Keep the keychain unlocker passphrase in locked memory (closes #36)

The passphrase protecting the keychain unlocker's age key was a plain
string passed through encoding/json, leaving copies in ordinary memory
when an unlocker was created and each time one was used.

It is now generated into a locked buffer, and KeychainData, moved to
keychaindata.go, which is not darwin-only so its tests run on Linux,
writes and reads the keychain JSON itself: encode copies the parts
straight into a locked buffer, and decodeKeychainData takes the
passphrase from a json.RawMessage that it wipes. The JSON field names
are unchanged. keychainunlocker.go only calls this code and stores the
item from the locked buffer without a string copy.

Model: opus-5-5

TODO.md

@@ -25,6 +25,11 @@ Bring the repo into policy compliance in one commit:
 
 # Completed Steps
 
+- 2026-10-03: The keychain unlocker's age key passphrase stays in
+  locked memory: it is generated into a locked buffer, and the
+  keychain JSON is written and read by `KeychainData` code in
+  `internal/secret/keychaindata.go` (tested on Linux) without
+  `encoding/json` holding it; the JSON field names are unchanged.
 - 2026-10-02: A plain `docker build .` builds again: the size tests
   skip a case that needs more locked memory than the process can
   lock, and run every case under `script/cibuild`. The image stamps the
@@ -88,12 +93,11 @@ Bring the repo into policy compliance in one commit:
     - Command injection: GPG key IDs passed unescaped to exec.Command
       (pgpunlocker.go:323-327); data.String() passed unescaped to the
       security command (keychainunlocker.go:472-476).
-    - Memory security: KeychainData stores AgePrivKeyPassphrase as a
+    - Memory security: age identity .String() creates unprotected
-      plain string (keychainunlocker.go:342,393-396); age identity
+      copies (keychainunlocker.go:356, pgpunlocker.go:256,
-      .String() creates unprotected copies (keychainunlocker.go:356,
+      version.go:155); age secret key held in a plain string in
-      pgpunlocker.go:256, version.go:155); age secret key held in a
+      cli/crypto.go:86,91,113; private keys exposed via buffer.Bytes()
-      plain string in cli/crypto.go:86,91,113; private keys exposed via
+      to GPGEncryptFunc and EncryptWithPassphrase.
-      buffer.Bytes() to GPGEncryptFunc and EncryptWithPassphrase.
     - Race conditions: no file locking in vault/secrets.go:142-176;
       non-atomic writes can leave the vault inconsistent.
     - Input validation: dots in secret names risk path traversal

internal/secret/helpers_darwin.go

@@ -1,29 +0,0 @@
-//go:build darwin
-
-package secret
-
-import (
-	"crypto/rand"
-	"fmt"
-	"math/big"
-)
-
-// generateRandomString generates a random string of the specified length using the given character set
-func generateRandomString(length int, charset string) (string, error) {
-	if length <= 0 {
-		return "", fmt.Errorf("length must be positive")
-	}
-
-	result := make([]byte, length)
-	charsetLen := big.NewInt(int64(len(charset)))
-
-	for i := range length {
-		randomIndex, err := rand.Int(rand.Reader, charsetLen)
-		if err != nil {
-			return "", fmt.Errorf("failed to generate random number: %w", err)
-		}
-		result[i] = charset[randomIndex.Int64()]
-	}
-
-	return string(result), nil
-}

internal/secret/keychaindata.go

@@ -0,0 +1,142 @@
+package secret
+
+import (
+	"bytes"
+	"encoding/hex"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"strings"
+
+	"github.com/awnumar/memguard"
+)
+
+var (
+	errPassphraseLength = errors.New(
+		"passphrase length must be a positive even number")
+	errPassphraseNotHex = errors.New(
+		"keychain passphrase must be lowercase hex")
+	errNoKeychainPassphrase = errors.New(
+		"keychain data has no agePrivKeyPassphrase string")
+)
+
+// KeychainData is what a keychain unlocker stores in the macOS keychain.
+// It is stored as JSON, but encode and decodeKeychainData keep the
+// passphrase out of encoding/json, which would leave copies of it in
+// ordinary memory.
+type KeychainData struct {
+	AgePublicKey         string
+	AgePrivKeyPassphrase *memguard.LockedBuffer
+	EncryptedLongtermKey string
+}
+
+// generateRandomPassphrase returns length random lowercase hex characters
+// in a locked buffer. The caller must destroy it.
+func generateRandomPassphrase(length int) (*memguard.LockedBuffer, error) {
+	// Each random byte becomes two hex characters.
+	randomBytes := hex.DecodedLen(length)
+	if length <= 0 || hex.EncodedLen(randomBytes) != length {
+		return nil, errPassphraseLength
+	}
+
+	random := memguard.NewBufferRandom(randomBytes)
+	defer random.Destroy()
+
+	passphrase := memguard.NewBuffer(length)
+	hex.Encode(passphrase.Bytes(), random.Bytes())
+	passphrase.Freeze()
+
+	return passphrase, nil
+}
+
+// encode returns d as JSON in a locked buffer:
+// {"agePublicKey":"...","agePrivKeyPassphrase":"...","encryptedLongtermKey":"..."}.
+// The passphrase is copied straight into the buffer, so it must be hex,
+// which JSON does not escape. The caller must destroy the returned buffer.
+func (d *KeychainData) encode() (*memguard.LockedBuffer, error) {
+	if d.AgePrivKeyPassphrase == nil {
+		return nil, errNilPassphraseBuffer
+	}
+
+	if d.AgePrivKeyPassphrase.Size() == 0 {
+		return nil, errEmptyPassphrase
+	}
+
+	for _, c := range d.AgePrivKeyPassphrase.Bytes() {
+		if strings.IndexByte("0123456789abcdef", c) < 0 {
+			return nil, errPassphraseNotHex
+		}
+	}
+
+	publicKey, err := json.Marshal(d.AgePublicKey)
+	if err != nil {
+		return nil, fmt.Errorf("failed to encode age public key: %w", err)
+	}
+
+	longtermKey, err := json.Marshal(d.EncryptedLongtermKey)
+	if err != nil {
+		return nil, fmt.Errorf("failed to encode long-term key: %w", err)
+	}
+
+	parts := [][]byte{
+		[]byte(`{"agePublicKey":`), publicKey,
+		[]byte(`,"agePrivKeyPassphrase":"`), d.AgePrivKeyPassphrase.Bytes(),
+		[]byte(`","encryptedLongtermKey":`), longtermKey,
+		[]byte(`}`),
+	}
+
+	size := 0
+	for _, part := range parts {
+		size += len(part)
+	}
+
+	encoded := memguard.NewBuffer(size)
+
+	written := 0
+	for _, part := range parts {
+		written += copy(encoded.Bytes()[written:], part)
+	}
+
+	encoded.Freeze()
+
+	return encoded, nil
+}
+
+// decodeKeychainData parses keychain data written by encode. The caller
+// must destroy the returned AgePrivKeyPassphrase.
+func decodeKeychainData(data *memguard.LockedBuffer) (*KeychainData, error) {
+	if data == nil {
+		return nil, errNilDataBuffer
+	}
+
+	// json.Unmarshal gives a json.RawMessage field the field's JSON text
+	// unchanged, in the one copy RawMessage makes; it is wiped on return.
+	var fields struct {
+		AgePublicKey         string          `json:"agePublicKey"`
+		AgePrivKeyPassphrase json.RawMessage `json:"agePrivKeyPassphrase"`
+		EncryptedLongtermKey string          `json:"encryptedLongtermKey"`
+	}
+
+	defer func() { memguard.WipeBytes(fields.AgePrivKeyPassphrase) }()
+
+	err := json.Unmarshal(data.Bytes(), &fields)
+	if err != nil {
+		return nil, fmt.Errorf("failed to parse keychain data: %w", err)
+	}
+
+	// json.Unmarshal accepted the JSON, so text that starts with a quote is
+	// a whole string. The passphrase is hex, so it is the text between the
+	// quotes.
+	quoted := fields.AgePrivKeyPassphrase
+	if !bytes.HasPrefix(quoted, []byte(`"`)) {
+		return nil, errNoKeychainPassphrase
+	}
+
+	return &KeychainData{
+		AgePublicKey: fields.AgePublicKey,
+		// NewBufferFromBytes wipes the bytes it copies.
+		AgePrivKeyPassphrase: memguard.NewBufferFromBytes(
+			quoted[1 : len(quoted)-1]),
+		EncryptedLongtermKey: fields.EncryptedLongtermKey,
+	}, nil
+}

internal/secret/keychaindata_test.go

@@ -0,0 +1,118 @@
+//nolint:testpackage // white-box test of unexported internals
+package secret
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/awnumar/memguard"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestGenerateRandomPassphrase(t *testing.T) {
+	t.Parallel()
+
+	first, err := generateRandomPassphrase(64)
+	require.NoError(t, err)
+
+	defer first.Destroy()
+
+	second, err := generateRandomPassphrase(64)
+	require.NoError(t, err)
+
+	defer second.Destroy()
+
+	assert.Regexp(t, `^[0-9a-f]{64}$`, first.String())
+	assert.NotEqual(t, first.String(), second.String())
+	assert.False(t, first.IsMutable())
+
+	for _, length := range []int{0, -2, 63} {
+		_, err := generateRandomPassphrase(length)
+		require.ErrorIs(t, err, errPassphraseLength, "length %d", length)
+	}
+}
+
+func TestKeychainDataEncodeDecode(t *testing.T) {
+	t.Parallel()
+
+	passphrase := memguard.NewBufferFromBytes([]byte("0a1b2c3d"))
+	defer passphrase.Destroy()
+
+	data := KeychainData{
+		AgePublicKey:         "age1example",
+		AgePrivKeyPassphrase: passphrase,
+		EncryptedLongtermKey: "beef",
+	}
+
+	encoded, err := data.encode()
+	require.NoError(t, err)
+
+	defer encoded.Destroy()
+
+	assert.JSONEq(t,
+		`{"agePublicKey":"age1example",`+
+			`"agePrivKeyPassphrase":"0a1b2c3d",`+
+			`"encryptedLongtermKey":"beef"}`,
+		encoded.String())
+	assert.False(t, encoded.IsMutable())
+
+	decoded, err := decodeKeychainData(encoded)
+	require.NoError(t, err)
+
+	defer decoded.AgePrivKeyPassphrase.Destroy()
+
+	assert.Equal(t, "age1example", decoded.AgePublicKey)
+	assert.Equal(t, "0a1b2c3d", decoded.AgePrivKeyPassphrase.String())
+	assert.Equal(t, "beef", decoded.EncryptedLongtermKey)
+}
+
+func TestKeychainDataEncodeRejectsBadPassphrase(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name       string
+		passphrase *memguard.LockedBuffer
+		wantErr    error
+	}{
+		{"nil", nil, errNilPassphraseBuffer},
+		{"empty", memguard.NewBuffer(0), errEmptyPassphrase},
+		{
+			"not hex",
+			memguard.NewBufferFromBytes([]byte(`abc"def`)),
+			errPassphraseNotHex,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			data := KeychainData{AgePrivKeyPassphrase: tt.passphrase}
+			_, err := data.encode()
+			require.ErrorIs(t, err, tt.wantErr)
+		})
+	}
+}
+
+func TestDecodeKeychainDataRejectsBadData(t *testing.T) {
+	t.Parallel()
+
+	for _, text := range []string{
+		`{"agePublicKey":"age1example"}`,
+		`{"agePrivKeyPassphrase":42}`,
+	} {
+		data := memguard.NewBufferFromBytes([]byte(text))
+		_, err := decodeKeychainData(data)
+		data.Destroy()
+		require.ErrorIs(t, err, errNoKeychainPassphrase, text)
+	}
+
+	notJSON := memguard.NewBufferFromBytes([]byte(`{"agePrivKeyPassphrase":`))
+	defer notJSON.Destroy()
+
+	_, err := decodeKeychainData(notJSON)
+
+	var syntaxError *json.SyntaxError
+	require.ErrorAs(t, err, &syntaxError)
+}

internal/secret/keychainunlocker.go

@@ -45,13 +45,6 @@ type KeychainUnlocker struct {
 	fs        afero.Fs
 }
 
-// KeychainData represents the data stored in the macOS keychain
-type KeychainData struct {
-	AgePublicKey         string `json:"agePublicKey"`
-	AgePrivKeyPassphrase string `json:"agePrivKeyPassphrase"`
-	EncryptedLongtermKey string `json:"encryptedLongtermKey"`
-}
-
 // GetIdentity implements Unlocker interface for Keychain-based unlockers
 func (k *KeychainUnlocker) GetIdentity() (*age.X25519Identity, error) {
 	DebugWith("Getting keychain unlocker identity",
@@ -81,13 +74,18 @@ func (k *KeychainUnlocker) GetIdentity() (*age.X25519Identity, error) {
 		slog.Int("data_length", len(keychainDataBytes)),
 	)
 
+	// Move the keychain data into locked memory; this wipes keychainDataBytes
+	keychainDataBuffer := memguard.NewBufferFromBytes(keychainDataBytes)
+	defer keychainDataBuffer.Destroy()
+
 	// Step 3: Parse keychain data
-	var keychainData KeychainData
+	keychainData, err := decodeKeychainData(keychainDataBuffer)
-	if err := json.Unmarshal(keychainDataBytes, &keychainData); err != nil {
+	if err != nil {
 		Debug("Failed to parse keychain data", "error", err, "unlocker_id", k.GetID())
 
 		return nil, fmt.Errorf("failed to parse keychain data: %w", err)
 	}
+	defer keychainData.AgePrivKeyPassphrase.Destroy()
 
 	Debug("Parsed keychain data successfully", "unlocker_id", k.GetID())
 
@@ -109,11 +107,7 @@ func (k *KeychainUnlocker) GetIdentity() (*age.X25519Identity, error) {
 
 	// Step 5: Decrypt the age private key using the passphrase from keychain
 	Debug("Decrypting age private key with keychain passphrase", "unlocker_id", k.GetID())
-	// Create secure buffer for the keychain passphrase
+	agePrivKeyBuffer, err := DecryptWithPassphrase(encryptedAgePrivKeyData, keychainData.AgePrivKeyPassphrase)
-	passphraseBuffer := memguard.NewBufferFromBytes([]byte(keychainData.AgePrivKeyPassphrase))
-	defer passphraseBuffer.Destroy()
-
-	agePrivKeyBuffer, err := DecryptWithPassphrase(encryptedAgePrivKeyData, passphraseBuffer)
 	if err != nil {
 		Debug("Failed to decrypt age private key with keychain passphrase", "error", err, "unlocker_id", k.GetID())
 
@@ -369,6 +363,7 @@ func CreateKeychainUnlocker(fs afero.Fs, stateDir string) (*KeychainUnlocker, er
 	if err != nil {
 		return nil, fmt.Errorf("failed to generate age private key passphrase: %w", err)
 	}
+	defer agePrivKeyPassphrase.Destroy()
 
 	// Step 3: Store age recipient as plaintext
 	ageRecipient := ageIdentity.Recipient().String()
@@ -378,15 +373,12 @@ func CreateKeychainUnlocker(fs afero.Fs, stateDir string) (*KeychainUnlocker, er
 	}
 
 	// Step 4: Encrypt age private key with the generated passphrase and store on disk
-	// Create secure buffers for both the private key and passphrase
+	// Create a secure buffer for the private key
 	agePrivKeyStr := ageIdentity.String()
 	agePrivKeyBuffer := memguard.NewBufferFromBytes([]byte(agePrivKeyStr))
 	defer agePrivKeyBuffer.Destroy()
 
-	passphraseBuffer := memguard.NewBufferFromBytes([]byte(agePrivKeyPassphrase))
+	encryptedAgePrivKey, err := EncryptWithPassphrase(agePrivKeyBuffer, agePrivKeyPassphrase)
-	defer passphraseBuffer.Destroy()
-
-	encryptedAgePrivKey, err := EncryptWithPassphrase(agePrivKeyBuffer, passphraseBuffer)
 	if err != nil {
 		return nil, fmt.Errorf("failed to encrypt age private key with passphrase: %w", err)
 	}
@@ -422,13 +414,10 @@ func CreateKeychainUnlocker(fs afero.Fs, stateDir string) (*KeychainUnlocker, er
 		EncryptedLongtermKey: hex.EncodeToString(encryptedLtPrivKeyToAge),
 	}
 
-	keychainDataBytes, err := json.Marshal(keychainData)
+	keychainDataBuffer, err := keychainData.encode()
 	if err != nil {
-		return nil, fmt.Errorf("failed to marshal keychain data: %w", err)
+		return nil, fmt.Errorf("failed to encode keychain data: %w", err)
 	}
-
-	// Create a secure buffer for keychain data
-	keychainDataBuffer := memguard.NewBufferFromBytes(keychainDataBytes)
 	defer keychainDataBuffer.Destroy()
 
 	// Step 8: Store data in keychain
@@ -501,7 +490,7 @@ func storeInKeychain(itemName string, data *memguard.LockedBuffer) error {
 	item.SetAccount(itemName)
 	item.SetLabel(fmt.Sprintf("%s - %s", KEYCHAIN_APP_IDENTIFIER, itemName))
 	item.SetDescription("Secret vault keychain data")
-	item.SetData([]byte(data.String()))
+	item.SetData(data.Bytes())
 	item.SetSynchronizable(keychain.SynchronizableNo)
 	// Use AccessibleWhenUnlockedThisDeviceOnly for better security and to trigger auth
 	item.SetAccessible(keychain.AccessibleWhenUnlockedThisDeviceOnly)
@@ -576,8 +565,3 @@ func deleteFromKeychain(itemName string) error {
 
 	return nil
 }
-
-// generateRandomPassphrase generates a random passphrase for encrypting the age private key
-func generateRandomPassphrase(length int) (string, error) {
-	return generateRandomString(length, "0123456789abcdef")
-}