feat: identity export/import (yaw-key-backup-1)

Portable encrypted identity backup compatible with the sister project's
format — argon2id KDF + nacl secretbox, peer ID visible in plaintext for
pre-import verification.

- crypto: ExportIdentity, ImportIdentity, SaveIdentity
- proto: export_identity / import_identity IPC commands + events
- ipc: export_identity returns encrypted blob; import_identity validates
  and decrypts (read-only — on-disk write requires daemon restart)
- netmgr: MasterIdentity() accessor
- daemon: --import-identity / --import-passphrase flags write identity.json
  and exit, enabling scripted account migration
- tests: roundtrip and wrong-passphrase rejection

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/crypto/crypto.go

@@ -21,10 +21,12 @@ import (
 	"time"
 
 	"filippo.io/edwards25519"
+	"golang.org/x/crypto/argon2"
 	"golang.org/x/crypto/chacha20poly1305"
 	"golang.org/x/crypto/curve25519"
 	"golang.org/x/crypto/hkdf"
 	"golang.org/x/crypto/nacl/box"
+	"golang.org/x/crypto/nacl/secretbox"
 
 	"github.com/waste-go/internal/proto"
 )
@@ -99,6 +101,21 @@ func LoadOrCreate(dataDir, alias string) (*Identity, error) {
 	return &Identity{privateKey: priv, PublicKey: pub, Alias: alias}, nil
 }
 
+// SaveIdentity writes the identity to dataDir/identity.json, overwriting any existing file.
+// Used by --import-identity to commit an imported backup to disk.
+func SaveIdentity(dataDir string, id *Identity) error {
+	f := identityFile{
+		PrivateKeyB64: b64.EncodeToString(id.privateKey),
+		Alias:         id.Alias,
+	}
+	raw, err := json.MarshalIndent(f, "", "  ")
+	if err != nil {
+		return err
+	}
+	path := filepath.Join(dataDir, "identity.json")
+	return os.WriteFile(path, raw, 0600)
+}
+
 // PeerID returns the lowercase hex encoding of the 32-byte Ed25519 public key (YAW/2 §2).
 func (id *Identity) PeerID() proto.PeerID {
 	return proto.PeerID(hex.EncodeToString(id.PublicKey))
@@ -263,6 +280,127 @@ func (ek *EphemeralKey) SharedSecret(theirPublicB64 string) ([32]byte, error) {
 	return result, nil
 }
 
+// ── Identity backup / restore ─────────────────────────────────────────────────
+
+// keyBackup is the on-disk / exportable format, matching the sister project's
+// yaw-key-backup-1 schema for cross-app portability.
+type keyBackup struct {
+	Yaw   string `json:"yaw"`
+	ID    string `json:"id"`
+	Alg   string `json:"alg"`
+	Ops   uint32 `json:"ops"`
+	Mem   uint32 `json:"mem"`
+	Salt  string `json:"salt"`
+	Nonce string `json:"nonce"`
+	Ct    string `json:"ct"`
+}
+
+// backupPlaintext is what gets sealed inside the backup.
+type backupPlaintext struct {
+	PrivateKey string `json:"priv"` // base64url Ed25519 private key (64 bytes)
+	Alias      string `json:"alias"`
+}
+
+const (
+	backupArgonOps = 2
+	backupArgonMem = 64 * 1024 * 1024 // 64 MiB
+)
+
+// ExportIdentity encrypts the identity with passphrase and returns a JSON
+// backup blob in the yaw-key-backup-1 format.
+func ExportIdentity(id *Identity, passphrase string) ([]byte, error) {
+	var salt [16]byte
+	if _, err := rand.Read(salt[:]); err != nil {
+		return nil, fmt.Errorf("generating salt: %w", err)
+	}
+
+	key := deriveKey(passphrase, salt[:])
+
+	pt, err := json.Marshal(backupPlaintext{
+		PrivateKey: base64.StdEncoding.EncodeToString(id.privateKey),
+		Alias:      id.Alias,
+	})
+	if err != nil {
+		return nil, err
+	}
+
+	var nonce [24]byte
+	if _, err := rand.Read(nonce[:]); err != nil {
+		return nil, fmt.Errorf("generating nonce: %w", err)
+	}
+
+	ct := secretbox.Seal(nil, pt, &nonce, &key)
+
+	backup := keyBackup{
+		Yaw:   "yaw-key-backup-1",
+		ID:    string(id.PeerID()),
+		Alg:   "argon2id-secretbox",
+		Ops:   backupArgonOps,
+		Mem:   backupArgonMem,
+		Salt:  base64.StdEncoding.EncodeToString(salt[:]),
+		Nonce: base64.StdEncoding.EncodeToString(nonce[:]),
+		Ct:    base64.StdEncoding.EncodeToString(ct),
+	}
+	return json.MarshalIndent(backup, "", "  ")
+}
+
+// ImportIdentity decrypts a yaw-key-backup-1 blob and returns the Identity.
+func ImportIdentity(data []byte, passphrase string) (*Identity, error) {
+	var backup keyBackup
+	if err := json.Unmarshal(data, &backup); err != nil {
+		return nil, fmt.Errorf("parsing backup: %w", err)
+	}
+	if backup.Yaw != "yaw-key-backup-1" {
+		return nil, fmt.Errorf("unsupported backup format %q", backup.Yaw)
+	}
+
+	salt, err := base64.StdEncoding.DecodeString(backup.Salt)
+	if err != nil {
+		return nil, fmt.Errorf("decoding salt: %w", err)
+	}
+	nonceBytes, err := base64.StdEncoding.DecodeString(backup.Nonce)
+	if err != nil || len(nonceBytes) != 24 {
+		return nil, fmt.Errorf("decoding nonce: %w", err)
+	}
+	ct, err := base64.StdEncoding.DecodeString(backup.Ct)
+	if err != nil {
+		return nil, fmt.Errorf("decoding ciphertext: %w", err)
+	}
+
+	key := deriveKey(passphrase, salt)
+
+	var nonce [24]byte
+	copy(nonce[:], nonceBytes)
+	pt, ok := secretbox.Open(nil, ct, &nonce, &key)
+	if !ok {
+		return nil, errors.New("decryption failed — wrong passphrase?")
+	}
+
+	var plain backupPlaintext
+	if err := json.Unmarshal(pt, &plain); err != nil {
+		return nil, fmt.Errorf("parsing backup plaintext: %w", err)
+	}
+
+	privBytes, err := base64.StdEncoding.DecodeString(plain.PrivateKey)
+	if err != nil || len(privBytes) != ed25519.PrivateKeySize {
+		return nil, fmt.Errorf("invalid private key in backup")
+	}
+
+	priv := ed25519.PrivateKey(privBytes)
+	return &Identity{
+		privateKey: priv,
+		PublicKey:  priv.Public().(ed25519.PublicKey),
+		Alias:      plain.Alias,
+	}, nil
+}
+
+func deriveKey(passphrase string, salt []byte) [32]byte {
+	raw := argon2.IDKey([]byte(passphrase), salt, backupArgonOps, backupArgonMem/1024, 1, 32)
+	var key [32]byte
+	copy(key[:], raw)
+	return key
+}
+
 // ── ChaCha20-Poly1305 AEAD ────────────────────────────────────────────────────
 
 // Session holds the symmetric key for an established peer session.