Re: modulo encrypt problem

From: Richard Bos (rlb_at_hoekstra-uitgeverij.nl)
Date: 10/19/04 

Date: Tue, 19 Oct 2004 14:38:33 GMT

john blackburn <john.blackburnNOSPAMPLS@lintonhealy.co.uk> wrote:

> for (count=0; count < (strlen(pattern)-4); count++) {
>
> digval = ((*(pattern+strlen(pattern)-count-1)-'0')+key[strlen(pattern)-count-1]+digval)%10;
> *(pattern+4+count) = digval+'0';

Assuming that strlen(pattern)>5, you're scribbling over your own input
buffer. When count==0, you read from pattern[strlen(pattern)-1], but you
write to pattern[4]. By the time count gets to strlen(pattern)-5, you
write to pattern[strlen(pattern)-1], but you read from pattern[4]...
which already contains its new value. The original values of the early
members of pattern never even get read. You need to use a second buffer
to hold the output while you still need the original value of pattern.

Richard

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