Zip Encryption Known Plain Text Attack

Posted on December 05, 2019

In this post I would like to highlight a really old flaw with the encryption used by the zip file format. This is a known text attack based on the pkcrack tool + A Paper by Dragos Barosan + The original vulnerability discovered back in 1994! By Eli Biham and Paul C. Koche

In a know plain text attack if you have the cipher text (encrypted zip file) and know some of plain text (a file inside the zip), they you can recover the key and extract all the files inside the zip!

A worked example:

I have this old zip file which I do not know the password for. I can use 7zip to open it and view its properties where we find it is encrypted using ZipCrypto and not the more common AES algorithm.

example_zip

Additionally and very conventionally the zip file also contains a very old version of putty.

example_zip

This will act as our known plain text. I don’t know exactly what version of putty this is but we can use the CRC checksum to find out! All we have todo is download lots of old versions of putty and compare their checksums with the putty inside the zip.

[tom@tomandtim zip_crack]$ 7za h

7-Zip (a) [64] 16.02 : Copyright (c) 1999-2016 Igor Pavlov : 2016-05-21
p7zip Version 16.02 (locale=en_US.UTF-8,Utf16=on,HugeFiles=on,64 bits,8 CPUs Intel(R) Xeon(R) CPU E3-1245 V2 @ 3.40GHz (306A9),ASM,AES-NI)

Scanning
1 folder, 23 files, 55198046 bytes (53 MiB)

CRC32             Size  Name
-------- -------------  ------------
                        lots_of_putty
53F55644       1096080  lots_of_putty/putty(20).exe
EE7F8E72        483328  lots_of_putty/putty(4).exe
49941324        524288  lots_of_putty/putty(7).exe
EDA9DDD1        829304  lots_of_putty/putty(11).exe
199F509B        495616  lots_of_putty/putty(5).exe
30B27D63       1090568  lots_of_putty/putty(16).exe
CAF59A4D       1098112  lots_of_putty/putty(18).exe
3E007457        483328  lots_of_putty/putty(3).exe
6CD97CDC        854072  lots_of_putty/putty(15).exe
F4C86100       1173000  lots_of_putty/putty(17).exe
153C4590        454656  lots_of_putty/putty(1).exe
692108A1        828400  lots_of_putty/putty(13).exe
1A573FCE        421888  lots_of_putty/putty.exe
07B01710        531368  lots_of_putty/putty(9).exe
A4C35A82        454656  lots_of_putty/putty(2).exe
2E8A61DB        524288  lots_of_putty/putty(6).exe
A5C15E01        524288  lots_of_putty/putty(8).exe  <----- That looks like the one! (Release 0.66)
383949C3       1181568  lots_of_putty/putty(19).exe
561B36DA        712176  lots_of_putty/putty(12).exe
DA2AB092       1179024  lots_of_putty/putty(21).exe
59307220        774200  lots_of_putty/putty(14).exe
E377B9D8        713592  lots_of_putty/putty(10).exe
A7A58BB8      38770246  test.zip
-------- -------------  ------------
94DD3875      55198046

Folders: 1
Files: 23
Size: 55198046

CRC32  for data:              94DD3875
CRC32  for data and names:    8259971F

Everything is Ok
[tom@tomandtim zip_crack]$

Now that we know the right putty version we need to zip the plain text putty using the same compression options as the encrypted version. It is very important that the two zips are as close to each other as possible. In my experience you need to use the same OS as well as the linux zip and windows zip act ever so slightly different.

Then we can use the pkcrack tool tool decrypt the encrypted zip. This can take a few minutes to run so I have sped up the gif:

cracking

Tada!

Is this still a Vulnerability?

If you use ZipCrypto yes, yes it is. ZipCrypto is still the default encryption mode for many pieces of software such as 7zip so to avoid this issue try to use AES instead. The attacker needs 100 bytes+ of plain text to mount an attack so if you are using a common file format such as word (.docx) it is likely the word document header might be enough.