Commit fe382afd820d05f7287959067680b3e1c043bb04
2020-03-05T22:37:59
Merge pull request #478 from MasterDuke17/make_exptmod_with_modulus_1_always_0 Give correct values for invmod with modulus of 1
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diff --git a/demo/test.c b/demo/test.c
index 98a8499..2d1d774 100644
--- a/demo/test.c
+++ b/demo/test.c
@@ -135,10 +135,7 @@ static int test_trivial_stuff(void)
mp_set(&b, 1u);
DO(mp_neg(&b, &b));
mp_set(&c, 1u);
- /* I expected this works, but somehow the computer sez no
- * DO(mp_exptmod(&a, &b, &c, &d));
- */
- EXPECT(mp_exptmod(&a, &b, &c, &d) != MP_OKAY);
+ DO(mp_exptmod(&a, &b, &c, &d));
mp_set(&c, 7u);
/* same here */
diff --git a/mp_invmod.c b/mp_invmod.c
index e19c067..2494acb 100644
--- a/mp_invmod.c
+++ b/mp_invmod.c
@@ -6,6 +6,12 @@
/* hac 14.61, pp608 */
mp_err mp_invmod(const mp_int *a, const mp_int *b, mp_int *c)
{
+ /* for all n in N and n > 0, n = 0 mod 1 */
+ if (!mp_isneg(a) && mp_cmp_d(b, 1uL) == MP_EQ) {
+ mp_zero(c);
+ return MP_OKAY;
+ }
+
/* b cannot be negative and has to be >1 */
if (mp_isneg(b) || (mp_cmp_d(b, 1uL) != MP_GT)) {
return MP_VAL;
diff --git a/tommath_class.h b/tommath_class.h
index 5e76e6a..0fe046f 100644
--- a/tommath_class.h
+++ b/tommath_class.h
@@ -440,6 +440,7 @@
#if defined(MP_INVMOD_C)
# define MP_CMP_D_C
+# define MP_ZERO_C
# define S_MP_INVMOD_C
# define S_MP_INVMOD_ODD_C
#endif