PicoCTF2018 Cryptography hertz 2

PicoCTF2018 Cryptography hertz 2

Objective:
This flag has been encrypted with some kind of cipher, can you decrypt it? Connect with nc 2018shell.picoctf.com 12521.

got this as the result of the nc

Let's decode this now!
Yws deruo caign pib qekvh ifsa yws ljzm tix. R ujn'y cslrsfs ywrh rh heuw jn sjhm vaiclsk rn Vrui. Ry'h jlkihy jh rp R hilfst j vaiclsk jlasjtm! Iojm, prns. Wsas'h yws pljx: vruiUYP{hechyryeyrin_urvwsah_jas_yii_sjhm_xknrcraynf}

the hint says "These kinds of problems are solved with a frequency that metric some analysis"

Solution:



so let's use letter frequency to try and crack it, some super high level analysis I don't see any double letters that would be like OO in English

but we can borrow some logic from the enigma crack, just like every message the Germans were sending ended with Heil Hitler, we know the format of the flag
the flag always starts with picoCTF

so we can be fairly certain that the letters  vruiUYP are actually picoCTF
we can also see there are Capital R by themselves at the beginning of sentences so that is most likely an I which matches up with the other guess we had

so just from picoCTF we know
v=p
r=i
u=c
i=o
U=c
Y=T
P=F


I used this site to help me keep track of my guesses
https://www.simonsingh.net/The_Black_Chamber/substitutioncrackingtool.html


the first word is only three letters long and we can guess its probably The

Y=T
w=h
s=e

so here is what we have decrypted so far

THE **IC* **O** FO* ***P* O*E* THE **** *O* I C**T *E*IE*E THI* I* **CH ** E*** P*O**E* I* PICO IT* ***O*T ** IF I *O**E* * P*O**E* ***E*** O*** FI*E HE*E* THE F*** PICOCTF****TIT*TIO*CIPHE****ETOOE******I*I*T**

The fourth word is most likely for lets try inserting
b=r
** it wasn't its an X for fox, after I got most of the rest of the deconding I recongized the text*****

The 13th word is THI* so most likely is and S
h=s

so now we are here

THE **IC* **O** FOR ***PS O*E* THE **** *O* I C**T *E*IE*E THIS IS S*CH ** E*S* P*O**E* I* PICO ITS ***OST *S IF I SO**E* * P*O**E* ***E*** O*** FI*E HE*ES THE F*** PICOCTFS**STIT*TIO*CIPHE*S**ETOOE*S****I*I*T**

I'm just going to look through here and start plugging in more substitutions to see if I can decode the rest

e=u
n=n
c=b


THE *UIC* B*O*N FOR *U*PS O*E* THE **** *O* I C*NT BE*IE*E THIS IS SUCH *N E*S* P*OB*E* IN PICO ITS ***OST *S IF I SO**E* * P*OB*E* ***E*** O*** FINE HE*ES THE F*** PICOCTFSUBSTITUTIONCIPHE*S**ETOOE*S***NIBI*TN*

j=a
l=l
m=y
x=g



Now I see that text is the quick brown fox jumps over the lazy dog, which coincidentally contains every letter in the alphabet.
so the key is

R A
X B
B C
Q D
U E
V F
W G
S H
O I
A J
M K
L L
Y M
N N
K O
F P
J Q
I R
E S
D T
C U
P V
H W
G X
T Y
Z Z

so decrypted message reads

THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG I CANT BELIEVE THIS IS SUCH AN EASY PROBLEM IN PICO ITS ALMOST AS IF I SOLVED A PROBLEM ALREADY OKAY FINE HERES THE FLAG PICOCTFSUBSTITUTIONCIPHERSARETOOEASYGMNIBIRTNV



picoCTF{SUBSTITUTION_CIPHERS_ARE_TOO_EASY_GMNIBIRTNV}