package cybersecurity

import (
	"crypto/rand"
	"fmt"
	"io"

	"golang.org/x/crypto/chacha20poly1305"
)

// SealAEAD encrypts plaintext with ChaCha20-Poly1305, returning a random nonce and ciphertext.
// key must be exactly 32 bytes. aad (additional authenticated data) may be nil.
// The returned nonce must be stored alongside the ciphertext and passed to OpenAEAD.
func SealAEAD(key, plaintext, aad []byte) (nonce, ciphertext []byte, err error) {
	if len(key) != chacha20poly1305.KeySize {
		return nil, nil, fmt.Errorf("seal_aead: key must be %d bytes, got %d", chacha20poly1305.KeySize, len(key))
	}

	aead, err := chacha20poly1305.New(key)
	if err != nil {
		return nil, nil, fmt.Errorf("seal_aead: create cipher: %w", err)
	}

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

	ciphertext = aead.Seal(nil, nonce, plaintext, aad)
	return nonce, ciphertext, nil
}