Cryptography Reference
In-Depth Information
Now the actual operation takes place. The loop runs over the digits of the shorter
of the two arguments.
while (s_l <= msdptrs_l)
{
*t_l++ = *r_l++
ˆ
*s_l++;
}
The remaining digits of the other argument are copied as above.
while (r_l <= msdptrr_l)
{
*t_l++ = *r_l++;
}
cpy_l (c_l, d_l);
}
The function
and_l()
can be used to reduce a number a modulo a power of two
2
k
by setting a
CLINT
variable
a_l
to the value
a
,a
CLINT
variable
b_l
to the value
2
k
1
, and executing
and_l(a_l, b_l, c_l)
. However, this operation executes
faster with the function
mod2_l()
created for this purpose, which takes into
account that the binary representation of
2
k
−
−
1
consists exclusively of ones (see
Section 4.3).
7.3 Direct Access to Individual Binary Digits
Occasionally, it is useful to be able to access individual binary digits of a number
in order to read or change them. As an example of this we might mention the
initialization of a
CLINT
object as a power of
2
, which can be accomplished easily
by setting a single bit.
In the following we shall develop three functions,
setbit_l()
,
testbit_l()
,
and
clearbit_l()
, which set an individual bit, test a particular bit, and delete a
single bit. The functions
setbit_l()
and
clearbit_l()
each return the state of the
specified bit before the operation. The bit positions are counted from
0
, and thus
the specified positions can be understood as logarithms of powers of two: If
n_l
is
equal to
0
,then
setbit_l(n_l, 0)
returns the value
0
, and afterwards,
n_l
has the
value
2
0
=1
;afteracallto
setbit_l(n_l, 512)
,
n_l
has the value
2
512
.
