National Cipher Challenge 1A

The National Cipher Challenge is an annual event organised by the University of Southampton.

Cipher Challenge Logo

This is the second year teams from our school have had a go at the competition. We have a keen but young group of pupils and I hope that by exploring how the ciphers could have been solved after the answers have been published, we will have a better prepared team ready to tackle next year's challenges.

Also, I thought this blog would be a good place to consider some of the educational benefits of tackling competitions like this. So here is how I solved Challenge 1A. I'll follow up in the next day or so with a consideration of the computing and maths skills that could be developed through cipher cracking activities.

The first challenge was pretty straightforward. The cipher text given was:

QEVO,

XLEROW JSV FVMRKMRK QI MR SR XLMW SRI, WIIQW PMOI E JEWGMREXMRK GEWI.

M LEZI XLVII UYIWXMSRW:
ALC ASYPH XLI JPEK HEC EWWSGMEXIW AERX E WLMT?
ALC ASYPH XLIC AERX XLMW WLMT?
ALC ASYPH XLIC AERX XLMW WLMT RSA?

LEZMRK VIEH XLI EXXEGLIH HSGYQIRX M WYWTIGX XLEX XLI ERWAIVW EVI EPP 
VIPEXIH XS XLI UYIWXMSR SJ ALEX IBEGXPC WLI ERH LIV JPEK HEC EWWSGMEXI 
GVIA AIVI XVCMRK XS WYVZIC.

M EQ KYIWWMRK XLEX CSY EPVIEHC GLIGOIH SYX XLI SRFSEVH KTW WCWXIQ JSV 
MRJSVQEXMSR EFSYX LIV QSZIQIRXW, FYX MJ CSY HMH JMRH ERCXLMRK M ASYPH 
FI JEWGMREXIH XS LIEV EFSYX MX. MR XLI QIERXMQI M EQ TVIXXC WYVI XLEX 
CSY ORSA QSVI EFSYX XLI JPEK HEC EWWSGMEXIW XLER CSY LEZI XSPH QI, WS 
E FVMIJMRK ASYPH FI QYGL ETTVIGMEXIH.

EPP XLI FIWX,

LEVVC

The passage looked like a letter between Harry and Mark (two people named in the introductory text related to the challenge) so an obvious crib presents itself if you guess that QEVO at the start of the cipher text is Mark and that LEVVC at the end is Harry. This suggests the following substitutions:

    

            
    
A
         
    
B
       
    
C
       
    
D
         
    
E
        
    
F
        
    
G
        
    
H
        
    
I
        
    
J
        
    
K
        
    
L
       
    
M
        
    
N
        
    
O
        
    
P
        
    
Q
        
    
R
        
    
S
        
    
T
        
    
U
        
    
V
        
    
W
        
    
X
       
    
Y
       
    
Z
      
  
        
    
.
        
    
.
        
    
y
       
    
.
        
    
a
        
    
.
        
    
.
       
    
.
         
    
.
        
    
.
        
    
.
        
    
h
        
    
.
        
    
.
        
    
k
        
    
.
        
    
m
        
    
.
        
    
.
        
    
.
        
    
.
        
    
r
        
    
.
        
    
.
        
    
.
        
    
.
     
  

(Note: from now on I will adopt the convention of showing Cipher text in capital letters and plain text in lowercase, even to the extent of showing proper names like "Harry" as "harry".)

Substituting these letters into the cipher text gives you:

mark,XhaRkW JSr FrMRKMRK mI MR SR XhMW SRI, WIImW PMkI a JaWGMRaXMRK 
GaWI. M haZI XhrII UYIWXMSRW: Ahy ASYPH XhI JPaK Hay aWWSGMaXIW AaRX 
a WhMT? Ahy ASYPH XhIy AaRX XhMW WhMT? Ahy ASYPH XhIy AaRX XhMW WhMT 
RSA? haZMRK rIaH XhI aXXaGhIH HSGYmIRX M WYWTIGX XhaX XhI aRWAIrW arI 
aPP rIPaXIH XS XhI UYIWXMSR SJ AhaX IBaGXPy WhI aRH hIr JPaK Hay 
aWWSGMaXI GrIA AIrI XryMRK XS WYrZIy. M am KYIWWMRK XhaX ySY aPrIaHy 
GhIGkIH SYX XhI SRFSarH KTW WyWXIm JSr MRJSrmaXMSR aFSYX hIr mSZImIRXW, 
FYX MJ ySY HMH JMRH aRyXhMRK M ASYPH FI JaWGMRaXIH XS hIar aFSYX MX. MR 
XhI mIaRXMmI M am TrIXXy WYrI XhaX ySY kRSA mSrI aFSYX XhI JPaK Hay 
aWWSGMaXIW XhaR ySY haZI XSPH mI, WS a FrMIJMRK ASYPH FI mYGh 
aTTrIGMaXIH. aPP XhI FIWX, harry

(Note: to make it easier to process in the spreadsheet I used to help me crack this cipher, I removed the extra lines, effectively turning it into a single paragraph, but otherwise left the punctuation and spacing intact.)

You now have a choice to make:

1. You can look at the partially deciphered text to see if any further substitutions suggest themselves. For example, the three letter grouping XhI appears many times in the text. Since "the" is a very common three letter word, there is a good chance that X=t and I=e. Similarly, the letters Ahy appear three times, which means A is probably w. Substitute these letters and then look at the text again. Keep looking for recognisable words and guessing letters until you have decoded the whole message.
    
2. In this case, the easier option is to guess what kind of cipher was used and see if your guess is right by trying it out on the cipher text. Since we have guessed that E=a, we could further guess that Mark and Harry are using a Caesar cipher where plain text letters are moved forward four places to get the cipher text letter. So a goes to E (b to c to d to E - four places). Use your code wheel, set A to E and check if the other letters we have chosen match our guesses. You should see that the guesses match all round the wheel. That is r goes to V (s to t to u to V - four places) and y goes to C (z to a to b to C) etc.

What ever method you choose, you should be able to decipher the rest of the message and read:

mark,
 
thanks for bringing me in on this one, seems like a fascinating case.
 
i have three questions:
why would the flag day associates want a ship?
why would they want this ship?
why would they want this ship now?
 
having read the attached document i suspect that the answers are all 
related to the question of what exactly she and her flag day associate 
crew were trying to survey.
 
i am guessing that you already checked out the onboard gps system for 
information about her movements, but if you did find anything i would be 
fascinated to hear about it. in the meantime i am pretty sure that you 
know more about the flag day associates than you have told me, so a 
briefing would be much appreciated.
 
all the best,
 
harry

So, Challenge 1A solved. Before going on to solve Challenge 1B, the next post will talk about the spreadsheets I developed to help crack the ciphers.