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November 25, 2005
A new thread
The other one is getting crowded, so feel free to use this one.
Posted by Harry at November 25, 2005 10:38 AM
Comments
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Harry, you said it was not a playfair or diagraph cipher - are you therefore not ruling out a trigraph cipher or another polygraph - or am I barking up the wrong tree?
[Keep barking. Harry]
Posted by: Stephen Harris at November 26, 2005 05:17 PM
one again can we get a new thread....it debatable whether reading the comments, or cracking the cipher is harder....
[Sure. Harry]
Posted by: chloe at November 26, 2005 04:57 PM
Just out of interest... Do most people use programme to help?
Posted by: Jenny at November 26, 2005 04:49 PM
How on earth did you guys figure out what kind of cipher it was?
Posted by: nick at November 26, 2005 04:44 PM
HELppp.... Is 7a a vigenere cipher? I still don get it....
Posted by: Jenny at November 26, 2005 04:35 PM
i built a program for 7b which which would be given a cipher _____ and what the equivalent plain block is, and then another couple of them, and it would automatically decode the cipher with the matrix and show me what it was like. As a result, i got two thirds of the text straight away when I used my crib.
Then its quite easy to work out the rest by hand :)
However, anyone who would like to read about cryptanalysis for the cipher could certainly do with the book:
Elementary Cryptanalysis: A mathematical approach
Abraham Sinkov
It also has systematical methods to crack monoalphabetic ciphers, polyalphabetic ciphers, transposition ciphers and the polygraphic cipher that this one is.
I also used the books methods to break 7a
chidders
Posted by: chidders at November 26, 2005 03:52 PM
hang on, surely 7a means that if we crack 8a and have built a fialka encoder/decoder, then we immediately have 8b. This would mean that we dont have to cryptanalyse 8b. Is this right? surely that takes the fun out it :S
chidders
[Firstly, it won't be a Fialka, that would be way too hard. Secondly, even if we give you the settings of the rotor machine we don't have to give you all of them at once (last year we drip fed the initial pack ordering for the Solitaire cipher). The crib will allow you to try and anticipate the settings before we give them to you. Harry]
Posted by: chidders at November 26, 2005 03:46 PM
maybe, but what equations?????????!
I'm extremely stuck here!
[I can't say much more than I have at the moment. Maybe later. Harry]
Posted by: Neel at November 26, 2005 03:26 PM
How can we work out the order of the matrix?
[Do you mean the number of rows and columns? Think about the size of the ciphertext and see what I said about the crib. Harry]
Posted by: Ruth at November 26, 2005 03:16 PM
lol, the achievement is the main factor im doing it, but ive got loads of gnvq ict to do etc so im wondering whether ill have time to make a fialka cracker.
I feel very annoyed. I finally cracked 7b, and found that i would have cracked it on thursday is i had realised that:
A = 0
B = 1
etc
instead of:
A = 1
B= 2
etc
so anyone having dificulty should check that they are doing the cipher right lol
and harry...
how many people have cracked it so far?
chidders
[Before you make a Fialka cracker read 7A carefully. Harry]
Posted by: chidders at November 26, 2005 03:15 PM
finally realized what kind of cipher this is, but solving it,
ARRRRRRRRRRRRRRGGGGGGGGGGGGHHHHHHHHHHHHHH!
Even the simple example shown on Wikipedia doesn't make much sense to me!
please, Harry, some clues!
[It's just a matter of solving some equations. Harry]
Posted by: Neel at November 26, 2005 02:56 PM
Harry,
How do you deal with a matrix that has a determinant of 0 (mod 26)?
In all the cribs I've tried, the determinant divides into 2 or it isn't in the plaintext.
[How do you know the determinant? I thought I'd set this up so the crib isn't quite long enough to determine the whole matrix. Harry]
Posted by: Alex at November 26, 2005 02:56 PM
Harry, how many people have cracked it so far - just so i know if there is any point continuing.
Has anyone who cracks it by 4 got a chance of winning or getting second?
chidders
[Chidders, I'm surprised at you. Isn't it worth continuing just for the achievement? Harry]
Posted by: chidders at November 26, 2005 01:55 PM
Phew! Got it! No brute force required either.
If you want to know exactly what maths I think you need to be able to do, here are the skills:
Multiplication of two matrices. (Wikipedia is a really great site!)
Modular arithmetic (Best way to think of this is the hours of a clock: round and round...)
Simultaneous equations (more than 2 though)
To crack the message you also need:
Knowledge of the cipher (obviously)
A little bit of known plaintext (wonder what that could be)
Even then, there are still multiple options for the decryption key. Finding the right one is not too difficult though.
[This post is full of crypto-goodness, you should all take it seriously. Harry]
Posted by: Jason at November 26, 2005 01:43 PM
harry, the enclosed note, are all parts of it relevant or just the numbers? also can you solve the cipher if your bad at algebra?
[The enclosed note is a crib for Challenge 8B as it says in 7A. You probably need to work quite hard if you find algebra difficult. Harry]
Posted by: help at November 26, 2005 01:02 PM
does this clue have something to do with the word PLAINTEXT? (I doubt it)
[No. Harry]
Posted by: Neel at November 26, 2005 12:55 PM
To Clever Code Cracker, bear in mind that Challenge 7 will be possible. If you know that much, try EVERYTHING. That's what I did :P And actually there is a bit of maths if you want to do it by hand but it's nothing major. I found brute-forcing everything with a computer easier though :P
Posted by: Ian at November 26, 2005 12:27 PM
I've decrypted just over a third of 7B, with only a bit of algebra and a crib (which is just one letter too short to make things easy). If I wasn't playing in a ceilidh last night I would have done it all. On Thursday, I didn't quite realise what the cipher was. Oh well.
Posted by: Jason at November 26, 2005 11:53 AM
Harry, does the cipher require a keyword? If yes has it been in either cipher 7a or previous ciphers?
[This one doesn't. Harry]
Posted by: hi at November 26, 2005 11:38 AM
Wow Can you believe it, a team from a Primary School are soooooooo good that they are in joint first place on the leader board and here am I Year 10 and struggling. Potential candidates for Southampton University alread, are they not?
[Anyone still in the Challenge is a potential candidate. Harry]
Posted by: richard at November 26, 2005 11:19 AM
gr8!
got the rite number of letters now, but its a long climb uphill?
Posted by: Insane (FHS) at November 26, 2005 11:12 AM
Harry, can I post this?
Have a look at http://en.wikipedia.org/wiki/Index_of_coincidence for a very good description of the index of coincidence. Imagine if you dumped all the letters in the text into a big bag, and drew two - the index of coincidence is the chance of drawing two the same. It is higher for English (and for a monoalphabetic substitution cioher - 0.06) than for random text (or a polyalphabetic cipher - 0.04), so you can use it to check what type of cipher a text is encrypted with. Also, if you split a Vigénere into the right number of columns, each one will be monoalphabetic.
Hope that helps you all!
Kati
Posted by: Katriel Cohn-Gordon at November 26, 2005 06:23 AM
Are the 10 letters at the end some sort of key word for part B or am i barking up the wrong tree here.
Posted by: Alex at November 25, 2005 10:52 PM
OK. Now I'm certain what type of cipher this is.
To those who have solved it is it possible without maths knowledge and how do you do it with out knowing the lengths of the blocks.
Posted by: Clever Code Cracker at November 25, 2005 10:35 PM
how many letters should there be?
i have 1134 but sum1 sed they had 1098
[You forgot to delete carriage returns before decoding the Morse! Harry]
Posted by: Insane (FHS) at November 25, 2005 10:10 PM
Ohhh.... i am still in part a.... I still don understand what "incidence of coincidence" is after i looked it up. What can i do? What can i do?
[Use Babbage Kasiski instead. Harry]
Posted by: Jenny at November 25, 2005 09:53 PM
The clue isn't too cryptic. I didn't get "Latin Five" until I had solved the cipher AND Harry had explained it, but this one I worked out the meaning of the clue quickly (not that I have actually broken the code yet). Try en.wikipedia.org for different types of cipher.
Posted by: Ruth at November 25, 2005 09:19 PM
its not the alice and bob cipher is it? only making a random guess...
[Aren't they all? Harry]
Posted by: gg4 at November 25, 2005 08:57 PM
Gah, I knew I should have followed that clue. It's ridiculously annoying when you neglect the right answer looking for a better one. Good luck to everybody else.
Posted by: OcelotIIX at November 25, 2005 08:33 PM
ok so...
can u plz suggest where in 7A the clues r?
beginning middle or end areas? :S plz :S
[Maybe I'll say on Monday. Harry]
Posted by: Insane (FHS) at November 25, 2005 08:29 PM
Harry, is what we are supposed to be reading in part a as cryptic as the 'latin five' thing? because if it, then like Jess said, it does not really work as a clue, but as an amusing thing once you have cracked the cipher.
Posted by: lara at November 25, 2005 08:06 PM
WOOOOOOOOOOOOOOOOOOOOOOH! i did 7a. i finally did it! i'm so surprised and sooooooooooo happy!!!!!!!!!! ok, its not as amazing as b, but for me its amazing!
[See! That's why I prefer not to give too many clues. Harry]
Posted by: lara at November 25, 2005 07:35 PM
Nope, not anything to do with Playfair.
I personally had heard of the cipher, but never investigated it. But when I read 7A, I knew instantly what it was. So I suppose I did know. If you've read 7A and don't know what it is, then you need to look at words which could potentially be clues. Like when people say "Are you playing fair?", that sounds weird so is probably a clue. Stuff like that.
Posted by: Ian at November 25, 2005 07:27 PM
Well, it's evidently not a Playfair cipher as it begins with 0002000 - Playfairs can't have the same letter in a digraph. Is it too much to ask if it's a variant of Playfair? I'm being pushy about hints here, I know...
Kati
[Nice observation. It is not a digraph cipher if that is what you mean. Harry]
Posted by: Katriel Cohn-Gordon at November 25, 2005 07:13 PM
What is the index of coincidence, i dont undertsnad. Is the word ****** significant!
[It is a number you can compute which helps you see what the length of a Vigenere keyword is. You might find it useful for part A. Harry
Posted by: What! at November 25, 2005 06:58 PM
Harry, when i search for incidence of coincidence, all i get is a load of maths which is gibberish to me. I'M NOT THAT GOOD AT MATHS YET, I'M ONLY IN YEAR 9... or have i just made a horrible mistake????????
[Maybe this is a but tricky, but you don't have to use the index of coincidence to do part A if you don't like the look of it. Part 7A is easier than 6B. Harry]
Posted by: lara at November 25, 2005 06:39 PM
To thos few people who have cracked it:
Had you heard of the cipher before or do we need to find ciphers that we've never heard of.
Posted by: Clever Code Cracker at November 25, 2005 06:35 PM
I really think you need to play fair at this stage harry, and not hold back any clues. or are you playing fair and giving clues out steadily?
[OK. It's not a Playfair cipher. How's that for a clue. Harry]
Posted by: rob at November 25, 2005 06:35 PM
Harry, is this cipher one that has a characteristic freq. anal. graph.
[I don't think so, but I haven't thought about it. Harry]
Posted by: Clever Code Cracker at November 25, 2005 06:34 PM
I meant as in not really Curriculum Maths :P Of course it is Maths :P
Posted by: Ian at November 25, 2005 06:27 PM
Referring to 7A, is it a Vigenere? And what's the test for the keyword length for a Vigenere?
Posted by: Vicky Bird at November 25, 2005 06:10 PM
WOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO!!!!!!!!!!!!!! I ROCK!! MY PROGRAMS ROCK!!!! WOOOOOOOOOOOOOOOOOOOOOOOOOOOO!!!!!!!!! YEEEEEEEEEEEEEEEEEEEEEAH!!!!!!
aka I did 7B.
Little or no maths is required for the challenges. From a programming point of view, you only have to be able to spot and implement algorithms.
[Doh. Algorithms are maths! Harry]
Posted by: Ian at November 25, 2005 06:01 PM
Harry, do you know how many people solved it before 8 o'clock this morning? They would be... very clever. (stares in awe)
[Around five. Harry]
Posted by: Jess at November 25, 2005 05:41 PM
do 2 letters represent the most common letters in part a e.g. e,t etc
[Frequency analysis shows it isn't a monoalphabetic substitution. Have you looked up "Incidence of coincidence"? Harry]
Posted by: lara at November 25, 2005 05:34 PM
Katriel - did the number 5 have any significance in part A? - I know what type of cipher it is, its just the keyword length: i know what is should be, but it doesn't seem to work....
Posted by: Stephen Harris at November 25, 2005 05:22 PM
The number of times I've read part A now I could probably say it off by heart. Is this reading carefully meant to give us the type of cipher? Or a potential keyword. Also, start middle or end of it? Have I read something and it just hasn't ticked?
Posted by: tim at November 25, 2005 05:14 PM
I am still on Part A... What can i do?
Posted by: Jenny at November 25, 2005 05:05 PM
harry what levl of maths do you require to crack these ciphers? is this chalenge aimed at the very top level a- Level maths students or should you not be requiring any highly complex matracies to solve the challenge?
Posted by: a mathematician at November 25, 2005 04:53 PM
how carefully and whereabouts is this clue in 7a???
Posted by: Neel at November 25, 2005 04:47 PM
I agree with Tim, what on earth is this cipher supposed to be??????
Some hint (even a small one) would really help here!
[Read 7A carefully. Harry]
Posted by: Neel at November 25, 2005 04:40 PM
Hurrah! I've got a really bad program I've just written brute-forcing the asterisk asterisk, and it might actually be done in a few hours :P
Posted by: Ian at November 25, 2005 04:34 PM
I've read part A over and over and the trusty inspiration has failed me. I see little help! Are trigraphs the way to go? Or is that not playing fair?
Tim
Posted by: Tim at November 25, 2005 04:05 PM
lbow this is your team-mates! DO NOT COMMUNICATE!....I REPEAT DO NOT COMMUNICTAE!....instead eat cake.
Harry, i think it was easier last year. How come its got harder?
[It's just different. It is slightly harder to work out which ciphers have been used this time, but last year the Challenges were mostly done within a day or so of being released, so I decided to make you work a little harded and do more research on different types of cipher this year. Everything we set is doable, just not immediately doable. Harry]
Posted by: eep at November 25, 2005 02:47 PM
Hi all -
Just figured out 7A finally - woo! As a general hint, (hope this is subtle enough) have a look at the index of coincidence of the message and see if that tells you anything. There are some other tests you could try too.
Kat
[Good hint. If you don't know what the index of coincidence is you should look it up. It is very easy and incredibly useful. Harry]
Posted by: Katriel Cohn-Gordon at November 25, 2005 02:33 PM
What's with all the asterisks in my comment?!
[Too big a hint for today. Harry]
Posted by: lbow at November 25, 2005 02:28 PM
Hmmm. Following up an idea I had, I got "Kick me" as the first six letters of the plaintext followed by gibberish. Somehow I guess that's not right.
Posted by: Danny White at November 25, 2005 02:21 PM
Come on guys. Google and part a could come in really handy when you try and solve it. *************************************
It is very hard Harry. Why did you make it this hard. We've solved it though... Good Luck
[It is supposed to be hard, this is the penultimate round in the Challenge. But I don't think it's very hard, just hard. Harry]
Posted by: lbow at November 25, 2005 01:36 PM
Hi Harry, I've just had a horrible thought relating to the no. of letters in the text.
Is the fact that there are exactly ten letters in "Love Agatha" relevant.
In fact is it even vaguely relevant at all.
Probably not, but I thought I would just check.
PS. thos who did the Alchemist and Babylon Stone Challenges have a better chance of understanding my question.
[Unfortunately no, for some reason Agatha does not appear in this year's adventure. Harry]
Posted by: Oh no!! at November 25, 2005 01:07 PM
how many people have actually managed to crack this &^#$%^& cipher yet!?!
[Seven I think. Harry
Posted by: Hmm at November 25, 2005 12:26 PM
I'm still stuck on the type of cipher. I can't find any vigenere keyword lengths that give sensible freq. anal. graphs. and I don't recognioze the freq. anal. graphs of the whole ciphertext.
Can ne1 suggest a good website with a list of older ciphers (ie. not digital ones for computers) and their corresponding Freq. Anal. graphs?
Thanks
Posted by: Micha el at November 25, 2005 12:25 PM