[Harry]

You need to use the Babbage-Kasiski method to work out the likely length of the keyword. In this case the answer is in the headline of this article:
https://edition.cnn.com/2018/11/25/opinions/nasa-insight-mars-landing-don-lincoln/index.html

(Trying to avoid spoilers if you want to work it out for yourself!)

That tells you the repeating pattern. If the answer is n then every nth letter is encrypted using the same alphabet so you can do frequency analysis on them to see what substitutions may have been made. Since the word structure is given this might help with finding cribs.

Does that help?

Hope so.

Happy New Year,

Harry

[Harry]

OK, here goes:

The frequencies of individual letters are too flat to be a mono-alphabetic cipher, and that also rules out a transposition cipher too.

Jodie hints that it might be a Playfair cipher or some sort of Vigenere. These are two different types of cipher that both produce flattened frequencies for individual letters and the best way to see which of these is likely is to try an index of coincidence attack. That should show you that it looks a lot like a Vigenere cipher with period a multiple of 7.

Unfortunately it does not appear to be a standard Vigenere, as trying the standard tabula recta and frequency analysis on every 7th letter does not crack the cipher.

That is because this is a keyed Vigenere cipher! Over to you for the rest of the solution!

[Harry]

I’ve spent about 4 solid hours working on this along with my team, we got A, but B is proving to be a dead end.

[EDIT, Harry: nothing dead about it! There is a reason we give you four weeks to do the last challenge! Keep at it.]

• This reply was modified 5 years, 4 months ago by Harry.
• This reply was modified 5 years, 4 months ago by Harry.

[Harry]

Sorry about this, some were postponed for timing issues, others got delayed while the elves wrapped presents. This one at least, got dealt with!

[Harry]

Really sorry, I can’t give much help with this at this stage, but you could try what you have learned about a standard columnar transposition cipher. We wrote about this in the codebreaking guide. The thing that makes it hard is to work out the length of the keyword, which you can do by looking for longer words you might expect to see!

Good luck,

Harry

[Harry]

We have posted the deadlines for Challenge 9 together with posting Challenge 8, so that should help with planning for next week.

Note that the first deadline will be midnight Thursday, rather than midnight Friday for that one.
We are likely to shorten them again for Challenge 10, but that will depend on how everyone is getting on.

Good luck!

Harry

[Harry]

All I can really say is that Jodie does her best to describe the sort of cipher used. It is very similar to Challenge 6B in spirit. But it is made harder by the way the manipulated text is read off. It is described in 7A, so I hope that will help. It might be a good idea to try to encrypt a short message using that recipe to see how it looks in practice.

Good luck, Harry

[Harry]

If people are really stuck on 6B try something a little different. The frequencies show you that the letters appear in the same proportion as they would in English so maybe the text has been jumbled using some form of transposition cipher, rather than a substitution cipher this time.

[Harry]

Yes!

[Harry]

There will be! Just give us a little time to make sure the merging is working. Harry

[Harry]

OK< will post a hint on Twitter in a minute. Take a look for me there! Good luck Harry

[Harry]

Just to confirm as our team joined late, the points for section B don’t hold value?

[EDIT, water_biscuit: For this challenge, and the next two which are also “Practice Challenges”, they have no impact on the final results. For the Competition Challenges (ie, 4-10) they will hold value, but the latest possible 100% solution is still better than a super quick 99.9% solution.]

• This reply was modified 5 years, 6 months ago by .
• This reply was modified 5 years, 6 months ago by .

[Harry]

Well, MrMintman was the 1,000 team marking around 2,000 registrations so far! It has been a big week for us, what with that and a retweet by Anthony Horowitz! Anyone got a favourite Alex Rider from their youth? He tells me he is writing a sequel!

Maybe someone should set up a poll!

Have a good weekend,

Harry

[Harry]

Yes, nationality doesn’t matter, you just have to be 18 or under in full time education in the UK. Welcome to the competition,

Harry

• This reply was modified 5 years, 6 months ago by Harry.

[Harry]

This is the old enemy of designers everywhere, the browser cache (or, if you are using a local proxy server, some local cache). We have cleared the caches in our server so those images have not been served recently and are a hold over in the ether. Could you try clearing your browser cache and get back to me? Thanks,

Harry

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