Rating:

Problem Statement(150 points):

Here's another simple cipher for you where we made a bunch of substitutions. Can you decrypt it? Connect with nc 2018shell2.picoctf.com 18581.

Solution :

I was given the following ciphertext :

-------------------------------------------------------------------------------
wgzivhot ymvm ut jglv achi - tlqtouolougz_wusymvt_hvm_tgcbhqcm_yhaxzqgmtb
-------------------------------------------------------------------------------
tohomcj, sclfs qlwr flccuihz whfm avgf oym tohuvymhx, qmhvuzi h qgnc ga
choymv gz nyuwy h fuvvgv hzx h vhpgv chj wvgttmx. h jmccgn xvmttuziignz,
lziuvxcmx, nht tltohuzmx imzocj qmyuzx yuf gz oym fucx fgvzuzi huv. ym
ymcx oym qgnc hcgao hzx uzogzmx:

-uzovguqg hx hcohvm xmu.

yhcomx, ym smmvmx xgnz oym xhvr nuzxuzi tohuvt hzx whccmx glo wghvtmcj:

-wgfm ls, ruzwy! wgfm ls, jgl amhvalc emtluo!

tgcmfzcj ym whfm agvnhvx hzx fglzomx oym vglzx ilzvmto. ym ahwmx hqglo
hzx qcmttmx ivhbmcj oyvuwm oym ognmv, oym tlvvglzxuzi chzx hzx oym
hnhruzi fglzohuzt. oymz, whowyuzi tuiyo ga tomsymz xmxhclt, ym qmzo
ognhvxt yuf hzx fhxm vhsux wvgttmt uz oym huv, ilvicuzi uz yut oyvgho
hzx tyhruzi yut ymhx. tomsymz xmxhclt, xutscmhtmx hzx tcmmsj, cmhzmx
yut hvft gz oym ogs ga oym tohuvwhtm hzx cggrmx wgcxcj ho oym tyhruzi
ilvicuzi ahwm oyho qcmttmx yuf, mdluzm uz uot cmzioy, hzx ho oym cuiyo
lzogztlvmx yhuv, ivhuzmx hzx ylmx curm shcm ghr.

qlwr flccuihz smmsmx hz uztohzo lzxmv oym fuvvgv hzx oymz wgbmvmx oym
qgnc tfhvocj.

-qhwr og qhvvhwrt! ym thux tomvzcj.

ym hxxmx uz h svmhwymvt ogzm:

-agv oyut, g xmhvcj qmcgbmx, ut oym imzluzm wyvutouzm: qgxj hzx tglc
hzx qcggx hzx glzt. tcgn fltuw, scmhtm. tylo jglv mjmt, imzot. gzm
fgfmzo. h cuoocm ovglqcm hqglo oygtm nyuom wgvsltwcmt. tucmzwm, hcc.

ym smmvmx tuxmnhjt ls hzx ihbm h cgzi tcgn nyutocm ga whcc, oymz shltmx
hnyucm uz vhso hoomzougz, yut mbmz nyuom ommoy icutomzuzi ymvm hzx oymvm
nuoy igcx sguzot. wyvjtgtogfgt. ong tovgzi tyvucc nyutocmt hztnmvmx
oyvgliy oym whcf.

-oyhzrt, gcx wyhs, ym wvumx qvutrcj. oyho nucc xg zuwmcj. tnuowy gaa
oym wlvvmzo, nucc jgl?

ym trussmx gaa oym ilzvmto hzx cggrmx ivhbmcj ho yut nhowymv, ihoymvuzi
hqglo yut cmit oym cggtm agcxt ga yut ignz. oym sclfs tyhxgnmx ahwm hzx
tlccmz gbhc egnc vmwhccmx h svmchom, shovgz ga hvot uz oym fuxxcm himt.
h scmhthzo tfucm qvgrm dlumocj gbmv yut cust.

As the question name tells hertz which means frequency.So we have to do frequency analysis.I decoded it with

http://crypto.interactive-maths.com/frequency-analysis-breaking-the-code.html and make following substitution

M H O Z T G C U Y X V L I W S N F Q J A R B D E P K
| | | | | | | | | | | | | | | | | | | | | | | | | |
e a t n s o l i h d r u g c p w m b y f k v z

Using the above substitution gave me the flag :

congrats here is your flag - substitution_ciphers_are_solvable_hafdnboesv

picoCTF{substitution_ciphers_are_solvable_hafdnboesv}

Original writeup (https://github.com/d4rkvaibhav/PICOCTF-2018/tree/master/Cryptography/hertz).