Tags: crc
Rating:
# CRC
In this task we were given 26 encrypted zip files. It is easy to notice though, that they have
very small size - they uncompress to 5 byte files. That means we should be able to brute force
the actual contents, and check their validity using crc, since ZIPs contain CRC32 of
their uncompressed contents:
```
[adam@adam-Y510P ~/CTF/backdoor/zipcrc]λ unzip -lv 0.zip
Archive: 0.zip
Length Method Size Cmpr Date Time CRC-32 Name
-------- ------ ------- ---- ---------- ----- -------- ----
5 Stored 5 0% 2015-11-26 19:49 a36bb2ae 0.txt
-------- ------- --- -------
5 5 0% 1 file
```
We listed all CRCs in `list`, and wrote a brute forcer - `check.cpp`. It looped over all printable
5-character strings and calculated its CRC, after which it checked whether it fits any zip.
Unfortunately there were many collisions, so we wrote a quick visualizer showing possibilites
for each zip (`parse.py`). Output:
```
[ p,id] [ func] [h&3o4] [func(] [) { e] [.ERM*] [H'9!R] [!LZW(] ["; fo] [ngxh4]
[t|g{] [r($i ]
[PL]l4] [ESV$!] [^y#Hz] [Qp+Rx]
[cho "] [lag: ]
['3!6] [p.;;4]
[!ol:4] [$i < ] [/}g!0] [8IJE<] [echo ] [.EOE8] [)VPL7] [ (($i]
[= 0; ] [8&|=4] [32; $] [Hu;@l] [y,4n4] [B6bA|] [E%}Hs] [<gt%}]
[lBQW<] [TUQ9p] [bPZL8] [T:gAx] [^y>@h] [Yj!Ig] [qJIHu]
[i++) ] [chr(0] [d{m!?]
[udw(4] ['.)$] [x41 +]
[ ^ 0x] [-}u!1] [ 0x19] [)); e] [.ERM*] [,RS>p] [ao:tz] [2JHW<] [*4DeR]
[PbQ5(] [12) %] [<$0-] [5fg!q] [B6In] [@!~:4] [} fun] [Bv9Rl]
[q
