My New Cipher

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My New Cipher; Quite Interesting!!
Topic Started: Apr 15 2008, 07:17 AM (900 Views)
jdege	May 1 2008, 11:58 AM Post #31
Elite member Posts: 263 Group: Admin Member #246 Joined: December 7, 2006	Paarth Dave May 1 2008, 01:09 AM How will Eve come to know if she has reached the substitution cipher if the keyword itself is not mentioned? That's where we get back to statistical tests like the Index of Coincidence. If she has shifted the text by the right amount, the result of the subtraction will have an IC very much like that of plain text, if she has not, it will not.
	When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.


Paarth Dave	May 3 2008, 05:23 AM Post #32
Advanced Member Posts: 70 Group: Members Member #2,712 Joined: March 26, 2008	What exactly is this Index of Coincidence? How is it calculated? Please explain it in simple terms.
	Cryptography Vanquished....


Donald	May 4 2008, 02:41 AM Post #33
Elite member Posts: 454 Group: Trusted Member Member #8 Joined: September 2, 2005	Have you found <this> stickied thread at the top of the Cryptanalysis section? It has several entries about cracking Vigenere ciphers, and most of them discuss the Index Of Coincidence, and some of them concentrate on it in detail. Take a look at those and if they don't answer your questions, ask again and we'll provide some more details!



jdege	May 5 2008, 01:27 AM Post #34
Elite member Posts: 263 Group: Admin Member #246 Joined: December 7, 2006	Paarth Dave May 2 2008, 11:23 PM What exactly is this Index of Coincidence? How is it calculated? Please explain it in simple terms. I've written up a description at: http://z13.invisionfree.com/Crypto/index.php?showtopic=456
	When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.


Donald	May 5 2008, 02:51 AM Post #35
Elite member Posts: 454 Group: Trusted Member Member #8 Joined: September 2, 2005	Quote: I've written up a description at: http://z13.invisionfree.com/Crypto/index.php?showtopic=456 NICE! thanks! I added it to the stickied list of tutorials. If there have been some nice tutorials that I missed while I was gone, let me know and I'll add them to the list. OR, just add them in a new post.



Paarth Dave	May 5 2008, 07:26 AM Post #36
Advanced Member Posts: 70 Group: Members Member #2,712 Joined: March 26, 2008	A very nice tutorial, indeed! So let's take a Vigenere example and let's see how effective is the IOC on it. UIRVYPUEAMISIHJ Now, someone with the knowledge of IOC can decipher this. I hope that when a person calculates the IOC in the next post, I can obviously clear myself from any doubts.....
	Cryptography Vanquished....


Revelation	May 5 2008, 09:20 AM Post #37
Administrator Posts: 386 Group: Admin Member #1 Joined: August 23, 2005	That's a very short ciphertext. I doubt the IOC is of any use. You see, the IOC is about a chance that two letters coincide. If the ciphertext is too short, you won't have an English distribution. Even short English texts haven't got an English distribution: A baffled madam standing on a catwalk. The example above hasn't even got an e in it.
	RRRREJMEEEEEPVKLWENFNVJKEEEEEAOLKAFKLXCFZAASDJXZTTTTTTTLSIOWJXMOKLAFJNNKFNXN RAGRBAQEMHIGDJVDSEOXVIYCELFHWLELJFIENXLRATALSJFSLCYTKLASJDKMHGOVOKAJDNMNUITN RRRRLJVEEEEECLYVYHNVPFTAEEEEEMWLMEIRNGLARWJAKJDFLWNTIERJMIPQWOTZEOCXKNUBNXCN RJIRPOWEANFUSNCZVDVZNMSFEKLOEPZLDKDJWSAAAAAAAOERHJCTNCKFRIMVKSOFOMKMANREWNBN RZUDRGXEEEEENFQIDVLQNCKNEEEEEDGLLLLLLAWIOSNCDARLODMTOEJXMILDFJROTKJSDNLVCZNN


Donald	May 5 2008, 11:31 AM Post #38
Elite member Posts: 454 Group: Trusted Member Member #8 Joined: September 2, 2005	"revelation" That's a very short ciphertext. I doubt the IOC is of any use Exactly. Give us a longer example and we'll be happy to break it for you. There is an example <here> where a vigenere style challenge of mine was cracked REALLY fast.



jdege	May 5 2008, 12:15 PM Post #39
Elite member Posts: 263 Group: Admin Member #246 Joined: December 7, 2006	Revelation May 5 2008, 03:20 AM That's a very short ciphertext. I doubt the IOC is of any use. It's not only that a ciphertext that short has few statistical cues to help break it, but also that a ciphertext that short has thousands of perfectly valid decryptions. It's an idea called the Unicity Distance - the length of text less than which it's probable that there will be multiple valid decryptions. Consider simple substitution, with the ciphertext 'QXG'. What is the plaintext? Well it could be 'cat', or it could be 'dog', or it could be any of the hundreds of valid three letter words, or it could be any one letter word followed by a two letter word, or a two letter word followed by a one letter word, or it could be a three letter acronym, or whatever. There are thousands of possibilities, and unless we have some context from outside the text itself, we have no means of eliminating any of them. The unicity distance of a simple substitution cipher on English text is 28 characters - messages shorter than that are very likely to have multiple equally valid decryptions. The Vig does a better job of hiding information than does the simple substitution, the unicity distance of English text encrypted with the Vig is longer than that of a simple substitution. The longer the keyword, the longer the unicity distance. If the keyword is longer than the message, there is a key that could generate every possible message. Code: `UIRVY PUEAM ISIHJ - BBJDQ XHQHI VEOBC = THISI SNOTE NOUGH` The ACA guidelines state that a Vig challenge should have a length ten to fifteen times that of the keyword.
	When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.


jdege	May 7 2008, 03:36 AM Post #40
Elite member Posts: 263 Group: Admin Member #246 Joined: December 7, 2006	Paarth Dave May 5 2008, 01:26 AM A very nice tutorial, indeed! So let's take a Vigenere example and let's see how effective is the IOC on it. How about this?
	When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.


Paarth Dave	May 9 2008, 07:13 AM Post #41
Advanced Member Posts: 70 Group: Members Member #2,712 Joined: March 26, 2008	Quote: The ACA guidelines state that a Vig challenge should have a length ten to fifteen times that of the keyword. So how do you decypt short ciphertexts like the one given above? Are there any statistical tests for that?
	Cryptography Vanquished....


jdege	May 9 2008, 11:32 AM Post #42
Elite member Posts: 263 Group: Admin Member #246 Joined: December 7, 2006	Paarth Dave May 9 2008, 01:13 AM Quote: The ACA guidelines state that a Vig challenge should have a length ten to fifteen times that of the keyword. So how do you decypt short ciphertexts like the one given above? Are there any statistical tests for that? You use the statistics you have, and then try out guesses until you find something that looks right. But there is, for every cipher type. a minimum length below which there isn't a single valid decryption. If the message is short enough, you simply can't break it. Here's a message, what does it mean? Code: `ABCDE FGHIJK`
	When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.


Donald	May 9 2008, 12:05 PM Post #43
Elite member Posts: 454 Group: Trusted Member Member #8 Joined: September 2, 2005	"Paarth Dave" So how do you decypt short ciphertexts like the one given above? As Jdege pointed out above, the main problem with really short text is the unicity distance. With a very short message, I can come up with a key to decrypt it into anything you want. I could even come up with monoalphabetic substitutions that would decrypt it into intelligible messages. For example: your message: UIRVYPUEAMISIHJ could be validly decrypted to: launcelot hazard or forgetful nobody by simple monoalphabetic substitution. And there are dozens, probably hundreds of valid decryptions, many of which will make perfect sense. So no kind of statistical analysis would be of any use. There just isn't enough there to guarantee a unique decryption.



Paarth Dave	May 9 2008, 02:16 PM Post #44
Advanced Member Posts: 70 Group: Members Member #2,712 Joined: March 26, 2008	So can you call it an unbreakable cipher which is quite similar to one-time pads?
	Cryptography Vanquished....


jdege	May 9 2008, 03:29 PM Post #45
Elite member Posts: 263 Group: Admin Member #246 Joined: December 7, 2006	Paarth Dave May 9 2008, 08:16 AM So can you call it an unbreakable cipher which is quite similar to one-time pads? A one-time pad is a Vig with a random key that is longer than the plaintext, where you use the key only once. If the key isn't random, it's a running-key cipher, and they can be sometimes be broken, if you have some information about the keyspace. If you gave me very short message, and told me that it was Vig encrypted with a word that is in the dictionary, I could try decrypting it with every word in the dictionary, and looking at the results to see how many would make any sense. Odds are only a few would. If the key could be anything, then there is a key that could generate every possible message. Absent external information, there is no way to distinguish which one of them is right.
	When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.


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