Tags: rc4
Rating:
## Hackover CTF: Racer
----------
## Challenge details
| Contest | Challenge | Category | Points |
|:---------------|:--------------|:----------|-------:|
| Hackover CTF | Racer | Crypto | 300 |
*Description*
> Join the race and make a round. Don't forget to view the winner animation!
----------
## Write-up
For this challenge we need to choose Red/Blue correct about 40 times in a row to win a race, if we get it wrong at any time we are moved back to the beginning. So we need to figure out something to base our choice on.
We can download a 'pills' file, in which there are 2048 files of appearantly random data.
In the source code we find:
>```go
> var fn func([]byte)
> if s.GetColor() == "red" {
> fn = fillRc4
> } else {
> fn = fillRandom
> }
> w.Header().Set("Content-Type", "application/x-gtar")
> buildArchive(w, fn)
>}
>
>func buildArchive(w io.Writer, fillRandom func([]byte)) {
> size := 16
> randombytes := make([]byte, size)
> z := gzip.NewWriter(w)
> tw := tar.NewWriter(z)
> createTime := time.Now()
> for i := 0; i < NUM_CHEMICALS; i++ {
> tw.WriteHeader(&tar.Header{Name: CHEMICALS[i],
> Mode: 0666,
> Uid: 1000,
> Gid: 1000,
> Size: int64(size),
> ModTime: createTime,
> Typeflag: tar.TypeReg,
> })
> if i == 42 {
> tw.Write(HINT)
> } else {
> fillRandom(randombytes)
> tw.Write(randombytes)
> }
> }
> tw.Close()
> z.Close()
>}
>```
This reveals that if the we need to choose "Red" the data is actully RC4 encrypted.
>```
>the second output byte of the cipher was biased toward zero with probability 1/128 (instead of 1/256)
>```
Thus we can distinguish the pills file:
>```python
>#!/usr/bin/python
>
>import os
>import tarfile
>import shutil
>
>tfile = tarfile.open("pills.tar.gz", 'r:gz')
>tfile.extractall('./pills')
>
>count = 0
>for filename in os.listdir("./pills"):
> with open("pills/" + filename, 'rb') as f:
> f.read(1)
> byte = f.read(1)
> if byte == "\x00":
> count += 1
>print count
>if count < 8:
> print "Seems random? -> blue"
>if count > 16:
> print "Seems RC4? -> red"
>
>shutil.rmtree("./pills")
>```
Thankfully we can redownload the file and we get a different one.
The flag was something like (I didn't write it down):
>```
>hackover15{juprc4istotallybroken}
>```
