golang illegal base64 data at input byte 0

I have a go test program to read encrypted content from file and decrypt it, but it get output like below:

illegal base64 data at input byte 0

if I hard code the encrypted content in a golang string variable, it can decrypt it fine. what I am missing here? I searched similar error in stackoverflow, there is similar report, but not exact the same problem I have. the test code like below:

package main
import (
"crypto/aes"
"crypto/cipher"
"crypto/rand"
"encoding/base64"
"errors"
"fmt"
"io"
"bufio"
"os"
"log"
)
func check(e error) {
if e != nil {
panic(e)
}
}
func main() {
plaintext := []byte("textstring")
key := []byte("a very very very very very secre")
fmt.Printf("%s\n", plaintext)
fh, err := os.Open("./test.txt")
check(err)
scanner := bufio.NewScanner(fh)
var encrypted_text string
if scanner.Scan() { //<==========READ FROM FILE
encrypted_text = scanner.Text()
fmt.Println("encrypted_text from file: ", encrypted_text)
} else { //<===========HARD CODE HERE
encrypted_text  = "\xf2F\xbc\x15\x9d\xaf\xceϘ\xa3L(>%\xa2\x94\x03_\x99\u007fG\xd8\v\xbf\t#u\xf8:\xc0D\u007f"
fmt.Println("encrypted_text hard coded: ", encrypted_text)
}
encrypted_byte  := []byte(encrypted_text)
fmt.Printf("encrypted_byte: %s\n", encrypted_byte)
result, err := decrypt(key, encrypted_byte)
if err != nil {
log.Fatal(err)
}
fmt.Printf("result %s\n", string(result))
}
func encrypt(key, text []byte) ([]byte, error) {
block, err := aes.NewCipher(key)
if err != nil {
return nil, err
}
b := base64.StdEncoding.EncodeToString(text)
ciphertext := make([]byte, aes.BlockSize+len(b))
iv := ciphertext[:aes.BlockSize]
if _, err := io.ReadFull(rand.Reader, iv); err != nil {
return nil, err
}
cfb := cipher.NewCFBEncrypter(block, iv)
cfb.XORKeyStream(ciphertext[aes.BlockSize:], []byte(b))
return ciphertext, nil
}
func decrypt(key, text []byte) ([]byte, error) {
block, err := aes.NewCipher(key)
if err != nil {
return nil, err
}
if len(text) < aes.BlockSize {
return nil, errors.New("ciphertext too short")
}
iv := text[:aes.BlockSize]
text = text[aes.BlockSize:]
cfb := cipher.NewCFBDecrypter(block, iv)
cfb.XORKeyStream(text, text)
data, err := base64.StdEncoding.DecodeString(string(text))
if err != nil {
return nil, err
}
return data, nil
}

You need to unquote the encrypted_text returned from the scanner.
Here's a minimal example

Modify your scanner.Scan() if block to look like this

	if scanner.Scan() { //<==========READ FROM FILE
encrypted_text = scanner.Text()
fmt.Println("encrypted_text from file: ", encrypted_text)
// Unquoting, don't forget to import strconv !
encrypted_text, err := strconv.Unquote(`"` + encrypted_text + `"`)
check(err)
}

> why you need to unquote

I'm guessing your file test.txt contains the raw string

\xf2F\xbc\x15\x9d\xaf\xceQ\xa3L(>%\xa2\x94\x03_\x99\u007fG\xd8\v\xbf\t#u\xf8:\xc0D\u007f

When scanner reads this from the file, it is reading a \ as a \.

However, when you hardcode it in your code like this

encrypted_text  = "\xf2F\xbc\x15\x9d\xaf\xceϘ\xa3L(>%\xa2\x94\x03_\x99\u007fG\xd8\v\xbf\t#u\xf8:\xc0D\u007f"

You are using double quotes ", so a \ isn't a \. It interprets the escape sequences. If you were to use a backquote as follows

encrypted_text  = `\xf2F\xbc\x15\x9d\xaf\xceϘ\xa3L(>%\xa2\x94\x03_\x99\u007fG\xd8\v\xbf\t#u\xf8:\xc0D\u007f`

you would face the same issue.

The solution is to unquote this string using strconv.Unquote

Also, take a look at This SO question