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Only allow ephemeral RSA keys in export ciphersuites.

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OpenSSL clients would tolerate temporary RSA keys in non-export
ciphersuites. It also had an option SSL_OP_EPHEMERAL_RSA which
enabled this server side. Remove both options as they are a
protocol violation.

Thanks to Karthikeyan Bhargavan for reporting this issue.
(CVE-2015-0204)
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tim Hudson <tjh@openssl.org>
This commit is contained in:
Dr. Stephen Henson 2014-10-23 17:09:57 +01:00
parent 1cfd7cf3cc
commit 4b4c1fcc88
7 changed files with 38 additions and 57 deletions
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8
CHANGES
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@ -380,6 +380,14 @@
(CVE-2014-3572)
[Steve Henson]
*) Remove non-export ephemeral RSA code on client and server. This code
violated the TLS standard by allowing the use of temporary RSA keys in
non-export ciphersuites and could be used by a server to effectively
downgrade the RSA key length used to a value smaller than the server
certificate. Thanks for Karthikeyan Bhargavan for reporting this issue.
(CVE-2015-0204)
[Steve Henson]
*) Ensure that the session ID context of an SSL is updated when its
SSL_CTX is updated via SSL_set_SSL_CTX.

10
doc/ssl/SSL_CTX_set_options.pod
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@ -158,15 +158,7 @@ temporary/ephemeral DH parameters are used.
=item SSL_OP_EPHEMERAL_RSA
Always use ephemeral (temporary) RSA key when doing RSA operations
(see L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>).
According to the specifications this is only done, when a RSA key
can only be used for signature operations (namely under export ciphers
with restricted RSA keylength). By setting this option, ephemeral
RSA keys are always used. This option breaks compatibility with the
SSL/TLS specifications and may lead to interoperability problems with
clients and should therefore never be used. Ciphers with DHE (ephemeral
Diffie-Hellman) key exchange should be used instead.
This option is no longer implemented and is treated as no op.
=item SSL_OP_CIPHER_SERVER_PREFERENCE

23
doc/ssl/SSL_CTX_set_tmp_rsa_callback.pod
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@ -74,21 +74,14 @@ exchange and use DHE (Ephemeral Diffie-Hellman) key exchange instead
in order to achieve forward secrecy (see
L<SSL_CTX_set_tmp_dh_callback(3)|SSL_CTX_set_tmp_dh_callback(3)>).
On OpenSSL servers ephemeral RSA key exchange is therefore disabled by default
and must be explicitly enabled using the SSL_OP_EPHEMERAL_RSA option of
L<SSL_CTX_set_options(3)|SSL_CTX_set_options(3)>, violating the TLS/SSL
standard. When ephemeral RSA key exchange is required for export ciphers,
it will automatically be used without this option!
An application may either directly specify the key or can supply the key via
a callback function. The callback approach has the advantage, that the
callback may generate the key only in case it is actually needed. As the
generation of a RSA key is however costly, it will lead to a significant
delay in the handshake procedure. Another advantage of the callback function
is that it can supply keys of different size (e.g. for SSL_OP_EPHEMERAL_RSA
usage) while the explicit setting of the key is only useful for key size of
512 bits to satisfy the export restricted ciphers and does give away key length
if a longer key would be allowed.
An application may either directly specify the key or can supply the key via a
callback function. The callback approach has the advantage, that the callback
may generate the key only in case it is actually needed. As the generation of a
RSA key is however costly, it will lead to a significant delay in the handshake
procedure. Another advantage of the callback function is that it can supply
keys of different size while the explicit setting of the key is only useful for
key size of 512 bits to satisfy the export restricted ciphers and does give
away key length if a longer key would be allowed.
The B<tmp_rsa_callback> is called with the B<keylength> needed and
the B<is_export> information. The B<is_export> flag is set, when the

21
ssl/d1_srvr.c
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@ -472,24 +472,15 @@ int dtls1_accept(SSL *s)
case SSL3_ST_SW_KEY_EXCH_B:
alg_k = s->s3->tmp.new_cipher->algorithm_mkey;
/* clear this, it may get reset by
* send_server_key_exchange */
if ((s->options & SSL_OP_EPHEMERAL_RSA)
#ifndef OPENSSL_NO_KRB5
&& !(alg_k & SSL_kKRB5)
#endif /* OPENSSL_NO_KRB5 */
)
/* option SSL_OP_EPHEMERAL_RSA sends temporary RSA key
* even when forbidden by protocol specs
* (handshake may fail as clients are not required to
* be able to handle this) */
s->s3->tmp.use_rsa_tmp=1;
else
s->s3->tmp.use_rsa_tmp=0;
/*
* clear this, it may get reset by
* send_server_key_exchange
*/
s->s3->tmp.use_rsa_tmp=0;
/* only send if a DH key exchange or
* RSA but we have a sign only certificate */
if (s->s3->tmp.use_rsa_tmp
if (0
/* PSK: send ServerKeyExchange if PSK identity
* hint if provided */
#ifndef OPENSSL_NO_PSK

7
ssl/s3_clnt.c
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@ -1623,6 +1623,13 @@ int ssl3_get_key_exchange(SSL *s)
#ifndef OPENSSL_NO_RSA
if (alg_k & SSL_kRSA)
{
/* Temporary RSA keys only allowed in export ciphersuites */
if (!SSL_C_IS_EXPORT(s->s3->tmp.new_cipher))
{
al=SSL_AD_UNEXPECTED_MESSAGE;
SSLerr(SSL_F_SSL3_GET_SERVER_CERTIFICATE,SSL_R_UNEXPECTED_MESSAGE);
goto f_err;
}
if ((rsa=RSA_new()) == NULL)
{
SSLerr(SSL_F_SSL3_GET_KEY_EXCHANGE,ERR_R_MALLOC_FAILURE);

21
ssl/s3_srvr.c
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@ -446,20 +446,11 @@ int ssl3_accept(SSL *s)
case SSL3_ST_SW_KEY_EXCH_B:
alg_k = s->s3->tmp.new_cipher->algorithm_mkey;
/* clear this, it may get reset by
* send_server_key_exchange */
if ((s->options & SSL_OP_EPHEMERAL_RSA)
#ifndef OPENSSL_NO_KRB5
&& !(alg_k & SSL_kKRB5)
#endif /* OPENSSL_NO_KRB5 */
)
/* option SSL_OP_EPHEMERAL_RSA sends temporary RSA key
* even when forbidden by protocol specs
* (handshake may fail as clients are not required to
* be able to handle this) */
s->s3->tmp.use_rsa_tmp=1;
else
s->s3->tmp.use_rsa_tmp=0;
/*
* clear this, it may get reset by
* send_server_key_exchange
*/
s->s3->tmp.use_rsa_tmp=0;
/* only send if a DH key exchange, fortezza or
@ -473,7 +464,7 @@ int ssl3_accept(SSL *s)
* server certificate contains the server's
* public key for key exchange.
*/
if (s->s3->tmp.use_rsa_tmp
if (0
/* PSK: send ServerKeyExchange if PSK identity
* hint if provided */
#ifndef OPENSSL_NO_PSK

5
ssl/ssl.h
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@ -622,9 +622,8 @@ struct ssl_session_st
#define SSL_OP_SINGLE_ECDH_USE 0x00080000L
/* If set, always create a new key when using tmp_dh parameters */
#define SSL_OP_SINGLE_DH_USE 0x00100000L
/* Set to always use the tmp_rsa key when doing RSA operations,
* even when this violates protocol specs */
#define SSL_OP_EPHEMERAL_RSA 0x00200000L
/* Does nothing: retained for compatibiity */
#define SSL_OP_EPHEMERAL_RSA 0x0
/* Set on servers to choose the cipher according to the server's
* preferences */
#define SSL_OP_CIPHER_SERVER_PREFERENCE 0x00400000L