---
title: peaCTF 2019 (Round1) - [Crypto] Crack The Key (450pts)
author: Maltemo
tags: CTF, peaCTF, Crypto, Vigenere
---
peaCTF 2019 (Round1) - [Crypto] Crack The Key (450pts)
===
Written by [Maltemo](https://twitter.com/Maltemo), member of team [SinHack](https://sinhack.blog/).

[TOC]

## Statement of the challenge

### Description
On one of my **frequent** walks through the woods, I stumbled upon this old French scroll with the title "le chiffre indéchiffrable." Remember to submit as peaCTF{plaintext_key}.

### Encrypted file :lock:
The file attached was `enc.txt` and contained :

DVMDVRWOUISIERRRGNNVMWPOPGTOHSBUIHTCSSMJIVUWEXHTCTKZKFXIENWTDDOVMEOWDZRQEBQPVLFWKJBGL
EEDALGCIVLQGLTWTCMFXSIAQTLTUGZQZZWOPVGIRCSLRUZRJUZBQSXSPXGJMGTPRPUGRSIVRGUDAFXHTNLVVBMF
ZMQSFUWTWTFSWHIGXHTQLGCUSRGLEIWWXXWWCJDAIFXGAPDWGWFHTZSVOBISITRVUTTVRTWTDGMCPHGGNRDB
POIZZWZPGHTTDQPHOYIUTUEWJDCPWORWDAZREDNHYSJZRJPAFSOCPDXZVPLVPGMNIWPFWUVRDUJINIDFXLYIUTE
NWAHITVJZRJPVQEFAJEXWIMQVIYPTWGZYYYXKTNNVMQJTPVZRJHEBVDWPOKGEIUDCAHDJGTRYKLHSILXHPIZPVDE
MDZGLEEGTDWDMGSTRAHXIPFGRVKPLUEDPHEVSEKHSZREMDCELWGVHKQBYSCXRLLRRGLQFLESIZGGDQXCQPETT
XEXGKLHDBUIRPCTQSCWLIPNHBTTYEYIIHSBUETIWPCKYSXALNPLBTPXAEXKTJVKBPGYEKJSRCIFQRYDYIKNEVHISILN
DFXGWXKTENCOASXEBFVVDPRAAHPWASPWFPTYIDIWZYYYXKTVNQEJCOIJNLLRPUIHPSMIWEIAWQOMTTSHEKNMOAQ
AKDDCMISLXBLIFWOWXRLDPVHVIEHESDYXZVJDGUGLAITGIJPSQTENWQJXEIJVEGNBBPOHTLRZFYUHAYIEEXYSJUIUI
WUIAGLSELYIKPLGSSPNLXGEIHCLBJTWTMMYSEUCWAESDGESXIELHMQTLPIQSJDQDYWEAAHPWVWRHBTVFGOCRPH
GELLHJRHOUHEVSNYQSMEELPCEIJEAKXKULUCVQVGDEETIZLELPDXOVPYTGRERHDWHSEHKPLYETTAJKJFAQGIGLEG
HESMKFXIPRAAHHEMDCEPPRRWTXRWSGBMQVXVKWXISEOZWHPVQFECTGSDVRWPXCIAGPYGWZRVEQGIOUISIXRG
WIPNXHXHEYKYIVWIQREKTCFWVRFJBOIFDGPPGEKWWMBXHTGLRADEOHJRKACIZEJIMYTIAHMPZPXZVQVTTIISRDXJGI
XDQTREFITCXZVMUSQSJEGTYXXRWKXWAWFXGDXURQHIPRXHGTPHGXWEACRFEAAUIKJMHPVQTICRSIJRRGIPRRTWT
AMYJAKDARXTATOHGNRLCBUISIGLAADQHSQNXEANTRXISQIWSXHTEWELWSUBBUIHTCDTWIGKTLGLEBHPPNVWRCBU
IWXCOSOJMOAAGLEEXRIGEWIACGXEGTOYHKSWWMEEFITCWLYIVWMRTACSNSOJPDNLBANQTSMFUXKTXVKSPCOFW
XEQIWPLELISIULHWWMGAORPCXZFVVTAOSXTGLRVTPRKMEGABTTRLFKHIPRVWPAVMFXZHGGFPOLAJEFUWHIBVRGS
DHRLYILGDNWTWPTVQYSRUAJMTWVCISKGDGMYISIISIJVWKDCYHBTHZQWJQDATNRIBPWGGEGHPTRHICISIKKVDLKYS
VTGHEKRWWDCGQOIWPVDPQDGMNTPGDLGZZRJBQQHLTATJWNLRWIQREKTCUMZXHVWGLEGUTKMIIEPKXEFITCLWIJ
RJZGLFDPWFGOIULIFENTCZVEFYVQMNWTCTLVDPILVPGIECWLRVJLLVPNRDPHDXJFRJPANRYILZSJUMQPZLLOGHPWH
LXWDORXHTGLAZZXHHBEMPTSZAFYMVCWFIGPKPLADEVDURAHPIDXMGMGPXCIAGPYGWRRGXVSECIWPASJRRIWSJI
GHEVSKILCBRPLXVPRUVFXIPRAAHJYMNVVVPTYCRTHAIUKIGUWELIHHEISUMQTAFSFRWLVSTXHGIAHTGTXIFUSXHXBA
EGHZJOFVNPNGIRIWPLGIWHHKNQEBJCMWCXKTEUMTTVZELRRGQMANABXYXZVHRCSRCBTCUEEZRZPAGLEDAOIKKE
QXUNPOCISIXRVPPVQXHTLZVKKXHBXRVESWPWWCHRBBNPKTSLRVNLHCPRHISXEASJYVJIYPYIDXECVWRBMPCNXRL
PJVQDGSSSRXCDXSEGHWMJSUASDEQKLDIOBHHPSRMNVRKXUNXAXAESCVISIPRJLXTDSXWFXIBUETWTHSMCHVDWA
IRWPGIZRHQDBNMLPCORGWPLTANPOCTLQGEKWWMNRIBPWWGEXKTNNVMWTYINVVOPCTLESXQEKBIGLPLLELDFPVJ
EBIPNXHTHLAFFXKXVTXOAPFKZRXQTDRVTWTWIKJALIPBYTDEPRDPEGBQGXICVTXZVADHLRZOITOXGSSATZGLEILZSX
KLHBCFYAAAJWHVRWIPRMRHJYHSPWWDORXHTGTRLYIVBIYPPPSOSUBFHNWAHTWTZVUYEUSOEEZXCRWAUIENAVH
EPCORWMIUHXREKXCR

### Hint
The text is guaranteed to be in modern English with regular letter frequencies.

## Analysis :mag:

When I started this challenge, I went directly to the encrypted file and tried it with different cypher and with mono alphabetical substitution.

I tried guessing, giving a possible word "PEACTF", but I stopped after reading 90 wrong results.

:::warning
That wasn't the right call.
:::

Do you remember what the teacher always said before tests ?
>No one : ""
>Every single teacher : "Don't forget to read the entire statement !"

After wasting my time, I finaly decided to read the statement and I found a lot of informations :

* **frequent** in bold made me thought about frequency attacks.
* **“le chiffre indéchiffrable.”** when you search this in google, you immediatly find Vigenere Cypher.
* I toggled the hint, and got the information that the text was in English.

:::success
That's much better, isn't it ?
:::

## Solution :unlock:

My favorite website resolving cyphers (dcode.fr) didn't work with english frequency analysis.

So I made new searches and found this awesome website :
* https://www.guballa.de/vigenere-solver

I tried with our cyphered text (enc.txt),
Chose classical Vigenere and English Language.
Clicked on the button `Break Cypher`

I got this result :

The text was readable and we also gained the cypher key.
As I wasn't shure if the flag was in the text, I read the text ...

MR JONES OF THE MANOR FARM HAD LOCKED THE HEN HOUSES FOR THE NIGHT BUT WAS TOO DRUNK TO REMEMBER TO SHUT THE
POP HOLES WITH THE RING OF LIGHT FROM HIS LANTERN DANCING FROM SIDE TO SIDE HE LURCHED ACROSS THE YARD KICKED OFF 
HIS BOOTS AT THE BACKDOOR DREW HIMSELF A LAST GLASS OF BEER FROM THE BARREL IN THE SCULL ERYAND
MADE HIS WAY UP TO BED WHERE MRS JONES WAS ALREADY SNORING AS SOON AS THE LIGHT IN THE BEDROOM WENT OUT THERE 
WAS A STIRRING AND A FLUTTERING ALL THROUGH THEFARMBUILDINGSWORDHADGONEROUNDDURINGTHEDAYTHATOLDMA
JORTHEPRIZEMIDDLEWHITEBOARHADHADASTRANGEDREAMONTHEPREVIOUSNIGHTANDWISHEDTOCOMMUNICATE
ITTOTHEOTHERANIMALSITHADBEENAGREEDTHATTHEYSHOULDALLMEETINTHEBIGBARNASSOONASMRJONESWASSAFELY
OUTOFTHEWAYOLDMAJORSOHEWASALWAYSCALLEDTHOUGHTHENAMEUNDERWHICHHEHADBEENEXHIBITEDWASWILLI
NGDONBEAUTYWASSOHIGHLYREGARDEDONTHEFARMTHATEVERYONEWASQUITEREADYTOLOSEANHOURSSLEEPINORDERT
OHEARWHATHEHADTOSAYATONEENDOFTHEBIGBARNONASORTOFRAISEDPLATFORMMAJORWASALREADYENSCONCED
ONHISBEDOFSTRAWUNDERALANTERNWHICHHUNGFROMABEAMHEWASTWELVEYEARSOLDANDHADLATELYGROWNRATHER
STOUTBUTHEWASSTILLAMAJESTICLOOKINGPIGWITHAWISEANDBENEVOLENTAPPEARANCEINSPITEOFTHEFAC
TTHATHISTUSHESHADNEVERBEENCUTBEFORELONGTHEOTHERANIMALSBEGANTOARRIVEANDMAKETHEMSELVESCOMFOR
TABLEAFTERTHEIRDIFFERENTFASHIONSFIRSTCAMETHETHREEDOGSBLUEBELLJESSIEANDPINCHERANDTHENTHE
PIGSWHOSETTLEDDOWNINTHESTRAWIMMEDIATELYINFRONTOFTHEPLATFORMTHEHENSPERCHEDTHEMSELVESONTH
EWINDOWSILLSTHEPIGEONSFLUTTEREDUPTOTHERAFTERSTHESHEEPANDCOWSLAYDOWNBEHINDTHEPIGSANDBE
GANTOCHEWTHECUDTHETWOCARTHORSESBOXERANDCLOVERCAMEINTOGETHERWALKINGVERYSLOWLYANDSETTINGD
OWNTHEIRVASTHAIRYHOOFSWITHGREATCARELESTTHERESHOULDBESOMESMALLANIMALCONCEALEDINTHESTRAWCLO
VERWASASTOUTMOTHERLYMAREAPPROACHINGMIDDLELIFEWHOHADNEVERQUITEGOTHERFIGUREBACKAFTERHERF
OURTHFOALBOXERWASANENORMOUSBEASTNEARLYEIGHTEENHANDSHIGHANDASSTRONGASANYTWOORDINARYHORSESP
UTTOGETHERAWHITESTRIPEDOWNHISNOSEGAVEHIMASOMEWHATSTUPIDAPPEARANCEANDINFACTHEWASNOTOFFI
RSTRATEINTELLIGENCEBUTHEWASUNIVERSALLYRESPECTEDFORHISSTEADINESSOFCHARACTERANDTREMENDOUSPOW
ERSOFWORKAFTERTHEHORSESCAMEMURIELTHEWHITEGOATANDBENJAMINTHEDONKEYBENJAMINWASTHEOLDESTA
NIMALONTHEFARMANDTHEWORSTTEMPEREDHESELDOMTALKEDANDWHENHEDIDITWASUSUALLYTOMAKESOMECYNICA
LREMARKFORINSTANCEHEWOULDSAYTHATGODHADGIVENHIMATAILTOKEEPTHEFLIESOFFBUTTHATHEWOULDSOON
ERHAVEHADNOTAILANDNOFLIESALONEAMONGTHEANIMALSONTHEFARMHENEVERLAUGHEDIFASKEDWHYHEWOULDS
AYTHATHESAWNOTHINGTOLAUGHATNEVERTHELESSWITHOUTOPENLYADMITTINGITHEWASDEVOTEDTOBOXERTHETWOOF
THEMUSUALLYSPENTTHEIRSUNDAYSTOGETHERINTHESMALLPADDOCKBEYONDTHEORCHARDGRAZINGSIDEBYSID
EANDNEVERSPEAKING

... and stopped quickly because I saw that the flag wasn't there.

I tried to use the key as the flag, and it worked.

### TL;DR
The text was cyphered with **Vigenere Cypher**.
I did a **frequency** attack using this website: https://www.guballa.de/vigenere-solver

### Flag :white_check_mark:

The flag is **peaCTF{redpineapples}**

___
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