package authproxy

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

// box seals and opens small JSON payloads with AES-256-GCM. The purpose
// string is bound in as additional authenticated data so a token minted for
// one use (e.g. OAuth state) can never be replayed as another (e.g. a
// session cookie).
type box struct {
	aead cipher.AEAD
}

func newBox(secret string) (*box, error) {
	key := sha256.Sum256([]byte(secret))
	block, err := aes.NewCipher(key[:])
	if err != nil {
		return nil, err
	}
	aead, err := cipher.NewGCM(block)
	if err != nil {
		return nil, err
	}
	return &box{aead: aead}, nil
}

func (b *box) seal(purpose string, v any) (string, error) {
	plain, err := json.Marshal(v)
	if err != nil {
		return "", err
	}
	nonce := make([]byte, b.aead.NonceSize())
	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
		return "", err
	}
	out := b.aead.Seal(nonce, nonce, plain, []byte(purpose))
	return base64.RawURLEncoding.EncodeToString(out), nil
}

func (b *box) open(purpose, token string, v any) error {
	raw, err := base64.RawURLEncoding.DecodeString(token)
	if err != nil {
		return fmt.Errorf("malformed token: %w", err)
	}
	ns := b.aead.NonceSize()
	if len(raw) <= ns {
		return fmt.Errorf("malformed token: too short")
	}
	plain, err := b.aead.Open(nil, raw[:ns], raw[ns:], []byte(purpose))
	if err != nil {
		return fmt.Errorf("token failed authentication: %w", err)
	}
	return json.Unmarshal(plain, v)
}

func randToken() string {
	buf := make([]byte, 16)
	if _, err := io.ReadFull(rand.Reader, buf); err != nil {
		panic(err) // crypto/rand failure is unrecoverable
	}
	return hex.EncodeToString(buf)
}