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Avoid future chance of out-of-bounds error #540

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Mar 31, 2023
Merged

Avoid future chance of out-of-bounds error #540

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9 changes: 6 additions & 3 deletions s_mp_div_school.c
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Original file line number Diff line number Diff line change
Expand Up @@ -19,6 +19,7 @@
mp_err s_mp_div_school(const mp_int *a, const mp_int *b, mp_int *c, mp_int *d)
{
mp_int q, x, y, t1, t2;
mp_digit xdpi;
int n, t, i, norm;
bool neg;
mp_err err;
Expand Down Expand Up @@ -68,14 +69,16 @@ mp_err s_mp_div_school(const mp_int *a, const mp_int *b, mp_int *c, mp_int *d)
if (i > x.used) {
continue;
}
/* Do not assume that more than enough memory is automatically allocated and set to '0' */
xdpi = (i == x.used) ? 0u : x.dp[i];

/* step 3.1 if xi == yt then set q{i-t-1} to b-1,
* otherwise set q{i-t-1} to (xi*b + x{i-1})/yt */
if (x.dp[i] == y.dp[t]) {
if (xdpi == y.dp[t]) {
q.dp[(i - t) - 1] = ((mp_digit)1 << (mp_digit)MP_DIGIT_BIT) - (mp_digit)1;
} else {
mp_word tmp;
tmp = (mp_word)x.dp[i] << (mp_word)MP_DIGIT_BIT;
tmp = (mp_word)xdpi << (mp_word)MP_DIGIT_BIT;
tmp |= (mp_word)x.dp[i - 1];
tmp /= (mp_word)y.dp[t];
if (tmp > (mp_word)MP_MASK) {
Expand Down Expand Up @@ -103,7 +106,7 @@ mp_err s_mp_div_school(const mp_int *a, const mp_int *b, mp_int *c, mp_int *d)
/* find right hand */
t2.dp[0] = ((i - 2) < 0) ? 0u : x.dp[i - 2];
t2.dp[1] = x.dp[i - 1]; /* i >= 1 always holds */
t2.dp[2] = x.dp[i];
t2.dp[2] = xdpi;
t2.used = 3;
} while (mp_cmp_mag(&t1, &t2) == MP_GT);

Expand Down