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Always generate DH keys for ephemeral DH cipher suites

Browse source 
Modified version of the commit ffaef3f15 in the master branch by Stephen
Henson. This makes the SSL_OP_SINGLE_DH_USE option a no-op and always
generates a new DH key for every handshake regardless.

This is a follow on from CVE-2016-0701. This branch is not impacted by
that CVE because it does not support X9.42 style parameters. It is still
possible to generate parameters based on primes that are not "safe",
although by default OpenSSL does not do this. The documentation does
sign post that using such parameters is unsafe if the private DH key is
reused. However to avoid accidental problems or future attacks this commit
has been backported to this branch.

Issue reported by Antonio Sanso

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
This commit is contained in:
Matt Caswell 2015-12-17 02:57:20 +00:00
parent 126ac21c80
commit 8bc643efc8
4 changed files with 9 additions and 53 deletions
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29
doc/ssl/SSL_CTX_set_tmp_dh_callback.pod
View file

@ -48,25 +48,8 @@ even if he gets hold of the normal (certified) key, as this key was
only used for signing.
In order to perform a DH key exchange the server must use a DH group
(DH parameters) and generate a DH key.
The server will always generate a new DH key during the negotiation
if either the DH parameters are supplied via callback or the
SSL_OP_SINGLE_DH_USE option of SSL_CTX_set_options(3) is set (or both).
It will immediately create a DH key if DH parameters are supplied via
SSL_CTX_set_tmp_dh() and SSL_OP_SINGLE_DH_USE is not set.
In this case,
it may happen that a key is generated on initialization without later
being needed, while on the other hand the computer time during the
negotiation is being saved.
If "strong" primes were used to generate the DH parameters, it is not strictly
necessary to generate a new key for each handshake but it does improve forward
secrecy. If it is not assured that "strong" primes were used,
SSL_OP_SINGLE_DH_USE must be used in order to prevent small subgroup
attacks. Always using SSL_OP_SINGLE_DH_USE has an impact on the
computer time needed during negotiation, but it is not very large, so
application authors/users should consider always enabling this option.
The option is required to implement perfect forward secrecy (PFS).
(DH parameters) and generate a DH key. The server will always generate
a new DH key during the negotiation.
As generating DH parameters is extremely time consuming, an application
should not generate the parameters on the fly but supply the parameters.
@ -93,10 +76,9 @@ can supply the DH parameters via a callback function.
Previous versions of the callback used B<is_export> and B<keylength>
parameters to control parameter generation for export and non-export
cipher suites. Modern servers that do not support export ciphersuites
are advised to either use SSL_CTX_set_tmp_dh() in combination with
SSL_OP_SINGLE_DH_USE, or alternatively, use the callback but ignore
B<keylength> and B<is_export> and simply supply at least 2048-bit
parameters in the callback.
are advised to either use SSL_CTX_set_tmp_dh() or alternatively, use
the callback but ignore B<keylength> and B<is_export> and simply
supply at least 2048-bit parameters in the callback.
=head1 EXAMPLES
@ -128,7 +110,6 @@ partly left out.)
if (SSL_CTX_set_tmp_dh(ctx, dh_2048) != 1) {
/* Error. */
}
SSL_CTX_set_options(ctx, SSL_OP_SINGLE_DH_USE);
...
=head1 RETURN VALUES

14
ssl/s3_lib.c
View file

@ -3164,13 +3164,6 @@ long ssl3_ctrl(SSL *s, int cmd, long larg, void *parg)
SSLerr(SSL_F_SSL3_CTRL, ERR_R_DH_LIB);
return (ret);
}
if (!(s->options & SSL_OP_SINGLE_DH_USE)) {
if (!DH_generate_key(dh)) {
DH_free(dh);
SSLerr(SSL_F_SSL3_CTRL, ERR_R_DH_LIB);
return (ret);
}
}
if (s->cert->dh_tmp != NULL)
DH_free(s->cert->dh_tmp);
s->cert->dh_tmp = dh;
@ -3482,13 +3475,6 @@ long ssl3_ctx_ctrl(SSL_CTX *ctx, int cmd, long larg, void *parg)
SSLerr(SSL_F_SSL3_CTX_CTRL, ERR_R_DH_LIB);
return 0;
}
if (!(ctx->options & SSL_OP_SINGLE_DH_USE)) {
if (!DH_generate_key(new)) {
SSLerr(SSL_F_SSL3_CTX_CTRL, ERR_R_DH_LIB);
DH_free(new);
return 0;
}
}
if (cert->dh_tmp != NULL)
DH_free(cert->dh_tmp);
cert->dh_tmp = new;

17
ssl/s3_srvr.c
View file

@ -1729,20 +1729,9 @@ int ssl3_send_server_key_exchange(SSL *s)
}
s->s3->tmp.dh = dh;
if ((dhp->pub_key == NULL ||
dhp->priv_key == NULL ||
(s->options & SSL_OP_SINGLE_DH_USE))) {
if (!DH_generate_key(dh)) {
SSLerr(SSL_F_SSL3_SEND_SERVER_KEY_EXCHANGE, ERR_R_DH_LIB);
goto err;
}
} else {
dh->pub_key = BN_dup(dhp->pub_key);
dh->priv_key = BN_dup(dhp->priv_key);
if ((dh->pub_key == NULL) || (dh->priv_key == NULL)) {
SSLerr(SSL_F_SSL3_SEND_SERVER_KEY_EXCHANGE, ERR_R_DH_LIB);
goto err;
}
if (!DH_generate_key(dh)) {
SSLerr(SSL_F_SSL3_SEND_SERVER_KEY_EXCHANGE, ERR_R_DH_LIB);
goto err;
}
r[0] = dh->p;
r[1] = dh->g;

2
ssl/ssl.h
View file

@ -602,7 +602,7 @@ struct ssl_session_st {
# define SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION 0x00040000L
/* If set, always create a new key when using tmp_ecdh parameters */
# define SSL_OP_SINGLE_ECDH_USE 0x00080000L
/* If set, always create a new key when using tmp_dh parameters */
/* Does nothing: retained for compatibility */
# define SSL_OP_SINGLE_DH_USE 0x00100000L
/* Does nothing: retained for compatibiity */
# define SSL_OP_EPHEMERAL_RSA 0x0