package crypto

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

const appSecret = "ai-expert-sidebar-v1-local-©2026"

// deriveKey produces a 32-byte AES key from the application secret.
func deriveKey() []byte {
	sum := sha256.Sum256([]byte(appSecret))
	return sum[:]
}

// EncryptAPIKey encrypts a plaintext API key. Returns "" for empty input.
func EncryptAPIKey(plaintext string) (string, error) {
	if plaintext == "" {
		return "", nil
	}
	block, err := aes.NewCipher(deriveKey())
	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
	}
	sealed := gcm.Seal(nonce, nonce, []byte(plaintext), nil)
	return base64.StdEncoding.EncodeToString(sealed), nil
}

// DecryptAPIKey decrypts a base64-encoded AES-256-GCM ciphertext. Returns "" for empty input.
func DecryptAPIKey(ciphertext64 string) (string, error) {
	if ciphertext64 == "" {
		return "", nil
	}
	data, err := base64.StdEncoding.DecodeString(ciphertext64)
	if err != nil {
		return "", fmt.Errorf("base64 decode: %w", err)
	}
	block, err := aes.NewCipher(deriveKey())
	if err != nil {
		return "", err
	}
	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return "", err
	}
	ns := gcm.NonceSize()
	if len(data) < ns {
		return "", fmt.Errorf("ciphertext too short")
	}
	plain, err := gcm.Open(nil, data[:ns], data[ns:], nil)
	return string(plain), err
}