Split ec_point_mul_scalar into two operations.
While it appears that we internally support constant-time
dual multiplication, it is not actually constant-time. Integrating the
two operations means we hit the doubling branch. Instead, replace the
constant-time functions with single multiplication functions, one for
arbitrary points and one for the base point. This simplifies timing
analysis of the EC_METHODs.
This CL only changes the wrapper functions. A subsequent CL will change
the EC_METHOD hooks. We conservatively assume EC_POINT_mul callers
expect secret scalars and split it into two multiplications and an
addition if needed.
Update-Note: EC_POINT_mul may get slower if called with both g_scalar
and p_scalar non-NULL. If the scalars were secret, this plugs a timing
leak (note neither ECDH nor ECDSA signing use such an operation). If
acting on public scalars, notably ECDSA verify, this slowdown is not
inherently necessary. If necessary, we can expose a public version of
ec_point_mul_scalar_public, but callers should be using BoringSSL's
ECDSA verify API instead.
Change-Id: I9c20b660ce4b58dc633588cfd5b2e97a40203ec3
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/36224
Commit-Queue: Adam Langley <agl@google.com>
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/crypto/ec_extra/ec_asn1.c b/crypto/ec_extra/ec_asn1.c
index 6e21275..31988f3 100644
--- a/crypto/ec_extra/ec_asn1.c
+++ b/crypto/ec_extra/ec_asn1.c
@@ -159,8 +159,8 @@
(point_conversion_form_t)(CBS_data(&public_key)[0] & ~0x01);
} else {
// Compute the public key instead.
- if (!ec_point_mul_scalar(group, &ret->pub_key->raw, &ret->priv_key->scalar,
- NULL, NULL)) {
+ if (!ec_point_mul_scalar_base(group, &ret->pub_key->raw,
+ &ret->priv_key->scalar)) {
goto err;
}
// Remember the original private-key-only encoding.
diff --git a/crypto/ecdh_extra/ecdh_extra.c b/crypto/ecdh_extra/ecdh_extra.c
index 1e08099..b8a099a 100644
--- a/crypto/ecdh_extra/ecdh_extra.c
+++ b/crypto/ecdh_extra/ecdh_extra.c
@@ -95,7 +95,7 @@
EC_RAW_POINT shared_point;
uint8_t buf[EC_MAX_BYTES];
size_t buf_len;
- if (!ec_point_mul_scalar(group, &shared_point, NULL, &pub_key->raw, priv) ||
+ if (!ec_point_mul_scalar(group, &shared_point, &pub_key->raw, priv) ||
!ec_point_get_affine_coordinate_bytes(group, buf, NULL, &buf_len,
sizeof(buf), &shared_point)) {
OPENSSL_PUT_ERROR(ECDH, ECDH_R_POINT_ARITHMETIC_FAILURE);
diff --git a/crypto/fipsmodule/ec/ec.c b/crypto/fipsmodule/ec/ec.c
index a0305a6..705d45f 100644
--- a/crypto/fipsmodule/ec/ec.c
+++ b/crypto/fipsmodule/ec/ec.c
@@ -892,8 +892,6 @@
}
int ret = 0;
- EC_SCALAR g_scalar_storage, p_scalar_storage;
- EC_SCALAR *g_scalar_arg = NULL, *p_scalar_arg = NULL;
BN_CTX *new_ctx = NULL;
if (ctx == NULL) {
new_ctx = BN_CTX_new();
@@ -903,27 +901,43 @@
ctx = new_ctx;
}
+ // If both |g_scalar| and |p_scalar| are non-NULL,
+ // |ec_point_mul_scalar_public| would share the doublings between the two
+ // products, which would be more efficient. However, we conservatively assume
+ // the caller needs a constant-time operation. (ECDSA verification does not
+ // use this function.)
+ //
+ // Previously, the low-level constant-time multiplication function aligned
+ // with this function's calling convention, but this was misleading. Curves
+ // which combined the two multiplications did not avoid the doubling case
+ // in the incomplete addition formula and were not constant-time.
+
if (g_scalar != NULL) {
- if (!arbitrary_bignum_to_scalar(group, &g_scalar_storage, g_scalar, ctx)) {
+ EC_SCALAR scalar;
+ if (!arbitrary_bignum_to_scalar(group, &scalar, g_scalar, ctx) ||
+ !ec_point_mul_scalar_base(group, &r->raw, &scalar)) {
goto err;
}
- g_scalar_arg = &g_scalar_storage;
}
if (p_scalar != NULL) {
- if (!arbitrary_bignum_to_scalar(group, &p_scalar_storage, p_scalar, ctx)) {
+ EC_SCALAR scalar;
+ EC_RAW_POINT tmp;
+ if (!arbitrary_bignum_to_scalar(group, &scalar, p_scalar, ctx) ||
+ !ec_point_mul_scalar(group, &tmp, &p->raw, &scalar)) {
goto err;
}
- p_scalar_arg = &p_scalar_storage;
+ if (g_scalar == NULL) {
+ OPENSSL_memcpy(&r->raw, &tmp, sizeof(EC_RAW_POINT));
+ } else {
+ group->meth->add(group, &r->raw, &r->raw, &tmp);
+ }
}
- ret = ec_point_mul_scalar(group, &r->raw, g_scalar_arg,
- p == NULL ? NULL : &p->raw, p_scalar_arg);
+ ret = 1;
err:
BN_CTX_free(new_ctx);
- OPENSSL_cleanse(&g_scalar_storage, sizeof(g_scalar_storage));
- OPENSSL_cleanse(&p_scalar_storage, sizeof(p_scalar_storage));
return ret;
}
@@ -941,15 +955,24 @@
}
int ec_point_mul_scalar(const EC_GROUP *group, EC_RAW_POINT *r,
- const EC_SCALAR *g_scalar, const EC_RAW_POINT *p,
- const EC_SCALAR *p_scalar) {
- if ((g_scalar == NULL && p_scalar == NULL) ||
- (p == NULL) != (p_scalar == NULL)) {
+ const EC_RAW_POINT *p, const EC_SCALAR *scalar) {
+ if (p == NULL || scalar == NULL) {
OPENSSL_PUT_ERROR(EC, ERR_R_PASSED_NULL_PARAMETER);
return 0;
}
- group->meth->mul(group, r, g_scalar, p, p_scalar);
+ group->meth->mul(group, r, NULL, p, scalar);
+ return 1;
+}
+
+int ec_point_mul_scalar_base(const EC_GROUP *group, EC_RAW_POINT *r,
+ const EC_SCALAR *scalar) {
+ if (scalar == NULL) {
+ OPENSSL_PUT_ERROR(EC, ERR_R_PASSED_NULL_PARAMETER);
+ return 0;
+ }
+
+ group->meth->mul(group, r, scalar, NULL, NULL);
return 1;
}
diff --git a/crypto/fipsmodule/ec/ec_key.c b/crypto/fipsmodule/ec/ec_key.c
index 3ef17d9..3851c19 100644
--- a/crypto/fipsmodule/ec/ec_key.c
+++ b/crypto/fipsmodule/ec/ec_key.c
@@ -322,8 +322,8 @@
if (eckey->priv_key != NULL) {
point = EC_POINT_new(eckey->group);
if (point == NULL ||
- !ec_point_mul_scalar(eckey->group, &point->raw,
- &eckey->priv_key->scalar, NULL, NULL)) {
+ !ec_point_mul_scalar_base(eckey->group, &point->raw,
+ &eckey->priv_key->scalar)) {
OPENSSL_PUT_ERROR(EC, ERR_R_EC_LIB);
goto err;
}
@@ -440,8 +440,7 @@
// Generate the private key by testing candidates (FIPS 186-4 B.4.2).
!ec_random_nonzero_scalar(key->group, &priv_key->scalar,
kDefaultAdditionalData) ||
- !ec_point_mul_scalar(key->group, &pub_key->raw, &priv_key->scalar, NULL,
- NULL)) {
+ !ec_point_mul_scalar_base(key->group, &pub_key->raw, &priv_key->scalar)) {
EC_POINT_free(pub_key);
ec_wrapped_scalar_free(priv_key);
return 0;
diff --git a/crypto/fipsmodule/ec/ec_test.cc b/crypto/fipsmodule/ec/ec_test.cc
index 1219e2b..c0ad61f 100644
--- a/crypto/fipsmodule/ec/ec_test.cc
+++ b/crypto/fipsmodule/ec/ec_test.cc
@@ -764,7 +764,15 @@
ASSERT_TRUE(BN_set_word(bn32.get(), 32));
ASSERT_TRUE(EC_POINT_mul(group(), ret.get(), bn32.get(), p.get(), bn31.get(),
nullptr));
+ EXPECT_EQ(0, EC_POINT_cmp(group(), ret.get(), g, nullptr));
+ // Repeat the computation with |ec_point_mul_scalar_public|, which ties the
+ // additions together.
+ EC_SCALAR sc31, sc32;
+ ASSERT_TRUE(ec_bignum_to_scalar(group(), &sc31, bn31.get()));
+ ASSERT_TRUE(ec_bignum_to_scalar(group(), &sc32, bn32.get()));
+ ASSERT_TRUE(
+ ec_point_mul_scalar_public(group(), &ret->raw, &sc32, &p->raw, &sc31));
EXPECT_EQ(0, EC_POINT_cmp(group(), ret.get(), g, nullptr));
}
diff --git a/crypto/fipsmodule/ec/internal.h b/crypto/fipsmodule/ec/internal.h
index 05175a5..a29468f 100644
--- a/crypto/fipsmodule/ec/internal.h
+++ b/crypto/fipsmodule/ec/internal.h
@@ -325,13 +325,15 @@
int ec_scalar_inv_montgomery_vartime(const EC_GROUP *group, EC_SCALAR *r,
const EC_SCALAR *a);
-// ec_point_mul_scalar sets |r| to generator * |g_scalar| + |p| *
-// |p_scalar|. Unlike other functions which take |EC_SCALAR|, |g_scalar| and
-// |p_scalar| need not be fully reduced. They need only contain as many bits as
-// the order.
+// ec_point_mul_scalar sets |r| to |p| * |scalar|. Both inputs are considered
+// secret.
int ec_point_mul_scalar(const EC_GROUP *group, EC_RAW_POINT *r,
- const EC_SCALAR *g_scalar, const EC_RAW_POINT *p,
- const EC_SCALAR *p_scalar);
+ const EC_RAW_POINT *p, const EC_SCALAR *scalar);
+
+// ec_point_mul_scalar_base sets |r| to generator * |scalar|. |scalar| is
+// treated as secret.
+int ec_point_mul_scalar_base(const EC_GROUP *group, EC_RAW_POINT *r,
+ const EC_SCALAR *scalar);
// ec_point_mul_scalar_public performs the same computation as
// ec_point_mul_scalar. It further assumes that the inputs are public so
diff --git a/crypto/fipsmodule/ecdh/ecdh.c b/crypto/fipsmodule/ecdh/ecdh.c
index b9dc237..a7b2f08 100644
--- a/crypto/fipsmodule/ecdh/ecdh.c
+++ b/crypto/fipsmodule/ecdh/ecdh.c
@@ -93,7 +93,7 @@
EC_RAW_POINT shared_point;
uint8_t buf[EC_MAX_BYTES];
size_t buflen;
- if (!ec_point_mul_scalar(group, &shared_point, NULL, &pub_key->raw, priv) ||
+ if (!ec_point_mul_scalar(group, &shared_point, &pub_key->raw, priv) ||
!ec_point_get_affine_coordinate_bytes(group, buf, NULL, &buflen,
sizeof(buf), &shared_point)) {
OPENSSL_PUT_ERROR(ECDH, ECDH_R_POINT_ARITHMETIC_FAILURE);
diff --git a/crypto/fipsmodule/ecdsa/ecdsa.c b/crypto/fipsmodule/ecdsa/ecdsa.c
index 010ee02..38771d5 100644
--- a/crypto/fipsmodule/ecdsa/ecdsa.c
+++ b/crypto/fipsmodule/ecdsa/ecdsa.c
@@ -232,7 +232,7 @@
ec_scalar_from_montgomery(group, out_kinv_mont, out_kinv_mont);
// Compute r, the x-coordinate of generator * k.
- if (!ec_point_mul_scalar(group, &tmp_point, &k, NULL, NULL) ||
+ if (!ec_point_mul_scalar_base(group, &tmp_point, &k) ||
!ec_get_x_coordinate_as_scalar(group, out_r, &tmp_point)) {
goto err;
}
