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[ec] Match built-in curves on EC_GROUP_new_from_ecparameters 

Description
-----------

Upon `EC_GROUP_new_from_ecparameters()` check if the parameters match any
of the built-in curves. If that is the case, return a new
`EC_GROUP_new_by_curve_name()` object instead of the explicit parameters
`EC_GROUP`.

This affects all users of `EC_GROUP_new_from_ecparameters()`:
- direct calls to `EC_GROUP_new_from_ecparameters()`
- direct calls to `EC_GROUP_new_from_ecpkparameters()` with an explicit
  parameters argument
- ASN.1 parsing of explicit parameters keys (as it eventually
  ends up calling `EC_GROUP_new_from_ecpkparameters()`)

A parsed explicit parameter key will still be marked with the
`OPENSSL_EC_EXPLICIT_CURVE` ASN.1 flag on load, so, unless
programmatically forced otherwise, if the key is eventually serialized
the output will still be encoded with explicit parameters, even if
internally it is treated as a named curve `EC_GROUP`.

Before this change, creating any `EC_GROUP` object using
`EC_GROUP_new_from_ecparameters()`, yielded an object associated with
the default generic `EC_METHOD`, but this was never guaranteed in the
documentation.
After this commit, users of the library that intentionally want to
create an `EC_GROUP` object using a specific `EC_METHOD` can still
explicitly call `EC_GROUP_new(foo_method)` and then manually set the
curve parameters using `EC_GROUP_set_*()`.

Motivation
----------

This has obvious performance benefits for the built-in curves with
specialized `EC_METHOD`s and subtle but important security benefits:
- the specialized methods have better security hardening than the
  generic implementations
- optional fields in the parameter encoding, like the `cofactor`, cannot
  be leveraged by an attacker to force execution of the less secure
  code-paths for single point scalar multiplication
- in general, this leads to reducing the attack surface

Check the manuscript at https://arxiv.org/abs/1909.01785 for an in depth
analysis of the issues related to this commit.

It should be noted that `libssl` does not allow to negotiate explicit
parameters (as per RFC 8422), so it is not directly affected by the
consequences of using explicit parameters that this commit fixes.
On the other hand, we detected external applications and users in the
wild that use explicit parameters by default (and sometimes using 0 as
the cofactor value, which is technically not a valid value per the
specification, but is tolerated by parsers for wider compatibility given
that the field is optional).
These external users of `libcrypto` are exposed to these vulnerabilities
and their security will benefit from this commit.

Related commits
---------------

While this commit is beneficial for users using built-in curves and
explicit parameters encoding for serialized keys, commit
b783beeadf (and its equivalents for the
1.0.2, 1.1.0 and 1.1.1 stable branches) fixes the consequences of the
invalid cofactor values more in general also for other curves
(CVE-2019-1547).

The following list covers commits in `master` that are related to the
vulnerabilities presented in the manuscript motivating this commit:

- d2baf88c43 [crypto/rsa] Set the constant-time flag in multi-prime RSA too
- 311e903d84 [crypto/asn1] Fix multiple SCA vulnerabilities during RSA key validation.
- b783beeadf [crypto/ec] for ECC parameters with NULL or zero cofactor, compute it
- 724339ff44 Fix SCA vulnerability when using PVK and MSBLOB key formats

Note that the PRs that contributed the listed commits also include other
commits providing related testing and documentation, in addition to
links to PRs and commits backporting the fixes to the 1.0.2, 1.1.0 and
1.1.1 branches.

This commit includes a partial backport of
https://github.com/openssl/openssl/pull/8555
(commit 8402cd5f75)
for which the main author is Shane Lontis.

Responsible Disclosure
----------------------

This and the other issues presented in https://arxiv.org/abs/1909.01785
were reported by Cesar Pereida García, Sohaib ul Hassan, Nicola Tuveri,
Iaroslav Gridin, Alejandro Cabrera Aldaya and Billy Bob Brumley from the
NISEC group at Tampere University, FINLAND.

The OpenSSL Security Team evaluated the security risk for this
vulnerability as low, and encouraged to propose fixes using public Pull
Requests.

_______________________________________________________________________________

Co-authored-by: Shane Lontis <shane.lontis@oracle.com>

(Backport from https://github.com/openssl/openssl/pull/9808)

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9810)
This commit is contained in:
Nicola Tuveri 2019-09-07 18:05:31 +03:00
parent d374a61bb7
commit d4a5dac9f9
4 changed files with 236 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

11
CHANGES
View File

@ -9,6 +9,17 @@
Changes between 1.1.0k and 1.1.0l [xx XXX xxxx]
*) For built-in EC curves, ensure an EC_GROUP built from the curve name is
used even when parsing explicit parameters, when loading a serialized key
or calling `EC_GROUP_new_from_ecpkparameters()`/
`EC_GROUP_new_from_ecparameters()`.
This prevents bypass of security hardening and performance gains,
especially for curves with specialized EC_METHODs.
By default, if a key encoded with explicit parameters is loaded and later
serialized, the output is still encoded with explicit parameters, even if
internally a "named" EC_GROUP is used for computation.
[Nicola Tuveri]
*) Compute ECC cofactors if not provided during EC_GROUP construction. Before
this change, EC_GROUP_set_generator would accept order and/or cofactor as
NULL. After this change, only the cofactor parameter can be NULL. It also

73
crypto/ec/ec_asn1.c
View File

@ -599,10 +599,12 @@ ECPKPARAMETERS *EC_GROUP_get_ecpkparameters(const EC_GROUP *group,
EC_GROUP *EC_GROUP_new_from_ecparameters(const ECPARAMETERS *params)
{
int ok = 0, tmp;
EC_GROUP *ret = NULL;
EC_GROUP *ret = NULL, *dup = NULL;
BIGNUM *p = NULL, *a = NULL, *b = NULL;
EC_POINT *point = NULL;
long field_bits;
int curve_name = NID_undef;
BN_CTX *ctx = NULL;
if (!params->fieldID || !params->fieldID->fieldType ||
!params->fieldID->p.ptr) {
@ -815,19 +817,80 @@ EC_GROUP *EC_GROUP_new_from_ecparameters(const ECPARAMETERS *params)
goto err;
}
/*
* Check if the explicit parameters group just created matches one of the
* built-in curves.
*
* We create a copy of the group just built, so that we can remove optional
* fields for the lookup: we do this to avoid the possibility that one of
* the optional parameters is used to force the library into using a less
* performant and less secure EC_METHOD instead of the specialized one.
* In any case, `seed` is not really used in any computation, while a
* cofactor different from the one in the built-in table is just
* mathematically wrong anyway and should not be used.
*/
if ((ctx = BN_CTX_new()) == NULL) {
ECerr(EC_F_EC_GROUP_NEW_FROM_ECPARAMETERS, ERR_R_BN_LIB);
goto err;
}
if ((dup = EC_GROUP_dup(ret)) == NULL
|| EC_GROUP_set_seed(dup, NULL, 0) != 1
|| !EC_GROUP_set_generator(dup, point, a, NULL)) {
ECerr(EC_F_EC_GROUP_NEW_FROM_ECPARAMETERS, ERR_R_EC_LIB);
goto err;
}
if ((curve_name = ec_curve_nid_from_params(dup, ctx)) != NID_undef) {
/*
* The input explicit parameters successfully matched one of the
* built-in curves: often for built-in curves we have specialized
* methods with better performance and hardening.
*
* In this case we replace the `EC_GROUP` created through explicit
* parameters with one created from a named group.
*/
EC_GROUP *named_group = NULL;
#ifndef OPENSSL_NO_EC_NISTP_64_GCC_128
/*
* NID_wap_wsg_idm_ecid_wtls12 and NID_secp224r1 are both aliases for
* the same curve, we prefer the SECP nid when matching explicit
* parameters as that is associated with a specialized EC_METHOD.
*/
if (curve_name == NID_wap_wsg_idm_ecid_wtls12)
curve_name = NID_secp224r1;
#endif /* !def(OPENSSL_NO_EC_NISTP_64_GCC_128) */
if ((named_group = EC_GROUP_new_by_curve_name(curve_name)) == NULL) {
ECerr(EC_F_EC_GROUP_NEW_FROM_ECPARAMETERS, ERR_R_EC_LIB);
goto err;
}
EC_GROUP_free(ret);
ret = named_group;
/*
* Set the flag so that EC_GROUPs created from explicit parameters are
* serialized using explicit parameters by default.
*/
EC_GROUP_set_asn1_flag(ret, OPENSSL_EC_EXPLICIT_CURVE);
}
ok = 1;
err:
if (!ok) {
EC_GROUP_clear_free(ret);
EC_GROUP_free(ret);
ret = NULL;
}
EC_GROUP_free(dup);
BN_free(p);
BN_free(a);
BN_free(b);
EC_POINT_free(point);
return (ret);
BN_CTX_free(ctx);
return ret;
}
EC_GROUP *EC_GROUP_new_from_ecpkparameters(const ECPKPARAMETERS *params)
@ -887,7 +950,7 @@ EC_GROUP *d2i_ECPKParameters(EC_GROUP **a, const unsigned char **in, long len)
}
if (a) {
EC_GROUP_clear_free(*a);
EC_GROUP_free(*a);
*a = group;
}
@ -935,7 +998,7 @@ EC_KEY *d2i_ECPrivateKey(EC_KEY **a, const unsigned char **in, long len)
ret = *a;
if (priv_key->parameters) {
EC_GROUP_clear_free(ret->group);
EC_GROUP_free(ret->group);
ret->group = EC_GROUP_new_from_ecpkparameters(priv_key->parameters);
}

155
crypto/ec/ec_curve.c
View File

@ -3167,3 +3167,158 @@ int EC_curve_nist2nid(const char *name)
}
return NID_undef;
}
static ossl_inline
int ec_group_get_curve(const EC_GROUP *group, BIGNUM *p, BIGNUM *a,
BIGNUM *b, BN_CTX *ctx)
{
int field_nid;
field_nid = EC_METHOD_get_field_type(EC_GROUP_method_of(group));
#ifndef OPENSSL_NO_EC2M
if (field_nid == NID_X9_62_characteristic_two_field) {
return EC_GROUP_get_curve_GF2m(group, p, a, b, ctx);
} else
#endif /* !def(OPENSSL_NO_EC2M) */
if (field_nid == NID_X9_62_prime_field) {
return EC_GROUP_get_curve_GFp(group, p, a, b, ctx);
} else {
/* this should never happen */
return 0;
}
}
static ossl_inline
int ec_point_get_affine_coordinates(const EC_GROUP *group,
const EC_POINT *point, BIGNUM *x,
BIGNUM *y, BN_CTX *ctx)
{
int field_nid;
field_nid = EC_METHOD_get_field_type(EC_GROUP_method_of(group));
#ifndef OPENSSL_NO_EC2M
if (field_nid == NID_X9_62_characteristic_two_field) {
return EC_POINT_get_affine_coordinates_GFp(group, point, x, y, ctx);
} else
#endif /* !def(OPENSSL_NO_EC2M) */
if (field_nid == NID_X9_62_prime_field) {
return EC_POINT_get_affine_coordinates_GF2m(group, point, x, y, ctx);
} else {
/* this should never happen */
return 0;
}
}
#define NUM_BN_FIELDS 6
/*
* Validates EC domain parameter data for known named curves.
* This can be used when a curve is loaded explicitly (without a curve
* name) or to validate that domain parameters have not been modified.
*
* Returns: The nid associated with the found named curve, or NID_undef
* if not found. If there was an error it returns -1.
*/
int ec_curve_nid_from_params(const EC_GROUP *group, BN_CTX *ctx)
{
int ret = -1, nid, len, field_type, param_len;
size_t i, seed_len;
const unsigned char *seed, *params_seed, *params;
unsigned char *param_bytes = NULL;
const EC_CURVE_DATA *data;
const EC_POINT *generator = NULL;
const EC_METHOD *meth;
const BIGNUM *cofactor = NULL;
/* An array of BIGNUMs for (p, a, b, x, y, order) */
BIGNUM *bn[NUM_BN_FIELDS] = {NULL, NULL, NULL, NULL, NULL, NULL};
meth = EC_GROUP_method_of(group);
if (meth == NULL)
return -1;
/* Use the optional named curve nid as a search field */
nid = EC_GROUP_get_curve_name(group);
field_type = EC_METHOD_get_field_type(meth);
seed_len = EC_GROUP_get_seed_len(group);
seed = EC_GROUP_get0_seed(group);
cofactor = EC_GROUP_get0_cofactor(group);
BN_CTX_start(ctx);
/*
* The built-in curves contains data fields (p, a, b, x, y, order) that are
* all zero-padded to be the same size. The size of the padding is
* determined by either the number of bytes in the field modulus (p) or the
* EC group order, whichever is larger.
*/
param_len = BN_num_bytes(group->order);
len = BN_num_bytes(group->field);
if (len > param_len)
param_len = len;
/* Allocate space to store the padded data for (p, a, b, x, y, order) */
param_bytes = OPENSSL_malloc(param_len * NUM_BN_FIELDS);
if (param_bytes == NULL)
goto end;
/* Create the bignums */
for (i = 0; i < NUM_BN_FIELDS; ++i) {
if ((bn[i] = BN_CTX_get(ctx)) == NULL)
goto end;
}
/*
* Fill in the bn array with the same values as the internal curves
* i.e. the values are p, a, b, x, y, order.
*/
/* Get p, a & b */
if (!(ec_group_get_curve(group, bn[0], bn[1], bn[2], ctx)
&& ((generator = EC_GROUP_get0_generator(group)) != NULL)
/* Get x & y */
&& ec_point_get_affine_coordinates(group, generator, bn[3], bn[4], ctx)
/* Get order */
&& EC_GROUP_get_order(group, bn[5], ctx)))
goto end;
/*
* Convert the bignum array to bytes that are joined together to form
* a single buffer that contains data for all fields.
* (p, a, b, x, y, order) are all zero padded to be the same size.
*/
for (i = 0; i < NUM_BN_FIELDS; ++i) {
if (BN_bn2binpad(bn[i], &param_bytes[i*param_len], param_len) <= 0)
goto end;
}
for (i = 0; i < curve_list_length; i++) {
const ec_list_element curve = curve_list[i];
data = curve.data;
/* Get the raw order byte data */
params_seed = (const unsigned char *)(data + 1); /* skip header */
params = params_seed + data->seed_len;
/* Look for unique fields in the fixed curve data */
if (data->field_type == field_type
&& param_len == data->param_len
&& (nid <= 0 || nid == curve.nid)
/* check the optional cofactor (ignore if its zero) */
&& (BN_is_zero(cofactor)
|| BN_is_word(cofactor, (const BN_ULONG)curve.data->cofactor))
/* Check the optional seed (ignore if its not set) */
&& (data->seed_len == 0 || seed_len == 0
|| ((size_t)data->seed_len == seed_len
&& memcmp(params_seed, seed, seed_len) == 0))
/* Check that the groups params match the built-in curve params */
&& memcmp(param_bytes, params, param_len * NUM_BN_FIELDS)
== 0) {
ret = curve.nid;
goto end;
}
}
/* Gets here if the group was not found */
ret = NID_undef;
end:
OPENSSL_free(param_bytes);
BN_CTX_end(ctx);
return ret;
}

2
crypto/ec/ec_lcl.h
View File

@ -582,6 +582,8 @@ int ec_key_simple_generate_key(EC_KEY *eckey);
int ec_key_simple_generate_public_key(EC_KEY *eckey);
int ec_key_simple_check_key(const EC_KEY *eckey);
int ec_curve_nid_from_params(const EC_GROUP *group, BN_CTX *ctx);
/* EC_METHOD definitions */
struct ec_key_method_st {