package secretbox

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

func DeriveKey(secret string) []byte {
	sum := sha256.Sum256([]byte(secret))
	return sum[:]
}

func Encrypt(key []byte, plaintext string) (string, error) {
	if plaintext == "" {
		return "", nil
	}
	block, err := aes.NewCipher(key)
	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
	}
	out := gcm.Seal(nonce, nonce, []byte(plaintext), nil)
	return base64.StdEncoding.EncodeToString(out), nil
}

func Decrypt(key []byte, encoded string) (string, error) {
	if encoded == "" {
		return "", nil
	}
	raw, err := base64.StdEncoding.DecodeString(encoded)
	if err != nil {
		return "", err
	}
	block, err := aes.NewCipher(key)
	if err != nil {
		return "", err
	}
	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return "", err
	}
	if len(raw) < gcm.NonceSize() {
		return "", errors.New("ciphertext too short")
	}
	nonce, ciphertext := raw[:gcm.NonceSize()], raw[gcm.NonceSize():]
	plain, err := gcm.Open(nil, nonce, ciphertext, nil)
	if err != nil {
		return "", err
	}
	return string(plain), nil
}

func RandomToken(n int) (string, error) {
	b := make([]byte, n)
	if _, err := io.ReadFull(rand.Reader, b); err != nil {
		return "", err
	}
	return base64.RawURLEncoding.EncodeToString(b), nil
}