Re: Newbie - How can I figure out which cipher was used...
From: X (x_at_x.x)
Date: 08/26/04
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Date: Wed, 25 Aug 2004 22:14:41 -0400
On Thu, 26 Aug 2004 01:09:45 GMT, Jim Gillogly <jim@acm.org> wrote:
>For English, as you say (with the version of IC you and Sinkov use
>rather than the normalized version preferred by Government cryppies),
>expect 0.066.
I'll look into that. I didn't know there was a normalized version. I
was simply "sliding" the CT against itself and counting matching
letters, then dividing the total by the length. It works fine for my
"test" ciphers. Then again, it's a lot easier to "crack" a cipher when
you already know the answer you're looking for.
>However, the average IC you're seeing with the long keys is *higher*
>than you'd expect with English. You may be seeing some kind of artifact
>resulting from a (for example) 2->3 expansion from plaintext to
>ciphertext. If it's a little redundant that might help explain the
>higher-than-expected IC.
Interesting.
>With a sufficiently long Vig key the IC will be the same as for random
>text: 0.0385. Short Vig keys, like period 3, will be between the two.
Wouldn't that be like trying to break a OTP? If the IC is equal to
random text?
>You can eliminate a number of ciphers by looking at the mono frequency
>count -- for example, there are i's, j's, k's, q's and an x, so it's
>unlikely that it's based on a Polybius square (like Playfair, Trisquare,
>Phillips, Bifid, etc.).
That's what I was looking for. Some kind of "atlas of ciphers" that
would give some hints on how to identify the cipher from the CT. Since
many classical "pen and papers" ciphers have weaknesses, I was
thinking that it would be possible to narrow down the field of
possible ciphers by examining some characteristics of the CT. I just
wasn't able to find that "atlas" anywhere.
>> I've also wondered about a "triple" Vigenere (because of the 3). Would
>> ciphering 3 times with different keys alter the IC? How hard would a
>> triple vig (with 3 different keys) be?
>
>Not impossible, depending on the key lengths. The complexity of a triple
>Vig with relatively prime key lengths p, q and r is the same as decrypting
>one with an incoherent key of length p + q + r -- not the p*q*r that the
>effective period would suggest. Another possibility with the "3" is
>Quagmire III, which is the type used for the first two parts of Kryptos.
Quagmire. Sounds lovely. :-) When you say "incoherent key", I
understand that to mean a key that's not a word but just some random
jumble of letters?
>Again, the IC is too high for Enigma: Enigma would be closer to
>that 0.0385 you'd see for random text.
I've pretty much ruled out Enigma. There are many tools available to
analyze it and crack it (I cheated a little with that one, went
straight to the tools...)
>> Maybe. Haven't seen much on F. Morse (except how to cipher with it) on
>> the internet. Not much about attacking it.
>
>I find that the most interesting thing to do with classical ciphers
>is figure out how to mount an attack on them... after that it's more
>like engineering.
I agree. I'm just trying to make sure I'm mounting my attack against
the right enemy.
>Oh -- that's rather poopy. Is this one of those challenge sites like
>the pc-facile contest? I prefer coherent text -- it's annoying to spend
>a month or so working on a hard cipher, only to have it say "This is a
>cipher that will not be read unless I give the keys" (yes, this happened
>to me). For the word list thing, it's annoying because it removes some
>of the contextual clues that make solving more enjoyable, like a
>crossword.
You don't miss much, let me tell you. It's one of those site (but
shhh! I'm not supposed to get help). And I agree that the random words
are a major pain. Then again, I'm not sure about those random words,
but from my experience with the previous "easier" ciphers, I'm
inclined to believe that's what I'll end up with. At least, it's not
as boring as brute force attacks...
I think I'll also try calculating ICs for lists for words picked
randomly from a dictionnary. That would probably differ from "normal"
text.
>> I guess even with random words, some letter combinations must show up
>> more often than others.
>
>Yes -- you can look at trigraphs and tetragraphs from samples and get
>some insight, perhaps.
I've started writing a program to analyze di-, tri- and tetragraphs.
Hopefully that'll give me some new angle on it. Writing the tools is
fun. Changes me from writing database apps... :-)
>> And then I remembered seeing your name a few times while searching on
>> the web and on Googling it found out about how you solved most of the
>> CIA's Kryptos monument. Color me impressed!
>
>One of my high points. :)
I'll say. Are you still trying to decrypt the last part?
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