flit/cli/internal/crypto/crypto.go

// Package crypto implements flit's identity and signaling crypto: Ed25519
// device identity, X25519 ECDH (static, derived from Ed25519; and ephemeral,
// yaw/2.1 forward-secret signaling), and the nacl/box signaling seal.
// Ported and trimmed from waste-go/internal/crypto — dropped chat/backup
// helpers not needed for 1:1 file transfer.
package crypto

import (
	"crypto/ed25519"
	"crypto/rand"
	"crypto/sha512"
	"encoding/base64"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"os"
	"path/filepath"

	"filippo.io/edwards25519"
	"golang.org/x/crypto/curve25519"
	"golang.org/x/crypto/nacl/box"
)

var b64 = base64.RawURLEncoding

// ── Identity ──────────────────────────────────────────────────────────────────

type Identity struct {
	privateKey ed25519.PrivateKey
	PublicKey  ed25519.PublicKey
}

type identityFile struct {
	PrivateKeyB64 string `json:"private_key"`
}

// LoadOrCreate loads the identity from dataDir/identity.json, or generates a
// fresh keypair if none exists yet.
func LoadOrCreate(dataDir string) (*Identity, error) {
	if err := os.MkdirAll(dataDir, 0700); err != nil {
		return nil, fmt.Errorf("creating data dir: %w", err)
	}
	path := filepath.Join(dataDir, "identity.json")

	data, err := os.ReadFile(path)
	if err == nil {
		var f identityFile
		if err := json.Unmarshal(data, &f); err != nil {
			return nil, fmt.Errorf("parsing identity file: %w", err)
		}
		privBytes, err := b64.DecodeString(f.PrivateKeyB64)
		if err != nil {
			return nil, fmt.Errorf("decoding private key: %w", err)
		}
		priv := ed25519.PrivateKey(privBytes)
		return &Identity{privateKey: priv, PublicKey: priv.Public().(ed25519.PublicKey)}, nil
	}
	if !errors.Is(err, os.ErrNotExist) {
		return nil, fmt.Errorf("reading identity file: %w", err)
	}

	pub, priv, err := ed25519.GenerateKey(rand.Reader)
	if err != nil {
		return nil, fmt.Errorf("generating keypair: %w", err)
	}
	f := identityFile{PrivateKeyB64: b64.EncodeToString(priv)}
	raw, _ := json.MarshalIndent(f, "", "  ")
	if err := os.WriteFile(path, raw, 0600); err != nil {
		return nil, fmt.Errorf("saving identity: %w", err)
	}
	return &Identity{privateKey: priv, PublicKey: pub}, nil
}

// PeerID returns the lowercase hex encoding of the 32-byte Ed25519 public key.
func (id *Identity) PeerID() string { return hex.EncodeToString(id.PublicKey) }

func (id *Identity) Sign(data []byte) string {
	return hex.EncodeToString(ed25519.Sign(id.privateKey, data))
}

func Verify(publicKeyHex string, data []byte, sigHex string) error {
	pubBytes, err := hex.DecodeString(publicKeyHex)
	if err != nil {
		return fmt.Errorf("decoding public key: %w", err)
	}
	sigBytes, err := hex.DecodeString(sigHex)
	if err != nil {
		return fmt.Errorf("decoding signature: %w", err)
	}
	if !ed25519.Verify(ed25519.PublicKey(pubBytes), data, sigBytes) {
		return errors.New("signature verification failed")
	}
	return nil
}

// CurvePublicKey returns the X25519 public key derived from this Ed25519 identity.
func (id *Identity) CurvePublicKey() *[32]byte {
	edPoint, _ := new(edwards25519.Point).SetBytes(id.PublicKey)
	mont := edPoint.BytesMontgomery()
	var out [32]byte
	copy(out[:], mont)
	return &out
}

// CurvePrivateKey returns the X25519 private key derived from this Ed25519 identity.
func (id *Identity) CurvePrivateKey() *[32]byte {
	h := sha512.Sum512(id.privateKey[:32])
	h[0] &= 248
	h[31] &= 127
	h[31] |= 64
	var out [32]byte
	copy(out[:], h[:32])
	return &out
}

// CurveFromPeerID derives the X25519 public key from a peer's hex Ed25519 id.
func CurveFromPeerID(idHex string) (*[32]byte, error) {
	pubBytes, err := hex.DecodeString(idHex)
	if err != nil || len(pubBytes) != 32 {
		return nil, fmt.Errorf("invalid peer id %q", idHex)
	}
	edPoint, err := new(edwards25519.Point).SetBytes(pubBytes)
	if err != nil {
		return nil, fmt.Errorf("ed25519 point: %w", err)
	}
	mont := edPoint.BytesMontgomery()
	var out [32]byte
	copy(out[:], mont)
	return &out, nil
}

// ── Signaling seal (nacl/box, base64 std-padded) ──────────────────────────────

func SignalingBox(plaintext []byte, recipientPub, senderPriv *[32]byte) string {
	var nonce [24]byte
	rand.Read(nonce[:]) //nolint:errcheck
	ct := box.Seal(nonce[:], plaintext, &nonce, recipientPub, senderPriv)
	return base64.StdEncoding.EncodeToString(ct)
}

func SignalingOpen(b64box string, senderPub, recipientPriv *[32]byte) ([]byte, error) {
	raw, err := base64.StdEncoding.DecodeString(b64box)
	if err != nil || len(raw) < 24 {
		return nil, errors.New("invalid box")
	}
	var nonce [24]byte
	copy(nonce[:], raw[:24])
	out, ok := box.Open(nil, raw[24:], &nonce, senderPub, recipientPriv)
	if !ok {
		return nil, errors.New("box open failed")
	}
	return out, nil
}

// ── X25519 ephemeral keys (yaw/2.1 forward secrecy) ───────────────────────────

type EphemeralKey struct {
	private [32]byte
	public  [32]byte
}

func GenerateEphemeral() (*EphemeralKey, error) {
	ek := &EphemeralKey{}
	if _, err := rand.Read(ek.private[:]); err != nil {
		return nil, fmt.Errorf("generating ephemeral key: %w", err)
	}
	ek.private[0] &= 248
	ek.private[31] &= 127
	ek.private[31] |= 64
	pub, err := curve25519.X25519(ek.private[:], curve25519.Basepoint)
	if err != nil {
		return nil, fmt.Errorf("computing public key: %w", err)
	}
	copy(ek.public[:], pub)
	return ek, nil
}

func (ek *EphemeralKey) PublicRaw() *[32]byte  { return &ek.public }
func (ek *EphemeralKey) PrivateRaw() *[32]byte { return &ek.private }

func (ek *EphemeralKey) Wipe() {
	for i := range ek.private {
		ek.private[i] = 0
	}
}