The result of a ^ 0 mod -1 is 0 not 1
Thanks to Guido Vranken and OSSFuzz for finding this issue.
@@ -178,8 +178,8 @@ int BN_mod_exp_recp(BIGNUM *r, const BIGNUM *a, const BIGNUM *p,
bits = BN_num_bits(p);
if (bits == 0) {
- /* x**0 mod 1 is still zero. */
- if (BN_is_one(m)) {
+ /* x**0 mod 1, or x**0 mod -1 is still zero. */
+ if (BN_abs_is_word(m, 1)) {
ret = 1;
BN_zero(r);
} else {
@@ -320,8 +320,8 @@ int BN_mod_exp_mont(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
}
bits = BN_num_bits(p);
if (bits == 0) {
- /* x**0 mod 1 is still zero. */
- if (BN_is_one(m)) {
+ /* x**0 mod 1, or x**0 mod -1 is still zero. */
+ if (BN_abs_is_word(m, 1)) {
ret = 1;
BN_zero(rr);
} else {
@@ -629,8 +629,8 @@ int BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
*/
bits = p->top * BN_BITS2;
if (bits == 0) {
- /* x**0 mod 1 is still zero. */
- if (BN_is_one(m)) {
+ /* x**0 mod 1, or x**0 mod -1 is still zero. */
+ if (BN_abs_is_word(m, 1)) {
ret = 1;
BN_zero(rr);
} else {
@@ -1143,8 +1143,8 @@ int BN_mod_exp_mont_word(BIGNUM *rr, BN_ULONG a, const BIGNUM *p,
bits = BN_num_bits(p);
if (bits == 0) {
- /* x**0 mod 1 is still zero. */
- if (BN_is_one(m)) {
+ /* x**0 mod 1, or x**0 mod -1 is still zero. */
+ if (BN_abs_is_word(m, 1)) {
ret = 1;
BN_zero(rr);
} else {
@@ -1265,8 +1265,8 @@ int BN_mod_exp_simple(BIGNUM *r, const BIGNUM *a, const BIGNUM *p,
bits = BN_num_bits(p);
if (bits == 0) {
- /* x**0 mod 1 is still zero. */
- if (BN_is_one(m)) {
+ /* x**0 mod 1, or x**0 mod -1 is still zero. */
+ if (BN_abs_is_word(m, 1)) {
ret = 1;
BN_zero(r);
} else {
