32c3CTF 2015 bruteforce tool for "config.bin"

bruteforce-cfg1.go

/*
 * bruteforce-cfg1.go
 * to bruteforce CFG1 encryption passwords for 32c3CTF "config.bin"
 *
 * @zxcvgm
 */
package main

import (
	brutedict "github.com/dieyushi/golang-brutedict"
	"crypto/aes"
	"crypto/md5"
	"io/ioutil"
	"encoding/binary"
	"bytes"
	"sync"
	"fmt"
	"os"
)

func main() {
	file, err := os.Open(os.Args[1])
	if err != nil {
		fmt.Println(err)
		return
	}
	defer file.Close()

	file.Seek(4 + 4 + 8 + 7 + 1 + 2, os.SEEK_SET)

	// padding size. not used in the end
	var padding uint16
	binary.Read(file, binary.BigEndian, &padding)

	data_hash := make([]byte, 16)
	file.Seek(4, os.SEEK_CUR)
	file.Read(data_hash)

	// read header to compute checksum
	headerBytes := make([]byte, 0x30)
	file.ReadAt(headerBytes, 0)
	for i := 8; i < 16; i++ {
		headerBytes[i] = 0
	}
	fmt.Printf("hdr hash = %x\n", md5.Sum(headerBytes))
	

	// header is 0x30 bytes
	file.Seek(0x30, os.SEEK_SET)
	src, err := ioutil.ReadAll(file)

	fmt.Printf("data hash = %x\n", data_hash)

	try_pwds := make(chan string, 2048)
	var wg sync.WaitGroup
	for i := 0; i < 4; i++ {
		wg.Add(1)
		go func() {
			for pwd := range try_pwds {
				//fmt.Printf("trying %s\n", pwd)
				cksum := decryptHash(pwd, src)
				if bytes.Equal(data_hash, cksum[:]) {
					fmt.Printf("found password! %s\n", pwd)
					os.Exit(0)
				}
			}
			wg.Done()
		}()
	}

	// generate passwords here
	try_pwds <- "dummy"

	// password len is hardcoded here
	bd := brutedict.New(true, true, true, 5, 5)
	defer bd.Close()
	for {
		id := bd.Id()
		if id != "" {
			try_pwds <- id
		} else {
			break
		}
	}

	close(try_pwds)
	wg.Wait()

	//cksum := decryptHash("dummy", src)
	//fmt.Printf("computed cksum %x\n", cksum)
	//fmt.Printf("matches? %s\n", bytes.Equal(data_hash, cksum[:]))
}

// decrypt and hash at the same time
func decryptHash(keyStr string, data []byte)[]byte {
	key := make([]byte, 32)
	dst := make([]byte, aes.BlockSize)

	copy(key, keyStr)
	//fmt.Printf("key: %x\n", key)
	c, err := aes.NewCipher(key)
	if err != nil {
		fmt.Println(err)
		return nil
	}

	hash := md5.New()
	n := 0
	l := len(data)
	for n < l {
		c.Decrypt(dst, data[n:n+aes.BlockSize])
		//fmt.Printf("offset %d\n", n)
		//fmt.Printf("dec  %x\n", dst)

		hash.Write(dst)
		n += aes.BlockSize
	}

	return hash.Sum(nil)
}