oauth: opt-in AES-256-GCM encrypted-at-rest token storage

internal/cli/auth.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"os"
 	"strings"
 	"time"
 
@@ -124,9 +125,24 @@ func validateAuthFlags(sub string, a authArgs) error {
 
 // newAuthManager builds an oauth.Manager backed by the file store, printing the
 // authorization URL / device code to stdout. The store path honors
-// ZERO_OAUTH_TOKENS_PATH (env), so callers/tests can redirect it.
+// ZERO_OAUTH_TOKENS_PATH (env), so callers/tests can redirect it. Setting
+// ZERO_OAUTH_STORAGE=encrypted-file selects the AES-256-GCM encrypted-at-rest
+// backend (a per-user secret is created beside the token file).
 func newAuthManager(deps appDeps, out io.Writer) (*oauth.Manager, error) {
-	store, err := oauth.NewStore(oauth.StoreOptions{Now: deps.now})
+	// Validate ZERO_OAUTH_STORAGE up front: a mistyped non-empty value must fail
+	// fast rather than silently fall back to plaintext while the user believes
+	// encryption is on. Empty = default (plaintext 0600); "encrypted-file" = AES.
+	encrypted := false
+	if mode := strings.TrimSpace(os.Getenv("ZERO_OAUTH_STORAGE")); mode != "" {
+		if !strings.EqualFold(mode, "encrypted-file") {
+			return nil, fmt.Errorf("invalid ZERO_OAUTH_STORAGE %q (supported: encrypted-file)", mode)
+		}
+		encrypted = true
+	}
+	store, err := oauth.NewStore(oauth.StoreOptions{
+		Now:       deps.now,
+		Encrypted: encrypted,
+	})
 	if err != nil {
 		return nil, err
 	}
@@ -333,6 +349,10 @@ built in). For a provider named <name>, set:
   ZERO_OAUTH_<NAME>_SCOPES          ZERO_OAUTH_<NAME>_FLOW (loopback|device)
 Endpoint URLs must be https (loopback exempt).
 
+Storage: tokens are written 0600 under $XDG_CONFIG_HOME/zero (override with
+ZERO_OAUTH_TOKENS_PATH). Set ZERO_OAUTH_STORAGE=encrypted-file to encrypt them
+at rest with AES-256-GCM (a per-user secret is created beside the token file).
+
 Flags:
       --device   Use the device-code flow (headless/SSH; no browser)
       --scope    Add an OAuth scope (repeatable)

internal/cli/auth_test.go

@@ -17,6 +17,20 @@ func withAuthStore(t *testing.T) string {
 	return path
 }
 
+func TestRunAuthRejectsInvalidStorageMode(t *testing.T) {
+	withAuthStore(t)
+	// A mistyped value must fail fast, not silently fall back to plaintext while
+	// the user believes encryption is active.
+	t.Setenv("ZERO_OAUTH_STORAGE", "encryptd")
+	var stdout, stderr bytes.Buffer
+	if code := runWithDeps([]string{"auth", "status"}, &stdout, &stderr, appDeps{}); code == exitSuccess {
+		t.Fatalf("invalid ZERO_OAUTH_STORAGE should fail, got success; stdout=%q", stdout.String())
+	}
+	if !strings.Contains(stderr.String(), "ZERO_OAUTH_STORAGE") {
+		t.Fatalf("error should name the offending env var, stderr=%q", stderr.String())
+	}
+}
+
 func TestRunAuthStatusEmpty(t *testing.T) {
 	withAuthStore(t)
 	var stdout, stderr bytes.Buffer

internal/oauth/encrypt.go

@@ -0,0 +1,147 @@
+package oauth
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/rand"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"path/filepath"
+	"time"
+)
+
+// secretBytes is the AES-256 key length kept in the per-user secret file.
+const secretBytes = 32
+
+// aesGCMCrypter encrypts the token file at rest with AES-256-GCM under a
+// per-user random secret persisted (0600) beside the token file. The on-disk
+// blob is nonce || ciphertext; GCM provides confidentiality AND tamper
+// detection, so a corrupted/forged file fails closed on open. This is the
+// opt-in "encrypted-file" storage backend; the default backend writes the
+// 0600 plaintext JSON unchanged.
+type aesGCMCrypter struct {
+	secretPath string
+}
+
+func newAESGCMCrypter(secretPath string) *aesGCMCrypter {
+	return &aesGCMCrypter{secretPath: secretPath}
+}
+
+// aead loads (or, when create is set, generates) the secret and returns the GCM
+// AEAD. open passes create=false so a missing secret is a hard error rather than
+// silently minting a new key that could never decrypt the existing file.
+func (c *aesGCMCrypter) aead(create bool) (cipher.AEAD, error) {
+	secret, err := loadOrCreateSecret(c.secretPath, create)
+	if err != nil {
+		return nil, err
+	}
+	block, err := aes.NewCipher(secret)
+	if err != nil {
+		return nil, fmt.Errorf("oauth: build cipher: %w", err)
+	}
+	return cipher.NewGCM(block)
+}
+
+// seal encrypts plaintext, prefixing a fresh random nonce. It creates the secret
+// on first use.
+func (c *aesGCMCrypter) seal(plaintext []byte) ([]byte, error) {
+	gcm, err := c.aead(true)
+	if err != nil {
+		return nil, err
+	}
+	nonce := make([]byte, gcm.NonceSize())
+	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
+		return nil, fmt.Errorf("oauth: generate nonce: %w", err)
+	}
+	return gcm.Seal(nonce, nonce, plaintext, nil), nil
+}
+
+// open decrypts a nonce||ciphertext blob, failing closed on a missing secret,
+// a short blob, or a failed authentication tag (tampering / wrong key).
+func (c *aesGCMCrypter) open(blob []byte) ([]byte, error) {
+	gcm, err := c.aead(false)
+	if err != nil {
+		return nil, err
+	}
+	if len(blob) < gcm.NonceSize() {
+		return nil, errors.New("oauth: encrypted token file is too short")
+	}
+	nonce, ciphertext := blob[:gcm.NonceSize()], blob[gcm.NonceSize():]
+	plaintext, err := gcm.Open(nil, nonce, ciphertext, nil)
+	if err != nil {
+		return nil, fmt.Errorf("oauth: decrypt token file (wrong secret or tampered): %w", err)
+	}
+	return plaintext, nil
+}
+
+// loadOrCreateSecret reads the 32-byte secret at path. When create is set and
+// the file is absent, it generates a random secret and creates the file
+// exclusively (0600). A wrong-sized existing secret fails closed (corruption).
+func loadOrCreateSecret(path string, create bool) ([]byte, error) {
+	if data, err := readSecretFile(path); err == nil {
+		return data, nil
+	} else if !errors.Is(err, os.ErrNotExist) {
+		return nil, err
+	}
+	if !create {
+		return nil, fmt.Errorf("oauth: token secret %s is missing; cannot decrypt the token file", path)
+	}
+	secret := make([]byte, secretBytes)
+	if _, err := io.ReadFull(rand.Reader, secret); err != nil {
+		return nil, fmt.Errorf("oauth: generate token secret: %w", err)
+	}
+	if err := os.MkdirAll(filepath.Dir(path), 0o700); err != nil {
+		return nil, err
+	}
+	// Create exclusively: an os.Rename publish would clobber on POSIX, so two
+	// concurrent first-run processes could each generate a secret and the loser
+	// would silently orphan the tokens it encrypts. O_EXCL lets exactly one
+	// process win; everyone else adopts the winner's on-disk secret.
+	f, err := os.OpenFile(path, os.O_CREATE|os.O_EXCL|os.O_WRONLY, 0o600)
+	if err != nil {
+		if errors.Is(err, os.ErrExist) {
+			// The winner may not have finished writing yet, so a read here can see a
+			// short/absent file. Retry briefly so concurrent first-run invocations
+			// converge on the winner's secret instead of failing transiently.
+			var lastErr error
+			for attempt := 0; attempt < 50; attempt++ {
+				secret, rerr := readSecretFile(path)
+				if rerr == nil {
+					return secret, nil
+				}
+				lastErr = rerr
+				time.Sleep(2 * time.Millisecond)
+			}
+			return nil, lastErr
+		}
+		return nil, fmt.Errorf("oauth: create token secret: %w", err)
+	}
+	if _, werr := f.Write(secret); werr != nil {
+		_ = f.Close()
+		_ = os.Remove(path)
+		return nil, fmt.Errorf("oauth: write token secret: %w", werr)
+	}
+	if cerr := f.Close(); cerr != nil {
+		_ = os.Remove(path)
+		return nil, fmt.Errorf("oauth: write token secret: %w", cerr)
+	}
+	return secret, nil
+}
+
+// readSecretFile reads and validates the secret at path, returning a wrapped
+// os.ErrNotExist when it is absent so callers can branch on creation.
+func readSecretFile(path string) ([]byte, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		if errors.Is(err, os.ErrNotExist) {
+			return nil, err
+		}
+		return nil, fmt.Errorf("oauth: read token secret: %w", err)
+	}
+	if len(data) != secretBytes {
+		return nil, fmt.Errorf("oauth: token secret at %s has unexpected size %d", path, len(data))
+	}
+	return data, nil
+}

internal/oauth/encrypt_test.go

@@ -0,0 +1,180 @@
+package oauth
+
+import (
+	"bytes"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"sync"
+	"testing"
+)
+
+func TestLoadOrCreateSecretConcurrentConverges(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "tok.json.secret")
+	const n = 16
+	secrets := make([][]byte, n)
+	errs := make([]error, n)
+	var wg sync.WaitGroup
+	for i := 0; i < n; i++ {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			secrets[i], errs[i] = loadOrCreateSecret(path, true)
+		}(i)
+	}
+	wg.Wait()
+	// Exactly one creator wins; every racer must converge on the same on-disk
+	// secret rather than orphaning its own (the O_EXCL + bounded-retry path).
+	for i := 0; i < n; i++ {
+		if errs[i] != nil {
+			t.Fatalf("goroutine %d: %v", i, errs[i])
+		}
+		if !bytes.Equal(secrets[i], secrets[0]) {
+			t.Fatalf("goroutine %d got a divergent secret; concurrent create did not converge", i)
+		}
+	}
+}
+
+func TestAESGCMCrypterRoundTripAndTamper(t *testing.T) {
+	secretPath := filepath.Join(t.TempDir(), "tok.json.secret")
+	c := newAESGCMCrypter(secretPath)
+	plaintext := []byte(`{"schemaVersion":1,"tokens":{}}`)
+	blob, err := c.seal(plaintext)
+	if err != nil {
+		t.Fatalf("seal: %v", err)
+	}
+	if bytes.Contains(blob, plaintext) {
+		t.Fatal("sealed blob must not contain the plaintext")
+	}
+	got, err := c.open(blob)
+	if err != nil {
+		t.Fatalf("open: %v", err)
+	}
+	if !bytes.Equal(got, plaintext) {
+		t.Fatalf("round-trip mismatch: %q", got)
+	}
+	// Tamper: flipping any byte breaks the GCM tag.
+	tampered := append([]byte(nil), blob...)
+	tampered[len(tampered)-1] ^= 0xff
+	if _, err := c.open(tampered); err == nil {
+		t.Fatal("open must reject a tampered blob (GCM auth)")
+	}
+}
+
+func TestLoadOrCreateSecret(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "tok.json.secret")
+	// Missing + create=false => fail closed (can't decrypt without the secret).
+	if _, err := loadOrCreateSecret(path, false); err == nil {
+		t.Fatal("missing secret with create=false must error")
+	}
+	secret, err := loadOrCreateSecret(path, true)
+	if err != nil {
+		t.Fatalf("create: %v", err)
+	}
+	if len(secret) != secretBytes {
+		t.Fatalf("secret length = %d, want %d", len(secret), secretBytes)
+	}
+	// Stable across reads.
+	again, err := loadOrCreateSecret(path, false)
+	if err != nil || !bytes.Equal(again, secret) {
+		t.Fatalf("secret not stable: %v", err)
+	}
+	if runtime.GOOS != "windows" {
+		info, err := os.Stat(path)
+		if err != nil {
+			t.Fatalf("stat secret: %v", err)
+		}
+		if perm := info.Mode().Perm(); perm != 0o600 {
+			t.Fatalf("secret file mode = %o, want 600", perm)
+		}
+	}
+	// Wrong-sized secret fails closed.
+	if err := os.WriteFile(path, []byte("short"), 0o600); err != nil {
+		t.Fatalf("corrupt secret: %v", err)
+	}
+	if _, err := loadOrCreateSecret(path, true); err == nil {
+		t.Fatal("wrong-sized secret must error")
+	}
+}
+
+func newEncryptedStore(t *testing.T) (*Store, string) {
+	t.Helper()
+	path := filepath.Join(t.TempDir(), "oauth-tokens.json")
+	s, err := NewStore(StoreOptions{FilePath: path, Encrypted: true})
+	if err != nil {
+		t.Fatalf("NewStore(encrypted): %v", err)
+	}
+	return s, path
+}
+
+func TestEncryptedStoreRoundTripAndCiphertextOnDisk(t *testing.T) {
+	s, path := newEncryptedStore(t)
+	tok := Token{AccessToken: "super-secret-access", RefreshToken: "super-secret-refresh", Account: "me@x"}
+	if err := s.Save(ProviderKey("demo"), tok); err != nil {
+		t.Fatalf("Save: %v", err)
+	}
+	// On-disk file must be ciphertext: not valid JSON, no plaintext token.
+	raw, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("read file: %v", err)
+	}
+	if strings.Contains(string(raw), "super-secret-access") || strings.Contains(string(raw), "schemaVersion") {
+		t.Fatalf("token file is not encrypted at rest:\n%s", raw)
+	}
+	// The secret file exists beside it.
+	if _, err := os.Stat(path + ".secret"); err != nil {
+		t.Fatalf("secret file missing: %v", err)
+	}
+	// A fresh Store (same path) decrypts and reads it back.
+	s2, err := NewStore(StoreOptions{FilePath: path, Encrypted: true})
+	if err != nil {
+		t.Fatalf("NewStore 2: %v", err)
+	}
+	got, ok, err := s2.Load(ProviderKey("demo"))
+	if err != nil || !ok || got.AccessToken != "super-secret-access" || got.Account != "me@x" {
+		t.Fatalf("Load = %+v ok=%v err=%v", got, ok, err)
+	}
+	// Delete + Status work through the encrypted backend too.
+	statuses, err := s2.Status(KeyPrefixProvider)
+	if err != nil || len(statuses) != 1 {
+		t.Fatalf("Status = %+v err=%v", statuses, err)
+	}
+	if removed, err := s2.Delete(ProviderKey("demo")); err != nil || !removed {
+		t.Fatalf("Delete = %v %v", removed, err)
+	}
+}
+
+func TestEncryptedStoreTamperFailsClosed(t *testing.T) {
+	s, path := newEncryptedStore(t)
+	if err := s.Save(ProviderKey("demo"), Token{AccessToken: "a"}); err != nil {
+		t.Fatalf("Save: %v", err)
+	}
+	raw, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("read file: %v", err)
+	}
+	if len(raw) == 0 {
+		t.Fatal("encrypted token file is empty")
+	}
+	raw[len(raw)-1] ^= 0xff
+	if err := os.WriteFile(path, raw, 0o600); err != nil {
+		t.Fatalf("rewrite: %v", err)
+	}
+	if _, _, err := s.Load(ProviderKey("demo")); err == nil {
+		t.Fatal("a tampered encrypted store must fail closed")
+	}
+}
+
+func TestEncryptedStoreMissingSecretFailsClosed(t *testing.T) {
+	s, path := newEncryptedStore(t)
+	if err := s.Save(ProviderKey("demo"), Token{AccessToken: "a"}); err != nil {
+		t.Fatalf("Save: %v", err)
+	}
+	if err := os.Remove(path + ".secret"); err != nil {
+		t.Fatalf("remove secret: %v", err)
+	}
+	if _, _, err := s.Load(ProviderKey("demo")); err == nil {
+		t.Fatal("missing secret must fail closed, not return empty")
+	}
+}