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A Thanksgiving Day Playfair; Rating: Very Hard
Topic Started: Nov 24 2005, 03:50 PM (2,100 Views)
rot13
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I'm about to head out to go see my mom, I thought I'd drop off a nice Playfair challenge:

Code:
 

TM NX LR QG CR XE EW EG VK GS MH XM EV EK TV GV SU GZ KH IC NH NB TM SA VS KN BH AN KT GI VO VA SF VA AR BV NI VE IV AV HX IQ NK IS EU LE BM HA LX VC BF ST CN RX MI AS HV AS HB CI HY BM AR BU NX RU IS EU LE VA SF GZ KN HG GC RC IK BS ES BP VA HU RE IR XE TY AB IU


I will leave two tips. The first is: eatingtitle
And the second one is the word that comes just before the first: turkey

Good luck!
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Donald
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This is my first time to seriously attack a playfair. I'm having to do a lot of reading... :)

Donald
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Revelation
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I will surely give this a try. :)

Is the first hint a crib?

Addition: I have got a present for you all: http://www.umich.edu/~umich/fm-34-40-2/appd.pdf :)
RRRREJMEEEEEPVKLWENFNVJKEEEEEAOLKAFKLXCFZAASDJXZTTTTTTTLSIOWJXMOKLAFJNNKFNXN
RAGRBAQEMHIGDJVDSEOXVIYCELFHWLELJFIENXLRATALSJFSLCYTKLASJDKMHGOVOKAJDNMNUITN
RRRRLJVEEEEECLYVYHNVPFTAEEEEEMWLMEIRNGLARWJAKJDFLWNTIERJMIPQWOTZEOCXKNUBNXCN
RJIRPOWEANFUSNCZVDVZNMSFEKLOEPZLDKDJWSAAAAAAAOERHJCTNCKFRIMVKSOFOMKMANREWNBN
RZUDRGXEEEEENFQIDVLQNCKNEEEEEDGLLLLLLAWIOSNCDARLODMTOEJXMILDFJROTKJSDNLVCZNN
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rot13
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Revelation
Nov 26 2005, 09:56 PM
I will surely give this a try. :)

Is the first hint a crib?

Yes, they are both cribs. I chose it because the pattern of the words makes it fairly easy to place.
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cows
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revelation
 
Addition: I have got a present for you all: http://www.umich.edu/~umich/fm-34-40-2/appd.pdf


Thanks - It's like my birthday and christmas rolled up into one :lol:
Everything is possible,
The impossible just takes longer

If we do not know what a particle is doing then it is allowed t do everything possible simultaneously.
"Anyone who can contemplate Quantum Mechanics without getting dizzy, didn't understand it."
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Donald
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Well, I have been slaving away at this in my spare moments. I've made some progress, but still have a long way to go.

In the meantime I thought I'd pass on some things I learned about playfairs. Hopefully this will acomplish two things!
1: Someone else struggling with their first playfair might be able to benefit from something I've learned.
2: Someone may find a serious mistake in my assumptions, and thereby help me out by putting me back on the straight and narrow! :thumbsup: Rot13 already had to correct me about playfairs being symetrical when rotated 90 degrees. (And thank goodness he did!)

- - -

because of the playfair rules, a letter can ONLY translate to the other 4 letters on the same row, or the letter directly beneath it in the same col. Of the 24 possible digrams with any particular letter:
20 (83%): encrypt to one of the 4 letters on the same row
8 (33%): encrypt to the letter directly to the right.
4 (17%): encrypt to the letter directly below in the same col

For a "same row" digram, there is only one possible encryption for each letter.
For a "same col" digram, there is only one possible encryption for each letter.
For a "diagonal" digram, there are four possible encryptions for each letter.

No letter can encrypt to itself.

For any digram pair, no matter which rule it was encrypted by, reversing the plain letters results in the same crypt letters, but also reversed. So if AB=CD then BA=DC.

If the same letter apears in the plain and the crypt of a digram, then all 3 letters must be in the same row or column, and must be directly connected. (this is true because the Diagonal rule always encrypts to two different letters).
So if AB=BC then you know that ABC is in one row or col of your playfair square.

If two different plain letters both encrypt to the same crypt letter at any point, then all three letters must be in the same ROW (not col), OR one plain and the crypt are in the same row, and the other crypt is in the row above, positioned directly over the crypt char. Under no circumstances can the two plains be in the same col.
So if AB=CD and EF=CG, both A and E have encrypted to C, and you know know that either A, E and C are all in the same row, OR they appear in the playfair square as:
Code:
 

_E  or _A
AC     EC

but no matter which configuration, A and E can not be in the same col.

The playfair square is equivilent when shifted by cols or rows. so:
Code:
 

this: and this: and this: and this:
abcde     fghik     bcdea     ghikf
fghik     lmnop     ghikf     mnopl
lmnop     qrstu     mnopl     rstuq
qrstu     vwxyz     rstuq     wxyzv
vwxyz     abcde     wxyzv     bcdea

will all produce the exact same encryptions/decryptions. They are, for all intents and purposes, the same playfair square.

BUT, you can NOT rotate the playfair square 90 degrees and get the same results because the DIAGONAL rule is NOT symetrical between columns and rows.

Donald
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Revelation
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I am having a fight with Playfair too :) This is how far I got:

SPOILER
VO VA SF VA AR B: this is most likely 'eatingtitle'. Notice the two VAs (plaintext ti). We also have AR, which is tl. Let's make a rectangle!

t_A
V_i

Now we include the other one. There aren't that many possibilities because an A is also included there. I figured they must be on the same row, because the t turns into an A. Correct me if I'm wrong :)

talr
v_i_

RRRREJMEEEEEPVKLWENFNVJKEEEEEAOLKAFKLXCFZAASDJXZTTTTTTTLSIOWJXMOKLAFJNNKFNXN
RAGRBAQEMHIGDJVDSEOXVIYCELFHWLELJFIENXLRATALSJFSLCYTKLASJDKMHGOVOKAJDNMNUITN
RRRRLJVEEEEECLYVYHNVPFTAEEEEEMWLMEIRNGLARWJAKJDFLWNTIERJMIPQWOTZEOCXKNUBNXCN
RJIRPOWEANFUSNCZVDVZNMSFEKLOEPZLDKDJWSAAAAAAAOERHJCTNCKFRIMVKSOFOMKMANREWNBN
RZUDRGXEEEEENFQIDVLQNCKNEEEEEDGLLLLLLAWIOSNCDARLODMTOEJXMILDFJROTKJSDNLVCZNN
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rot13
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SPOILER:

There are two places in the ciphertext where you have an AB CD AB pattern. One is the VO VA SF VA AR B that Revelation mentioned. When you put the letters in there, though, you get some odd-looking results. These are some potential rectangles:

EV TV NS TA
OA AI FG RL


If E and V are in the same row, then A appears below V. The other possibility is that E,V,O and A are all in the same row or column. Suppose for a minute that it is just A and V in the same row. Look at the next rectangle. If T and V are in the same row, A should have been under V not under T. If you assume that either E,V,O and A are in the same column or row, then the TV/AI pair says that T and I should be thrown in there as well and you have more than 5 items in the row or column. I think the arrangement of letters here rules this position out as a possibility for the crib placement.

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Donald
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Spoiler: click to toggle

I'm finding that with my skill level the crib can only take me so far. I managed to lock down the positions of 5 letters in the square using only the crib, and have limited the possiblities for quite a few others, but then you have to take those results out to the full message and start making guesses and looking for impossible letter combinations in order to eliminate the bad guesses. My first guess pass pinned down another 2 letters, so I've got 7 now. Hopefully a few more lucky guesses will pin down another 4 hopefuls I'm working on. And that MAY be enough to crack this nut open. :)

There are several repeated sequances in the crypt text that are VERY tantilizing...

Donald
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Donald
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So, anyone else making progress? I've got 8 letters mapped into the playfair square now with pretty high certainty. I need a bit more time and then hopefully I can place one or two more letters, which should be enough to start breaking it open.

Cracking a playfair is the hardest pen and paper cryptological challenge I've ever worked on!

Donald
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rot13
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Donald
Dec 9 2005, 04:05 PM
Cracking a playfair is the hardest pen and paper cryptological challenge I've ever worked on!

It is fun, though, right? I spent about 2-3 hours last night cracking a two-square, which is like a playfair but with two cipher squares (and thus two keywords). I think that the more you do them, the better you get.
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Donald
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"rot13"
 
It is fun, though, right?

Oh, ABSOLUTELY! The difficulty level is what makes it fun! :)

My 6 year old has now grown bored with passing Caesars back and forth and want's more Vigenere's where he can use his code wheel. :) No, he's not CRACKING Vigenere's yet, he just likes passing messages in them. For cracking he's still working with Caesars and simple monosubs (with cribs and help). The point of this snuck in proud daddy brag is that HARD, but doable, is what makes a challenge FUN.

I just need a few more letters. Just one piece of the cryptogram (besides the cribs) to start matching with an obvious word. I'm up against a conceptual barrier that I've got to find a way over, then I think this whole thing will fall apart before my eyes.

So, I'll keep working on what-if scenarios until I manage to get that one more critical break through...

Sweating, but loving every minute of it. :)

Donald
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rot13
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Donald
Dec 11 2005, 05:20 AM
I just need a few more letters. Just one piece of the cryptogram (besides the cribs) to start matching with an obvious word. I'm up against a conceptual barrier that I've got to find a way over, then I think this whole thing will fall apart before my eyes.

I find that I frequently spend a lot of time attacking the key instead of the ciphertext. This isn't just for playfair, it is for most ciphers. Since the keys aren't random and often have something to do with the subject, I find I am often able to make the key fall apart or at least enough to make huge breaks in the ciphertext.
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Donald
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Ok guys, I am seriously stuck on the playfair.
I could ask rot13 for more hints, but I KNOW I should be able to get this without any more hints, and besides, I have an idea that might actually be more educational and fun than more hints.

Would anyone be interested in working this as a team effort? I'd be happy to post all of my research so far, then everyone else could go over it and either find where I've made a stupid mistake or missed the next step. Together I'm betting we could beat this thing, and come out much better armed to attack the next playfair individually.

(besides, it might get our daily traffic level back up again!) :)

Donald

[edit]
Dang, I had a break through 15 min after I posted this. Ain't it always that way? I may have this thing by tomorrow.
[/edit]
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Donald
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At LAST!!!!! I busted the playfair!!!

VG HN XR UT YR RC WR NH LX VC AO HV VK SA

I had made a few mistakes and that was what was hampering me. First I had some typos in my deciphering. Once I detected and corrected those, the cipher started falling apart before my eyes. But it wasn't RIGHT. There were places where it was perfect, and other places where I could tell what it was supposed to say, but the message was all garbled up. Digrams reversed and shifted etc. VERY frustrating. BUT, I had enough of the message now that I could identify the source and confirm that yes, the message WAS supposed to say what I thought it did, so my square was off. I went back to the very begining of my analysis and started double checking every step. And there it was. I had fixed two cols as being adjacent without having proof that they were. Stupid amature mistake. rot13 had tried to warn me that my cols weren't necessarily in the right sequance, but I couldn't find the error at that time. <sigh>

Well, since the error was VERY early in my research, I had to start over and re-do every step. I did it BETTER this time, and worked it all the way to the correct conclusion. (and no, having the original plain text didn't actually help any, I was simply recreating my original research without the same flaw, I didn't use ANY words this second time through that I hadn't already correctly identified on the first attempt)

That was an extremely educational experiance, and very satisfying to complete. Thanks rot13!

I have a step by step log of exactly how I solved the playfair. If anyone is interested, (and if rot13 doesn't object), I could post it as an example of how to attack one of these monsters. I would post it in a different thread with a clear warning label at the top so anyone who was still working on the challenge could avoid it. But, like I said, only if someone might be interested and only if rot13 doesn't object.

Donald
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