I thought I’d keep a little record of my progress on the 2013 GCHQ challenge since I just managed to crack the first stage!
The GCHQ challenge site is at: https://canyoufindit.co.uk/
The ciphertext presented there is as follows:
AWVLI QIQVT QOSQO ELGCV IIQWD LCUQE EOENN WWOAO
LTDNU QTGAW TSMDO QTLAO QSDCH PQQIQ DQQTQ OOTUD
BNIQH BHHTD UTEET FDUEA UMORE SQEQE MLTME TIREC
LICAI QATUN QRALT ENEIN RKG
I started by doing a quick frequency analysis of the letters, and by googling a little bit about how to identify ciphers. The frequency analysis suggested that this was likely to be something simple like a substitution or cipher but simple subsitutions of, e.g. “Q” for “E”, etc, for the most common letters didn’t reveal anything, so I took a step back an just stared at the ciphertext a bit!
Stage 1 A couple of things stood out to me about the ciphertext:
The grouping of letters into 5’s seems to suggest either some “modulo 5” operation in the encryption, or more likely it’s just a presentation/obfuscation technique to obscure the word length in the plaintext.
Given the frequency analysis suggested something simple, I assumed the latter and I wondered if the extraordinary preponderance of the letter Q in the text actually signified a space character, all the more likely since it seems to crop up at very frequent intervals in the ciphertext that simply couldn’t match to a regular letter.
If you ignore the “original” spaces in the ciphertext there are 143 characters which is 11*13, and if you arrange the characters in just such a grid you can pretty much just read out the plaintext:
AWVLIQIQVTQOS
QOELGCVIIQWDL
CUQEEOENNWWOA
OLTDNUQTGAWTS
MDOQTLAOQSDCH
PQQIQDQQTQOOT
UDBNIQHBHHTDU
TEETFDUEAUMOR
ESQEQEMLTMETI
RECLICAIQATUN
QRALTENEINRKG
replacing Q’s with spaces makes it more obvious:
AWVLI I VT OS
OELGCVII WDL
CU EEOENNWWOA
OLTDNU TGAWTS
MDO TLAO SDCH
P I D T OOT
UDBNI HBHHTDU
TEETFDUEAUMOR
ES E EMLTMETI
RECLICAI ATUN
RALTENEINRKG
Reading down the columns: “A COMPUTER WOULD DESERVE TO BE CALLED INTELLIGENT IF IT COULD DECEIVE A HUMAN INTO BELIEVING THAT IT WAS HUMAN WWWDOTMETRODOTCODOTUKSLASHTURING”
Or slightly more easily: “A COMPUTER WOULD DESERVE TO BE CALLED INTELLIGENT IF IT COULD DECEIVE A HUMAN INTO BELIEVING THAT IT WAS HUMAN WWW.METRO.CO.UK/TURING”
Following the link takes me to stage 2