QuoteForge/internal/localdb/encryption.go

package localdb

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"crypto/sha256"
	"encoding/base64"
	"errors"
	"fmt"
	"io"
	"os"
	"path/filepath"
	"strings"

	"git.mchus.pro/mchus/quoteforge/internal/appstate"
)

const encryptionKeyFileName = "local_encryption.key"

// getEncryptionKey resolves the active encryption key.
// Preference order:
// 1. QUOTEFORGE_ENCRYPTION_KEY env var
// 2. application-managed random key file in the user state directory
func getEncryptionKey() ([]byte, error) {
	key := os.Getenv("QUOTEFORGE_ENCRYPTION_KEY")
	if key != "" {
		hash := sha256.Sum256([]byte(key))
		return hash[:], nil
	}

	stateDir, err := resolveEncryptionStateDir()
	if err != nil {
		return nil, fmt.Errorf("resolve encryption state dir: %w", err)
	}

	return loadOrCreateEncryptionKey(filepath.Join(stateDir, encryptionKeyFileName))
}

func resolveEncryptionStateDir() (string, error) {
	configPath, err := appstate.ResolveConfigPath("")
	if err != nil {
		return "", err
	}
	return filepath.Dir(configPath), nil
}

func loadOrCreateEncryptionKey(path string) ([]byte, error) {
	if data, err := os.ReadFile(path); err == nil {
		return parseEncryptionKeyFile(data)
	} else if !errors.Is(err, os.ErrNotExist) {
		return nil, fmt.Errorf("read encryption key: %w", err)
	}

	if err := os.MkdirAll(filepath.Dir(path), 0700); err != nil {
		return nil, fmt.Errorf("create encryption key dir: %w", err)
	}

	raw := make([]byte, 32)
	if _, err := io.ReadFull(rand.Reader, raw); err != nil {
		return nil, fmt.Errorf("generate encryption key: %w", err)
	}

	encoded := base64.StdEncoding.EncodeToString(raw)
	if err := writeKeyFile(path, []byte(encoded+"\n")); err != nil {
		if errors.Is(err, os.ErrExist) {
			data, readErr := os.ReadFile(path)
			if readErr != nil {
				return nil, fmt.Errorf("read concurrent encryption key: %w", readErr)
			}
			return parseEncryptionKeyFile(data)
		}
		return nil, err
	}

	return raw, nil
}

func writeKeyFile(path string, data []byte) error {
	file, err := os.OpenFile(path, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0600)
	if err != nil {
		return err
	}
	defer file.Close()

	if _, err := file.Write(data); err != nil {
		return err
	}

	return file.Sync()
}

func parseEncryptionKeyFile(data []byte) ([]byte, error) {
	trimmed := strings.TrimSpace(string(data))
	decoded, err := base64.StdEncoding.DecodeString(trimmed)
	if err != nil {
		return nil, fmt.Errorf("decode encryption key file: %w", err)
	}
	if len(decoded) != 32 {
		return nil, fmt.Errorf("invalid encryption key length: %d", len(decoded))
	}
	return decoded, nil
}

func getLegacyEncryptionKey() []byte {
	hostname, _ := os.Hostname()
	key := hostname + "quoteforge-salt-2024"
	hash := sha256.Sum256([]byte(key))
	return hash[:]
}

// Encrypt encrypts plaintext using AES-256-GCM
func Encrypt(plaintext string) (string, error) {
	if plaintext == "" {
		return "", nil
	}

	key, err := getEncryptionKey()
	if err != nil {
		return "", err
	}
	block, err := aes.NewCipher(key)
	if err != nil {
		return "", err
	}

	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return "", err
	}

	nonce := make([]byte, gcm.NonceSize())
	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
		return "", err
	}

	ciphertext := gcm.Seal(nonce, nonce, []byte(plaintext), nil)
	return base64.StdEncoding.EncodeToString(ciphertext), nil
}

// Decrypt decrypts ciphertext that was encrypted with Encrypt
func Decrypt(ciphertext string) (string, error) {
	if ciphertext == "" {
		return "", nil
	}

	key, err := getEncryptionKey()
	if err != nil {
		return "", err
	}
	plaintext, legacy, err := decryptWithKeys(ciphertext, key, getLegacyEncryptionKey())
	if err != nil {
		return "", err
	}
	_ = legacy
	return plaintext, nil
}

func DecryptWithMetadata(ciphertext string) (string, bool, error) {
	if ciphertext == "" {
		return "", false, nil
	}

	key, err := getEncryptionKey()
	if err != nil {
		return "", false, err
	}
	return decryptWithKeys(ciphertext, key, getLegacyEncryptionKey())
}

func decryptWithKeys(ciphertext string, primaryKey, legacyKey []byte) (string, bool, error) {
	data, err := base64.StdEncoding.DecodeString(ciphertext)
	if err != nil {
		return "", false, err
	}

	plaintext, err := decryptWithKey(data, primaryKey)
	if err == nil {
		return plaintext, false, nil
	}

	legacyPlaintext, legacyErr := decryptWithKey(data, legacyKey)
	if legacyErr == nil {
		return legacyPlaintext, true, nil
	}

	return "", false, err
}

func decryptWithKey(data, key []byte) (string, error) {
	block, err := aes.NewCipher(key)
	if err != nil {
		return "", err
	}

	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return "", err
	}

	nonceSize := gcm.NonceSize()
	if len(data) < nonceSize {
		return "", errors.New("ciphertext too short")
	}

	nonce, ciphertextBytes := data[:nonceSize], data[nonceSize:]
	plaintext, err := gcm.Open(nil, nonce, ciphertextBytes, nil)
	if err != nil {
		return "", err
	}

	return string(plaintext), nil
}