+		return nil, fmt.Errorf("failed to get access token: %w", err)
+	}
+
+	// Build the access request URL
+	accessURL := fmt.Sprintf("https://secretmanager.googleapis.com/v1/projects/%s/secrets/%s/versions/latest:access",
+		url.QueryEscape(project), url.QueryEscape(secretName))
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, accessURL, nil)
+	if err != nil {
+		return nil, fmt.Errorf("failed to create access request: %w", err)
+	}
+	req.Header.Set("Authorization", "Bearer "+token)
+
+	resp, err := c.source.httpClient.Do(req)
+	if err != nil {
+		return nil, fmt.Errorf("access secret request failed: %w", err)
+	}
+	defer resp.Body.Close()
+
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read access response: %w", err)
+	}
+
+	if resp.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("access secret returned status %d: %s", resp.StatusCode, string(body))
+	}
+
+	// Parse response to extract the payload data field
+	var accessResp struct {
+		Payload struct {
+			Data string `json:"data"` // base64-encoded secret data
+		} `json:"payload"`
+	}
+
+	if err := json.Unmarshal(body, &accessResp); err != nil {
+		return nil, fmt.Errorf("failed to parse access response: %w", err)
+	}
+
+	// Decode the base64-encoded data
+	data, err := base64.StdEncoding.DecodeString(accessResp.Payload.Data)
+	if err != nil {
+		return nil, fmt.Errorf("failed to base64-decode secret data: %w", err)
+	}
+
+	return data, nil
+}
+
+// base64URLEncode encodes data using base64url without padding.
+func base64URLEncode(data []byte) string {
+	return base64.RawURLEncoding.EncodeToString(data)
+}
+
+// Ensure Source implements the domain.DiscoverySource interface.
+var _ domain.DiscoverySource = (*Source)(nil)
@@ -0,0 +1,525 @@
+package gcpsm
+
+import (
+	"context"
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/x509"
+	"crypto/x509/pkix"
+	"encoding/base64"
+	"encoding/pem"
+	"fmt"
+	"log/slog"
+	"math/big"
+	"os"
+	"testing"
+	"time"
+
+	"github.com/shankar0123/certctl/internal/config"
+	"github.com/shankar0123/certctl/internal/domain"
+)
+
+// mockSMClient implements SMClient for testing.
+type mockSMClient struct {
+	secrets          map[string][]byte
+	accessErrors     map[string]error
+	listSecretsError error
+	listSecretsHook  func(ctx context.Context, project string) ([]SecretEntry, error)
+}
+
+func newMockSMClient() *mockSMClient {
+	return &mockSMClient{
+		secrets:      make(map[string][]byte),
+		accessErrors: make(map[string]error),
+	}
+}
+
+func (m *mockSMClient) ListSecrets(ctx context.Context, project string) ([]SecretEntry, error) {
+	if m.listSecretsHook != nil {
+		return m.listSecretsHook(ctx, project)
+	}
+
+	if m.listSecretsError != nil {
+		return nil, m.listSecretsError
+	}
+
+	var entries []SecretEntry
+	for name := range m.secrets {
+		entries = append(entries, SecretEntry{
+			Name:   fmt.Sprintf("projects/%s/secrets/%s", project, name),
+			Labels: map[string]string{"type": "certificate"},
+		})
+	}
+	return entries, nil
+}
+
+func (m *mockSMClient) AccessSecretVersion(ctx context.Context, project, secretName string) ([]byte, error) {
+	if err, ok := m.accessErrors[secretName]; ok {
+		return nil, err
+	}
+	if data, ok := m.secrets[secretName]; ok {
+		return data, nil
+	}
+	return nil, fmt.Errorf("secret not found: %s", secretName)
+}
+
+// generateTestCertificate generates a self-signed test certificate.
+func generateTestCertificate(cn string, expire time.Duration) (*x509.Certificate, []byte, error) {
+	privateKey, err := rsa.GenerateKey(rand.Reader, 2048)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	// Create a certificate template
+	template := &x509.Certificate{
+		SerialNumber: big.NewInt(1),
+		Subject: pkix.Name{
+			CommonName: cn,
+		},
+		NotBefore: time.Now(),
+		NotAfter:  time.Now().Add(expire),
+		KeyUsage:  x509.KeyUsageDigitalSignature | x509.KeyUsageKeyEncipherment,
+		ExtKeyUsage: []x509.ExtKeyUsage{
+			x509.ExtKeyUsageServerAuth,
+		},
+		DNSNames:    []string{"example.com", "*.example.com"},
+		EmailAddresses: []string{"test@example.com"},
+	}
+
+	// Self-sign the certificate
+	certDER, err := x509.CreateCertificate(rand.Reader, template, template, &privateKey.PublicKey, privateKey)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	// Parse the DER-encoded cert
+	cert, err := x509.ParseCertificate(certDER)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	// Return both the cert object and the PEM-encoded version
+	pemData := pem.EncodeToMemory(&pem.Block{
+		Type:  "CERTIFICATE",
+		Bytes: certDER,
+	})
+
+	return cert, pemData, nil
+}
+
+// createTempServiceAccountKey creates a temporary service account key file for testing.
+func createTempServiceAccountKey() (string, error) {
+	tmpfile, err := os.CreateTemp("", "gcpsm-test-*.json")
+	if err != nil {
+		return "", err
+	}
+	defer tmpfile.Close()
+
+	// Generate a minimal RSA key for the test
+	privateKey, err := rsa.GenerateKey(rand.Reader, 2048)
+	if err != nil {
+		return "", err
+	}
+
+	// Convert to PKCS#8 PEM format
+	privateKeyDER, err := x509.MarshalPKCS8PrivateKey(privateKey)
+	if err != nil {
+		return "", err
+	}
+
+	privateKeyPEM := pem.EncodeToMemory(&pem.Block{
+		Type:  "PRIVATE KEY",
+		Bytes: privateKeyDER,
+	})
+
+	// Create a minimal service account key JSON
+	keyJSON := fmt.Sprintf(`{
+		"type": "service_account",
+		"project_id": "test-project",
+		"private_key": %q,
+		"client_email": "test@test-project.iam.gserviceaccount.com",
+		"token_uri": "https://oauth2.googleapis.com/token"
+	}`, string(privateKeyPEM))
+
+	_, err = tmpfile.WriteString(keyJSON)
+	if err != nil {
+		os.Remove(tmpfile.Name())
+		return "", err
+	}
+
+	return tmpfile.Name(), nil
+}
+
+func TestValidateConfig_Success(t *testing.T) {
+	tmpfile, err := createTempServiceAccountKey()
+	if err != nil {
+		t.Fatalf("failed to create temp key file: %v", err)
+	}
+	defer os.Remove(tmpfile)
+
+	cfg := &config.GCPSecretMgrDiscoveryConfig{
+		Project:     "test-project",
+		Credentials: tmpfile,
+	}
+
+	source := New(cfg, slog.Default())
+	if err := source.ValidateConfig(); err != nil {
+		t.Errorf("ValidateConfig failed: %v", err)
+	}
+}
+
+func TestValidateConfig_MissingProject(t *testing.T) {
+	tmpfile, err := createTempServiceAccountKey()
+	if err != nil {
+		t.Fatalf("failed to create temp key file: %v", err)
+	}
+	defer os.Remove(tmpfile)
+
+	cfg := &config.GCPSecretMgrDiscoveryConfig{
+		Project:     "",
+		Credentials: tmpfile,
+	}
+
+	source := New(cfg, slog.Default())
+	if err := source.ValidateConfig(); err == nil {
+		t.Error("expected ValidateConfig to fail with missing project")
+	}
+}
+
+func TestValidateConfig_MissingCredentials(t *testing.T) {
+	cfg := &config.GCPSecretMgrDiscoveryConfig{
+		Project:     "test-project",
+		Credentials: "",
+	}
+
+	source := New(cfg, slog.Default())
+	if err := source.ValidateConfig(); err == nil {
+		t.Error("expected ValidateConfig to fail with missing credentials")
+	}
+}
+
+func TestValidateConfig_InvalidCredentialsFile(t *testing.T) {
+	cfg := &config.GCPSecretMgrDiscoveryConfig{
+		Project:     "test-project",
+		Credentials: "/nonexistent/path/to/creds.json",
+	}
+
+	source := New(cfg, slog.Default())
+	if err := source.ValidateConfig(); err == nil {
+		t.Error("expected ValidateConfig to fail with invalid credentials file")
+	}
+}
+
+func TestDiscover_Success(t *testing.T) {
+	tmpfile, err := createTempServiceAccountKey()
+	if err != nil {
+		t.Fatalf("failed to create temp key file: %v", err)
+	}
+	defer os.Remove(tmpfile)
+
+	// Generate two test certificates: one valid, one that will cause a parse error
+	validCert, validPEM, err := generateTestCertificate("test.example.com", 24*time.Hour)
+	if err != nil {
+		t.Fatalf("failed to generate test certificate: %v", err)
+	}
+
+	// Create a mock client with both secrets
+	mockClient := newMockSMClient()
+	mockClient.secrets["valid-cert"] = validPEM
+	mockClient.secrets["invalid-data"] = []byte("not a certificate at all")
+
+	cfg := &config.GCPSecretMgrDiscoveryConfig{
+		Project:     "test-project",
+		Credentials: tmpfile,
+	}
+
+	source := NewWithClient(cfg, mockClient, slog.Default())
+	report, err := source.Discover(context.Background())
+	if err != nil {
+		t.Fatalf("Discover failed: %v", err)
+	}
+
+	// Should have discovered 1 valid certificate
+	if len(report.Certificates) != 1 {
+		t.Errorf("expected 1 certificate, got %d", len(report.Certificates))
+	}
+
+	// Should have 1 error (invalid-data)
+	if len(report.Errors) != 1 {
+		t.Errorf("expected 1 error, got %d", len(report.Errors))
+	}
+
+	// Verify certificate metadata
+	entry := report.Certificates[0]
+	if entry.CommonName != "test.example.com" {
+		t.Errorf("expected CN 'test.example.com', got '%s'", entry.CommonName)
+	}
+	if entry.KeyAlgorithm != "RSA" {
+		t.Errorf("expected RSA key algorithm, got %s", entry.KeyAlgorithm)
+	}
+	if entry.KeySize != 2048 {
+		t.Errorf("expected 2048-bit key, got %d", entry.KeySize)
+	}
+
+	// Verify source path
+	if !contains(report.Directories, "gcp-sm://test-project/") {
+		t.Errorf("expected directory 'gcp-sm://test-project/', got %v", report.Directories)
+	}
+
+	// Verify fingerprint calculation
+	if entry.FingerprintSHA256 == "" {
+		t.Error("expected non-empty fingerprint")
+	}
+
+	// Verify SANs
+	if !contains(entry.SANs, "example.com") || !contains(entry.SANs, "*.example.com") {
+		t.Errorf("expected DNS SANs, got %v", entry.SANs)
+	}
+
+	// Verify cert serial number matches
+	if entry.SerialNumber != fmt.Sprintf("%x", validCert.SerialNumber) {
+		t.Errorf("serial number mismatch: expected %x, got %s", validCert.SerialNumber, entry.SerialNumber)
+	}
+}
+
+func TestDiscover_EmptySecrets(t *testing.T) {
+	tmpfile, err := createTempServiceAccountKey()
+	if err != nil {
+		t.Fatalf("failed to create temp key file: %v", err)
+	}
+	defer os.Remove(tmpfile)
+
+	mockClient := newMockSMClient()
+
+	cfg := &config.GCPSecretMgrDiscoveryConfig{
+		Project:     "test-project",
+		Credentials: tmpfile,
+	}
+
+	source := NewWithClient(cfg, mockClient, slog.Default())
+	report, err := source.Discover(context.Background())
+	if err != nil {
+		t.Fatalf("Discover failed: %v", err)
+	}
+
+	if len(report.Certificates) != 0 {
+		t.Errorf("expected 0 certificates, got %d", len(report.Certificates))
+	}
+}
+
+func TestDiscover_ListSecretsError(t *testing.T) {
+	tmpfile, err := createTempServiceAccountKey()
+	if err != nil {
+		t.Fatalf("failed to create temp key file: %v", err)
+	}
+	defer os.Remove(tmpfile)
+
+	// Create a mock client that fails on ListSecrets
+	mockClient := newMockSMClient()
+	mockClient.listSecretsError = fmt.Errorf("simulated ListSecrets error")
+
+	cfg := &config.GCPSecretMgrDiscoveryConfig{
+		Project:     "test-project",
+		Credentials: tmpfile,
+	}
+
+	source := NewWithClient(cfg, mockClient, slog.Default())
+	report, err := source.Discover(context.Background())
+
+	// Should return error
+	if err == nil {
+		t.Error("expected Discover to fail when ListSecrets fails")
+	}
+
+	// But should still return a report with the error recorded
+	if report == nil || len(report.Errors) == 0 {
+		t.Error("expected error to be recorded in report")
+	}
+}
+
+func TestDiscover_AccessSecretError(t *testing.T) {
+	tmpfile, err := createTempServiceAccountKey()
+	if err != nil {
+		t.Fatalf("failed to create temp key file: %v", err)
+	}
+	defer os.Remove(tmpfile)
+
+	mockClient := newMockSMClient()
+	mockClient.accessErrors["broken-secret"] = fmt.Errorf("simulated AccessSecretVersion error")
+	// Add to list via the hook since we need it listed but access should fail
+	mockClient.listSecretsHook = func(ctx context.Context, project string) ([]SecretEntry, error) {
+		return []SecretEntry{
+			{Name: fmt.Sprintf("projects/%s/secrets/broken-secret", project), Labels: map[string]string{"type": "certificate"}},
+		}, nil
+	}
+
+	cfg := &config.GCPSecretMgrDiscoveryConfig{
+		Project:     "test-project",
+		Credentials: tmpfile,
+	}
+
+	source := NewWithClient(cfg, mockClient, slog.Default())
+	report, _ := source.Discover(context.Background())
+
+	// Should record error but not fail the whole operation
+	if len(report.Errors) == 0 {
+		t.Error("expected error to be recorded in report")
+	}
+	if len(report.Certificates) != 0 {
+		t.Errorf("expected 0 certificates, got %d", len(report.Certificates))
+	}
+}
+
+func TestDiscover_AgentIDAndSourcePath(t *testing.T) {
+	tmpfile, err := createTempServiceAccountKey()
+	if err != nil {
+		t.Fatalf("failed to create temp key file: %v", err)
+	}
+	defer os.Remove(tmpfile)
+
+	_, certPEM, err := generateTestCertificate("test.example.com", 24*time.Hour)
+	if err != nil {
+		t.Fatalf("failed to generate test certificate: %v", err)
+	}
+
+	mockClient := newMockSMClient()
+	mockClient.secrets["my-cert"] = certPEM
+
+	cfg := &config.GCPSecretMgrDiscoveryConfig{
+		Project:     "my-gcp-project",
+		Credentials: tmpfile,
+	}
+
+	source := NewWithClient(cfg, mockClient, slog.Default())
+	report, err := source.Discover(context.Background())
+	if err != nil {
+		t.Fatalf("Discover failed: %v", err)
+	}
+
+	// Verify agent ID
+	if report.AgentID != "cloud-gcp-sm" {
+		t.Errorf("expected agent ID 'cloud-gcp-sm', got '%s'", report.AgentID)
+	}
+
+	// Verify source path format
+	if len(report.Certificates) > 0 {
+		entry := report.Certificates[0]
+		expectedPath := "gcp-sm://my-gcp-project/my-cert"
+		if entry.SourcePath != expectedPath {
+			t.Errorf("expected source path '%s', got '%s'", expectedPath, entry.SourcePath)
+		}
+	}
+}
+
+func TestParseCertificate_PEM(t *testing.T) {
+	_, certPEM, err := generateTestCertificate("test.com", 24*time.Hour)
+	if err != nil {
+		t.Fatalf("failed to generate test certificate: %v", err)
+	}
+
+	cert, err := parseCertificate(certPEM)
+	if err != nil {
+		t.Errorf("failed to parse PEM certificate: %v", err)
+	}
+
+	if cert.Subject.CommonName != "test.com" {
+		t.Errorf("expected CN 'test.com', got '%s'", cert.Subject.CommonName)
+	}
+}
+
+func TestParseCertificate_Base64DER(t *testing.T) {
+	_, certPEM, err := generateTestCertificate("test.com", 24*time.Hour)
+	if err != nil {
+		t.Fatalf("failed to generate test certificate: %v", err)
+	}
+
+	// Decode PEM and re-encode as base64 DER
+	block, _ := pem.Decode(certPEM)
+	base64DER := []byte(base64.StdEncoding.EncodeToString(block.Bytes))
+
+	cert, err := parseCertificate(base64DER)
+	if err != nil {
+		t.Errorf("failed to parse base64 DER certificate: %v", err)
+	}
+
+	if cert.Subject.CommonName != "test.com" {
+		t.Errorf("expected CN 'test.com', got '%s'", cert.Subject.CommonName)
+	}
+}
+
+func TestParseCertificate_RawDER(t *testing.T) {
+	_, certPEM, err := generateTestCertificate("test.com", 24*time.Hour)
+	if err != nil {
+		t.Fatalf("failed to generate test certificate: %v", err)
+	}
+
+	// Decode PEM to get raw DER
+	block, _ := pem.Decode(certPEM)
+
+	cert, err := parseCertificate(block.Bytes)
+	if err != nil {
+		t.Errorf("failed to parse raw DER certificate: %v", err)
+	}
+
+	if cert.Subject.CommonName != "test.com" {
+		t.Errorf("expected CN 'test.com', got '%s'", cert.Subject.CommonName)
+	}
+}
+
+func TestParseCertificate_Invalid(t *testing.T) {
+	invalidData := []byte("not a certificate at all")
+
+	_, err := parseCertificate(invalidData)
+	if err == nil {
+		t.Error("expected parseCertificate to fail on invalid data")
+	}
+}
+
+// Helper function to check if a slice contains a string
+func contains(slice []string, item string) bool {
+	for _, s := range slice {
+		if s == item {
+			return true
+		}
+	}
+	return false
+}
+
+// TestSourceImplementsInterface ensures Source implements domain.DiscoverySource
+func TestSourceImplementsInterface(t *testing.T) {
+	var _ domain.DiscoverySource = (*Source)(nil)
+}
+
+// BenchmarkDiscover provides basic performance metrics for discovery
+func BenchmarkDiscover(b *testing.B) {
+	tmpfile, err := createTempServiceAccountKey()
+	if err != nil {
+		b.Fatalf("failed to create temp key file: %v", err)
+	}
+	defer os.Remove(tmpfile)
+
+	// Generate 10 test certificates
+	mockClient := newMockSMClient()
+	for i := 0; i < 10; i++ {
+		_, certPEM, err := generateTestCertificate(fmt.Sprintf("test%d.example.com", i), 24*time.Hour)
+		if err != nil {
+			b.Fatalf("failed to generate test certificate: %v", err)
+		}
+		mockClient.secrets[fmt.Sprintf("cert-%d", i)] = certPEM
+	}
+
+	cfg := &config.GCPSecretMgrDiscoveryConfig{
+		Project:     "test-project",
+		Credentials: tmpfile,
+	}
+
+	source := NewWithClient(cfg, mockClient, slog.Default())
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_, err := source.Discover(context.Background())
+		if err != nil {
+			b.Fatalf("Discover failed: %v", err)
+		}
+	}
+}