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SP math: sp_invmod changed to not call itself

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When the modulus is even, calculate m^-1 mod a instead and fixup after.
Don't call self to do inverse.
This commit is contained in:
Sean Parkinson
2022-01-18 09:19:12 +10:00
parent ec44747029
commit fc861f3d6d
2 changed files with 49 additions and 32 deletions
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65
wolfcrypt/src/sp_int.c
View File

@ -8109,7 +8109,7 @@ int sp_mulmod(sp_int* a, sp_int* b, sp_int* m, sp_int* r)
*
* @param [in] a SP integer to find inverse of.
* @param [in] m SP integer this is the modulus.
* @param [out] r SP integer to hold result.
* @param [out] r SP integer to hold result. r cannot be m.
*
* @return MP_OKAY on success.
* @return MP_VAL when a, m or r is NULL; a or m is zero; a and m are even or
@ -8125,9 +8125,11 @@ int sp_invmod(sp_int* a, sp_int* m, sp_int* r)
sp_int* c = NULL;
int used = ((m == NULL) || (a == NULL)) ? 1 :
((m->used >= a->used) ? m->used + 1 : a->used + 1);
int evenMod = 0;
DECL_SP_INT_ARRAY(t, used, 4);
(void)used;
if ((a == NULL) || (m == NULL) || (r == NULL)) {
if ((a == NULL) || (m == NULL) || (r == NULL) || (r == m)) {
err = MP_VAL;
}
@ -8143,19 +8145,18 @@ int sp_invmod(sp_int* a, sp_int* m, sp_int* r)
v = t[1];
b = t[2];
c = t[3];
sp_init_size(v, used + 1);
if (_sp_cmp_abs(a, m) != MP_LT) {
err = sp_mod(a, m, v);
a = v;
err = sp_mod(a, m, r);
a = r;
}
}
#ifdef WOLFSSL_SP_INT_NEGATIVE
if ((err == MP_OKAY) && (a->sign == MP_NEG)) {
/* Make 'a' positive */
err = sp_add(m, a, v);
a = v;
err = sp_add(m, a, r);
a = r;
}
#endif
@ -8174,29 +8175,29 @@ int sp_invmod(sp_int* a, sp_int* m, sp_int* r)
}
else if (err != MP_OKAY) {
}
else if (sp_iseven(m)) {
/* a^-1 mod m = m + (1 - m*(m^-1 % a)) / a
* = m - (m*(m^-1 % a) - 1) / a
*/
err = sp_invmod(m, a, r);
if (err == MP_OKAY) {
err = sp_mul(r, m, r);
}
if (err == MP_OKAY) {
_sp_sub_d(r, 1, r);
err = sp_div(r, a, r, NULL);
if (err == MP_OKAY) {
sp_sub(m, r, r);
}
}
}
else {
sp_init_size(u, m->used + 1);
sp_init_size(v, 2*m->used + 1);
sp_init_size(b, m->used + 1);
sp_init_size(c, m->used + 1);
sp_copy(m, u);
sp_copy(a, v);
if (sp_iseven(m)) {
sp_int* ts;
/* a^-1 mod m = m + (1 - m*(m^-1 % a)) / a
* = m - (m*(m^-1 % a) - 1) / a
*/
/* Reverse a and m and perform invmod. */
ts = a;
a = m;
m = ts;
sp_copy(m, u);
sp_mod(a, m, v);
evenMod = 1;
}
else {
sp_copy(m, u);
sp_copy(a, v);
}
_sp_zero(b);
sp_set(c, 1);
@ -8233,6 +8234,20 @@ int sp_invmod(sp_int* a, sp_int* m, sp_int* r)
if (sp_iszero(u)) {
err = MP_VAL;
}
else if (evenMod) {
/* a and m were reversed and now we need to finish operation.
* m - ((m*r - 1) / a) (reverse a and m)
* => a - ((a*r - 1) / m)
*/
err = sp_mul(c, a, v);
if (err == MP_OKAY) {
_sp_sub_d(v, 1, v);
err = sp_div(v, m, v, NULL);
}
if (err == MP_OKAY) {
sp_sub(a, v, r);
}
}
else {
err = sp_copy(c, r);
}

16
wolfcrypt/test/test.c
View File

@ -36660,18 +36660,20 @@ static int mp_test_invmod(mp_int* a, mp_int* m, mp_int* r)
ret = mp_invmod(a, m, r);
if (ret != MP_OKAY)
return -13175;
if (mp_cmp_d(r, 3))
return -13176;
mp_set(a, 3);
mp_set(m, 5);
ret = mp_invmod(a, m, r);
if (ret != MP_OKAY)
return -13176;
return -13177;
#if !defined(WOLFSSL_SP_MATH) || defined(WOLFSSL_SP_INT_NEGATIVE)
mp_read_radix(a, "-3", 16);
ret = mp_invmod(a, m, r);
if (ret != MP_OKAY)
return -13177;
return -13178;
#endif
#if defined(WOLFSSL_SP_MATH_ALL) && defined(HAVE_ECC)
@ -36679,28 +36681,28 @@ static int mp_test_invmod(mp_int* a, mp_int* m, mp_int* r)
mp_set(m, 3);
ret = mp_invmod_mont_ct(a, m, r, 1);
if (ret != MP_VAL)
return -13178;
return -13179;
mp_set(a, 1);
mp_set(m, 0);
ret = mp_invmod_mont_ct(a, m, r, 1);
if (ret != MP_VAL)
return -13179;
return -13180;
mp_set(a, 1);
mp_set(m, 1);
ret = mp_invmod_mont_ct(a, m, r, 1);
if (ret != MP_VAL)
return -13180;
return -13181;
mp_set(a, 1);
mp_set(m, 2);
ret = mp_invmod_mont_ct(a, m, r, 1);
if (ret != MP_VAL)
return -13181;
return -13182;
mp_set(a, 1);
mp_set(m, 3);
ret = mp_invmod_mont_ct(a, m, r, 1);
if (ret != MP_OKAY)
return -13182;
return -13183;
#endif
return 0;