Deep Dive [misc]
SarCTF2020

Deep Dive

Worth digging into these tricks.

Recon

The txt file is a matroshka archive.

Code

Automatically unpack based on file signatures. We need (at least) cargo install zipcat.

unpack.sh:

#!/bin/bash

INPUT="$1"

FTYPE=`file ${INPUT}`

if [[ "$FTYPE" == *"POSIX tar"* ]]; then
    echo "TAR"
    tar -xOvf "$INPUT" > tmp
    cp tmp $INPUT
elif [[ "$FTYPE" == *"Zip archive data"* ]]; then
    echo "ZIP"
    zipcat -s "$INPUT" > tmp
    cp tmp $INPUT
elif [[ "$FTYPE" == *"bzip2 compressed"* ]]; then
    echo "ZIP"
    bzcat "$INPUT" > tmp
    cp tmp $INPUT
elif [[ "$FTYPE" == *"gzip compressed data"* ]]; then
    echo "ZIP"
    zcat "$INPUT" > tmp
    cp tmp $INPUT
elif [[ "$FTYPE" == *"XZ compressed data"* ]]; then
    echo "ZIP"
    xzcat "$INPUT" > tmp
    cp tmp $INPUT
else
    echo "NOT RECOGNIZED"
    echo ${FTYPE}
fi

Let it run for a while:

while true; do ./unpack.sh sar2020_flag.txt; done

Untill you see:

sar2020_flag.txt: ASCII text, with no line terminators

Cat the flag:

cat sar2020_flag.txt 
FLAG{matri0sha256}

Flag

FLAG{matri0sha256}