sekur julius

3 minutes to read

We are given the Python source code to encrypt the flag:

from random import choices
import os

def julius_encrypt(msg, shift):
    ct = ''
    for p in msg:
        if p == ' ':
            ct += '0'
        elif not ord('A') <= ord(p) <= ord('Z'):  
            ct += p
        else:
            o = ord(p) - 65
            ct += chr(65 + (o + shift) % 26)
    return ct

def encrypt(msg, key):
    for shift in key:
        msg = julius_encrypt(msg, shift)
    return msg

msg = open('secret.txt').read().upper()
secure_key = os.urandom(1337)

with open('output.txt', 'w') as f:
    f.write(encrypt(msg, secure_key))

And this is the output of the script:

JRYPBZR0GB0UNPXGUROBB0GJBGUBHFNAQGJRAGLSBHE!0GUVF0VF0N0CEBBS0BS0PBAPRCG0GB0CEBIR0LBH0GUNG0GUR0PNRFNE0PVCURE0VF0VAFRPHER0AB0ZNGGRE0UBJ0ZNAL0GVZRF0LBH0NCCYL0VG.0GUR0FRPHEVGL0BS0N0GUBHFNAQ0QVFGVAPG0FUVSGF0VF0RIRAGHNYYL0GUR0FNZR0NF0GUNG0BS0N0FVATYR0FUVSG.0RABHTU0ZHZOYVAT,0GNXR0LBHE0SYNT0NAQ0RAWBL0GUR0ERFG0BS0GUR0PBAGRFG.0ZNXR0FHER0LBH0JENC0GUR0SBYYBJVAT0GRKG0JVGU0GUR0UGO0SYNT0SBEZNG0GURRSSRPGVIRXRLFCNPRBSPNRFNEQRCRAQFBAGURFVMRBSGURNYCUNORG.

Source code analysis

A quick look at julius_encrypt tells us that it is using a Caesar cipher (the name of the challenge is kind of a hint):

def julius_encrypt(msg, shift):
    ct = ''
    for p in msg:
        if p == ' ':
            ct += '0'
        elif not ord('A') <= ord(p) <= ord('Z'):  
            ct += p
        else:
            o = ord(p) - 65
            ct += chr(65 + (o + shift) % 26)
    return ct

This means that the encryption is just a substitution cipher where each plaintext letter is shifted by a fix number to get the ciphertext letter:

$ python3 -q
>>> def julius_encrypt(msg, shift):
...     ct = ''
...     for p in msg:
...         if p == ' ':
...             ct += '0'
...         elif not ord('A') <= ord(p) <= ord('Z'):  
...             ct += p
...         else:
...             o = ord(p) - 65
...             ct += chr(65 + (o + shift) % 26)
...     return ct
... 
>>> julius_encrypt('ASDF', 3)
'DVGI'

However, there is an encrypt function that wraps the use of julius_encrypt:

def encrypt(msg, key):
    for shift in key:
        msg = julius_encrypt(msg, shift)  
    return msg

It implements an iterated Caesar cipher… Is this more secure?

Solution

Well, since Caesar cipher is just a shift, it happens that a shift of 26 leaves the plaintext untouched:

>>> julius_encrypt('ASDF', 26)  
'ASDF'

As a result, no matter how many times we apply julius_encrypt, because the shift can be mapped to a value ranging from 0 to 25.

Therefore, we can simply brute force this shift and find the correct plaintext:

>>> ciphertext = 'JRYPBZR0GB0UNPXGUROBB0GJBGUBHFNAQGJRAGLSBHE!0GUVF0VF0N0CEBBS0BS0PBAPRCG0GB0CEBIR0LBH0GUNG0GUR0PNRFNE0PVCURE0VF0VAFRPHER0AB0ZNGGRE0UBJ0ZNAL0GVZRF0LBH0NCCYL0VG.0GUR0FRPHEVGL0BS0N0GUBHFNAQ0QVFGVAPG0\
FUVSGF0VF0RIRAGHNYYL0GUR0FNZR0NF0GUNG0BS0N0FVATYR0FUVSG.0RABHTU0ZHZOYVAT,0GNXR0LBHE0SYNT0NAQ0RAWBL0GUR0ERFG0BS0GUR0PBAGRFG.0ZNXR0FHER0LBH0JENC0GUR0SBYYBJVAT0GRKG0JVGU0GUR0UGO0SYNT0SBEZNG0GURRSSRPGVIRXRLFCNPRBSPNRF\
NEQRCRAQFBAGURFVMRBSGURNYCUNORG.'
>>> 
>>> for shift in range(26):
...     print(shift, julius_encrypt(ciphertext, shift))
... 
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20 DLSJVTL0AV0OHJRAOLIVV0ADVAOVBZHUKADLUAFMVBY!0AOPZ0PZ0H0WYVVM0VM0JVUJLWA0AV0WYVCL0FVB0AOHA0AOL0JHLZHY0JPWOLY0PZ0PUZLJBYL0UV0THAALY0OVD0THUF0APTLZ0FVB0HWWSF0PA.0AOL0ZLJBYPAF0VM0H0AOVBZHUK0KPZAPUJA0ZOPMAZ0PZ0LCLUABHSSF0AOL0ZHTL0HZ0AOHA0VM0H0ZPUNSL0ZOPMA.0LUVBNO0TBTISPUN,0AHRL0FVBY0MSHN0HUK0LUQVF0AOL0YLZA0VM0AOL0JVUALZA.0THRL0ZBYL0FVB0DYHW0AOL0MVSSVDPUN0ALEA0DPAO0AOL0OAI0MSHN0MVYTHA0AOLLMMLJAPCLRLFZWHJLVMJHLZHYKLWLUKZVUAOLZPGLVMAOLHSWOHILA.
21 EMTKWUM0BW0PIKSBPMJWW0BEWBPWCAIVLBEMVBGNWCZ!0BPQA0QA0I0XZWWN0WN0KWVKMXB0BW0XZWDM0GWC0BPIB0BPM0KIMAIZ0KQXPMZ0QA0QVAMKCZM0VW0UIBBMZ0PWE0UIVG0BQUMA0GWC0IXXTG0QB.0BPM0AMKCZQBG0WN0I0BPWCAIVL0LQABQVKB0APQNBA0QA0MDMVBCITTG0BPM0AIUM0IA0BPIB0WN0I0AQVOTM0APQNB.0MVWCOP0UCUJTQVO,0BISM0GWCZ0NTIO0IVL0MVRWG0BPM0ZMAB0WN0BPM0KWVBMAB.0UISM0ACZM0GWC0EZIX0BPM0NWTTWEQVO0BMFB0EQBP0BPM0PBJ0NTIO0NWZUIB0BPMMNNMKBQDMSMGAXIKMWNKIMAIZLMXMVLAWVBPMAQHMWNBPMITXPIJMB.
22 FNULXVN0CX0QJLTCQNKXX0CFXCQXDBJWMCFNWCHOXDA!0CQRB0RB0J0YAXXO0XO0LXWLNYC0CX0YAXEN0HXD0CQJC0CQN0LJNBJA0LRYQNA0RB0RWBNLDAN0WX0VJCCNA0QXF0VJWH0CRVNB0HXD0JYYUH0RC.0CQN0BNLDARCH0XO0J0CQXDBJWM0MRBCRWLC0BQROCB0RB0NENWCDJUUH0CQN0BJVN0JB0CQJC0XO0J0BRWPUN0BQROC.0NWXDPQ0VDVKURWP,0CJTN0HXDA0OUJP0JWM0NWSXH0CQN0ANBC0XO0CQN0LXWCNBC.0VJTN0BDAN0HXD0FAJY0CQN0OXUUXFRWP0CNGC0FRCQ0CQN0QCK0OUJP0OXAVJC0CQNNOONLCRENTNHBYJLNXOLJNBJAMNYNWMBXWCQNBRINXOCQNJUYQJKNC.
23 GOVMYWO0DY0RKMUDROLYY0DGYDRYECKXNDGOXDIPYEB!0DRSC0SC0K0ZBYYP0YP0MYXMOZD0DY0ZBYFO0IYE0DRKD0DRO0MKOCKB0MSZROB0SC0SXCOMEBO0XY0WKDDOB0RYG0WKXI0DSWOC0IYE0KZZVI0SD.0DRO0COMEBSDI0YP0K0DRYECKXN0NSCDSXMD0CRSPDC0SC0OFOXDEKVVI0DRO0CKWO0KC0DRKD0YP0K0CSXQVO0CRSPD.0OXYEQR0WEWLVSXQ,0DKUO0IYEB0PVKQ0KXN0OXTYI0DRO0BOCD0YP0DRO0MYXDOCD.0WKUO0CEBO0IYE0GBKZ0DRO0PYVVYGSXQ0DOHD0GSDR0DRO0RDL0PVKQ0PYBWKD0DROOPPOMDSFOUOICZKMOYPMKOCKBNOZOXNCYXDROCSJOYPDROKVZRKLOD.
24 HPWNZXP0EZ0SLNVESPMZZ0EHZESZFDLYOEHPYEJQZFC!0ESTD0TD0L0ACZZQ0ZQ0NZYNPAE0EZ0ACZGP0JZF0ESLE0ESP0NLPDLC0NTASPC0TD0TYDPNFCP0YZ0XLEEPC0SZH0XLYJ0ETXPD0JZF0LAAWJ0TE.0ESP0DPNFCTEJ0ZQ0L0ESZFDLYO0OTDETYNE0DSTQED0TD0PGPYEFLWWJ0ESP0DLXP0LD0ESLE0ZQ0L0DTYRWP0DSTQE.0PYZFRS0XFXMWTYR,0ELVP0JZFC0QWLR0LYO0PYUZJ0ESP0CPDE0ZQ0ESP0NZYEPDE.0XLVP0DFCP0JZF0HCLA0ESP0QZWWZHTYR0EPIE0HTES0ESP0SEM0QWLR0QZCXLE0ESPPQQPNETGPVPJDALNPZQNLPDLCOPAPYODZYESPDTKPZQESPLWASLMPE.
25 IQXOAYQ0FA0TMOWFTQNAA0FIAFTAGEMZPFIQZFKRAGD!0FTUE0UE0M0BDAAR0AR0OAZOQBF0FA0BDAHQ0KAG0FTMF0FTQ0OMQEMD0OUBTQD0UE0UZEQOGDQ0ZA0YMFFQD0TAI0YMZK0FUYQE0KAG0MBBXK0UF.0FTQ0EQOGDUFK0AR0M0FTAGEMZP0PUEFUZOF0ETURFE0UE0QHQZFGMXXK0FTQ0EMYQ0ME0FTMF0AR0M0EUZSXQ0ETURF.0QZAGST0YGYNXUZS,0FMWQ0KAGD0RXMS0MZP0QZVAK0FTQ0DQEF0AR0FTQ0OAZFQEF.0YMWQ0EGDQ0KAG0IDMB0FTQ0RAXXAIUZS0FQJF0IUFT0FTQ0TFN0RXMS0RADYMF0FTQQRRQOFUHQWQKEBMOQAROMQEMDPQBQZPEAZFTQEULQARFTQMXBTMNQF.

It looks like 13 is the correct shift. We can replace 0s by spaces to enhance readability:

>>> print(julius_encrypt(ciphertext, 13).replace('0', ' '))
WELCOME TO HACKTHEBOO TWOTHOUSANDTWENTYFOUR! THIS IS A PROOF OF CONCEPT TO PROVE YOU THAT THE CAESAR CIPHER IS INSECURE NO MATTER HOW MANY TIMES YOU APPLY IT. THE SECURITY OF A THOUSAND DISTINCT SHIFTS IS EVENTUALLY THE SAME AS THAT OF A SINGLE SHIFT. ENOUGH MUMBLING, TAKE YOUR FLAG AND ENJOY THE REST OF THE CONTEST. MAKE SURE YOU WRAP THE FOLLOWING TEXT WITH THE HTB FLAG FORMAT THEEFFECTIVEKEYSPACEOFCAESARDEPENDSONTHESIZEOFTHEALPHABET.

Flag

So, the flag is:

HTB{THEEFFECTIVEKEYSPACEOFCAESARDEPENDSONTHESIZEOFTHEALPHABET}