Test that a ^ 0 mod -1 is always 0

Check all functions that do this.
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6355)

test/bntest.c

@@ -2063,6 +2063,53 @@ err:
     return st;
 }
 
+static int test_expmodone(void)
+{
+    int ret = 0, i;
+    BIGNUM *r = BN_new();
+    BIGNUM *a = BN_new();
+    BIGNUM *p = BN_new();
+    BIGNUM *m = BN_new();
+
+    if (!TEST_ptr(r)
+            || !TEST_ptr(a)
+            || !TEST_ptr(p)
+            || !TEST_ptr(p)
+            || !TEST_ptr(m)
+            || !TEST_true(BN_set_word(a, 1))
+            || !TEST_true(BN_set_word(p, 0))
+            || !TEST_true(BN_set_word(m, 1)))
+        goto err;
+
+    /* Calculate r = 1 ^ 0 mod 1, and check the result is always 0 */
+    for (i = 0; i < 2; i++) {
+        if (!TEST_true(BN_mod_exp(r, a, p, m, NULL))
+                || !TEST_BN_eq_zero(r)
+                || !TEST_true(BN_mod_exp_mont(r, a, p, m, NULL, NULL))
+                || !TEST_BN_eq_zero(r)
+                || !TEST_true(BN_mod_exp_mont_consttime(r, a, p, m, NULL, NULL))
+                || !TEST_BN_eq_zero(r)
+                || !TEST_true(BN_mod_exp_mont_word(r, 1, p, m, NULL, NULL))
+                || !TEST_BN_eq_zero(r)
+                || !TEST_true(BN_mod_exp_simple(r, a, p, m, NULL))
+                || !TEST_BN_eq_zero(r)
+                || !TEST_true(BN_mod_exp_recp(r, a, p, m, NULL))
+                || !TEST_BN_eq_zero(r))
+            goto err;
+        /* Repeat for r = 1 ^ 0 mod -1 */
+        if (i == 0)
+            BN_set_negative(m, 1);
+    }
+
+    ret = 1;
+err:
+    BN_free(r);
+    BN_free(a);
+    BN_free(p);
+    BN_free(m);
+    return ret;
+}
+
 static int test_smallprime(void)
 {
     static const int kBits = 10;
@@ -2189,6 +2236,7 @@ int setup_tests(void)
         ADD_TEST(test_negzero);
         ADD_TEST(test_badmod);
         ADD_TEST(test_expmodzero);
+        ADD_TEST(test_expmodone);
         ADD_TEST(test_smallprime);
         ADD_TEST(test_swap);
 #ifndef OPENSSL_NO_EC2M