Fix: removed sign operation in s_mp_invmod_odd

Changed a check for <0 with mp_isneg() in s_mp_invmod

Additional tests for mp_invmod() in demo/test.c

Author: czurnieden

demo/test.c

@@ -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 */
@@ -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:

s_mp_invmod.c

@@ -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;
    }
 

s_mp_invmod_odd.c

@@ -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   */
@@ -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! */
@@ -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;
    }
@@ -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: