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

Frequency analysisA more methodical attack is frequency analysis. One counts the number ofoccurrences of each character in the cipher text and compares it with an expectedfrequency for the standard English alphabet. In the cipher text above a character countgives us the following table of occurrences:a b c d e f g h i j k l m7 0 12 0 26 3 0 32 6 9 11 20 18n o p q r s t u v w x y z16 5 55 0 14 17 2 17 35 4 4 11 4Compare this to a table of expected frequencies, taken from Simon Singh’s “TheCode Book”:a 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 0.1Using this and information about common one, two and three letter words we haveenough to begin to tackle the cipher.Disguising the word structureThe chink in the armour of our ciphers so far has been the preservation of wordstructure. This allows one to spot common words. In order to avoid such weaknesscryptographers usually remove punctuation and block the characters together ingroups of four or five, so our previous cipher text looks likeVEPHY XHLVH TPMOA WFJYF LTHRF NEPSH JNEHA PVFLV EFUZHCFUVE HVFVF UPHUC VMKPK MSFUP VEPIP CZMSY MSIPC NESHUPHLYE PLRPV EPRFN EPSHJ NEHAP VVEFU FUFKN MSVHL VAPRHWUPFO VEPUP LYPSE HUVMI PPNVE PRFNE PSHJN EHAPV MLHNFPRPMO NHNPS VEPPL PKCRH LRHNV WSPVE PNHNP SYFUR MXPSVEPIPC HLYSP HYHLC RMKKW LFRHV FMLUV EHVEH XPAPP LPLRSCNVPY ZFVEF VEMZP XPSFO VEPIP CRHLA PRMKK FVVPY VMKPKMSCFV FUJPU UJFIP JCVMO HJJFL VMPLP KCEHL YUUsually the length of the text groups doesn’t matter, however, in analysing a Vigenèrecipher (see below) a carelessly chosen block length may make the length of thekeyword more apparent, since it can reveal the repetitions more easily.13