A simple example with Golang that uses AES-128 to encrypt and decrypt messages.

encrypt_decrypt.go

package mycrypto

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

func EncryptMessage(key []byte, message string) (string, error) {
	byteMsg := []byte(message)
	block, err := aes.NewCipher(key)
	if err != nil {
		return "", fmt.Errorf("could not create new cipher: %v", err)
	}

	cipherText := make([]byte, aes.BlockSize+len(byteMsg))
	iv := cipherText[:aes.BlockSize]
	if _, err = io.ReadFull(rand.Reader, iv); err != nil {
		return "", fmt.Errorf("could not encrypt: %v", err)
	}

	stream := cipher.NewCFBEncrypter(block, iv)
	stream.XORKeyStream(cipherText[aes.BlockSize:], byteMsg)

	return base64.StdEncoding.EncodeToString(cipherText), nil
}

func DecryptMessage(key []byte, message string) (string, error) {
	cipherText, err := base64.StdEncoding.DecodeString(message)
	if err != nil {
		return "", fmt.Errorf("could not base64 decode: %v", err)
	}

	block, err := aes.NewCipher(key)
	if err != nil {
		return "", fmt.Errorf("could not create new cipher: %v", err)
	}

	if len(cipherText) < aes.BlockSize {
		return "", fmt.Errorf("invalid ciphertext block size")
	}

	iv := cipherText[:aes.BlockSize]
	cipherText = cipherText[aes.BlockSize:]

	stream := cipher.NewCFBDecrypter(block, iv)
	stream.XORKeyStream(cipherText, cipherText)

	return string(cipherText), nil
}

encrypt_decrypt_test.go

package mycrypto

import (
	"testing"

	"github.com/stretchr/testify/require"
)

const isBase64 = "^(?:[A-Za-z0-9+/]{4})*(?:[A-Za-z0-9+/]{2}==|[A-Za-z0-9+/]{3}=|[A-Za-z0-9+/]{4})$"

func TestEncryptDecryptMessage(t *testing.T) {
	key := []byte("0123456789abcdef") // must be of 16 bytes for this example to work
	message := "Lorem ipsum dolor sit amet"

	encrypted, err := EncryptMessage(key, message)
	require.Nil(t, err)
	require.Regexp(t, isBase64, encrypted)

	decrypted, err := DecryptMessage(key, encrypted)
	require.Nil(t, err)
	require.Equal(t, message, decrypted)
}

Bro thank you, I've been struggling with understanding the ciphertext block size and this code has cleared it out for me