// Package cryptutil provides authenticated symmetric encryption (AES-256-GCM)
// for secrets that must be stored at rest but read back in plaintext at
// runtime — currently the LDAP service-bind password (internal/ldapstore).
//
// The 256-bit key is derived via HKDF-SHA256 from the application's existing
// master/JWT secret (config.API.Secret), so no additional secret needs to be
// provisioned. A distinct HKDF info label keeps this key independent from the
// JWT signing key even though both originate from the same input secret.
package cryptutil

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"crypto/sha256"
	"fmt"
	"io"

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

// hkdfInfo domain-separates the secretbox key from every other key derived
// from the same master secret (e.g. the JWT signing key uses "archivdms-jwt-v1").
const hkdfInfo = "archivdms-ldap-secretbox-v1"

// Box performs AES-256-GCM encrypt/decrypt with a key derived from a secret.
type Box struct {
	gcm cipher.AEAD
}

// NewBox derives a 256-bit AES key from secret via HKDF-SHA256 and returns a
// ready-to-use Box. secret must be non-empty.
func NewBox(secret string) (*Box, error) {
	if secret == "" {
		return nil, fmt.Errorf("cryptutil: empty secret")
	}
	key := make([]byte, 32)
	if _, err := io.ReadFull(hkdf.New(sha256.New, []byte(secret), nil, []byte(hkdfInfo)), key); err != nil {
		return nil, fmt.Errorf("cryptutil: derive key: %w", err)
	}
	block, err := aes.NewCipher(key)
	if err != nil {
		return nil, fmt.Errorf("cryptutil: new cipher: %w", err)
	}
	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return nil, fmt.Errorf("cryptutil: new gcm: %w", err)
	}
	return &Box{gcm: gcm}, nil
}

// Encrypt seals plaintext, returning the ciphertext and the freshly generated
// nonce (stored separately in the DB). Callers persist both.
func (b *Box) Encrypt(plaintext []byte) (ciphertext, nonce []byte, err error) {
	nonce = make([]byte, b.gcm.NonceSize())
	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
		return nil, nil, fmt.Errorf("cryptutil: nonce: %w", err)
	}
	ciphertext = b.gcm.Seal(nil, nonce, plaintext, nil)
	return ciphertext, nonce, nil
}

// Decrypt opens ciphertext using nonce, returning the original plaintext.
func (b *Box) Decrypt(ciphertext, nonce []byte) ([]byte, error) {
	if len(nonce) != b.gcm.NonceSize() {
		return nil, fmt.Errorf("cryptutil: bad nonce length %d", len(nonce))
	}
	plaintext, err := b.gcm.Open(nil, nonce, ciphertext, nil)
	if err != nil {
		return nil, fmt.Errorf("cryptutil: decrypt: %w", err)
	}
	return plaintext, nil
}