bn_exp: also special case -1 modulus

Anything taken to the power of 0 is 1, and then reduced mod 1 or mod -1 it
will be 0. If "anything" includes 0 or not is a matter of convention, but
it should not depend on the sign of the modulus...

Reported by Guido Vranken

ok jsing (who had the same diff)

diff --git a/lib/libcrypto/bn/bn_exp.c b/lib/libcrypto/bn/bn_exp.c
index ff99335..9e5d1fd 100644 (file)
--- a/lib/libcrypto/bn/bn_exp.c
+++ b/lib/libcrypto/bn/bn_exp.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: bn_exp.c,v 1.45 2023/03/30 14:21:10 tb Exp $ */
+/* $OpenBSD: bn_exp.c,v 1.46 2023/05/09 05:38:11 tb Exp $ */
 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
  * All rights reserved.
  *
@@ -194,7 +194,7 @@ BN_mod_exp_simple(BIGNUM *r, const BIGNUM *a, const BIGNUM *p, const BIGNUM *m,
        bits = BN_num_bits(p);
        if (bits == 0) {
                /* x**0 mod 1 is still zero. */
-               if (BN_is_one(m)) {
+               if (BN_abs_is_word(m, 1)) {
                        ret = 1;
                        BN_zero(r);
                } else
@@ -402,7 +402,7 @@ BN_mod_exp_mont_consttime(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)) {
+               if (BN_abs_is_word(m, 1)) {
                        ret = 1;
                        BN_zero(rr);
                } else
@@ -658,7 +658,7 @@ BN_mod_exp_mont_internal(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p, const BIG
        bits = BN_num_bits(p);
        if (bits == 0) {
                /* x**0 mod 1 is still zero. */
-               if (BN_is_one(m)) {
+               if (BN_abs_is_word(m, 1)) {
                        ret = 1;
                        BN_zero(rr);
                } else
@@ -843,7 +843,7 @@ BN_mod_exp_mont_word(BIGNUM *rr, BN_ULONG a, const BIGNUM *p, const BIGNUM *m,
        bits = BN_num_bits(p);
        if (bits == 0) {
                /* x**0 mod 1 is still zero. */
-               if (BN_is_one(m)) {
+               if (BN_abs_is_word(m, 1)) {
                        ret = 1;
                        BN_zero(rr);
                } else
@@ -968,7 +968,7 @@ BN_mod_exp_recp(BIGNUM *r, const BIGNUM *a, const BIGNUM *p, const BIGNUM *m,
        bits = BN_num_bits(p);
        if (bits == 0) {
                /* x**0 mod 1 is still zero. */
-               if (BN_is_one(m)) {
+               if (BN_abs_is_word(m, 1)) {
                        ret = 1;
                        BN_zero(r);
                } else