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Always generate DH keys for ephemeral DH cipher suites.

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Reviewed-by: Matt Caswell <matt@openssl.org>
This commit is contained in:
Dr. Stephen Henson 2015-12-17 02:57:20 +00:00
parent d938e8dfee
commit ffaef3f152
4 changed files with 14 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. only used for signing.
In order to perform a DH key exchange the server must use a DH group In order to perform a DH key exchange the server must use a DH group
(DH parameters) and generate a DH key. (DH parameters) and generate a DH key. The server will always generate
The server will always generate a new DH key during the negotiation 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).
As generating DH parameters is extremely time consuming, an application As generating DH parameters is extremely time consuming, an application
should not generate the parameters on the fly but supply the parameters. should not generate the parameters on the fly but supply the parameters.
@ -92,10 +75,9 @@ can supply the DH parameters via a callback function.
Previous versions of the callback used B<is_export> and B<keylength> Previous versions of the callback used B<is_export> and B<keylength>
parameters to control parameter generation for export and non-export parameters to control parameter generation for export and non-export
cipher suites. Modern servers that do not support export ciphersuites cipher suites. Modern servers that do not support export ciphersuites
are advised to either use SSL_CTX_set_tmp_dh() in combination with are advised to either use SSL_CTX_set_tmp_dh() or alternatively, use
SSL_OP_SINGLE_DH_USE, or alternatively, use the callback but ignore the callback but ignore B<keylength> and B<is_export> and simply
B<keylength> and B<is_export> and simply supply at least 2048-bit supply at least 2048-bit parameters in the callback.
parameters in the callback.
=head1 EXAMPLES =head1 EXAMPLES
@ -127,7 +109,6 @@ partly left out.)
if (SSL_CTX_set_tmp_dh(ctx, dh_2048) != 1) { if (SSL_CTX_set_tmp_dh(ctx, dh_2048) != 1) {
/* Error. */ /* Error. */
} }
SSL_CTX_set_options(ctx, SSL_OP_SINGLE_DH_USE);
... ...
=head1 RETURN VALUES =head1 RETURN VALUES

4
include/openssl/ssl.h
View File

@ -411,8 +411,8 @@ typedef int (*custom_ext_parse_cb) (SSL *s, unsigned int ext_type,
# define SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION 0x00040000U # define SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION 0x00040000U
/* Does nothing: retained for compatibility */ /* Does nothing: retained for compatibility */
# define SSL_OP_SINGLE_ECDH_USE 0x0 # define SSL_OP_SINGLE_ECDH_USE 0x0
/* If set, always create a new key when using tmp_dh parameters */ /* Does nothing: retained for compatibility */
# define SSL_OP_SINGLE_DH_USE 0x00100000U # define SSL_OP_SINGLE_DH_USE 0x0
/* Does nothing: retained for compatibiity */ /* Does nothing: retained for compatibiity */
# define SSL_OP_EPHEMERAL_RSA 0x0 # define SSL_OP_EPHEMERAL_RSA 0x0
/* /*

9
ssl/s3_lib.c
View File

@ -3499,13 +3499,6 @@ long ssl3_ctrl(SSL *s, int cmd, long larg, void *parg)
SSLerr(SSL_F_SSL3_CTRL, ERR_R_DH_LIB); SSLerr(SSL_F_SSL3_CTRL, ERR_R_DH_LIB);
return (ret); 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);
}
}
DH_free(s->cert->dh_tmp); DH_free(s->cert->dh_tmp);
s->cert->dh_tmp = dh; s->cert->dh_tmp = dh;
ret = 1; ret = 1;
@ -3887,13 +3880,11 @@ long ssl3_ctx_ctrl(SSL_CTX *ctx, int cmd, long larg, void *parg)
SSLerr(SSL_F_SSL3_CTX_CTRL, ERR_R_DH_LIB); SSLerr(SSL_F_SSL3_CTX_CTRL, ERR_R_DH_LIB);
return 0; return 0;
} }
if (!(ctx->options & SSL_OP_SINGLE_DH_USE)) {
if (!DH_generate_key(new)) { if (!DH_generate_key(new)) {
SSLerr(SSL_F_SSL3_CTX_CTRL, ERR_R_DH_LIB); SSLerr(SSL_F_SSL3_CTX_CTRL, ERR_R_DH_LIB);
DH_free(new); DH_free(new);
return 0; return 0;
} }
}
DH_free(cert->dh_tmp); DH_free(cert->dh_tmp);
cert->dh_tmp = new; cert->dh_tmp = new;
return 1; return 1;

11
ssl/statem/statem_srvr.c
View File

@ -1800,21 +1800,10 @@ int tls_construct_server_key_exchange(SSL *s)
} }
s->s3->tmp.dh = dh; 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)) { if (!DH_generate_key(dh)) {
SSLerr(SSL_F_TLS_CONSTRUCT_SERVER_KEY_EXCHANGE, ERR_R_DH_LIB); SSLerr(SSL_F_TLS_CONSTRUCT_SERVER_KEY_EXCHANGE, ERR_R_DH_LIB);
goto err; 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_TLS_CONSTRUCT_SERVER_KEY_EXCHANGE, ERR_R_DH_LIB);
goto err;
}
}
r[0] = dh->p; r[0] = dh->p;
r[1] = dh->g; r[1] = dh->g;
r[2] = dh->pub_key; r[2] = dh->pub_key;