modulo encrypt problem

From: john blackburn (john.blackburnNOSPAMPLS_at_lintonhealy.co.uk)
Date: 10/19/04 

Date: Tue, 19 Oct 2004 10:35:35 +0100

Hi,

  I am trying to get an extremely simple character string encryption function
to work using modulo 10 arithmetic.

void ccencode(UNSIGNED8* pattern) {

    UNSIGNED8 key[] = CCKEY;
    UNSIGNED8 digval = 0;
    UNSIGNED8 count;

    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';
    }

} /* ccencode */

The pattern to be encrypted is a string of ASCII numeric digits. This function
starts with the last digit and adds it to the corresponding digit in the key
(the key array of integers is #defined elsewhere) and then modulo divides it
by 10 to always end up with a digit in the range 0 - 9. The digit is then
added to '0' to make an ASCII character and written to the pattern. the digit
then accumulates into the next and so on.

Two other characteristcs are that the top 4 digits of the pattern are
deliberately unencrypted and the remainder are not only encrypted, but
also reversed in sequence.

My problem is that the first 6 digits are processed correctly but the
remainder do not. The next 3 encrypted digits are 1 greater than they should be
and the remaining 3 are then way out.

Surely this cannot be a rounding effect as % is an
integer division giving an integer remainder. The algorithm works fine
on a Borland C++ compiler but not on gcc. Note that I have to run gcc
in ISO mode for other reasons.

Any ideas what is wrong ?

Thanks in advance

John Blackburn

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