base64 decode then json decode: base64.NewDecoder EOF error and json invalid character error

Question

I'm trying to base64 decode HTTP request, then decode it using JSON decoder.

I tried two ways to implement base64 decoder:

func decode(encoded []byte) ([]byte, error) {
    buff := new(bytes.Buffer)
    decoder := base64.NewDecoder(base64.StdEncoding, buff)
    _, err := decoder.Read(encoded)

    return buff.Bytes(), err
}

This function returns an EOF error. Go playground link: https://play.golang.org/p/038rEXWYW6q

---

func decode(encoded []byte) ([]byte, error) {
    decoded := make([]byte, base64.StdEncoding.EncodedLen(len(encoded)))
    _, err := base64.StdEncoding.Decode(decoded, encoded)
    return decoded, err
}

This works but has extra x\00 characters so when decoding JSON we would have invalid character '\x00' after top-level value error.

Where is the problem with the first strategy?

Answer 1

In your code:

You are creating a new buffer and assigning it to buff, since you have not given any source of input for it, it is empty.

buff := new(bytes.Buffer)

and NewDecoder is reading from empty buff. Like everything in go, if you want to create a something new, you should use it's constructor which always starts with package.Newxxx

bytes.NewBuffer(src)

Then decoder is a variable which contains the actual decoded data, and it has a reader interface(Read method). So you can pass it to method which accepts reader interface, and ioutil.ReadAll() is one of them.

Added comments where ever necessary:

package main

import (
    "bytes"
    "encoding/base64"
    "fmt"
    "io/ioutil"
    "log"
)

// encoded data
var data = "eyJhY3Rpdml0aWVzIjpbXSwic3VjY2VzcyI6ZmFsc2UsImNvZGUiOjk5OTl9"

func main() {
    dec, err := decode([]byte(data))
    if err != nil {
        log.Println(err)
    }
    fmt.Println(string(dec)) // print returned value
}

func decode(enc []byte) ([]byte, error) {
    // create new buffer from enc
    // you can also use bytes.NewBuffer(enc)
    r := bytes.NewReader(enc)
    // pass it to NewDecoder so that it can read data
    dec := base64.NewDecoder(base64.StdEncoding, r)
    // read decoded data from dec to res
    res, err := ioutil.ReadAll(dec)
    return res, err
}

Infact the whole thing can be written in one line:

func decode(enc []byte) ([]byte, error) {
    return ioutil.ReadAll(base64.NewDecoder(base64.StdEncoding, bytes.NewReader(enc)))
}

Output:

{"activities":[],"success":false,"code":9999}