picoCTF 2022 ~transposition-trial writeup~

#security #ctf #python

Description

Our data got corrupted on the way here. Luckily, nothing got replaced, but every block of 3 got scrambled around! The first word seems to be three letters long, maybe you can use that to recover the rest of the message. Download the corrupted message here.

First, open message.txt.

heTfl g as iicpCTo{7F4NRP051N5_16_35P3X51N3_V9AAB1F8}7%

To be honest, this challenge needs a bit of an inspiration, but if you replace spaces with underscores (_) and look carefully, you can see the encrypting rules.

"heT" : "The"
"fl_" : "_fl"
"g_a" : "ag_"
"s_i" : "is_"
"icp" : "pic"
"CTo" : "oCT"

If you connect them, you can read The_flag_is_picoCT. Let's separate message.txt by three characters (3-gram) and decode it.

def main():
    f = open("message.txt", "r", encoding="UTF-8")
    txt = f.read()

    n=3
    txt3gram = [txt[i:i+n] for i in range(0, len(txt), n)]
    decode_lst = []

    for i in range(len(txt3gram)):
        decode_lst.append(txt3gram[i][2]+txt3gram[i][0]+txt3gram[i][1])

    print(''.join(decode_lst))


if __name__ == '__main__':
    main()

You can get The flag is picoCTF{7R4N5P051N6_15_3XP3N51V3_A9AFB178}, submit it!

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