Secure Key Exchange over an Insecure Medium with Public Key Cryptography - Implementing SSL/TLS Using Cryptography and PKI - page 144

{
result_sign = h1->sign;
if ( h1->sign == h2->sign )
{
add_magnitude( h1, h2 );
}
else
{
subtract_magnitude( h1, h2 );
}
}
else
{
huge tmp;
// put h1 into tmp and h2 into h1 to swap the operands
set_huge( &tmp, 0 ); // initialize
copy_huge( &tmp, h1 );
copy_huge( h1, h2 );
if ( h1->sign == tmp.sign )
{
result_sign = h1->sign;
add_magnitude( h1, &tmp );
}
else
{
result_sign = h2->sign;
subtract_magnitude( h1, &tmp );
}
free_huge( &tmp );
}
// Use the stored sign to set the result
h1->sign = result_sign;
}
5. This routine embodies the signing rules described by the fi rst half of
Table 3-2. If h1 is greater than h2 (see Figure 3-7), add or subtract, depend-
ing on whether the signs of the two operands are the same or different,
and preserve the sign of h1 . h2 never changes regardless. If h1 is less
than h2 , swap them; you want to preserve h2 as before, so copy h1 into
a temporary object and h2 into h1 .
The net effect here is that you add or subtract h1 from h2 , overwriting h1 ,
just as if the operation had been called with the operators reversed, but
not touching h2 , which is what you want.