A simple AEAD implementation

program.go

package main

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"encoding/hex"
	"fmt"
	"io"
	"os"
)

// `key` is the AES key. Since it is 32 bytes (256 bits) long, aes.NewCipher will select AES-256
var key = []byte{
	0x42, 0x8d, 0xcd, 0xf7, 0x26, 0xc5, 0xf1, 0xc8,
	0x7c, 0xec, 0x50, 0xfa, 0xaa, 0xd1, 0xf7, 0x12,
	0x2e, 0xca, 0x58, 0x04, 0x7f, 0xdd, 0xec, 0x75,
	0x9b, 0x26, 0xe5, 0xb9, 0xb8, 0x9f, 0x39, 0xa3,
}

var (
	// block uses the AES encryption cipher with `key` as the encryption key
	block, _ = aes.NewCipher(key)

	// aesgcm wraps the AES cipher with the GCM authenticated encryption algorithm
	aesgcm, _ = cipher.NewGCM(block)
)

func main() {
	var (
		r   = os.Stdin  // read from STDIN
		w   = os.Stdout // write to STDOUT
		err error
	)

	switch os.Args[1] {
	case "seal":
		err = seal(r, w)

	case "open":
		err = open(r, w)

	default:
		err = fmt.Errorf("invalid command %q", os.Args[1])
	}

	if err != nil {
		fmt.Println(err)
		os.Exit(1)
	}
}

// seal reads plaintext from r and writes ciphertext to w
func seal(r io.Reader, w io.Writer) error {
	// wrap w with a hex encoder
	w = hex.NewEncoder(w)

	// generate a fresh random nonce
	nonce := make([]byte, aesgcm.NonceSize())
	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
		return err
	}

	// read all of the plaintext into memory
	plaintext, err := io.ReadAll(r)
	if err != nil {
		return err
	}

	ciphertext := aesgcm.Seal(nil, nonce, plaintext, nil)

	// first write the nonce
	if _, err := w.Write(nonce); err != nil {
		return err
	}

	// then write the ciphertext
	if _, err := w.Write(ciphertext); err != nil {
		return err
	}

	return nil
}

// open reads ciphertext from r and writes plaintext to w
func open(r io.Reader, w io.Writer) error {
	// wrap r with a hex decoder
	r = hex.NewDecoder(r)

	// read the nonce from r
	nonce := make([]byte, aesgcm.NonceSize())
	if _, err := io.ReadFull(r, nonce); err != nil {
		return err
	}

	// read the rest of the ciphertext from r
	ciphertext, err := io.ReadAll(r)
	if err != nil {
		return err
	}

	plaintext, err := aesgcm.Open(nil, nonce, ciphertext, nil)
	if err != nil {
		return err
	}

	// write the plaintext to w
	if _, err := w.Write(plaintext); err != nil {
		return err
	}

	return nil
}