cbenson@reed.UUCP (Jason E. Elliott) (05/14/84)
I don't know whether those that replied to my request
for a critique on my idea behind it. (I won't post the
program again unless asked to.)
Being a mere high school student, and not versed passed
pre-calculus, I wrote my program using ideas suggested by a
book titled "Cryptanlisis, A Mathematical Approach". I
looked at the Vigenere method primarily because it was sim-
ple to implement, but could be made complicated enough to
deter the pirating of my source by other students. However,
as you all know, the Vigenere method by itself, is not at
all hard to solve. However, with the twist that makes my
program a little more complicated, the key that the user
provides actually is permuted once for each time that it is
used.
When the key has been used once, the key is rotated
left one letter and used again. Therefore, there is a one-
to-one correspondance between the number of letters in the
key, and the number of keys used.
A dissemination interval equal to the length of the key
is used to make things even more messy.
In any case, although this program is primitive by
today's standards, it does protect my work from my class-
mates. (As well as impressing the computer teachers.)
I hope this makes things a little clearer (if there was
any question as to the program's operation). I would appre-
ciate any advice or constructive criticism that you in net-
land might have. Also, my request still stands with regard
to my desire to get some kind of estimate as to the relative
difficulty of deciphering a message coded with my program.
I would also like some help with finding the optimum key-
to-file-length ratio (that is, the best length of key to use
for a given file).
As they say: Thanks in advance
Jason E. Elliott
May 14, 1984gwyn@brl-vgr.ARPA (Doug Gwyn ) (05/23/84)
Look for references to "unicity distance" in the cryptography literature. I am not sure how much knowledge of this is in the public domain so I am reluctant to discuss it, but it is the answer to your question about appropriate key length.
henry@utzoo.UUCP (Henry Spencer) (06/01/84)
"Unicity distance" is discussed, as I recall, in some of the more
technical sections of Kahn's "The Codebreakers". That wouldn't be
a bad place to start. Note that you must be reading the hardcover
edition -- the paperback is abridged!
--
Henry Spencer @ U of Toronto Zoology
{allegra,ihnp4,linus,decvax}!utzoo!henrymab@cadvax (Matt Bishop) (06/12/84)
You can also look in Dorothy Denning's book, "Cryptography and Data Security", in the first chapter on cryptography (I think it's chapter 2, but I'm at home and my copy is at work, so I can't check that for you.) Also, I don't remember where, but I'm sure Konheim talks about it in his book "Cryptography". Unless you're REALLY mathematically inclined, try Dorothy's book first! -- Peace, Matt Bishop UUCP: decvax!decwrl!sun!megatest!mab ARPA: megatest!mab@lbl-csam
pjbk@hwcs.UUCP (Peter King) (06/13/84)
Unicity distance and many other cryptographic terms and concepts are defined and discussed in the following books Denning, D.E. "Cryptography and Data Security" Addison-Wesley 1982 Konheim, A.G. "Cryptography: A Primer" Wiley 1981 Meyer, C.H. and Matyas, S.M. "Cryptography: a New Dimension in Computer Data Security" Wiley 1982 They are a starting point for any attempt to measure the 'strength' of any encryption technique. P.S. In the UK even the hardback version of David Kahn's "The Codebreakers" is an abridged version!