diff options
author | Nicola Tuveri <nic.tuv@gmail.com> | 2019-09-07 18:05:31 +0300 |
---|---|---|
committer | Nicola Tuveri <nic.tuv@gmail.com> | 2019-09-09 14:03:25 +0300 |
commit | bacaa618c26411d212015493d0eb82076a3e76a1 (patch) | |
tree | 0369481e8cb646be05d6fa28b4a5e573a88bf658 /crypto | |
parent | bd9faf117db1e53b2ad2ee9cbfe8def8c98ecb7b (diff) |
[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
b783beeadf6b80bc431e6f3230b5d5585c87ef87 (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.
Responsible Disclosure
----------------------
This and the other issues presented in https://arxiv.org/abs/1909.01785
were reported by Cesar Pereida GarcĂa, 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.
_______________________________________________________________________________
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/9808)
Diffstat (limited to 'crypto')
-rw-r--r-- | crypto/ec/ec_asn1.c | 71 |
1 files changed, 67 insertions, 4 deletions
diff --git a/crypto/ec/ec_asn1.c b/crypto/ec/ec_asn1.c index 3a8128b755..e0d02bb01a 100644 --- a/crypto/ec/ec_asn1.c +++ b/crypto/ec/ec_asn1.c @@ -571,10 +571,12 @@ 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) { @@ -792,18 +794,79 @@ 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); + + BN_CTX_free(ctx); + return ret; } @@ -864,7 +927,7 @@ 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; } @@ -912,7 +975,7 @@ 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); } |