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Section E.3 Vigenère Analysis

We start by looking through this cipher text for repeated strings and counting the spaces between them:

PRMOUL FMJOZX JOLDLN KRHMFB QIEQOE WDKIJC KOGXRM 

ZXDLCI LNTRDR XORFYS VBIYWH VBLNCK ZIKMRU CNEEUY 

QESOFA LCHTYO UENKVV VBBLZD WLVLXS ZXHSJK WAEIWI 

DODNUZ UATDLC RVOYEY QERDDL CLHFFB HTYOHV VXLNXW 

UVVBOO TMDRVN EUKVLT KVHASY XTYSVO JDHNJS ELVLXS 

ZXHSJK QDDYUE FFHRYS VWZPHW RCLNTR DRXORF YSVBIY 

WHVBLN CKZTYO VHFZZA JCPACV DNUCRW RCWHVR RUJOLT 

NKVOEO RFKRRS VQUIDI ERZMNH FEVEJG KITRHX ZCWEUS 

QLRBJE HEDNKS WIVCEE WYUEKR HEIKRF IOFOEC WRLMWI 

FXGANX HDLZRN CYQDFX WHVCKO GGDSRC TURBHB FHRFRZ 

OATOZI KRWHVP UOEDJL RJHDZX VMRVOP RXHSZX WHVNDY 

KSPEKR HDFYUR VWDIEO GCCYVE USQTYO HVVXLN XSWSKY 

RDUSVC IOHTCI EUKCXS GSFIFE VLPKMA I

There are a lot of different repeated strings at different spacings, but the most common divisor for the spacings is 4. Thus, 4 is likely the length of the key.

  • LN: 32, 20, 96, 72, 20
  • TYO: 64, 104, 228
  • VB: 6, 36, 68, 78, 6
  • JO: 4
  • VO: 100
  • YQE: 60
  • LC: 45, 46, 13
  • DLC: 13, 91, 104
  • LVLXSZXHSJK: 96
  • FX: 16
  • LNTRDRXORFYSVBIYWHVBLNCKZ: 188

Dividing the cipher text into four subsets and finding frequencies for each we get:

The length of this subset is 134

Character Count Percent
H 20 14.93
W 14 10.45
V 12 8.955
R 12 8.955
D 11 8.209
L 10 7.463

And, the index of coincidence for this set is 0.06973

The length of this subset is 134

Character Count Percent
E 14 10.45
N 12 8.955
I 11 8.209
S 9 6.716
O 9 6.716
L 9 6.716

And, the index of coincidence for this set is 0.05558

The length of this subset is 134

Character Count Percent
V 16 11.94
K 12 8.955
R 11 8.209
F 11 8.209
Z 10 7.463
C 9 6.716

And, the index of coincidence for this set is 0.05814

The length of this subset is 133

Character Count Percent
O 15 11.28
X 13 9.774
S 12 9.023
C 12 9.023
Y 11 8.271
R 9 6.767

And, the index of coincidence for this set is 0.0597

Now looking in Kasiski's Key Table Figure 4.3.2 for the columns labeled with the most common characters in each subset we get a likely key of “DARK.” Testing this we get this deciphered message:

MRVERL OCGOIN GOUTIN THEMOR NINGLE FTHISS HOPNOM 

INALLY INCHAR GEOFHI SBROTH ERINLA WITCOU LDBEDO 

NEBECA USETHE REWASV ERYLIT TLEBUS INESSA TANYTI 

MEANDP RACTIC ALLYNO NEATAL LBEFOR ETHEEV ENINGM 

RVERLO CCARED BUTLIT TLEABO UTHISO STENSI BLEBUS 

INESSA NDMORE OVERHI SWIFEW ASINCH ARGEOF HISBRO 

THERIN LAWTHE SHOPWA SSMALL ANDSOW ASTHEH OUSEIT 

WASONE OFTHOS EGRIMY BRICKH OUSESW HICHEX ISTEDI 

NLARGE QUANTI TIESBE FORETH EERAOF RECONS TRUCTI 

ONDAWN EDUPON LONDON THESHO PWASAS QUAREB OXOFAP 

LACEWI THTHEF RONTGL AZEDIN SMALLP ANESIN THEDAY 

TIMETH EDOORR EMAINE DCLOSE DINTHE EVENIN GITSTO 

ODDISC REETLY BUTSUS PICIOU SLYAJA R

Once we clean this up and do a quick search for the text online we find this passage:

“Mr Verloc, going out in the morning, left his shop nominally in charge of his brother-in-law. It could be done, because there was very little business at any time, and practically none at all before the evening. Mr Verloc cared but little about his ostensible business. And, moreover, his wife was in charge of his brother-in-law.”

“The shop was small, and so was the house. It was one of those grimy brick houses which existed in large quantities before the era of reconstruction dawned upon London. The shop was a square box of a place, with the front glazed in small panes. In the daytime the door remained closed; in the evening it stood discreetly but suspiciously ajar.” - The Secret Agent: A Simple Tale by Joseph Conrad

Finally, here is the sort of hand scrap work you would expect to do.

Figure E.3.1. Sample Vigenere Analysis Page 1:
Figure E.3.2. Sample Vigenere Analysis Page 2: