Cryptography Reference
In-Depth Information
leftover carry at the end, it is stored as the most-significant digit of the sum. The
output of this digit is suppressed if it has the value zero.
Steps 2 and 4 of the algorithm appear in a similar form in the case of
subtraction, multiplication, and division. The associated code, which is illustrated
by the following lines, is typical for arithmetic functions:
1
s = (USHORT)(carry = (ULONG)a + (ULONG)b + (ULONG)(USHORT)(carry >> BITPERDGT));
The intermediate value
t
that appears in the algorithm is represented by the
variable
carry
, of type
ULONG
, which holds the sum of the digits
a
i
and
b
i
as well
as the carry of the previous operation. The new summation digit
s
i
is stored in
the less-significant part of
carry
, from where it is taken by means of a cast as a
USHORT
. The resulting carry from this operation is held in the more-significant part
of
carry
for the next operation.
The implementation of this algorithm by our function
add_l()
deals with a
possible overflow of the sum, where in this case a reduction of the sum modulo
(
N
max
+1)
is carried out.
Function:
addition
int add_l (CLINT a_l, CLINT b_l, CLINT s_l);
Syntax:
a_l, b_l
(summands)
Input:
s_l
(sum)
Output:
E_CLINT_OK
if all is ok
E_CLINT_OFL
in the case of overflow
Return:
int
add_l (CLINT a_l, CLINT b_l, CLINT s_l)
{
clint ss_l[CLINTMAXSHORT + 1];
clint *msdptra_l, *msdptrb_l;
clint *aptr_l, *bptr_l, *sptr_l = LSDPTR_L (ss_l);
ULONG carry = 0L;
int OFL = E_CLINT_OK;
1
The C expression in this compact form is due to my colleague Robert Hammelrath.
