5
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So here's a little puzzle for all of the IT crowd out there.

It demonstrates the Vigenere cipher, the unexpectedly strong 'chiffre indéchiffrable'. The Wikipedia description explains how you can use the tabula recta to decode it.

Each row starts with a key letter. The remainder of the row holds the letters A to Z (in shifted order). Although there are 26 key rows shown, you will only use as many keys (different alphabets) as there are unique letters in the key string, here just 5 keys, {L, E, M, O, N}. For successive letters of the message, we are going to take successive letters of the key string, and encipher each message letter using its corresponding key row. Choose the next letter of the key, go along that row to find the column heading that matches the message character; the letter at the intersection of [key-row, msg-col] is the enciphered letter.

If you can't be bothered to do it by hand then there's a handy converter here at cryptii.com, which can handle a bunch of common ciphers.

Your ciphertext is:

WEMGO ZJLD EGH XJCD ELW

Let the countdown begin!

  • The solution is a short series of English dictionary words.

  • The key length is 9 characters.


OK, well evidently one of the things to watch for when setting this type of puzzle is that if you don't specify enough details of the plaintext or the key then the plaintext can be anything which fits the word boundaries. ;) Thank you Sp3000 for pointing this out in such an entertaining way.

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    $\begingroup$ How long is the key? What if the key length was as long as the message? $\endgroup$ – Sp3000 Nov 19 '14 at 12:24
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    $\begingroup$ In that case I'm going to have fun picking a key :D $\endgroup$ – Sp3000 Nov 19 '14 at 12:46
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    $\begingroup$ Is the key a "common" word too? $\endgroup$ – Andrea Gottardi Nov 19 '14 at 14:05
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    $\begingroup$ I think I'll have to stop for today - a key length of 9 is actually quite long, since each letter of the key only affects 2/3 letters in the cipher text (making frequency counting a lot less useful)... $\endgroup$ – Sp3000 Nov 19 '14 at 14:06
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    $\begingroup$ @AE I wrote one, but the search space is big :( $\endgroup$ – Sp3000 Nov 19 '14 at 14:11
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Well, I AM a giddy goat.

As a fan of

The IT Crowd,

I knew there was some connection but couldn't flipping put it together, until I realized

Let the countdown begin!

is a reference to

The Final Countdown (Season 4 Episode 2), in which (as Wikipedia describes it) "Moss stuns everyone by declaring that the 9 letter string TNETENNBA is in fact a word."

Obviously, that makes the key:

TNETENNBA

and the deciphered text:

DRINK MILK AND KICK ASS (from a Moss quote from the same episode: "I came here to drink milk and kick ass. And I've just finished my milk.")

I regret not asking if the 9-letter key could be used in a sentence.

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  • $\begingroup$ HOORAY! Well done! :) $\endgroup$ – A E Nov 20 '14 at 17:10
  • $\begingroup$ (I checked that it was also possible to get that by GoogleFu, one didn't have to be a fan). $\endgroup$ – A E Nov 20 '14 at 17:17
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This was posted before A.E. specified the key length, just to highlight what happens if you don't :P

The near-pangram "key" is:

qtmowadvxsrcgupfjhznbly

And the "message", revealing what we should do if the Earth ever meets its impending doom, is...

GLASS WOOL CAN SAVE DAY

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    $\begingroup$ Argh! Well that is definitely a solution! :) Perhaps the correct answer should give all the possible solutions? ;) $\endgroup$ – A E Nov 19 '14 at 13:11
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    $\begingroup$ I've specified the key length - 9 characters. ;) $\endgroup$ – A E Nov 19 '14 at 13:13

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