Data Encryption Techniques

[Doug Gwyn]

(You should probably also address questions like this to net.crypt.)

Vigenere encryption of a long file with a short key is pretty easy to
break.  A Kasiski analysis will show the key length, then each of the
monalphabetic encryptions can be attacked by frequency analysis.  The
"l*" in your algorithm may delay more advanced attacks if the analyst
does not know the algorithm you used, but he would almost certainly
be able to break the scheme with a modest amount of work.  If the
encryption algorithm is known to or discovered by the analyst, then
there are some more powerful techniques that make an automatic
cryptanalytic attack possible.

The real question is, are you trying to hide your sources from users
or from dedicated cryptanalysts?  If the former, several simple
methods are sufficient; if the latter, no known method is perfectly
safe (excepting those that involve secure storage of a large amount
of key).  One simplification you could make would be to use exclusive-
or of the key bits and the text bits; this is easy in C and faster
than the modulo method you used.