invest

Invest (Forensics, 50points)

tl;dr dump http objects, use scheme to get the key, decode the encrypted file using aes-256-cbc

We're given a pcap file that contains some internet traffic, there are lots of downloaded files so we decide to export them using Export Objects -> HTTP.

There are 2 folders: chall and key. Chall contains a lot of base64 encoded files which turn out to be a split encrypted message. We can concact them simply using cat * > out

Key folder has some 'funny' pictures but what interests us is 12767348_10208095326368148_1014857467_n.jpeg: (my annotations)

And key.txt:

010001110101111001100011011011100100100100111001010111100100011101000111001110010100011100111001010001110011100101000111001110010101111001100011011011100100100101101110010010010011100100110101010111100110001100111001001101010110111001001001011011100100100101000111010111100011100100110101011011100100100101011110011000110100011101011110001110010011010101011110011000110101111001100011010111100110001101000111010111100101111001100011011011100100100101000111010111100011100100110101010001110101111001101110010010010101111001100011010111100110001101101110010010010101111001100011010111100110001100111001001101010100011101011110010111100110001101011110011000110101111001100011010001110101111001000111010111100101111001100011011011100100100101101110010010010101111001100011

We cannot use the key as the password (as well as decrypted binary). Let's try to run it through the graph byte by byte.

decode.py does the job for us, we get 001101000101010101101011011110100011100100110101010001100011001001011001011100010101000001101001 or 4Ukz95F2YqPi back, pretty good so far.

Looking up the message header (Salted__) we come across this website. So we probably have to use openssl to decrypt the flag, okay but which encryption algorithm?...

Let's brute it!

code=(aes-128-cbc aes-128-ecb aes-192-cbc aes-192-ecb aes-256-cbc aes-256-ecb base64 bf bf-cbc bf-cfb bf-ecb bf-ofb camellia-128-cbc camellia-128-ecb camellia-192-cbc camellia-192-ecb camellia-256-cbc camellia-256-ecb cast cast-cbc cast5-cbc cast5-cfb cast5-ecb cast5-ofb des des-cbc des-cfb des-ecb des-ede des-ede-cbc des-ede-cfb des-ede-ofb des-ede3 des-ede3-cbc des-ede3-cfb des-ede3-ofb des-ofb des3 desx rc2 rc2-40-cbc rc2-64-cbc rc2-cbc rc2-cfb rc2-ecb rc2-ofb rc4 rc4-40 seed seed-cbc seed-cfb seed-ecb seed-ofb)

for c in ${code[@]};
do
	echo $c;
	openssl $c -d -in out.bin -pass file:pass.txt -out decoded/$c;
	echo $c;
done;

It turns out that aes-256-cbc runs without any problems, it creates a Microsoft Word 2007+ document. Since I don't have Microsoft Office(lol) let's just treat it as a zip and see what's inside.

Bingo!