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November 27, 2005
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Posted by Harry at November 27, 2005 07:01 PM
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Harry, why dont you ever give us a hint about what the KEYWORD is, it is so mean of you not to tell us.
Slight subtle hint there for part A, got it at last, avit
[Sorry about the outbreak of editing, but .... Harry]
Posted by: Hugh Bird at November 29, 2005 01:45 PM
I may be very thick BUT In deciphering the morse code I failed to find any equations or even a single number. Does that mean my translation of the morse is wrong? or are you talking equations to fool the rest of us?
[I'm not sure how to answer this! The point is that the text you see after decoding the Morse has been encrypred using a Hill cipher. This is like a high powered version of a Caesar shift cipher which encrypts (in this case) triples of letters as triples. You can decrypt it by solving some equations you get from cribs (the "known plaintext attack"). Hope that helps. Harry
Posted by: richard at November 28, 2005 10:06 PM
like how to find the Keyword for it please
[Have you tried using Babbage's method? Simon Singh describes it in his book. Harry]
Posted by: freaks at November 28, 2005 08:51 PM
can you please give another hint for cipher 7a
[What sort of hint do you want? Harry]
Posted by: freaks at November 28, 2005 08:25 PM
Yes, there may be 26^10 combinations, but with the clues we would gain from 8A I'm sure that would be cut down to much less (a feasible number to search through the possible ones). A Fialka is just like any other mechanical machine - it follows specific instructions, one after the other, in a predictable way. Creating a program to encrypt a message is particularly easy, granted however that some of the basic information is available. I think I shall make a Fialka program anyway, just to test myself - decryption seems like a hard challenge to overcome, but it won't be impossible.
Posted by: Richard at November 28, 2005 06:42 PM
i think when harry said it wouldnt be a fialka then he meant it would be a simplified fialka.
also, you cant brute force a fiakla. try working out how many combinations there are ;) i think you'll find it'll take a supercomputer to brute force it, and that would take a few hundred/thousand years.
thats why its 8b. it'll take strategy, and a lot of logical thinking. it wont be a case of whose computer can cycle through the combinations fastest.
am i right harry :P
chidders
[I hope so, though I can't make any promises. Harry]
Posted by: chidders at November 28, 2005 04:29 PM
i cracked 7B using mod simultaneous equations.
i managed to reduce it to being only 16 possibles and then only 2 were actually feasibly possible. just remember to watch your Ps and Qs and it is not that difficult. Even to do by hand.
Posted by: miol at November 28, 2005 04:16 PM
Yes! We finally finished it and and we are so pleased with ourselves. I probably could have finished it earlier had this week not been my busiest week of the whole term (grrr). I wrote a program to brute-force the equations (all 17576 of them) to find solutions, then mixed and matched the possibilities for each different row of the matrix. I had more possibilities than I would have liked because I could only guess the first 8 letters of the plaintext, not 9, which would have given me a single solution straight away. Oh well.
About using a Fialka for part 8 - why not? Writing a program to emulate a fialka is extraordinarily easy (if you were having trouble working it out before you were told not to bother, the way I would think about creating a program would be to define *specific* functions that would happen, eg a keystroke) and to brute-force it would require a little more skill than the basics. What can we expect instead then Harry?
[I already told you what to expect in 7A ;) Harry]
Posted by: Richard at November 28, 2005 03:19 PM
Yes!! Finally got 7B, after spending almost all my weekend on it ;). The trick? A mix of brute force, luck, and intuition. Tips:
1. Don't go into modular simultaneous equations - it's a waste of time. You can brute force them.
2. There are many (832 - can I say it?) possible solutions to the equations - you need to find some way of distinguishing between them.
3. Brute force everything you can find - my computer can crack it from scratch given two cribs in about 2 minutes. indexOf in Javascript is very useful for this.
That's all folks!
Kati
Posted by: Katriel Cohn-Gordon at November 28, 2005 09:39 AM
Ahhhhhhhhhhhh! Darn autocorrect in Microsoft Office!! Make sure you turn it off. I thought that i'd finished it last night and got it all right but i just got the email and it said that one letter was wrong, one that the autocorrect changed! Ahhhhhhhhhhhhhhhhhhhh!
[/soapbox]
Posted by: sephtonj at November 28, 2005 08:01 AM
You could always bruteforce it and see which of the possible matrices work!! Only about 17,000 combinations to try ;-)
[If you wanted to do this you might use the index of coincidence to try and spot successes automatically. Anyone care to write a short description of the idea, or does that give away one of your key advantages? Harry]
Posted by: Josh at November 27, 2005 10:47 PM
Finally done it! - I've left the 'T's as they are and have not corrected it - we can't expect the operaters to be word perfect in english, considering it's not their native language!
Is there anyway of deciphering this cipher, without knowledge of a crib - I don't know about anyone else but I almost guessed the first sentence!
[Frequency analysis can still work, but not on individual characters. You need to do trigram analysis (etc.) instead and it means you need a much longer text than you would need for a substitution cipher. Harry]
Posted by: Stephen Harris at November 27, 2005 10:14 PM
harry, is it a **** cipher with a *******?
and i still do not really understand properly. any hints, anyone?
for those still stuck on 7A, it is really simple, easier than 6b. hint, hint.
[No nasty extra twists this week, just a single type of cipher. Harry]
Posted by: laura at November 27, 2005 09:44 PM
Harry. Just purely out of curiosity. Did you work out what values you wanted for the crib so that it worked or did you just chose a key grid and hope that the numbers came out nicely.
Posted by: Clever Code Cracker at November 27, 2005 08:51 PM
Are the errors some Ts that should be Rs?
[Like I said, the radio operators were under stress. Harry]
Posted by: Dave at November 27, 2005 08:42 PM
Spent so long trying to work out the damn inverse of matrices and impossible simultaneous equasions, got bored and decided to bring out the l33t programming skills to brute force it and had it done within 30 minutes :/
[Ho ho. Harry]
Posted by: Captain Cool at November 27, 2005 08:16 PM