Known Plaintext Attack - Worked Example

Over the past few days we’ve been working on this puzzle:

VKMHG QFVMO IJOII OHNSN IZXSS CSZEA WWEXU
LIOZB AGEKQ UHRDH IKHWE OBNSQ RVIES LISYK
BIOVF IEWEO BQXIE UUIXK EKTUH NSZIB SWJIZ
BSKFK YWSXS EIDSQ INTBD RKOZD QELUM AAAEV
MIDMD GKJXR UKTUH TSBGI EQRVF XBAYG UBTCS
XTBDR SLYKW AFHMM TYCKU JHBWV TUHRQ XYHWM
IJBXS LSXUB BAYDI OFLPO XBULU OZAHE JOBDT
ATOUT GLPKO FHNSO KBHMW XKTWX SX

We established that the key was 6 letters long, and the first letter of the key was ‘W’

Is there some other way we can crack this?

There is always the known plaintext attack.

You may have noticed that I have a tendency to refer to the cipher itself in the message. So an astute observer might have tried assuming that the plaintext contained ‘BEAUFORT’, and slid this along the message in order to try and work out what the key is.

 Ciphertext: VKMHG QFVMO IJOII OHNSN...
     Groups: BEAUF ORT...
Implied Key: WOMBL EWO...

In the first position we find that a key of ‘WOMBLEWO’ is applied, or ‘WOMBLE’ repeating after six characters.

The message may be easily found from here!