Category: Crypto Points: 100 Description:
Let’s extract the provided cry100.zip file:
$ unzip cry100.zip
Archive: cry100.zip
inflating: cry100The extracted cry100 file is a text document:
$ file cry100
cry100: ASCII text, with CRLF line terminatorsIt contains the following ciphertext:
XMVZGC RGC AMG RVMG HGFGMQYCD VT VWM BYNO, NSVWDS NSGO RAO XG UWFN AF
HACDGMVWF. AIRVFN AII AMG JVRRVC-XVMC, FYRBIG TVIZ ESV SAH CGQGM XGGC
RVMG NSAC A RYIG TMVR NSG SVWFG ESGMG NSGO EGMG XVMC WCNYI NSG HAO
FVRG IVMH JARG MVWCH NV NAZG NSGR VTT NV EAM. OVWM TIAD YF "CV NSYF
YF CVN JMOBNV RO HGAM", YC IVEGMJAFG, EYNS WCHGMFJVMGF YCFNGAH VT
FBAJGF, FWMMVWCHGH XO NSG WFWAI "TIAD" NAD ACH JWMIO XMAJGF. GCUVO.
This string is encoded using a substitution cipher. Specifically, it is a cryptogram, a cipher in which each letter is replaced by some other letter. One way to identify a substitution cipher is to run character frequency analysis on the ciphertext.
Using a cryptogram solver like quipqiup.com (and assuming that the cleartext is in English) reveals that the key is AXJHGTDSYUZIRCVBPMFNWQEKOL. The cleartext is:
BROKEN MEN ARE MORE DESERVING OF OUR PITY, THOUGH THEY MAY BE JUST AS
DANGEROUS. ALMOST ALL ARE COMMON-BORN, SIMPLE FOLK WHO HAD NEVER BEEN
MORE THAN A MILE FROM THE HOUSE WHERE THEY WERE BORN UNTIL THE DAY
SOME LORD CAME ROUND TO TAKE THEM OFF TO WAR. YOUR FLAG IS "NO THIS
IS NOT CRYPTO MY DEAR", IN LOWERCASE, WITH UNDERSCORES INSTEAD OF
SPACES, SURROUNDED BY THE USUAL "FLAG" TAG AND CURLY BRACES. ENJOY.
The flag is flag{no_this_is_not_crypto_my_dear}.