picoCTF 2019 - la cifra de writeup

08 Nov, 2020

Description

I found this cipher in an old book. Can you figure out what it says? Connect with nc 2019shell1.picoctf.com 61559

Category: Cryptography

Points: 200

Problem

Connecting to the above server gives us the following message:

Encrypted message:
Ne iy nytkwpsznyg nth it mtsztcy vjzprj zfzjy rkhpibj nrkitt ltc tnnygy ysee itd tte cxjltk

Ifrosr tnj noawde uk siyyzre, yse Bnretèwp Cousex mls hjpn xjtnbjytki xatd eisjd

Iz bls lfwskqj azycihzeej yz Brftsk ip Volpnèxj ls oy hay tcimnyarqj dkxnrogpd os 1553 my Mnzvgs Mazytszf Merqlsu ny hox moup Wa inqrg ipl. Ynr. Gotgat Gltzndtg Gplrfdo

Ltc tnj tmvqpmkseaznzn uk ehox nivmpr g ylbrj ts ltcmki my yqtdosr tnj wocjc hgqq ol fy oxitngwj arusahje fuw ln guaaxjytrd catizm tzxbkw zf vqlckx hizm ceyupcz yz tnj fpvjc hgqqpohzCZK{m311a50_0x_a1rn3x3_h1ah3xg6ndl651}

Yse lncsz bplr-izcarpnzjo dkxnroueius zf g uzlefwpnfmeznn cousex mzwkapr, cfd mgip axtfnj 1467 gj Lkty Bgyeiyyl Argprzn.

Ehk Atgksèce Inahkw ts zmprkkzrk xzmkytmkx narqpd zmp Argprzn Oiyh zr Gqmexyt Cousex.

Ny 1508, Jumlntjd Txnehkrtuy nyvkseej yse yt-narqpd zfmurf ceiyl (a sferoc zf ymtfzjo arusahjes) zmlt ctflj qltkw me g hciznnar hzmvtyety zf zmp Volpnèxj Nivmpr.

Hjwlgxz’s yjnoti moupwez fapkfcej ny 1555 ay f notytnafeius zf zmp fowdt. Zmp lubpr nfwvkx zf zmp arusahjes gwp nub dhokeej wpgaqlrrd, muz yse gqahggpty fyd zmp itipx rjetkwd axj xidjo be rpatx zf g ryestyii ppy vmcayj, hhohs cgs me jnqfkwpnz bttn jlcn hzrxjdpusoety.

Solution

I went to Cyber Chef and searched for a decoder.

Since the title of this challenge is in French, I assumed we would need to use a French titled decoder. I searched through the available options and chose the Vigenere cipher because it sounded the most French.

This decoder requires a key. I tried the keys “pico” and “picoCTF” but was unsuccessful. After several more guesses I found the correct key was “flag”:

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