package auth

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"fmt"
)

// Sealer is the encryption seam for TOTP secrets at rest (A9). The key comes
// from config later (D-035); this package only defines the mechanism.
//
// A Sealer must return ciphertext that never equals — and never contains — the
// plaintext.
type Sealer interface {
	Seal(plaintext []byte) ([]byte, error)
	Open(ciphertext []byte) ([]byte, error)
}

// KeySize is the AES-256 key length required by AEADSealer.
const KeySize = 32

// AEADSealer encrypts with AES-256-GCM. The stored blob is
//
//	nonce || ciphertext || tag
//
// A fresh random nonce is generated on every Seal, so sealing the same secret
// twice yields different ciphertext.
type AEADSealer struct {
	aead cipher.AEAD
}

// NewAEADSealer builds a Sealer from a 32-byte key.
func NewAEADSealer(key []byte) (*AEADSealer, error) {
	if len(key) != KeySize {
		return nil, fmt.Errorf("auth: TOTP encryption key must be %d bytes, got %d", KeySize, len(key))
	}
	block, err := aes.NewCipher(key)
	if err != nil {
		return nil, fmt.Errorf("auth: new cipher: %w", err)
	}
	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return nil, fmt.Errorf("auth: new GCM: %w", err)
	}
	return &AEADSealer{aead: gcm}, nil
}

// Seal encrypts plaintext.
func (s *AEADSealer) Seal(plaintext []byte) ([]byte, error) {
	nonce := make([]byte, s.aead.NonceSize())
	if _, err := rand.Read(nonce); err != nil {
		return nil, fmt.Errorf("auth: seal nonce: %w", err)
	}
	// Seal appends to nonce, producing nonce || ciphertext||tag.
	return s.aead.Seal(nonce, nonce, plaintext, nil), nil
}

// Open decrypts a blob produced by Seal.
func (s *AEADSealer) Open(ciphertext []byte) ([]byte, error) {
	ns := s.aead.NonceSize()
	if len(ciphertext) < ns {
		return nil, ErrInvalidCiphertext
	}
	nonce, ct := ciphertext[:ns], ciphertext[ns:]
	plaintext, err := s.aead.Open(nil, nonce, ct, nil)
	if err != nil {
		return nil, fmt.Errorf("%w: %v", ErrInvalidCiphertext, err)
	}
	return plaintext, nil
}

var _ Sealer = (*AEADSealer)(nil)