The Defence Signals Directorate (Australia’s SIGINT and INFOSEC Agency) has a puzzle corner on its website. This little code-breaking one has me stumped :mad:
Authorities intercepted the message LJPPV KOUYK OIRWQ HKIQC DPAKB RXHJI, believed transmitted by a gang of smugglers. This was decrypted to: “Password for next month is Bogeyman”. About a month later the message KUVMF PPVLO RVDII EUPUK QLKQS UPRFX was intercepted. This was decrypted to: “New password will be sent on Tuesday”. The following Tuesday the single word EFGMRIHQ was intercepted.
The change is the key. You need to work out the passwords for the first two messages. Knowing these will suggest an obvious next password. Checking this paaword for the third message should confirm your guess.
Well, I was hoping the password for the second message was actually “bogeyman” but I can’t make it fit yet. Don’t have too much time to spend on it just now, but I’ll look later.
This is the complete solution, so don’t call me a spoilsport if you click on it:
Natural alphabet: ABCDEFGHIJKLMNOPQRSTUVWXYZ
First code: JA URY CD HIKL OPQ V X
Second Code: FE RU D I KLM OPQS V X
We can tentatively assume that the cipher consists of a key word
followed by the remaining letters of the alphabet in proper order;
the consecutive keys appear to be 'JANUARY' and 'FEBRUARY',
shortened to 'JANURY' and 'FEBRUAY' (as no letter can be repeated in the key)
The next cipher should therefore be:
Natural alphabet: ABCDEFGHIJKLMNOPQRSTUVWXYZ
Third code: MARCHBDEFGIJKLNOPQSTUVWXYZ
The coded word EFGMRIHQ decodes thus:
E>H
F>I
G>J
M>A
R>C
I>K
H>E
Q>R
spoilsport! hehehe. No, acually thanks very much to Jabba and Mangetout .
More important than getting the correct answer I would love to have an action replay of the logical mental steps you took to solve the problem. Did the key words just jump out at you after you did the letter substitutions?
first:
noted that the code is the same length as the message
noticed the double-letter sequences in the code matched by double-letter sequences in the same positions in the message
made a quick substitution grid on paper
noticed that the substituted letters were pretty much still in sequence, apart from this chunk at the beginning
then I noticed some of the gaps in the coded sequence (for example, HIKL OPQ - the J is missing because it is elsewhere and the single space in the middle is too small to accomodate both M and N
so maybe one or the other of those was elsewhere too… then I noticed that if it was the N, it almost spells JANUARY
then I realised that the best you can do with single-use letters is JANURY, which would be spot on.
then I started on the second message and it was very obvious that the key word was FEBRUARY and it was all downhill from there.
