package main

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"encoding/base64"
	"flag"
	"fmt"
	"io"
)

func main() {
	k := flag.String("key", "", "AES-256 compatible encryption/decryption key")
	t := flag.String("text", "", "The text that should be encrypted/decrypted")
	d := flag.Bool("decrypt", false, "Should I decrypt or encrypt (default)")

	flag.Parse()

	if k == nil || t == nil || d == nil {
		panic("One of the flags is nil")
	}

	key := []byte(*k)
	text := []byte(*t)

	if len(key) < 32 {
		panic("Key too short, needs at least 32 bytes")
	}

	c, err := aes.NewCipher(key)
	if err != nil {
		panic(err)
	}

	gcm, err := cipher.NewGCM(c)
	if err != nil {
		panic(err)
	}

	if *d { // decrypt
		ns := gcm.NonceSize()

		text, err := base64.StdEncoding.DecodeString(*t)
		if err != nil {
			panic(err)
		}

		if len(text) < ns {
			panic("Invalid text, too short for decryption")
		}

		nonce, ciphertext := text[:ns], text[ns:]
		plain, err := gcm.Open(nil, nonce, ciphertext, nil)
		if err != nil {
			panic(err)
		}

		fmt.Println(string(plain))
	} else { // encrypt
		nonce := make([]byte, gcm.NonceSize())

		if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
			panic(err)
		}

		fmt.Println(base64.StdEncoding.EncodeToString(gcm.Seal(nonce, nonce, text, nil)))
	}
}