encryption with pi

vedaal_at_hush.com
Date: 07/29/04 

Date: 29 Jul 2004 13:31:56 -0700

can the following simple substitution cipher using pi, be made secure
?

C = P rot [ s(l+n) mod 256 ]

where 's' is a substring of pi of length k, beginning at digit 'l',
and 'n' is the sequenced character of the plaintext

example:

if 'l' is chosen to be 599, and 'k' to be 10,
then for the first character of the plaintext, (n=1) ,
a 10 digit string of pi beginning with the 600th digit, would be;
2000568127

for the second character, (n=2),
s(l+n) would be:
0005681271

as s(l+n) is not periodic,
[s(l+n) mod 256] would also expected not to be periodic.

(assume k would arbitrarily be greater than 12,
as there are several areas of pi that have strings of 0000000,
but none known (to 100 million digits, anyway, ;-) ) to have eight
consecutive zeroes)

would this be secure enough,
or would it need the string length 'k' to change by some agreed upon
f(n),
so that k also changes with each character of the plaintext?

the correspondents would include instructions in each message, for new
values of 'k', 'l', and f(n), to be used for the next message,
to thwart known-plaintext attacks.

the attackers would know the algortithm, but not the chosen values of
k and l
(or of f(n) if it is necessary).

tia,

vedaal

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