Golang - Packing and hashing binary data

Question

I'm attempting to learn Golang and have a background in Python. I'm currently trying to get my head around how to pack variables into a binary format (with a checksum). In Python I'd use something like:

import struct
import hashlib

a = 100
b = "foo\x00\x00"  # Padded to fixed length
packet = struct.pack('<B5s', a, b)
digest = hashlib.sha256(packet).digest()
packet += digest

To do the same thing in Go, I'm trying code like this:

package main

import (
    "crypto/sha256"
    "fmt"
    "encoding/binary"
    "bytes"
)

type packet struct {
    a uint8
    b string
}

func main() {
    var p = packet{}
    p.a = 1
    p.b = "foo\x00\x00"
    buf := new(bytes.Buffer)
    binary.Write(buf, binary.LittleEndian, &p)
    h := sha256.New()
    h.Write(buf.String())
    fmt.Printf("% x\n", p)
}

Unfortunately, however I attack it I seem to get into a nightmare of clashing variable types (buffers, byte arrays and strings). I'd appreciate some guidance as to whether I'm taking even remotely the right approach.

Answer 1

Updated to something that works.

package main

import (
   "bytes"
   "crypto/sha256"
   "encoding/binary"
   "fmt"
)

type packet struct {
   a uint8
   b []byte
}

func main() {
   var p = packet{}
   p.a = 1
   p.b = []byte("foo\x00\x00")
   buf := bytes.Buffer{}
   err := binary.Write(&buf, binary.BigEndian, p.a)
   if err != nil {
       fmt.Println(err)
   }
   _, err = buf.Write(p.b)
   if err != nil {
       fmt.Println(err)
   }
   h := sha256.New()
   h.Write(buf.Bytes())
   hash := h.Sum([]byte{})
   fmt.Printf("% x\n", hash)
}

http://play.golang.org/p/t8ltu_WCpe

You're right that it's a bit painful to write structs with possibly dynamic length items in them (slices and strings) using encoding/binary. You might be interested in checking out the "encoding/gob" package that encodes strings automatically (although it isn't compatible with the padded string you've got here).