Teachers' pack - National Cipher Challenge - University of ...

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OHP Slide 2 for lesson 2Occurrences tablea b c d e f g h i j k l mn o p q r s t u v w x y zExpected Frequency tablea b c d e f g h i j k l m8.2 1.5 2.8 4.3 12.7 2.2 2.0 6.1 7.0 0.2 0.8 4.0 2.4n o p q r s t u v w x y z6.7 7.5 1.9 0.1 6.0 6.3 9.1 2.8 1.0 2.4 0.2 2.0 0.1This table was taken from “The Code Book” by Simon Singh, and gives expectedfrequencies as a percentage. To accurately compare it to the actual frequencies aboveyou should compute the actual frequencies as percentages.Actual Frequencies as percentagesa b c d e f g h i j k l mn o p q r s t u v w x y z30 email:cipher@soton.ac.uk
University of Southampton National Cipher Challenge Teachers’ pack Handout for lesson 2STAGE 1 – P is the commonest letter in the ciphertext so could stand for e - maybethe first word is the:the HYXHLtHTe MO AWFJYFLT H RFNheS HJNhHAet FL thFU ZHC FU thHt FtFU eHUC tM KeKMSFUe the IeCZMSY MS IeCNhSHUe, HLY heLRe the RFNheSHJNhHAet. thFU FU FKNMStHLt, AeRHWUe FO the UeLYeS hHU tM IeeN theRFNheS HJNhHAet ML H NFeRe MO NHNeS, the eLeKC RHL RHNtWSe theNHNeS, YFURMXeS the IeC, HLY SeHY HLC RMKKWLFRHtFMLU thHt hHXeAeeL eLRSCNteY ZFth Ft. hMZeXeS FO the IeC RHL Ae RMKKFtteY tMKeKMSC Ft FU JeUU JFIeJC tM OHJJ FLtM eLeKC hHLYU.STAGE 2 We see the single letter word H, and the four letter word thHt in the firstline - guess that H encodes the letter a.the aYXaLtaTe MO AWFJYFLT a RFNheS aJNhaAet FL thFU ZaC FU that Ft FUeaUC tM KeKMSFUe the IeCZMSY MS IeCNhSaUe, aLY heLRe the RFNheSaJNhaAet. thFU FU FKNMStaLt, AeRaWUe FO the UeLYeS haU tM IeeN theRFNheS aJNhaAet ML a NFeRe MO NaNeS, the eLeKC RaL RaNtWSe the NaNeS,YFURMXeS the IeC, aLY SeaY aLC RMKKWLFRatFMLU that haXe AeeLeLRSCNteY ZFth Ft. hMZeXeS FO the IeC RaL Ae RMKKFtteY tM KeKMSC FtFU JeUU JFIeJC tM OaJJ FLtM eLeKC haLYU.STAGE 3 The two 2 letter words Ft FU are probably it is meaning that F encodes iand U encodes s:the aYXaLtaTe MO AWiJYiLT a RiNheS aJNhaAet iL this ZaC is that it is easC tMKeKMSise the IeCZMSY MS IeCNhSase, aLY heLRe the RiNheS aJNhaAet. this isiKNMStaLt, AeRaWse iO the seLYeS has tM IeeN the RiNheS aJNhaAet ML aNieRe MO NaNeS, the eLeKC RaL RaNtWSe teh NaNeS, YisRMXeS the IeC, aLYSeaY aLC RMKKWLiRatiMLs that haXe AeeL eLRSCNteY Zith it. hMZeXeS iOthe IeC RaL Ae RMKKitteY tM KeKMSC it is Jess JiIeJC tM OaJJ iLtM eLeKChaLYs.STAGE 4: haXe = have, easC = easy and so on - we get the following extract fromSimon Singh’s excellent history of codes and ciphers, The Code Book:“The advantage of building a cipher alphabet in this way is that it is easy to memorisethe keyword or keyphrase, and hence the cipher alphabet. This is important, becauseif the sender has to keep the cipher alphabet on a piece of paper, the enemy cancapture the paper, discover the key, and read any communications that have beenencrypted with it. However if the key can be committed to memory it is less likely tofall into enemy hands."
Page 1 and 2: email:cipher@soton.ac.uk
Page 3 and 4: Points are awarded for speed and ac
Page 5 and 6: Getting helpWe offer online feedbac
Page 7 and 8: Frequently Asked QuestionsWhat does
Page 9 and 10: 9AZYXWVUTSRQPONMLKJIHGFEDCBAZYXWVUT
Page 11: Keyword substitution ciphersTo incr
Page 14 and 15: To attack cipher text that has been
Page 16 and 17: to 26. If we choose a carelessly (s
Page 18 and 19: On transposition ciphersSometimes w
Page 20 and 21: second columns have been transposed
Page 22 and 23: Handout for lesson 1.GROUP ACode: C
Page 24: Teachers’ solutions to encryption
Page 27 and 28: University of Southampton Nation
Page 29: University of Southampton Nation

Teachers' pack - National Cipher Challenge - University of ...

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