Rating: 3.5
# Volga CTF Quals 2017 Keypass
### Category: Reverse, 100 points
>For reasons unknown an amature cryptographer wrote an application to generate
>"strong encryption keys". One of these keys was used to encrypt a tar archive
>with the flag. They used openssl command line util with -aes-128-cbc.
>Could you please get the flag? It shouldn't take much time...
### Write-up
After messing about with the keypass program it was obvious that the passphrase generation was weak
```
$ ./keypass A
Ah=j&FQpX).Y!x2?H
$ ./keypass AA
BWf81zF%SdkxL*jL.
$ ./keypass AAA
Ah=j&FQpX).Y!x2?H
$ ./keypass AAAA
BWf81zF%SdkxL*jL.
```
Oops.
Lets generate some keys
```
for x in {0..255}
do
export x
./keypass $(perl -le 'print chr $ENV{x}')
done
```
>$sh generatekeys.sh > keys
Some of these inputs are invalid, the keyfile can be cleaned up or not, doesn't matter.
Now let's check how many unique keys there are.
```
with open("keys", "r") as ins:
array = []
a = 0;
for line in ins:
if line not in array:
array.append(line)
a = a + 1
print(a)
```
>$python uniquekeys.py
>251
>$ wc keys
>251 251 4518 keys
Cool, seems like all these keys are unique atleast.
Now let's try to crack that zip-file using the following script.
```
KEYS=`cat keys`
for k in $KEYS; do
openssl aes-128-cbc -d -in flag.zip.enc -out a -pass pass:"$k"
if [ $? -eq 0 ]
then
exit
fi
done
```
However, no luck. After this, generation of keys of all possible inputs with 2 bytes was tested but this didn't
generate any more unique keys and took a long time.
After flailing around for a bit this hint was dropped
>$ openssl
>OpenSSL> version
>OpenSSL 1.1.0e 16 Feb 2017
However, when we check this on the running machine
>$ openssl
>OpenSSL> version
>OpenSSL 1.0.2k 26 Jan 2017
Oops. Lets try this operation on a kali machine instead.
```
root@kali:~# apt-get update
root@kali:~# apt-get install openssl
root@kali:~# sh decrypt.sh
root@kali:~# unzip a
Archive: a
extracting: flag.txt
root@kali:~# cat flag.txt
VolgaCTF{L0ve_a11_trust_@_few_d0_not_reinvent_the_wh33l}
```
Much better.
