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Deny 2k-reduce if lsd is zero #564

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Mar 11, 2024
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Deny 2k-reduce if lsd is zero #564

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9 changes: 9 additions & 0 deletions demo/test.c
View file
Original file line number Diff line number Diff line change
@@ -1229,11 +1229,20 @@ static int test_mp_cnt_lsb(void)
static int test_mp_reduce_2k(void)
{
int ix, cnt;
bool is2k;

mp_int a, b, c, d;
DOR(mp_init_multi(&a, &b, &c, &d, NULL));

/* test mp_reduce_2k */

/* Algorithm as implemented does not work if the least significant digit is zero */
DO(mp_2expt(&a, 100));
DO(mp_sub_d(&a, 1, &a));
DO(mp_sub_d(&a, MP_MASK, &a));
is2k = mp_reduce_is_2k(&a);
EXPECT(!is2k);

for (cnt = 3; cnt <= 128; ++cnt) {
mp_digit tmp;

11 changes: 9 additions & 2 deletions mp_reduce_is_2k.c
View file
Original file line number Diff line number Diff line change
@@ -11,9 +11,16 @@ bool mp_reduce_is_2k(const mp_int *a)
} else if (a->used == 1) {
return true;
} else if (a->used > 1) {
int ix, iy = mp_count_bits(a), iw = 1;
mp_digit iz = 1;
int ix, iy, iw = 1;
mp_digit iz;
/* Algorithm as implemented does not work if the least significant digit is zero */
iz = a->dp[0] & MP_MASK;
if (iz == 0u) {
return false;
}

iy = mp_count_bits(a);
iz = 1;
/* Test every bit from the second digit up, must be 1 */
for (ix = MP_DIGIT_BIT; ix < iy; ix++) {
if ((a->dp[iw] & iz) == 0u) {