diff --git a/tools/dwh-auth/internal/registry/store.go b/tools/dwh-auth/internal/registry/store.go
new file mode 100644
index 00000000..fd054b31
--- /dev/null
+++ b/tools/dwh-auth/internal/registry/store.go
@@ -0,0 +1,584 @@
+//go:build linux
+
+// Package registry stores protected DWH credential records in active and
+// revoked directories. It never exposes credential digests through its public
+// listing type.
+package registry
+
+import (
+	"bytes"
+	"crypto/rand"
+	"encoding/base64"
+	"encoding/hex"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"sort"
+	"strings"
+	"syscall"
+	"time"
+
+	"github.com/aritmolab/thothii/tools/dwh-auth/internal/record"
+	"github.com/aritmolab/thothii/tools/dwh-auth/internal/securefile"
+)
+
+const maxRecordBytes = 4096
+
+// State describes which registry directory owns a public record.
+type State string
+
+const (
+	StateActive  State = "active"
+	StateRevoked State = "revoked"
+)
+
+var (
+	// ErrNotFound means no protected record exists for the requested key ID.
+	ErrNotFound = errors.New("credential record not found")
+	// ErrRevoked means a well-formed revoked record exists for the key ID.
+	ErrRevoked = errors.New("credential record is revoked")
+	// ErrConflict means an active record cannot be replaced or resurrected.
+	ErrConflict = errors.New("credential record already exists")
+	// ErrIntegrity means an unsafe or malformed registry object was observed.
+	ErrIntegrity = errors.New("registry integrity failure")
+)
+
+// PublicRecord is the redacted inventory form of a persisted record.
+type PublicRecord struct {
+	Kind             record.Kind `json:"credential_kind"`
+	KeyID            string      `json:"key_id"`
+	InstallationID   string      `json:"installation_id"`
+	Description      string      `json:"description,omitempty"`
+	CreatedAt        time.Time   `json:"created_at"`
+	ExpiresAt        *time.Time  `json:"expires_at,omitempty"`
+	State            State       `json:"state"`
+	RevokedAt        *time.Time  `json:"revoked_at,omitempty"`
+	RevocationReason string      `json:"revocation_reason,omitempty"`
+}
+
+// Store owns protected descriptors for one registry root.
+type Store struct {
+	root    *securefile.Dir
+	active  *securefile.Dir
+	revoked *securefile.Dir
+}
+
+type storedRecord struct {
+	record record.Record
+	state  State
+}
+
+// Open opens one existing protected absolute registry root and creates only its
+// active and revoked child directories when absent.
+func Open(root string) (*Store, error) {
+	rootDir, err := securefile.OpenDir(root)
+	if err != nil {
+		return nil, integrity(err)
+	}
+	active, err := rootDir.OpenOrCreateDir(string(StateActive), 0o750)
+	if err != nil {
+		_ = rootDir.Close()
+		return nil, integrity(err)
+	}
+	revoked, err := rootDir.OpenOrCreateDir(string(StateRevoked), 0o750)
+	if err != nil {
+		_ = active.Close()
+		_ = rootDir.Close()
+		return nil, integrity(err)
+	}
+	return &Store{root: rootDir, active: active, revoked: revoked}, nil
+}
+
+// Close closes descriptors held by the store.
+func (s *Store) Close() error {
+	if s == nil {
+		return nil
+	}
+	var first error
+	for _, dir := range []*securefile.Dir{s.revoked, s.active, s.root} {
+		if dir == nil {
+			continue
+		}
+		if err := dir.Close(); err != nil && first == nil {
+			first = err
+		}
+	}
+	return first
+}
+
+// Add validates and atomically publishes one active record. Existing active or
+// revoked records cannot be overwritten or resurrected.
+func (s *Store) Add(value record.Record) error {
+	if err := validateForState(value, StateActive); err != nil {
+		return err
+	}
+	return s.withWriterLock(func() error {
+		active, revoked, err := s.scanAll()
+		if err != nil {
+			return err
+		}
+		for _, existing := range revoked {
+			if existing.record.KeyID == value.KeyID {
+				return ErrRevoked
+			}
+		}
+		for _, existing := range active {
+			if existing.record.KeyID == value.KeyID {
+				return ErrConflict
+			}
+		}
+		if value.Kind == record.KindLegacyRaw {
+			for _, existing := range append(active, revoked...) {
+				if existing.record.Kind == record.KindLegacyRaw {
+					return ErrConflict
+				}
+			}
+		}
+		return s.writeRecord(s.active, value)
+	})
+}
+
+// Find returns an active credential record. A revoked counterpart is checked
+// first and always wins, including during the safe revoke-publication overlap.
+func (s *Store) Find(keyID string) (record.Record, error) {
+	if !validKeyID(keyID) {
+		return record.Record{}, ErrNotFound
+	}
+	if _, err := s.load(s.revoked, StateRevoked, keyID); err == nil {
+		return record.Record{}, ErrRevoked
+	} else if !errors.Is(err, ErrNotFound) {
+		return record.Record{}, err
+	}
+	return s.load(s.active, StateActive, keyID)
+}
+
+// FindLegacy returns the sole active legacy_raw record. A revoked legacy record
+// wins; any multiple-legacy condition is an integrity fault.
+func (s *Store) FindLegacy() (record.Record, error) {
+	active, revoked, err := s.scanAll()
+	if err != nil {
+		return record.Record{}, err
+	}
+	if err := validateLegacyMultiplicity(active, revoked); err != nil {
+		return record.Record{}, err
+	}
+	for _, existing := range revoked {
+		if existing.record.Kind == record.KindLegacyRaw {
+			return record.Record{}, ErrRevoked
+		}
+	}
+	for _, existing := range active {
+		if existing.record.Kind == record.KindLegacyRaw {
+			return existing.record, nil
+		}
+	}
+	return record.Record{}, ErrNotFound
+}
+
+// List returns a stable, redacted inventory. A revoked record replaces any
+// same-key active record visible during a revoked-first transition.
+func (s *Store) List() ([]PublicRecord, error) {
+	active, revoked, err := s.scanAll()
+	if err != nil {
+		return nil, err
+	}
+	if err := validateLegacyMultiplicity(active, revoked); err != nil {
+		return nil, err
+	}
+	byKey := make(map[string]PublicRecord, len(active)+len(revoked))
+	for _, existing := range active {
+		byKey[existing.record.KeyID] = public(existing.record, StateActive)
+	}
+	for _, existing := range revoked {
+		byKey[existing.record.KeyID] = public(existing.record, StateRevoked)
+	}
+	keys := make([]string, 0, len(byKey))
+	for keyID := range byKey {
+		keys = append(keys, keyID)
+	}
+	sort.Strings(keys)
+	result := make([]PublicRecord, 0, len(keys))
+	for _, keyID := range keys {
+		result = append(result, byKey[keyID])
+	}
+	return result, nil
+}
+
+// Revoke publishes a validated revoked record and fsyncs it before removing the
+// active record. If deletion then fails, Find still returns ErrRevoked.
+func (s *Store) Revoke(keyID, reason string, at time.Time) error {
+	if !validKeyID(keyID) {
+		return ErrNotFound
+	}
+	return s.withWriterLock(func() error {
+		active, revoked, err := s.scanAll()
+		if err != nil {
+			return err
+		}
+		for _, existing := range revoked {
+			if existing.record.KeyID == keyID {
+				return ErrRevoked
+			}
+		}
+		var target *record.Record
+		for _, existing := range active {
+			if existing.record.KeyID == keyID {
+				candidate := existing.record
+				target = &candidate
+				break
+			}
+		}
+		if target == nil {
+			return ErrNotFound
+		}
+		target.RevokedAt = &at
+		target.RevocationReason = reason
+		if err := validateForState(*target, StateRevoked); err != nil {
+			return err
+		}
+		if err := s.writeRecord(s.revoked, *target); err != nil {
+			return err
+		}
+		if err := s.active.Remove(recordFileName(keyID)); err != nil {
+			return integrity(err)
+		}
+		if err := s.active.Sync(); err != nil {
+			return integrity(err)
+		}
+		return nil
+	})
+}
+
+// Check validates every record and protected directory without exposing any
+// digest data.
+func (s *Store) Check() error {
+	active, revoked, err := s.scanAll()
+	if err != nil {
+		return err
+	}
+	return validateLegacyMultiplicity(active, revoked)
+}
+
+func (s *Store) withWriterLock(fn func() error) error {
+	if s == nil || s.root == nil || s.active == nil || s.revoked == nil {
+		return integrity(errors.New("uninitialized store"))
+	}
+	lock, err := s.root.Lock(".writer.lock")
+	if err != nil {
+		return integrity(err)
+	}
+	defer lock.Close()
+	return fn()
+}
+
+func (s *Store) scanAll() ([]storedRecord, []storedRecord, error) {
+	if s == nil || s.active == nil || s.revoked == nil {
+		return nil, nil, integrity(errors.New("uninitialized store"))
+	}
+	active, err := s.scan(s.active, StateActive)
+	if err != nil {
+		return nil, nil, err
+	}
+	revoked, err := s.scan(s.revoked, StateRevoked)
+	if err != nil {
+		return nil, nil, err
+	}
+	return active, revoked, nil
+}
+
+func (s *Store) scan(dir *securefile.Dir, state State) ([]storedRecord, error) {
+	names, err := dir.Names()
+	if err != nil {
+		return nil, integrity(err)
+	}
+	sort.Strings(names)
+	result := make([]storedRecord, 0, len(names))
+	for _, name := range names {
+		if !strings.HasSuffix(name, ".json") {
+			return nil, integrity(fmt.Errorf("unexpected registry entry %q", name))
+		}
+		keyID := strings.TrimSuffix(name, ".json")
+		if !validKeyID(keyID) {
+			return nil, integrity(fmt.Errorf("invalid registry filename %q", name))
+		}
+		value, err := s.load(dir, state, keyID)
+		if err != nil {
+			return nil, err
+		}
+		result = append(result, storedRecord{record: value, state: state})
+	}
+	return result, nil
+}
+
+func (s *Store) load(dir *securefile.Dir, state State, keyID string) (record.Record, error) {
+	data, err := dir.ReadFile(recordFileName(keyID), maxRecordBytes)
+	if errors.Is(err, syscall.ENOENT) {
+		return record.Record{}, ErrNotFound
+	}
+	if err != nil {
+		return record.Record{}, integrity(err)
+	}
+	value, err := decodeRecord(data)
+	if err != nil {
+		return record.Record{}, integrity(err)
+	}
+	if value.KeyID != keyID {
+		return record.Record{}, integrity(fmt.Errorf("record key ID does not match filename"))
+	}
+	if err := validateForState(value, state); err != nil {
+		return record.Record{}, integrity(err)
+	}
+	return value, nil
+}
+
+func (s *Store) writeRecord(dir *securefile.Dir, value record.Record) (err error) {
+	data, err := json.Marshal(value)
+	if err != nil {
+		return err
+	}
+	data = append(data, '\n')
+	if len(data) > maxRecordBytes {
+		return fmt.Errorf("record exceeds %d byte bound", maxRecordBytes)
+	}
+	temporary, err := temporaryName()
+	if err != nil {
+		return err
+	}
+	file, err := dir.CreateExclusive(temporary, 0o600)
+	if err != nil {
+		return integrity(err)
+	}
+	published := false
+	defer func() {
+		if file != nil {
+			_ = file.Close()
+		}
+		if !published {
+			_ = dir.Remove(temporary)
+			_ = dir.Sync()
+		}
+	}()
+	if err := writeAll(file, data); err != nil {
+		return err
+	}
+	if err := file.Chmod(0o640); err != nil {
+		return err
+	}
+	if err := file.Sync(); err != nil {
+		return err
+	}
+	if err := file.Close(); err != nil {
+		return err
+	}
+	file = nil
+	name := recordFileName(value.KeyID)
+	exists, err := dir.Exists(name)
+	if err != nil {
+		return integrity(err)
+	}
+	if exists {
+		return ErrConflict
+	}
+	if err := dir.Rename(temporary, dir, name); err != nil {
+		return integrity(err)
+	}
+	published = true
+	if err := dir.Sync(); err != nil {
+		return integrity(err)
+	}
+	return nil
+}
+
+func validateForState(value record.Record, state State) error {
+	if err := record.Validate(value); err != nil {
+		return err
+	}
+	switch state {
+	case StateActive:
+		if value.RevokedAt != nil || value.RevocationReason != "" {
+			return errors.New("active record carries revocation data")
+		}
+	case StateRevoked:
+		if value.RevokedAt == nil || value.RevocationReason == "" {
+			return errors.New("revoked record lacks revocation data")
+		}
+	default:
+		return errors.New("unknown registry state")
+	}
+	return nil
+}
+
+func validateLegacyMultiplicity(active, revoked []storedRecord) error {
+	activeCount := 0
+	revokedCount := 0
+	for _, existing := range active {
+		if existing.record.Kind == record.KindLegacyRaw {
+			activeCount++
+		}
+	}
+	for _, existing := range revoked {
+		if existing.record.Kind == record.KindLegacyRaw {
+			revokedCount++
+		}
+	}
+	if activeCount > 1 || revokedCount > 1 || activeCount+revokedCount > 2 {
+		return integrity(errors.New("multiple legacy records"))
+	}
+	return nil
+}
+
+func decodeRecord(data []byte) (record.Record, error) {
+	if err := rejectDuplicateOrTrailingJSON(data); err != nil {
+		return record.Record{}, err
+	}
+	decoder := json.NewDecoder(bytes.NewReader(data))
+	decoder.DisallowUnknownFields()
+	var value record.Record
+	if err := decoder.Decode(&value); err != nil {
+		return record.Record{}, err
+	}
+	var extra any
+	if err := decoder.Decode(&extra); !errors.Is(err, io.EOF) {
+		if err == nil {
+			return record.Record{}, errors.New("trailing JSON value")
+		}
+		return record.Record{}, err
+	}
+	if err := record.Validate(value); err != nil {
+		return record.Record{}, err
+	}
+	return value, nil
+}
+
+func rejectDuplicateOrTrailingJSON(data []byte) error {
+	decoder := json.NewDecoder(bytes.NewReader(data))
+	if err := consumeJSONValue(decoder); err != nil {
+		return err
+	}
+	var extra any
+	if err := decoder.Decode(&extra); !errors.Is(err, io.EOF) {
+		if err == nil {
+			return errors.New("trailing JSON value")
+		}
+		return err
+	}
+	return nil
+}
+
+func consumeJSONValue(decoder *json.Decoder) error {
+	token, err := decoder.Token()
+	if err != nil {
+		return err
+	}
+	delimiter, isDelimiter := token.(json.Delim)
+	if !isDelimiter {
+		return nil
+	}
+	switch delimiter {
+	case '{':
+		seen := make(map[string]struct{})
+		for decoder.More() {
+			keyToken, err := decoder.Token()
+			if err != nil {
+				return err
+			}
+			key, ok := keyToken.(string)
+			if !ok {
+				return errors.New("JSON object key is not a string")
+			}
+			if _, duplicate := seen[key]; duplicate {
+				return fmt.Errorf("duplicate JSON field %q", key)
+			}
+			seen[key] = struct{}{}
+			if err := consumeJSONValue(decoder); err != nil {
+				return err
+			}
+		}
+		end, err := decoder.Token()
+		if err != nil {
+			return err
+		}
+		if end != json.Delim('}') {
+			return errors.New("unterminated JSON object")
+		}
+	case '[':
+		for decoder.More() {
+			if err := consumeJSONValue(decoder); err != nil {
+				return err
+			}
+		}
+		end, err := decoder.Token()
+		if err != nil {
+			return err
+		}
+		if end != json.Delim(']') {
+			return errors.New("unterminated JSON array")
+		}
+	default:
+		return errors.New("unexpected JSON delimiter")
+	}
+	return nil
+}
+
+func public(value record.Record, state State) PublicRecord {
+	return PublicRecord{
+		Kind:             value.Kind,
+		KeyID:            value.KeyID,
+		InstallationID:   value.InstallationID,
+		Description:      value.Description,
+		CreatedAt:        value.CreatedAt,
+		ExpiresAt:        value.ExpiresAt,
+		State:            state,
+		RevokedAt:        value.RevokedAt,
+		RevocationReason: value.RevocationReason,
+	}
+}
+
+func recordFileName(keyID string) string {
+	return keyID + ".json"
+}
+
+func validKeyID(keyID string) bool {
+	if keyID == record.LegacyKeyID {
+		return true
+	}
+	if len(keyID) != 16 {
+		return false
+	}
+	decoded, err := base64.RawURLEncoding.DecodeString(keyID)
+	return err == nil && len(decoded) == 12 && base64.RawURLEncoding.EncodeToString(decoded) == keyID
+}
+
+func temporaryName() (string, error) {
+	bytes := make([]byte, 16)
+	if _, err := io.ReadFull(rand.Reader, bytes); err != nil {
+		return "", err
+	}
+	return ".tmp-" + hex.EncodeToString(bytes), nil
+}
+
+func writeAll(file *os.File, data []byte) error {
+	for len(data) > 0 {
+		written, err := file.Write(data)
+		if err != nil {
+			return err
+		}
+		if written == 0 {
+			return io.ErrShortWrite
+		}
+		data = data[written:]
+	}
+	return nil
+}
+
+func integrity(err error) error {
+	if err == nil {
+		return nil
+	}
+	if errors.Is(err, ErrIntegrity) {
+		return err
+	}
+	return fmt.Errorf("%w: %v", ErrIntegrity, err)
+}
diff --git a/tools/dwh-auth/internal/registry/store_test.go b/tools/dwh-auth/internal/registry/store_test.go
new file mode 100644
index 00000000..f2edc5de
--- /dev/null
+++ b/tools/dwh-auth/internal/registry/store_test.go
@@ -0,0 +1,457 @@
+//go:build linux
+
+package registry
+
+import (
+	"bytes"
+	"crypto/sha256"
+	"encoding/base64"
+	"encoding/json"
+	"errors"
+	"os"
+	"path/filepath"
+	"reflect"
+	"sort"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/aritmolab/thothii/tools/dwh-auth/internal/record"
+)
+
+func TestAddFindAndListPublishCanonicalProtectedRecord(t *testing.T) {
+	root := t.TempDir()
+	store, err := Open(root)
+	if err != nil {
+		t.Fatalf("Open() error = %v", err)
+	}
+	record := syntheticRecord(1)
+	if err := store.Add(record); err != nil {
+		t.Fatalf("Add() error = %v", err)
+	}
+
+	path := filepath.Join(root, "active", record.KeyID+".json")
+	gotBytes, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("ReadFile() error = %v", err)
+	}
+	wantBytes, err := json.Marshal(record)
+	if err != nil {
+		t.Fatalf("Marshal() error = %v", err)
+	}
+	wantBytes = append(wantBytes, '\n')
+	if !bytes.Equal(gotBytes, wantBytes) {
+		t.Fatalf("published record = %s, want %s", gotBytes, wantBytes)
+	}
+	info, err := os.Stat(path)
+	if err != nil {
+		t.Fatalf("Stat() error = %v", err)
+	}
+	if got, want := info.Mode().Perm(), os.FileMode(0o640); got != want {
+		t.Fatalf("record mode = %04o, want %04o", got, want)
+	}
+
+	found, err := store.Find(record.KeyID)
+	if err != nil {
+		t.Fatalf("Find() error = %v", err)
+	}
+	if found != record {
+		t.Fatalf("Find() = %#v, want %#v", found, record)
+	}
+
+	listed, err := store.List()
+	if err != nil {
+		t.Fatalf("List() error = %v", err)
+	}
+	if got, want := len(listed), 1; got != want {
+		t.Fatalf("List() length = %d, want %d", got, want)
+	}
+	if got, want := listed[0], publicRecord(record, StateActive); got != want {
+		t.Fatalf("List()[0] = %#v, want %#v", got, want)
+	}
+	encoded, err := json.Marshal(listed[0])
+	if err != nil {
+		t.Fatalf("Marshal(PublicRecord) error = %v", err)
+	}
+	if bytes.Contains(encoded, []byte(record.SecretSHA256)) {
+		t.Fatalf("PublicRecord JSON exposed secret digest: %s", encoded)
+	}
+}
+
+func TestOpenRejectsSymlinkedRegistryRootAndDirectories(t *testing.T) {
+	t.Run("root", func(t *testing.T) {
+		parent := t.TempDir()
+		target := t.TempDir()
+		root := filepath.Join(parent, "registry")
+		if err := os.Symlink(target, root); err != nil {
+			t.Fatalf("Symlink() error = %v", err)
+		}
+		if _, err := Open(root); err == nil {
+			t.Fatal("Open() error = nil, want symlink refusal")
+		}
+	})
+
+	t.Run("active directory", func(t *testing.T) {
+		root := t.TempDir()
+		if err := os.Symlink(t.TempDir(), filepath.Join(root, "active")); err != nil {
+			t.Fatalf("Symlink() error = %v", err)
+		}
+		if _, err := Open(root); err == nil {
+			t.Fatal("Open() error = nil, want active-directory symlink refusal")
+		}
+	})
+
+	t.Run("revoked directory", func(t *testing.T) {
+		root := t.TempDir()
+		if err := os.Symlink(t.TempDir(), filepath.Join(root, "revoked")); err != nil {
+			t.Fatalf("Symlink() error = %v", err)
+		}
+		if _, err := Open(root); err == nil {
+			t.Fatal("Open() error = nil, want revoked-directory symlink refusal")
+		}
+	})
+}
+
+func TestCheckRejectsUnsafeModesAndSymlinkedRecords(t *testing.T) {
+	t.Run("unsafe active mode", func(t *testing.T) {
+		root := t.TempDir()
+		if err := os.Mkdir(filepath.Join(root, "active"), 0o770); err != nil {
+			t.Fatalf("Mkdir(active) error = %v", err)
+		}
+		if err := os.Chmod(filepath.Join(root, "active"), 0o770); err != nil {
+			t.Fatalf("Chmod(active) error = %v", err)
+		}
+		if _, err := Open(root); err == nil {
+			t.Fatal("Open() error = nil, want unsafe directory-mode refusal")
+		}
+	})
+
+	t.Run("unsafe record mode", func(t *testing.T) {
+		root := t.TempDir()
+		store := openStore(t, root)
+		record := syntheticRecord(2)
+		writeRecord(t, root, StateActive, record.KeyID+".json", marshalRecord(t, record), 0o660)
+		if err := store.Check(); err == nil {
+			t.Fatal("Check() error = nil, want unsafe record-mode refusal")
+		}
+	})
+
+	t.Run("symlinked record", func(t *testing.T) {
+		root := t.TempDir()
+		store := openStore(t, root)
+		record := syntheticRecord(3)
+		target := filepath.Join(root, "target.json")
+		writeRegistryFile(t, target, marshalRecord(t, record), 0o640)
+		if err := os.Symlink(target, filepath.Join(root, "active", record.KeyID+".json")); err != nil {
+			t.Fatalf("Symlink() error = %v", err)
+		}
+		if _, err := store.Find(record.KeyID); err == nil {
+			t.Fatal("Find() error = nil, want symlink refusal")
+		}
+	})
+}
+
+func TestCheckRejectsInvalidRecordJSONAndFilenameMismatches(t *testing.T) {
+	cases := []struct {
+		name     string
+		fileName func(record.Record) string
+		mutate   func([]byte) []byte
+	}{
+		{
+			name:     "unknown JSON field",
+			fileName: func(record.Record) string { return "AAAAAAAAAAAAAAAA.json" },
+			mutate: func(data []byte) []byte {
+				data = bytes.TrimSuffix(data, []byte{'\n'})
+				return append(append(data[:len(data)-1], []byte(`,"unknown":true}`)...), '\n')
+			},
+		},
+		{
+			name:     "duplicate JSON field",
+			fileName: func(record.Record) string { return "AAAAAAAAAAAAAAAA.json" },
+			mutate: func(data []byte) []byte {
+				return append([]byte(`{"schema_version":1,`), data[1:]...)
+			},
+		},
+		{
+			name:     "trailing JSON value",
+			fileName: func(record.Record) string { return "AAAAAAAAAAAAAAAA.json" },
+			mutate: func(data []byte) []byte {
+				return append(data, []byte(`{}`)...)
+			},
+		},
+		{
+			name:     "partial JSON",
+			fileName: func(record.Record) string { return "AAAAAAAAAAAAAAAA.json" },
+			mutate:   func([]byte) []byte { return []byte(`{"schema_version":`) },
+		},
+		{
+			name:     "filename mismatch",
+			fileName: func(record.Record) string { return "wrong.json" },
+			mutate:   func(data []byte) []byte { return data },
+		},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			root := t.TempDir()
+			store := openStore(t, root)
+			record := syntheticRecord(0)
+			writeRecord(t, root, StateActive, tc.fileName(record), tc.mutate(marshalRecord(t, record)), 0o640)
+			if err := store.Check(); err == nil {
+				t.Fatal("Check() error = nil, want integrity refusal")
+			}
+		})
+	}
+}
+
+func TestRevokePublishesRevokedStateAndFindRevokedWins(t *testing.T) {
+	root := t.TempDir()
+	store := openStore(t, root)
+	record := syntheticRecord(4)
+	if err := store.Add(record); err != nil {
+		t.Fatalf("Add() error = %v", err)
+	}
+	revokedAt := record.CreatedAt.Add(2 * time.Hour)
+	if err := store.Revoke(record.KeyID, "synthetic rotation", revokedAt); err != nil {
+		t.Fatalf("Revoke() error = %v", err)
+	}
+	if _, err := os.Stat(filepath.Join(root, "active", record.KeyID+".json")); !errors.Is(err, os.ErrNotExist) {
+		t.Fatalf("active record stat error = %v, want not-exist", err)
+	}
+	if _, err := os.Stat(filepath.Join(root, "revoked", record.KeyID+".json")); err != nil {
+		t.Fatalf("revoked record stat error = %v", err)
+	}
+	if _, err := store.Find(record.KeyID); !errors.Is(err, ErrRevoked) {
+		t.Fatalf("Find() error = %v, want ErrRevoked", err)
+	}
+
+	// A crash after revoked publication but before active removal must still deny the key.
+	writeRecord(t, root, StateActive, record.KeyID+".json", marshalRecord(t, record), 0o640)
+	if _, err := store.Find(record.KeyID); !errors.Is(err, ErrRevoked) {
+		t.Fatalf("Find() with active and revoked files error = %v, want ErrRevoked", err)
+	}
+	listed, err := store.List()
+	if err != nil {
+		t.Fatalf("List() error = %v", err)
+	}
+	if got, want := listed, []PublicRecord{publicRecord(revokedRecord(record, revokedAt, "synthetic rotation"), StateRevoked)}; !equalPublicRecords(got, want) {
+		t.Fatalf("List() = %#v, want %#v", got, want)
+	}
+}
+
+func TestFindLegacyAllowsOneAndRejectsMultipleRecords(t *testing.T) {
+	t.Run("one active legacy", func(t *testing.T) {
+		root := t.TempDir()
+		store := openStore(t, root)
+		legacy := syntheticLegacyRecord()
+		if err := store.Add(legacy); err != nil {
+			t.Fatalf("Add() error = %v", err)
+		}
+		got, err := store.FindLegacy()
+		if err != nil {
+			t.Fatalf("FindLegacy() error = %v", err)
+		}
+		if got != legacy {
+			t.Fatalf("FindLegacy() = %#v, want %#v", got, legacy)
+		}
+	})
+
+	t.Run("multiple legacy records", func(t *testing.T) {
+		root := t.TempDir()
+		store := openStore(t, root)
+		legacy := syntheticLegacyRecord()
+		if err := store.Add(legacy); err != nil {
+			t.Fatalf("Add() error = %v", err)
+		}
+		writeRecord(t, root, StateActive, "duplicate.json", marshalRecord(t, legacy), 0o640)
+		if _, err := store.FindLegacy(); err == nil {
+			t.Fatal("FindLegacy() error = nil, want multiple-legacy integrity refusal")
+		}
+	})
+}
+
+func TestConcurrentAddDoesNotOverwriteAndFindNeverReadsPartialRecord(t *testing.T) {
+	root := t.TempDir()
+	left := openStore(t, root)
+	right := openStore(t, root)
+	first := syntheticRecord(5)
+	second := first
+	second.Description = "second synthetic writer"
+
+	start := make(chan struct{})
+	errs := make(chan error, 2)
+	var writers sync.WaitGroup
+	for _, candidate := range []record.Record{first, second} {
+		writers.Add(1)
+		go func(candidate record.Record) {
+			defer writers.Done()
+			<-start
+			errs <- left.Add(candidate)
+		}(candidate)
+	}
+	close(start)
+	writers.Wait()
+	close(errs)
+
+	successes := 0
+	for err := range errs {
+		if err == nil {
+			successes++
+		}
+	}
+	if got, want := successes, 1; got != want {
+		t.Fatalf("concurrent Add() successes = %d, want %d", got, want)
+	}
+	found, err := right.Find(first.KeyID)
+	if err != nil {
+		t.Fatalf("Find() after concurrent Add() error = %v", err)
+	}
+	if found.Description != first.Description && found.Description != second.Description {
+		t.Fatalf("Find() description = %q, want one published writer", found.Description)
+	}
+
+	readStore := openStore(t, root)
+	readersDone := make(chan struct{})
+	readErrs := make(chan error, 16)
+	var readers sync.WaitGroup
+	for range 16 {
+		readers.Add(1)
+		go func() {
+			defer readers.Done()
+			for range 100 {
+				got, err := readStore.Find(first.KeyID)
+				if err == nil {
+					if got.KeyID != first.KeyID {
+						readErrs <- errors.New("Find returned an unexpected key ID")
+					}
+					continue
+				}
+				if !errors.Is(err, ErrRevoked) {
+					readErrs <- err
+				}
+			}
+		}()
+	}
+	if err := right.Revoke(first.KeyID, "synthetic rotation", first.CreatedAt.Add(time.Hour)); err != nil {
+		t.Fatalf("Revoke() during concurrent reads error = %v", err)
+	}
+	readers.Wait()
+	close(readersDone)
+	close(readErrs)
+	for err := range readErrs {
+		if err != nil {
+			t.Fatalf("concurrent Find() error = %v", err)
+		}
+	}
+	if _, err := readStore.Find(first.KeyID); !errors.Is(err, ErrRevoked) {
+		t.Fatalf("Find() after Revoke() error = %v, want ErrRevoked", err)
+	}
+}
+
+func syntheticRecord(seed byte) record.Record {
+	keyBytes := bytes.Repeat([]byte{seed}, 12)
+	digest := sha256.Sum256([]byte{seed, 's', 'y', 'n', 't', 'h', 'e', 't', 'i', 'c'})
+	return record.Record{
+		SchemaVersion:  record.SchemaVersion,
+		Kind:           record.KindV1,
+		KeyID:          base64.RawURLEncoding.EncodeToString(keyBytes),
+		InstallationID: "test-installation",
+		Description:    "synthetic test credential",
+		SecretSHA256:   base64.RawURLEncoding.EncodeToString(digest[:]),
+		CreatedAt:      time.Date(2026, 8, 20, 12, 0, int(seed), 0, time.UTC),
+	}
+}
+
+func syntheticLegacyRecord() record.Record {
+	value := syntheticRecord(9)
+	value.Kind = record.KindLegacyRaw
+	value.KeyID = record.LegacyKeyID
+	value.InstallationID = "legacy-shared"
+	return value
+}
+
+func revokedRecord(value record.Record, at time.Time, reason string) record.Record {
+	value.RevokedAt = &at
+	value.RevocationReason = reason
+	return value
+}
+
+func publicRecord(value record.Record, state State) PublicRecord {
+	return PublicRecord{
+		Kind:             value.Kind,
+		KeyID:            value.KeyID,
+		InstallationID:   value.InstallationID,
+		Description:      value.Description,
+		CreatedAt:        value.CreatedAt,
+		ExpiresAt:        value.ExpiresAt,
+		State:            state,
+		RevokedAt:        value.RevokedAt,
+		RevocationReason: value.RevocationReason,
+	}
+}
+
+func equalPublicRecords(got, want []PublicRecord) bool {
+	if len(got) != len(want) {
+		return false
+	}
+	for i := range got {
+		if !reflect.DeepEqual(got[i], want[i]) {
+			return false
+		}
+	}
+	return true
+}
+
+func openStore(t *testing.T, root string) *Store {
+	t.Helper()
+	store, err := Open(root)
+	if err != nil {
+		t.Fatalf("Open() error = %v", err)
+	}
+	return store
+}
+
+func marshalRecord(t *testing.T, value record.Record) []byte {
+	t.Helper()
+	data, err := json.Marshal(value)
+	if err != nil {
+		t.Fatalf("Marshal() error = %v", err)
+	}
+	return append(data, '\n')
+}
+
+func writeRecord(t *testing.T, root string, state State, name string, data []byte, mode os.FileMode) {
+	t.Helper()
+	writeRegistryFile(t, filepath.Join(root, string(state), name), data, mode)
+}
+
+func writeRegistryFile(t *testing.T, path string, data []byte, mode os.FileMode) {
+	t.Helper()
+	if err := os.WriteFile(path, data, mode); err != nil {
+		t.Fatalf("WriteFile(%q) error = %v", path, err)
+	}
+	if err := os.Chmod(path, mode); err != nil {
+		t.Fatalf("Chmod(%q) error = %v", path, err)
+	}
+}
+
+func TestListOrdersRecordsByKeyID(t *testing.T) {
+	root := t.TempDir()
+	store := openStore(t, root)
+	for _, seed := range []byte{8, 1, 5} {
+		if err := store.Add(syntheticRecord(seed)); err != nil {
+			t.Fatalf("Add(%d) error = %v", seed, err)
+		}
+	}
+	listed, err := store.List()
+	if err != nil {
+		t.Fatalf("List() error = %v", err)
+	}
+	keys := make([]string, len(listed))
+	for i, item := range listed {
+		keys[i] = item.KeyID
+	}
+	if !sort.StringsAreSorted(keys) {
+		t.Fatalf("List() key order = %v, want sorted order", keys)
+	}
+}
diff --git a/tools/dwh-auth/internal/securefile/securefile_linux.go b/tools/dwh-auth/internal/securefile/securefile_linux.go
new file mode 100644
index 00000000..1f57fc65
--- /dev/null
+++ b/tools/dwh-auth/internal/securefile/securefile_linux.go
@@ -0,0 +1,535 @@
+//go:build linux
+
+// Package securefile provides Linux-only, no-follow filesystem primitives for
+// protected credential material.
+package securefile
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"path/filepath"
+	"strings"
+	"syscall"
+)
+
+var (
+	// ErrUnsafe identifies a path, mode, or file type that cannot be trusted.
+	ErrUnsafe = errors.New("unsafe filesystem object")
+	// ErrTooLarge identifies a file that exceeds its caller-provided bound.
+	ErrTooLarge = errors.New("file exceeds size bound")
+	// ErrChanged identifies a file whose size changed while it was read.
+	ErrChanged = errors.New("file changed during read")
+)
+
+// Dir is an open, protected directory descriptor. Operations are rooted at the
+// descriptor rather than reopening attacker-controlled path prefixes.
+type Dir struct {
+	fd   int
+	path string
+}
+
+// Lock serializes cooperating writers using an advisory lock on a protected
+// regular file.
+type Lock struct {
+	fd int
+}
+
+// OpenDir opens an absolute directory without following any path component and
+// rejects unsafe modes on the protected final directory.
+func OpenDir(path string) (*Dir, error) {
+	clean, err := absolutePath(path)
+	if err != nil {
+		return nil, err
+	}
+	fd, err := openDirectoryPath(clean)
+	if err != nil {
+		return nil, err
+	}
+	if err := validateDirectoryFD(fd); err != nil {
+		_ = syscall.Close(fd)
+		return nil, err
+	}
+	if err := compareWithLstat(fd, clean, true); err != nil {
+		_ = syscall.Close(fd)
+		return nil, err
+	}
+	return &Dir{fd: fd, path: clean}, nil
+}
+
+// Close closes the directory descriptor.
+func (d *Dir) Close() error {
+	if d == nil || d.fd < 0 {
+		return nil
+	}
+	err := syscall.Close(d.fd)
+	d.fd = -1
+	return err
+}
+
+// OpenDir opens one direct child directory without following it.
+func (d *Dir) OpenDir(name string) (*Dir, error) {
+	if err := validName(name); err != nil {
+		return nil, err
+	}
+	if err := d.check(); err != nil {
+		return nil, err
+	}
+	fd, err := syscall.Openat(d.fd, name, directoryOpenFlags, 0)
+	if err != nil {
+		return nil, err
+	}
+	if err := validateDirectoryFD(fd); err != nil {
+		_ = syscall.Close(fd)
+		return nil, err
+	}
+	path := filepath.Join(d.path, name)
+	if err := compareWithLstat(fd, path, true); err != nil {
+		_ = syscall.Close(fd)
+		return nil, err
+	}
+	return &Dir{fd: fd, path: path}, nil
+}
+
+// OpenOrCreateDir opens a protected child directory or creates it with a
+// restrictive mode. A collision is reopened and fully revalidated.
+func (d *Dir) OpenOrCreateDir(name string, mode os.FileMode) (*Dir, error) {
+	child, err := d.OpenDir(name)
+	if err == nil {
+		return child, nil
+	}
+	if !errors.Is(err, syscall.ENOENT) {
+		return nil, err
+	}
+	if err := validMode(mode); err != nil {
+		return nil, err
+	}
+	if err := d.check(); err != nil {
+		return nil, err
+	}
+	if err := syscall.Mkdirat(d.fd, name, uint32(mode.Perm())); err != nil && !errors.Is(err, syscall.EEXIST) {
+		return nil, err
+	}
+	child, err = d.OpenDir(name)
+	if err != nil {
+		return nil, err
+	}
+	if err := d.Sync(); err != nil {
+		_ = child.Close()
+		return nil, err
+	}
+	return child, nil
+}
+
+// ReadFile reads one protected regular child file, bounded by max bytes. It
+// verifies file type and mode before and after the read and detects size races.
+func (d *Dir) ReadFile(name string, max int) ([]byte, error) {
+	return d.readFile(name, max, nil)
+}
+
+// ReadSecret reads an absolute secret input whose final parent is protected and
+// whose file mode is exactly 0600.
+func ReadSecret(path string, max int) ([]byte, error) {
+	clean, err := absolutePath(path)
+	if err != nil {
+		return nil, err
+	}
+	parent := filepath.Dir(clean)
+	name := filepath.Base(clean)
+	if err := validName(name); err != nil {
+		return nil, err
+	}
+	dir, err := OpenDir(parent)
+	if err != nil {
+		return nil, err
+	}
+	defer dir.Close()
+	mode := uint32(0o600)
+	return dir.readFile(name, max, &mode)
+}
+
+// CreateSecret creates a new absolute output file with O_CREAT|O_EXCL and an
+// exact mode of 0600. The caller owns and must close the returned file.
+func CreateSecret(path string) (*os.File, error) {
+	clean, err := absolutePath(path)
+	if err != nil {
+		return nil, err
+	}
+	parent := filepath.Dir(clean)
+	name := filepath.Base(clean)
+	if err := validName(name); err != nil {
+		return nil, err
+	}
+	dir, err := OpenDir(parent)
+	if err != nil {
+		return nil, err
+	}
+	defer dir.Close()
+	return dir.CreateExclusive(name, 0o600)
+}
+
+// CreateExclusive creates one direct child with O_CREAT|O_EXCL|O_NOFOLLOW and
+// the exact requested restrictive mode. The caller owns and must close it.
+func (d *Dir) CreateExclusive(name string, mode os.FileMode) (*os.File, error) {
+	if err := validName(name); err != nil {
+		return nil, err
+	}
+	if err := validMode(mode); err != nil {
+		return nil, err
+	}
+	if err := d.check(); err != nil {
+		return nil, err
+	}
+	fd, err := syscall.Openat(d.fd, name, syscall.O_WRONLY|syscall.O_CREAT|syscall.O_EXCL|syscall.O_NOFOLLOW|syscall.O_CLOEXEC, uint32(mode.Perm()))
+	if err != nil {
+		return nil, err
+	}
+	if err := syscall.Fchmod(fd, uint32(mode.Perm())); err != nil {
+		_ = syscall.Close(fd)
+		return nil, err
+	}
+	if err := validateRegularFD(fd, uint32(mode.Perm())); err != nil {
+		_ = syscall.Close(fd)
+		return nil, err
+	}
+	return os.NewFile(uintptr(fd), filepath.Join(d.path, name)), nil
+}
+
+// Exists reports whether a direct protected regular child exists. An unsafe
+// collision is an error rather than an absent file.
+func (d *Dir) Exists(name string) (bool, error) {
+	if err := validName(name); err != nil {
+		return false, err
+	}
+	if err := d.check(); err != nil {
+		return false, err
+	}
+	fd, err := syscall.Openat(d.fd, name, syscall.O_RDONLY|syscall.O_NOFOLLOW|syscall.O_CLOEXEC, 0)
+	if errors.Is(err, syscall.ENOENT) {
+		return false, nil
+	}
+	if err != nil {
+		return false, err
+	}
+	defer syscall.Close(fd)
+	if err := validateRegularFD(fd, 0); err != nil {
+		return false, err
+	}
+	if err := compareWithLstat(fd, filepath.Join(d.path, name), false); err != nil {
+		return false, err
+	}
+	return true, nil
+}
+
+// Rename atomically renames a direct child into another protected directory.
+// Callers that need no-replace semantics must serialize writers and verify the
+// destination is absent before calling Rename.
+func (d *Dir) Rename(oldName string, destination *Dir, newName string) error {
+	if err := validName(oldName); err != nil {
+		return err
+	}
+	if err := validName(newName); err != nil {
+		return err
+	}
+	if destination == nil {
+		return fmt.Errorf("%w: nil destination", ErrUnsafe)
+	}
+	if err := d.check(); err != nil {
+		return err
+	}
+	if err := destination.check(); err != nil {
+		return err
+	}
+	return syscall.Renameat(d.fd, oldName, destination.fd, newName)
+}
+
+// Remove removes one direct child file without following it.
+func (d *Dir) Remove(name string) error {
+	if err := validName(name); err != nil {
+		return err
+	}
+	if err := d.check(); err != nil {
+		return err
+	}
+	return syscall.Unlinkat(d.fd, name)
+}
+
+// Sync makes prior directory entry changes durable.
+func (d *Dir) Sync() error {
+	if err := d.check(); err != nil {
+		return err
+	}
+	return syscall.Fsync(d.fd)
+}
+
+// Names returns direct entry names from the protected directory descriptor.
+func (d *Dir) Names() ([]string, error) {
+	if err := d.check(); err != nil {
+		return nil, err
+	}
+	fd, err := syscall.Openat(d.fd, ".", directoryOpenFlags, 0)
+	if err != nil {
+		return nil, err
+	}
+	file := os.NewFile(uintptr(fd), d.path)
+	entries, err := file.ReadDir(-1)
+	closeErr := file.Close()
+	if err != nil {
+		return nil, err
+	}
+	if closeErr != nil {
+		return nil, closeErr
+	}
+	names := make([]string, 0, len(entries))
+	for _, entry := range entries {
+		names = append(names, entry.Name())
+	}
+	return names, nil
+}
+
+// Lock opens or creates a protected 0600 lock file and acquires an exclusive
+// advisory lock. Close releases the lock and descriptor.
+func (d *Dir) Lock(name string) (*Lock, error) {
+	if err := validName(name); err != nil {
+		return nil, err
+	}
+	if err := d.check(); err != nil {
+		return nil, err
+	}
+	for {
+		fd, err := syscall.Openat(d.fd, name, syscall.O_RDWR|syscall.O_NOFOLLOW|syscall.O_CLOEXEC, 0)
+		if errors.Is(err, syscall.ENOENT) {
+			fd, err = syscall.Openat(d.fd, name, syscall.O_RDWR|syscall.O_CREAT|syscall.O_EXCL|syscall.O_NOFOLLOW|syscall.O_CLOEXEC, 0o600)
+			if errors.Is(err, syscall.EEXIST) {
+				continue
+			}
+			if err != nil {
+				return nil, err
+			}
+			if err := syscall.Fchmod(fd, 0o600); err != nil {
+				_ = syscall.Close(fd)
+				return nil, err
+			}
+			if err := d.Sync(); err != nil {
+				_ = syscall.Close(fd)
+				return nil, err
+			}
+		}
+		if err != nil {
+			return nil, err
+		}
+		if err := validateRegularFD(fd, 0o600); err != nil {
+			_ = syscall.Close(fd)
+			return nil, err
+		}
+		if err := compareWithLstat(fd, filepath.Join(d.path, name), false); err != nil {
+			_ = syscall.Close(fd)
+			return nil, err
+		}
+		if err := syscall.Flock(fd, syscall.LOCK_EX); err != nil {
+			_ = syscall.Close(fd)
+			return nil, err
+		}
+		return &Lock{fd: fd}, nil
+	}
+}
+
+// Close releases an advisory lock and closes its descriptor.
+func (l *Lock) Close() error {
+	if l == nil || l.fd < 0 {
+		return nil
+	}
+	unlockErr := syscall.Flock(l.fd, syscall.LOCK_UN)
+	closeErr := syscall.Close(l.fd)
+	l.fd = -1
+	if unlockErr != nil {
+		return unlockErr
+	}
+	return closeErr
+}
+
+func (d *Dir) readFile(name string, max int, exactMode *uint32) ([]byte, error) {
+	if err := validName(name); err != nil {
+		return nil, err
+	}
+	if max < 0 {
+		return nil, fmt.Errorf("%w: negative size bound", ErrUnsafe)
+	}
+	if err := d.check(); err != nil {
+		return nil, err
+	}
+	fd, err := syscall.Openat(d.fd, name, syscall.O_RDONLY|syscall.O_NOFOLLOW|syscall.O_CLOEXEC, 0)
+	if err != nil {
+		return nil, err
+	}
+	defer syscall.Close(fd)
+
+	var before syscall.Stat_t
+	if err := syscall.Fstat(fd, &before); err != nil {
+		return nil, err
+	}
+	if err := validateRegularStat(&before, exactMode); err != nil {
+		return nil, err
+	}
+	if err := compareWithLstat(fd, filepath.Join(d.path, name), false); err != nil {
+		return nil, err
+	}
+	if before.Size < 0 || before.Size > int64(max) {
+		return nil, fmt.Errorf("%w: %d bytes", ErrTooLarge, before.Size)
+	}
+	size := int(before.Size)
+	data := make([]byte, size+1)
+	n := 0
+	for n < len(data) {
+		read, readErr := syscall.Read(fd, data[n:])
+		if read > 0 {
+			n += read
+		}
+		if errors.Is(readErr, syscall.EINTR) {
+			continue
+		}
+		if readErr != nil {
+			return nil, readErr
+		}
+		if read == 0 {
+			break
+		}
+	}
+
+	var after syscall.Stat_t
+	if err := syscall.Fstat(fd, &after); err != nil {
+		return nil, err
+	}
+	if err := validateRegularStat(&after, exactMode); err != nil {
+		return nil, err
+	}
+	if before.Dev != after.Dev || before.Ino != after.Ino || before.Size != after.Size {
+		return nil, ErrChanged
+	}
+	if n != size {
+		return nil, ErrChanged
+	}
+	return data[:n], nil
+}
+
+func (d *Dir) check() error {
+	if d == nil || d.fd < 0 {
+		return fmt.Errorf("%w: closed directory", ErrUnsafe)
+	}
+	return validateDirectoryFD(d.fd)
+}
+
+func absolutePath(path string) (string, error) {
+	if !filepath.IsAbs(path) {
+		return "", fmt.Errorf("%w: path must be absolute", ErrUnsafe)
+	}
+	return filepath.Clean(path), nil
+}
+
+func openDirectoryPath(path string) (int, error) {
+	fd, err := syscall.Open("/", directoryOpenFlags, 0)
+	if err != nil {
+		return -1, err
+	}
+	if path == "/" {
+		return fd, nil
+	}
+	for _, part := range strings.Split(strings.TrimPrefix(path, "/"), "/") {
+		next, err := syscall.Openat(fd, part, directoryOpenFlags, 0)
+		_ = syscall.Close(fd)
+		if err != nil {
+			return -1, err
+		}
+		var stat syscall.Stat_t
+		if err := syscall.Fstat(next, &stat); err != nil {
+			_ = syscall.Close(next)
+			return -1, err
+		}
+		if stat.Mode&syscall.S_IFMT != syscall.S_IFDIR {
+			_ = syscall.Close(next)
+			return -1, fmt.Errorf("%w: non-directory path component", ErrUnsafe)
+		}
+		fd = next
+	}
+	return fd, nil
+}
+
+func compareWithLstat(fd int, path string, directory bool) error {
+	var opened syscall.Stat_t
+	if err := syscall.Fstat(fd, &opened); err != nil {
+		return err
+	}
+	var linked syscall.Stat_t
+	if err := syscall.Lstat(path, &linked); err != nil {
+		return err
+	}
+	if directory {
+		if linked.Mode&syscall.S_IFMT != syscall.S_IFDIR {
+			return fmt.Errorf("%w: path is not a directory", ErrUnsafe)
+		}
+	} else if linked.Mode&syscall.S_IFMT != syscall.S_IFREG {
+		return fmt.Errorf("%w: path is not a regular file", ErrUnsafe)
+	}
+	if opened.Dev != linked.Dev || opened.Ino != linked.Ino {
+		return fmt.Errorf("%w: path changed while opening", ErrUnsafe)
+	}
+	return nil
+}
+
+func validateDirectoryFD(fd int) error {
+	var stat syscall.Stat_t
+	if err := syscall.Fstat(fd, &stat); err != nil {
+		return err
+	}
+	if stat.Mode&syscall.S_IFMT != syscall.S_IFDIR {
+		return fmt.Errorf("%w: not a directory", ErrUnsafe)
+	}
+	if stat.Mode&0o7022 != 0 {
+		return fmt.Errorf("%w: unsafe directory mode %04o", ErrUnsafe, stat.Mode&0o7777)
+	}
+	return nil
+}
+
+func validateRegularFD(fd int, exactMode uint32) error {
+	var stat syscall.Stat_t
+	if err := syscall.Fstat(fd, &stat); err != nil {
+		return err
+	}
+	var required *uint32
+	if exactMode != 0 {
+		required = &exactMode
+	}
+	return validateRegularStat(&stat, required)
+}
+
+func validateRegularStat(stat *syscall.Stat_t, exactMode *uint32) error {
+	if stat.Mode&syscall.S_IFMT != syscall.S_IFREG {
+		return fmt.Errorf("%w: not a regular file", ErrUnsafe)
+	}
+	if stat.Mode&0o7022 != 0 {
+		return fmt.Errorf("%w: unsafe file mode %04o", ErrUnsafe, stat.Mode&0o7777)
+	}
+	if exactMode != nil && stat.Mode&0o777 != *exactMode {
+		return fmt.Errorf("%w: file mode %04o is not %04o", ErrUnsafe, stat.Mode&0o777, *exactMode)
+	}
+	return nil
+}
+
+func validMode(mode os.FileMode) error {
+	if mode&^os.FileMode(0o777) != 0 || mode.Perm()&0o022 != 0 {
+		return fmt.Errorf("%w: unsafe creation mode %04o", ErrUnsafe, mode)
+	}
+	return nil
+}
+
+func validName(name string) error {
+	if name == "" || name == "." || name == ".." || strings.Contains(name, "/") || strings.ContainsRune(name, 0) {
+		return fmt.Errorf("%w: invalid path component", ErrUnsafe)
+	}
+	return nil
+}
+
+const directoryOpenFlags = syscall.O_RDONLY | syscall.O_DIRECTORY | syscall.O_NOFOLLOW | syscall.O_CLOEXEC
+
+var _ io.Closer = (*Dir)(nil)
diff --git a/tools/dwh-auth/internal/securefile/securefile_linux_test.go b/tools/dwh-auth/internal/securefile/securefile_linux_test.go
new file mode 100644
index 00000000..6a68140d
--- /dev/null
+++ b/tools/dwh-auth/internal/securefile/securefile_linux_test.go
@@ -0,0 +1,215 @@
+//go:build linux
+
+package securefile
+
+import (
+	"bytes"
+	"errors"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestOpenDirAndReadFileAcceptProtectedRegularFile(t *testing.T) {
+	root := t.TempDir()
+	path := filepath.Join(root, "record.json")
+	want := []byte(`{"record":"synthetic"}`)
+	writeFile(t, path, want, 0o640)
+
+	dir, err := OpenDir(root)
+	if err != nil {
+		t.Fatalf("OpenDir() error = %v", err)
+	}
+	t.Cleanup(func() { _ = dir.Close() })
+
+	got, err := dir.ReadFile("record.json", 4096)
+	if err != nil {
+		t.Fatalf("ReadFile() error = %v", err)
+	}
+	if !bytes.Equal(got, want) {
+		t.Fatalf("ReadFile() = %q, want %q", got, want)
+	}
+}
+
+func TestProtectedPathsRejectSymlinks(t *testing.T) {
+	t.Run("root", func(t *testing.T) {
+		parent := t.TempDir()
+		target := t.TempDir()
+		root := filepath.Join(parent, "registry")
+		if err := os.Symlink(target, root); err != nil {
+			t.Fatalf("Symlink() error = %v", err)
+		}
+		if _, err := OpenDir(root); err == nil {
+			t.Fatal("OpenDir() error = nil, want symlink refusal")
+		}
+	})
+
+	t.Run("directory", func(t *testing.T) {
+		root := t.TempDir()
+		target := t.TempDir()
+		if err := os.Symlink(target, filepath.Join(root, "active")); err != nil {
+			t.Fatalf("Symlink() error = %v", err)
+		}
+		dir, err := OpenDir(root)
+		if err != nil {
+			t.Fatalf("OpenDir() error = %v", err)
+		}
+		t.Cleanup(func() { _ = dir.Close() })
+		if _, err := dir.OpenDir("active"); err == nil {
+			t.Fatal("OpenDir(active) error = nil, want symlink refusal")
+		}
+	})
+
+	t.Run("record", func(t *testing.T) {
+		root := t.TempDir()
+		dir, err := OpenDir(root)
+		if err != nil {
+			t.Fatalf("OpenDir() error = %v", err)
+		}
+		t.Cleanup(func() { _ = dir.Close() })
+		target := filepath.Join(root, "target.json")
+		writeFile(t, target, []byte(`{"safe":false}`), 0o640)
+		if err := os.Symlink(target, filepath.Join(root, "record.json")); err != nil {
+			t.Fatalf("Symlink() error = %v", err)
+		}
+		if _, err := dir.ReadFile("record.json", 4096); err == nil {
+			t.Fatal("ReadFile() error = nil, want symlink refusal")
+		}
+	})
+
+	t.Run("secret", func(t *testing.T) {
+		root := t.TempDir()
+		target := filepath.Join(root, "target.secret")
+		writeFile(t, target, []byte("synthetic-secret"), 0o600)
+		secret := filepath.Join(root, "secret")
+		if err := os.Symlink(target, secret); err != nil {
+			t.Fatalf("Symlink() error = %v", err)
+		}
+		if _, err := ReadSecret(secret, 128); err == nil {
+			t.Fatal("ReadSecret() error = nil, want symlink refusal")
+		}
+	})
+}
+
+func TestProtectedPathsRejectUnsafeModes(t *testing.T) {
+	t.Run("root", func(t *testing.T) {
+		root := t.TempDir()
+		if err := os.Chmod(root, 0o770); err != nil {
+			t.Fatalf("Chmod() error = %v", err)
+		}
+		if _, err := OpenDir(root); err == nil {
+			t.Fatal("OpenDir() error = nil, want unsafe mode refusal")
+		}
+	})
+
+	t.Run("child directory", func(t *testing.T) {
+		root := t.TempDir()
+		child := filepath.Join(root, "active")
+		if err := os.Mkdir(child, 0o770); err != nil {
+			t.Fatalf("Mkdir() error = %v", err)
+		}
+		if err := os.Chmod(child, 0o770); err != nil {
+			t.Fatalf("Chmod() error = %v", err)
+		}
+		dir, err := OpenDir(root)
+		if err != nil {
+			t.Fatalf("OpenDir() error = %v", err)
+		}
+		t.Cleanup(func() { _ = dir.Close() })
+		if _, err := dir.OpenDir("active"); err == nil {
+			t.Fatal("OpenDir(active) error = nil, want unsafe mode refusal")
+		}
+	})
+
+	t.Run("record", func(t *testing.T) {
+		root := t.TempDir()
+		path := filepath.Join(root, "record.json")
+		writeFile(t, path, []byte(`{"unsafe":true}`), 0o660)
+		dir, err := OpenDir(root)
+		if err != nil {
+			t.Fatalf("OpenDir() error = %v", err)
+		}
+		t.Cleanup(func() { _ = dir.Close() })
+		if _, err := dir.ReadFile("record.json", 4096); err == nil {
+			t.Fatal("ReadFile() error = nil, want unsafe mode refusal")
+		}
+	})
+
+	t.Run("secret", func(t *testing.T) {
+		root := t.TempDir()
+		secret := filepath.Join(root, "secret")
+		writeFile(t, secret, []byte("synthetic-secret"), 0o640)
+		if _, err := ReadSecret(secret, 128); err == nil {
+			t.Fatal("ReadSecret() error = nil, want non-0600 refusal")
+		}
+	})
+}
+
+func TestReadFileRejectsOversizeContent(t *testing.T) {
+	root := t.TempDir()
+	path := filepath.Join(root, "record.json")
+	writeFile(t, path, bytes.Repeat([]byte{'a'}, 4097), 0o640)
+	dir, err := OpenDir(root)
+	if err != nil {
+		t.Fatalf("OpenDir() error = %v", err)
+	}
+	t.Cleanup(func() { _ = dir.Close() })
+
+	if _, err := dir.ReadFile("record.json", 4096); err == nil {
+		t.Fatal("ReadFile() error = nil, want bounded-read refusal")
+	}
+}
+
+func TestReadSecretAndCreateSecretUse0600AndExclusiveCreate(t *testing.T) {
+	root := t.TempDir()
+	input := filepath.Join(root, "input.secret")
+	want := []byte("synthetic-secret")
+	writeFile(t, input, want, 0o600)
+
+	got, err := ReadSecret(input, 128)
+	if err != nil {
+		t.Fatalf("ReadSecret() error = %v", err)
+	}
+	if !bytes.Equal(got, want) {
+		t.Fatalf("ReadSecret() = %q, want %q", got, want)
+	}
+
+	output := filepath.Join(root, "output.secret")
+	file, err := CreateSecret(output)
+	if err != nil {
+		t.Fatalf("CreateSecret() error = %v", err)
+	}
+	if _, err := file.Write(want); err != nil {
+		_ = file.Close()
+		t.Fatalf("Write() error = %v", err)
+	}
+	if err := file.Sync(); err != nil {
+		_ = file.Close()
+		t.Fatalf("Sync() error = %v", err)
+	}
+	if err := file.Close(); err != nil {
+		t.Fatalf("Close() error = %v", err)
+	}
+	info, err := os.Stat(output)
+	if err != nil {
+		t.Fatalf("Stat() error = %v", err)
+	}
+	if got, want := info.Mode().Perm(), os.FileMode(0o600); got != want {
+		t.Fatalf("output mode = %04o, want %04o", got, want)
+	}
+	if _, err := CreateSecret(output); err == nil {
+		t.Fatal("CreateSecret(existing) error = nil, want exclusive-create refusal")
+	} else if !errors.Is(err, os.ErrExist) {
+		t.Fatalf("CreateSecret(existing) error = %v, want os.ErrExist", err)
+	}
+}
+
+func writeFile(t *testing.T, path string, data []byte, mode os.FileMode) {
+	t.Helper()
+	if err := os.WriteFile(path, data, mode); err != nil {
+		t.Fatalf("WriteFile(%q) error = %v", path, err)
+	}
+	if err := os.Chmod(path, mode); err != nil {
+		t.Fatalf("Chmod(%q) error = %v", path, err)
+	}
+}