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Fix a ** 0 mod 1 = 0 for real this time.

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Commit 2b0180c37fa6ffc48ee40caa831ca398b828e680 attempted to do this but
only hit one of many BN_mod_exp codepaths. Fix remaining variants and add
a test for each method.

Thanks to Hanno Boeck for reporting this issue.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Dr. Stephen Henson <steve@openssl.org>
(cherry picked from commit d911097d7c93e4cfeab624b34d73fe51da158b69)
This commit is contained in:
Emilia Kasper 2015-12-14 16:38:15 +01:00
parent 474a53b3a3
commit 44e4f5b04b
2 changed files with 104 additions and 17 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

39
crypto/bn/bn_exp.c
View File

@ -282,9 +282,14 @@ int BN_mod_exp_recp(BIGNUM *r, const BIGNUM *a, const BIGNUM *p,
}
bits = BN_num_bits(p);
if (bits == 0) {
ret = BN_one(r);
/* x**0 mod 1 is still zero. */
if (BN_is_one(m)) {
ret = 1;
BN_zero(r);
} else {
ret = BN_one(r);
}
return ret;
}
@ -418,7 +423,13 @@ int BN_mod_exp_mont(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
}
bits = BN_num_bits(p);
if (bits == 0) {
ret = BN_one(rr);
/* x**0 mod 1 is still zero. */
if (BN_is_one(m)) {
ret = 1;
BN_zero(rr);
} else {
ret = BN_one(rr);
}
return ret;
}
@ -671,7 +682,13 @@ int BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
bits = BN_num_bits(p);
if (bits == 0) {
ret = BN_one(rr);
/* x**0 mod 1 is still zero. */
if (BN_is_one(m)) {
ret = 1;
BN_zero(rr);
} else {
ret = BN_one(rr);
}
return ret;
}
@ -1182,8 +1199,9 @@ int BN_mod_exp_mont_word(BIGNUM *rr, BN_ULONG a, const BIGNUM *p,
if (BN_is_one(m)) {
ret = 1;
BN_zero(rr);
} else
} else {
ret = BN_one(rr);
}
return ret;
}
if (a == 0) {
@ -1297,9 +1315,14 @@ int BN_mod_exp_simple(BIGNUM *r, const BIGNUM *a, const BIGNUM *p,
}
bits = BN_num_bits(p);
if (bits == 0) {
ret = BN_one(r);
if (bits == 0) {
/* x**0 mod 1 is still zero. */
if (BN_is_one(m)) {
ret = 1;
BN_zero(r);
} else {
ret = BN_one(r);
}
return ret;
}

82
crypto/bn/exptest.c
View File

@ -72,6 +72,25 @@
static const char rnd_seed[] =
"string to make the random number generator think it has entropy";
/*
* Test that r == 0 in test_exp_mod_zero(). Returns one on success,
* returns zero and prints debug output otherwise.
*/
static int a_is_zero_mod_one(const char *method, const BIGNUM *r,
const BIGNUM *a) {
if (!BN_is_zero(r)) {
fprintf(stderr, "%s failed:\n", method);
fprintf(stderr, "a ** 0 mod 1 = r (should be 0)\n");
fprintf(stderr, "a = ");
BN_print_fp(stderr, a);
fprintf(stderr, "\nr = ");
BN_print_fp(stderr, r);
fprintf(stderr, "\n");
return 0;
}
return 1;
}
/*
* test_exp_mod_zero tests that x**0 mod 1 == 0. It returns zero on success.
*/
@ -79,8 +98,9 @@ static int test_exp_mod_zero()
{
BIGNUM a, p, m;
BIGNUM r;
BN_ULONG one_word = 1;
BN_CTX *ctx = BN_CTX_new();
int ret = 1;
int ret = 1, failed = 0;
BN_init(&m);
BN_one(&m);
@ -92,21 +112,65 @@ static int test_exp_mod_zero()
BN_zero(&p);
BN_init(&r);
BN_mod_exp(&r, &a, &p, &m, ctx);
BN_CTX_free(ctx);
if (BN_is_zero(&r))
ret = 0;
else {
printf("1**0 mod 1 = ");
BN_print_fp(stdout, &r);
printf(", should be 0\n");
if (!BN_rand(&a, 1024, 0, 0))
goto err;
if (!BN_mod_exp(&r, &a, &p, &m, ctx))
goto err;
if (!a_is_zero_mod_one("BN_mod_exp", &r, &a))
failed = 1;
if (!BN_mod_exp_recp(&r, &a, &p, &m, ctx))
goto err;
if (!a_is_zero_mod_one("BN_mod_exp_recp", &r, &a))
failed = 1;
if (!BN_mod_exp_simple(&r, &a, &p, &m, ctx))
goto err;
if (!a_is_zero_mod_one("BN_mod_exp_simple", &r, &a))
failed = 1;
if (!BN_mod_exp_mont(&r, &a, &p, &m, ctx, NULL))
goto err;
if (!a_is_zero_mod_one("BN_mod_exp_mont", &r, &a))
failed = 1;
if (!BN_mod_exp_mont_consttime(&r, &a, &p, &m, ctx, NULL)) {
goto err;
}
if (!a_is_zero_mod_one("BN_mod_exp_mont_consttime", &r, &a))
failed = 1;
/*
* A different codepath exists for single word multiplication
* in non-constant-time only.
*/
if (!BN_mod_exp_mont_word(&r, one_word, &p, &m, ctx, NULL))
goto err;
if (!BN_is_zero(&r)) {
fprintf(stderr, "BN_mod_exp_mont_word failed:\n");
fprintf(stderr, "1 ** 0 mod 1 = r (should be 0)\n");
fprintf(stderr, "r = ");
BN_print_fp(stderr, &r);
fprintf(stderr, "\n");
return 0;
}
ret = failed;
err:
BN_free(&r);
BN_free(&a);
BN_free(&p);
BN_free(&m);
BN_CTX_free(ctx);
return ret;
}