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Fix: removed sign operation in s_mp_invmod_odd (issue #534) #535

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Merged
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Oct 28, 2022
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Fix: removed sign operation in s_mp_invmod_odd (issue #534) #535

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72 changes: 72 additions & 0 deletions demo/test.c
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Original file line number Diff line number Diff line change
Expand Up @@ -549,6 +549,54 @@ static int test_mp_and(void)
static int test_mp_invmod(void)
{
mp_int a, b, c, d;
int i, j, k;
int e;

int results[21][21] =
/* Table generated with Pari/GP

for(i=-10,10,
k=0;
d=0;
printf(" {");
for(j=-10,10,
iferr(
printf(lift(Mod(1/i, j)) ", "),
k,
printf("-1, "))
);
print("},")
)

Changes to the output: replaced j < 1 with -1 for now and added the result of 0^(-1) mod (1)

j = -10, -9, -8, -7, -6, -5, -4, -3, -2, -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 */

{
{-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 2, -1, -1, -1, 2, -1, 8, -1 }, /* i = -10 */
{-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 1, -1, 3, 1, -1, 3, 7, -1, 1 }, /* -9 */
{-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 1, -1, 3, -1, 6, -1, 1, -1 }, /* -8 */
{-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 1, 2, 1, 2, 5, -1, 1, 5, 7 }, /* -7 */
{-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 4, -1, 1, -1, -1, -1 }, /* -6 */
{-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 1, 1, 3, -1, 1, 4, 3, 7, -1 }, /* -5 */
{-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 2, -1, 1, -1, 5, -1, 2, -1 }, /* -4 */
{-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 1, -1, 1, 3, -1, 2, 5, -1, 3 }, /* -3 */
{-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 1, -1, 2, -1, 3, -1, 4, -1 }, /* -2 */
{-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 1, 2, 3, 4, 5, 6, 7, 8, 9 }, /* -1 */
{-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 0, -1, -1, -1, -1, -1, -1, -1, -1, -1 }, /* 0 */
{-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1 }, /* 1 */
{-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 0, -1, 2, -1, 3, -1, 4, -1, 5, -1 }, /* 2 */
{-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 0, 1, -1, 3, 2, -1, 5, 3, -1, 7 }, /* 3 */
{-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 0, -1, 1, -1, 4, -1, 2, -1, 7, -1 }, /* 4 */
{-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 0, 1, 2, 1, -1, 5, 3, 5, 2, -1 }, /* 5 */
{-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 0, -1, -1, -1, 1, -1, 6, -1, -1, -1 }, /* 6 */
{-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 0, 1, 1, 3, 3, 1, -1, 7, 4, 3 }, /* 7 */
{-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 0, -1, 2, -1, 2, -1, 1, -1, 8, -1 }, /* 8 */
{-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 0, 1, -1, 1, 4, -1, 4, 1, -1, 9 }, /* 9 */
{-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 0, -1, 1, -1, -1, -1, 5, -1, 1, -1 } /* 10 */
};


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

/* mp_invmod corner-case of https://github.com/libtom/libtommath/issues/118 */
Expand All @@ -564,6 +612,30 @@ static int test_mp_invmod(void)
EXPECT(mp_cmp(&c, &d) == MP_EQ);
}

/* Some small general tests https://github.com/libtom/libtommath/issues/534 */
for (i = -10; i < 11; i++) {
for (j = -10; j < 11; j++) {
mp_set_i32(&a, i);
mp_set_i32(&b, j);
e = mp_invmod(&a, &b, &c);
if (e != MP_OKAY) {
if (results[i+10][j+10] != -1) {
printf("error = %s from ", mp_error_to_string(e));
printf("error at i = %d, j =%d should be an error but gave ",i,j);
e = mp_fwrite(&c,10,stdout);
printf("\n");
goto LBL_ERR;
}
} else {
k = mp_get_i32(&c);
if (k != results[i+10][j+10]) {
printf("result at i = %d, j =%d is %d but should be %d \n", i,j,k,results[i+10][j+10]);
goto LBL_ERR;
}
}
}
}

mp_clear_multi(&a, &b, &c, &d, NULL);
return EXIT_SUCCESS;
LBL_ERR:
Expand Down
2 changes: 1 addition & 1 deletion s_mp_invmod.c
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Original file line number Diff line number Diff line change
Expand Up @@ -98,7 +98,7 @@ mp_err s_mp_invmod(const mp_int *a, const mp_int *b, mp_int *c)
}

/* if its too low */
while (mp_cmp_d(&C, 0uL) == MP_LT) {
while (mp_isneg(&C)) {
if ((err = mp_add(&C, b, &C)) != MP_OKAY) goto LBL_ERR;
}

Expand Down
5 changes: 1 addition & 4 deletions s_mp_invmod_odd.c
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Original file line number Diff line number Diff line change
Expand Up @@ -12,7 +12,6 @@
mp_err s_mp_invmod_odd(const mp_int *a, const mp_int *b, mp_int *c)
{
mp_int x, y, u, v, B, D;
mp_sign sign;
mp_err err;

/* 2. [modified] b must be odd */
Expand All @@ -28,7 +27,7 @@ mp_err s_mp_invmod_odd(const mp_int *a, const mp_int *b, mp_int *c)
/* x == modulus, y == value to invert */
if ((err = mp_copy(b, &x)) != MP_OKAY) goto LBL_ERR;

/* we need y = |a| */
/* y needs to be positive but the remainder d of mp_div(a,b,c,d) might be negative */
if ((err = mp_mod(a, b, &y)) != MP_OKAY) goto LBL_ERR;

/* if one of x,y is zero return an error! */
Expand Down Expand Up @@ -95,7 +94,6 @@ mp_err s_mp_invmod_odd(const mp_int *a, const mp_int *b, mp_int *c)
}

/* b is now the inverse */
sign = a->sign;
while (mp_isneg(&D)) {
if ((err = mp_add(&D, b, &D)) != MP_OKAY) goto LBL_ERR;
}
Expand All @@ -106,7 +104,6 @@ mp_err s_mp_invmod_odd(const mp_int *a, const mp_int *b, mp_int *c)
}

mp_exch(&D, c);
c->sign = sign;
err = MP_OKAY;

LBL_ERR:
Expand Down