Challenge Response in Go

client.go

package main

import (
	"bufio"
	"crypto/hmac"
	"crypto/sha256"
	"encoding/base64"
	"log"
	"net"
	"strings"
)

const secret = "wambo"

func main() {
	conn, err := net.Dial("tcp", "localhost:1337")
	if err != nil {
		log.Fatalln("Error: could not connect to server")
	}

	connbuf := bufio.NewReader(conn)

	// send username
	conn.Write([]byte("Alex\n"))

	// receive random string
	randstr, err := connbuf.ReadString('\n')
	if err != nil {
		log.Fatalln("Error: could not receive random string")
	}
	log.Println("random string:", randstr)
	randstr = strings.TrimSpace(randstr)
	randbytes, err := base64.StdEncoding.DecodeString(randstr)
	if err != nil {
		log.Fatalln("Error: could not decode random string")
	}

	// send HMAC
	mac := hmac.New(sha256.New, []byte(secret))
	mac.Write(randbytes)
	resultmac := mac.Sum(nil)
	conn.Write([]byte(base64.StdEncoding.EncodeToString(resultmac) + "\n"))

	conn.Close()
}

server.go

package main

import (
	"bufio"
	"crypto/hmac"
	"crypto/rand"
	"crypto/sha256"
	"encoding/base64"
	"log"
	"net"
	"strings"
)

const secret = "wambo"

func handleClient(conn net.Conn) {
	defer func() {
		conn.Close()
		log.Println("closing connection")
	}()

	log.Println("client connected")

	connbuf := bufio.NewReader(conn)

	// get username
	username, err := connbuf.ReadString('\n')
	if err != nil {
		log.Println("Error: could not receive username")
		return
	}
	username = strings.TrimSpace(username)
	log.Println("Client is", username)

	// send random string
	randbytes := make([]byte, 32)
	rand.Read(randbytes)
	conn.Write([]byte(base64.StdEncoding.EncodeToString(randbytes) + "\n"))

	// calc HMAC
	mac := hmac.New(sha256.New, []byte(secret))
	mac.Write(randbytes)
	expectedmac := mac.Sum(nil)

	// get HMAC
	clientmacstr, err := connbuf.ReadString('\n')
	if err != nil {
		log.Println("Error: could not receive response")
		return
	}
	clientmacstr = strings.TrimSpace(clientmacstr)
	clientmac, err := base64.StdEncoding.DecodeString(clientmacstr)
	if err != nil {
		log.Println("Error: response format incorrect")
		return
	}

	// compare
	if hmac.Equal(clientmac, expectedmac) {
		log.Println(username, "is valid")
	} else {
		log.Println(username, "is not valid")
	}
}

func main() {
	l, err := net.Listen("tcp", "localhost:1337")
	if err != nil {
		log.Fatalln("Error:", err.Error())
	}
	log.Println("server listening")
	defer l.Close()

	for {
		conn, err := l.Accept()
		if err != nil {
			log.Println("Error:", err.Error())
			continue;
		}
		go handleClient(conn)
	}
}