DownUnderCTF 2020: fix my pc

fix my pc

500 points

My boss's computer died recently. We managed to dump some of the drive, but can't figure out a way to unlock it.

Download (233MB) https://cloudstor.aarnet.edu.au/plus/s/lIZ7mV36US93DhA

Solution

We start with rescue.zip with two files within: system.bin and crash.bin

The first one is disk image, the second one looks like memory dump.

I’ve started with mounting the disk.

modprobe nbd max_part=8

qemu-nbd –connect=/dev/nbd0 /tmp/system.bin

But I’ve been stopped by disk encryption.

Ok, let’s try to retrieve the key to decrypt those partitions (/dev/nbd0p1 was boot with nothing interesting).

I downloaded findaes tool from https://sourceforge.net/projects/findaes/ and used it to find the keys in memory dump.

Looks like I’ve been lucky today ;-)

I’ve combined two parts of the key together and saved as binary.

echo 094e2adf58cfb17d85f0f6933f7b44efa00a3cda7bbe01873e09ff4ee7a60539ff98d76761147024ebb0c8d4e1141814214d2a83d7936609377755e5180a3c57 | xxd -r -p > /tmp/key

And then tried to use it.

cryptsetup luksAddKey /dev/nbd0p2 --master-key-file /tmp/key

cryptsetup luksOpen /dev/nbd0p2 rescue

lsblk

mount /dev/mapper/rescue /mnt

On mounted partition there were many files with corrupted names, but the content was ok and gave me the hint where’s the key for 2nd partition.

cryptsetup luksOpen /dev/nbd0p3 crypthome --key-file /mnt/etc/crypttab.d/home.key

Interesting parts were ssh keys and .ash_history

So I’ve used bob’s keys to clone the repo and have a look.

export GIT_SSH_COMMAND="ssh -o IdentitiesOnly=yes -i /mnt2/bob/.ssh/id_rsa"

git clone [email protected]:cornochips/configs

cd configs

Checked the content of files with no luck, then suddenly...

for i in `git log --all --oneline | awk -F ' ' '{print $1, $8}'`; do git diff ${i}; done

DUCTF{aT_l3ast_I_had_A_B3ck8p_y4n63xOVX4A}