如何在golang中使用AES256-GCM加密文件？

import (
    "crypto/aes"
    "crypto/cipher"
    "crypto/rand"
    "io"
    "os"
)

key := make([]byte, 32)
if _, err := rand.Read(key); err != nil {
    panic(err)
}

iv := make([]byte, 12)
if _, err := rand.Read(iv); err != nil {
    panic(err)
}

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

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

file, err := os.Open("plaintext.txt")
if err != nil {
    panic(err)
}
defer file.Close()

encryptedFile, err := os.Create("encrypted.txt")
if err != nil {
    panic(err)
}
defer encryptedFile.Close()

nonce := make([]byte, gcm.NonceSize())
if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
    panic(err)
}

if _, err := encryptedFile.Write(nonce); err != nil {
    panic(err)
}

stream := cipher.NewCTR(block, nonce)
writer := &cipher.StreamWriter{S: stream, W: encryptedFile}

if _, err := io.Copy(writer, file); err != nil {
    panic(err)
}

if err := writer.Close(); err != nil {
    panic(err)
}