Deny 2k-reduce if lsd is zero

libtom · Mar 11, 2024 · fa8f3cf · fa8f3cf
1 parent 8314bde
commit fa8f3cf
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Showing 2 changed files with 18 additions and 2 deletions.
diff --git a/demo/test.c b/demo/test.c
@@ -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;
 

diff --git a/mp_reduce_is_2k.c b/mp_reduce_is_2k.c
@@ -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) {