package backup

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"errors"
	"io"

	"golang.org/x/crypto/scrypt"
)

const (
	saltSize = 16
	keySize  = 32 // AES-256
)

// deriveKey turns a passphrase into a 32-byte AES key via scrypt, using the
// given salt. scrypt (N=32768, r=8, p=1) is deliberately expensive to slow
// down offline brute-force of a leaked archive.
func deriveKey(passphrase string, salt []byte) ([]byte, error) {
	return scrypt.Key([]byte(passphrase), salt, 32768, 8, 1, keySize)
}

// Encrypt symmetrically encrypts data with a passphrase using scrypt key
// derivation + AES-256-GCM. Output layout: [16-byte salt][12-byte
// nonce][GCM ciphertext+tag]. This is a small self-contained format
// specific to this app - NOT OpenPGP/gpg-compatible - chosen to avoid
// shelling out to an external gpg binary or adding a full PGP dependency.
func Encrypt(plaintext []byte, passphrase string) ([]byte, error) {
	salt := make([]byte, saltSize)
	if _, err := io.ReadFull(rand.Reader, salt); err != nil {
		return nil, err
	}
	key, err := deriveKey(passphrase, salt)
	if err != nil {
		return nil, err
	}
	block, err := aes.NewCipher(key)
	if err != nil {
		return nil, err
	}
	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return nil, err
	}
	nonce := make([]byte, gcm.NonceSize())
	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
		return nil, err
	}
	ciphertext := gcm.Seal(nil, nonce, plaintext, nil)

	out := make([]byte, 0, saltSize+len(nonce)+len(ciphertext))
	out = append(out, salt...)
	out = append(out, nonce...)
	out = append(out, ciphertext...)
	return out, nil
}

// Decrypt reverses Encrypt, for manual restore/recovery.
func Decrypt(data []byte, passphrase string) ([]byte, error) {
	if len(data) < saltSize {
		return nil, errors.New("backup: ciphertext too short")
	}
	salt := data[:saltSize]
	key, err := deriveKey(passphrase, salt)
	if err != nil {
		return nil, err
	}
	block, err := aes.NewCipher(key)
	if err != nil {
		return nil, err
	}
	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return nil, err
	}
	nonceSize := gcm.NonceSize()
	if len(data) < saltSize+nonceSize {
		return nil, errors.New("backup: ciphertext too short")
	}
	nonce := data[saltSize : saltSize+nonceSize]
	ciphertext := data[saltSize+nonceSize:]
	return gcm.Open(nil, nonce, ciphertext, nil)
}