From: jac54@cas.org ()
Subject: Re: Attack Methods
Organization: Chemical Abstracts Service, Columbus, Ohio
Date: Wed, 18 Nov 1992 19:44:35 GMT

In article <1992Nov18.190513.10997@cis.uab.edu> sloan@cis.uab.edu (Kenneth Sloan) writes:
>In article <1992Nov18.134243.24089@qiclab.scn.rain.com> Leonard.Erickson@f51.n105.z1.fidonet.org writes:
>>jdailey@dadd.ti.com (Jim Dailey) writes:
>>
>>>So what are some of the methods used to attack an encrypted text, when the
>>>encryption method is unknown?
>>
>>First you run frequency tests. This will tell you *immediately*
>>if a "reasonably sized" text was encrypted via *any* character
>>transposition cipher.
>
>This thread is at about my speed - so I'll toss in a question.  I'm sure
>that this is hopelessly naive - but I hope someone will tolerate the
>question long enough to answer it.
>
>Suppose that I have a character transposition cipher.  Will the
>following scheme make it appreciably stronger (or weaker?):
>
>To encrypt:
>
>    0) compose the message
>    1) add (to both the front, and the back) additional characters,
>       chosen to produce a flat histogram.  Pseudo-randomize, as needed.
>       Perhaps also pad to a standard size block?
>       Perhaps randomize the positioning of the message within the block?
>       Perhaps break the original message into bite-sized pieces and
>       include additional material between bites as well as at the
>       beginning and end?
>    2) apply the character transposition cipher.

	Read the article the"Two Soviet Spy Ciphers" by David Kahn in
	"Kahn on Codes".  The system used by Hayhanen did several of
	these things and, if I remember correctly, everything was
	susbtituted first.  Another common ploy is to split the message
	down the middle and put the last part first, thereby hiding
	the standard beginnings and endings.  Despite all this, I think
	the transposition would fall to multiple anagramming fairly
	quickly.  Somebody very patiently explained to me that
	transpositions are considered very weak these days.

	Incidentally, the Soviets seem to have put a lot of effort
	into putting field-expedient ciphers together so that they
	wouldn't forever be delivering one-time pads to their
	people.

>to decrypt:
>
>    0) reverse the transposition
>    1) rely on the reader (either human or otherwise) to recognize
>       the start and end of the original message (or the multiple starts
>       and ends of message bites).
>
>One weakness which springs to mind is that absolutely flattening the
>character histogram may cause an unacceptable increase in the size of
>the transmitted message.  Perhaps this is what was intended by the
>qualification ``"reasonably sized"'' above?
>
>A (probably foolish) complication which springs to mind is to:
>
>    *restrict words in the message to a pre-defined lexicon

	I wouldn't do this, it weakens the transposition considerably.

>    *break up the original message on word boundaries

	See above, the Hayhanen system used letter counts rather than
	word boundaries (safer).

>    *add "histogram-flattening" characters between words (instead of
>     whitespace?)

	Lots of nulls in this one! (To quote the Queene's Decypherer).


>    *ensure that no "allowed words" are generated accidently
>    *have the decrypting process find the "allowed words" among
>     the random garbage (after reversing the transposition)
>
>Assuming that the message has been appropriately un-transposed, it
>should be simple enough for the intended receiver to find the allowed
>words, and hence the original message.  The question is: does this give
>the enemy too large a lever on cracking the transposition cipher?
>
>       Yes, see above.
>
>       Alec Chambers
>
>--
>Kenneth Sloan                   Computer and Information Sciences
>sloan@cis.uab.edu               University of Alabama at Birmingham
>(205) 934-2213                  115A Campbell Hall, UAB Station
>(205) 934-5473 FAX              Birmingham, AL 35294-1170