Crypto challenge: from Cipher Deavours college course, 1976

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Crypto challenge: from Cipher Deavours college course, 1976
Topic Started: Jul 11 2009, 07:16 PM (138 Views)
mosher	Jul 11 2009, 07:16 PM Post #1
Member Posts: 41 Group: Moderators Member #3,268 Joined: May 26, 2009	In 1976 I corresponded with Professor Cipher Deavours about a bimonthly cryptologic journal he was trying to put together called "Polygraphia". It never materialized, transforming instead into the journal "Cryptologia". In that letter he sent me some assignment problems he had given his students in his Kean College course entitled "Cryptography and Cryptanalysis". Professor Deavours is definitely an original and effective educator (as evidenced by his technical articles) and his assignments have a nice real-life feel to them. Try your hand at one such assignment. Quote: Several months ago, the FCC [Federal Communications Commission] began intercepting clandestine encoded messages beamed from off the coast of a small Southern town. Not many of the messages could be solved due to their short length. Of the ones solved, all turned out to be polyalphabetic and periodic. All of the alphabets were keyword mixed using the same keyword but the alphabet keyword and the cipher key appear to be incoherent and seem to change irregularly. The messages solved all related to an illegal drug delivery which was to be made soon from an offshore ship. The last message was intercepted on 10 March and is thought to give the final delivery date and place. The message seems too short to solve by the usual methods. See if you can solve it. FINAL TRANSMISSION: FPTBI UGDGX POFDV MXSUF MTGQK UJFGM MGGPV HXLNB MPUVS GQENW TIME DAY DATE ALPHABET KEY CIPHER KEY 0130 Thursday 19 Feb. CTLSPAIYNR OJOFUFFOUI 1230 Monday 23 Feb. CRYPTANLSI UXFOUZUIJSE 1930 Saturday 28 Feb. CNSAITPYLR UXFOUZFJHIUI 2300 Monday 1 March CRYPTANLSI GJSTU 0340 Tuesday 9 March CYTNLSRPAI OJOFUI The challenge here is to find the overall patterns and, using that knowledge, solve the last message. Good luck! Moshe



mosher	Jul 11 2009, 09:28 PM Post #2
Member Posts: 41 Group: Moderators Member #3,268 Joined: May 26, 2009	The original assignment read "1930 / Saturday / 28 Feb. / CNSAITPYLR / UXFOUZFJHIUI" as listed in the previous posting. I believe this is a typo and should read "1930 / Saturday / 28 Feb. / CNSAITRLYP / UXFOUZFJHIUI". Moshe



tenpanaris	Nov 7 2009, 09:11 AM Post #3
Just registered Posts: 1 Group: Members Member #3,299 Joined: November 3, 2009	I don't think there is a typo at all. Here is how I calculate the Alphabet keys: X = 10, ! = skip (repetition) MON - STEP 1 CRYPTANALYSIS 1234567!8!9X TUE - STEP 2 CRYPTANALYSIS 1728394 5 6X! WED - STEP 3 CRYPTANALYSIS 19!2X63 74 85 THU - STEP 4 CRYPTANALYSIS 1 852 963 !74 FRI - STEP 5 CRYPTANALYSIS 1649527!X!38! SAT - STEP 6 CRYPTANALYSIS 1X!7642 98!53 MONDAY CRYPTANLSI TUESDAY CYTNLSRPAI WEDNESDAY CPNYSALIRT THURSDAY CTLSPAIYNR FRIDAY CASYTRNIPL SATURDAY CNSAITPYLR Cipher key is just a plain mono substitution on the written number of the day. Eg, 19th = NINETEENTH = OJOFUFFOUI There are two slightly odd things about this. One is the minor typo in the CT. The other is the fact that the second keyword is TENTH. Either the Prof screwed it up, or the reason the FCC couldn't decrpyt this message is because the criminals made the mistake and encrypted with the plaintext keyword TENTH, forgetting to apply the monosubstitution they had applied to the other cipher keys. This actually threw me off for ages, until I let Cryptool 1.4 have a go at finding the Vignere key, and it came up with TENTV, which is surprisingly good considering how short the CT is.



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