// Keywarden - Centralized SSH Key Management and Deployment
// Copyright (C) 2026 Patrick Asmus (scriptos)
// SPDX-License-Identifier: AGPL-3.0-or-later

package encryption

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"crypto/sha256"
	"encoding/base64"
	"fmt"
	"io"
)

// Service handles AES-256-GCM encryption/decryption
type Service struct {
	key []byte // 32 bytes for AES-256
}

// NewService creates a new encryption service from a passphrase.
//
// NOTE: Key derivation currently uses SHA-256 for backward compatibility with
// existing encrypted data. A migration to a proper KDF (e.g. Argon2id) would
// require re-encrypting all stored secrets and is tracked as a future improvement.
func NewService(passphrase string) *Service {
	hash := sha256.Sum256([]byte(passphrase))
	return &Service{key: hash[:]}
}

// Encrypt encrypts plaintext using AES-256-GCM and returns base64-encoded ciphertext
func (s *Service) Encrypt(plaintext string) (string, error) {
	block, err := aes.NewCipher(s.key)
	if err != nil {
		return "", fmt.Errorf("failed to create cipher: %w", err)
	}

	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return "", fmt.Errorf("failed to create GCM: %w", err)
	}

	nonce := make([]byte, gcm.NonceSize())
	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
		return "", fmt.Errorf("failed to generate nonce: %w", err)
	}

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

// Decrypt decrypts base64-encoded AES-256-GCM ciphertext
func (s *Service) Decrypt(encoded string) (string, error) {
	ciphertext, err := base64.StdEncoding.DecodeString(encoded)
	if err != nil {
		return "", fmt.Errorf("failed to decode base64: %w", err)
	}

	block, err := aes.NewCipher(s.key)
	if err != nil {
		return "", fmt.Errorf("failed to create cipher: %w", err)
	}

	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return "", fmt.Errorf("failed to create GCM: %w", err)
	}

	nonceSize := gcm.NonceSize()
	if len(ciphertext) < nonceSize {
		return "", fmt.Errorf("ciphertext too short")
	}

	nonce, ciphertext := ciphertext[:nonceSize], ciphertext[nonceSize:]
	plaintext, err := gcm.Open(nil, nonce, ciphertext, nil)
	if err != nil {
		return "", fmt.Errorf("failed to decrypt: %w", err)
	}

	return string(plaintext), nil
}