=pod

=head1 NAME

SSL_CTX_set_options, SSL_set_options, SSL_CTX_clear_options,

SSL_clear_options, SSL_CTX_get_options, SSL_get_options,

SSL_get_secure_renegotiation_support - manipulate SSL options

=head1 SYNOPSIS

 #include <openssl/ssl.h>

 long SSL_CTX_set_options(SSL_CTX *ctx, long options);

 long SSL_set_options(SSL *ssl, long options);

 long SSL_CTX_clear_options(SSL_CTX *ctx, long options);

 long SSL_clear_options(SSL *ssl, long options);

 long SSL_CTX_get_options(SSL_CTX *ctx);

 long SSL_get_options(SSL *ssl);

 long SSL_get_secure_renegotiation_support(SSL *ssl);

=head1 DESCRIPTION

SSL_CTX_set_options() adds the options set via bitmask in B<options> to B<ctx>.

Options already set before are not cleared!

SSL_set_options() adds the options set via bitmask in B<options> to B<ssl>.

Options already set before are not cleared!

SSL_CTX_clear_options() clears the options set via bitmask in B<options>

to B<ctx>.

SSL_clear_options() clears the options set via bitmask in B<options> to B<ssl>.

SSL_CTX_get_options() returns the options set for B<ctx>.

SSL_get_options() returns the options set for B<ssl>.

SSL_get_secure_renegotiation_support() indicates whether the peer supports

secure renegotiation.

Note, this is implemented via a macro.

=head1 NOTES

The behaviour of the SSL library can be changed by setting several options.

The options are coded as bitmasks and can be combined by a bitwise B<or>

operation (|).

SSL_CTX_set_options() and SSL_set_options() affect the (external)

protocol behaviour of the SSL library. The (internal) behaviour of

the API can be changed by using the similar

L<SSL_CTX_set_mode(3)> and SSL_set_mode() functions.

During a handshake, the option settings of the SSL object are used. When

a new SSL object is created from a context using SSL_new(), the current

option setting is copied. Changes to B<ctx> do not affect already created

SSL objects. SSL_clear() does not affect the settings.

The following B<bug workaround> options are available:

=over 4

=item SSL_OP_SAFARI_ECDHE_ECDSA_BUG

Don't prefer ECDHE-ECDSA ciphers when the client appears to be Safari on OS X.

OS X 10.8..10.8.3 has broken support for ECDHE-ECDSA ciphers.

=item SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS

Disables a countermeasure against a SSL 3.0/TLS 1.0 protocol

vulnerability affecting CBC ciphers, which cannot be handled by some

broken SSL implementations.  This option has no effect for connections

using other ciphers.

=item SSL_OP_TLSEXT_PADDING

Adds a padding extension to ensure the ClientHello size is never between

256 and 511 bytes in length. This is needed as a workaround for some

implementations.

=item SSL_OP_ALL

All of the above bug workarounds plus B<SSL_OP_LEGACY_SERVER_CONNECT> as

mentioned below.

=back

It is usually safe to use B<SSL_OP_ALL> to enable the bug workaround

options if compatibility with somewhat broken implementations is

desired.

The following B<modifying> options are available:

=over 4

=item SSL_OP_TLS_ROLLBACK_BUG

Disable version rollback attack detection.

During the client key exchange, the client must send the same information

about acceptable SSL/TLS protocol levels as during the first hello. Some

clients violate this rule by adapting to the server's answer. (Example:

the client sends a SSLv2 hello and accepts up to SSLv3.1=TLSv1, the server

only understands up to SSLv3. In this case the client must still use the

same SSLv3.1=TLSv1 announcement. Some clients step down to SSLv3 with respect

to the server's answer and violate the version rollback protection.)

=item SSL_OP_CIPHER_SERVER_PREFERENCE

When choosing a cipher, use the server's preferences instead of the client

preferences. When not set, the SSL server will always follow the clients

preferences. When set, the SSL/TLS server will choose following its

own preferences.

=item SSL_OP_NO_SSLv3, SSL_OP_NO_TLSv1, SSL_OP_NO_TLSv1_1,

SSL_OP_NO_TLSv1_2, SSL_OP_NO_TLSv1_3, SSL_OP_NO_DTLSv1, SSL_OP_NO_DTLSv1_2

These options turn off the SSLv3, TLSv1, TLSv1.1, TLSv1.2 or TLSv1.3 protocol

versions with TLS or the DTLSv1, DTLSv1.2 versions with DTLS,

respectively.

As of OpenSSL 1.1.0, these options are deprecated, use

L<SSL_CTX_set_min_proto_version(3)> and

L<SSL_CTX_set_max_proto_version(3)> instead.

=item SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION

When performing renegotiation as a server, always start a new session

(i.e., session resumption requests are only accepted in the initial

handshake). This option is not needed for clients.

=item SSL_OP_NO_COMPRESSION

Do not use compression even if it is supported.

=item SSL_OP_NO_QUERY_MTU

Do not query the MTU. Only affects DTLS connections.

=item SSL_OP_COOKIE_EXCHANGE

Turn on Cookie Exchange as described in RFC4347 Section 4.2.1. Only affects

DTLS connections.

=item SSL_OP_NO_TICKET

SSL/TLS supports two mechanisms for resuming sessions: session ids and stateless

session tickets.

When using session ids a copy of the session information is

cached on the server and a unique id is sent to the client. When the client

wishes to resume it provides the unique id so that the server can retrieve the

session information from its cache.

When using stateless session tickets the server uses a session ticket encryption

key to encrypt the session information. This encrypted data is sent to the

client as a "ticket". When the client wishes to resume it sends the encrypted

data back to the server. The server uses its key to decrypt the data and resume

the session. In this way the server can operate statelessly - no session

information needs to be cached locally.

The TLSv1.3 protocol only supports tickets and does not directly support session

ids. However OpenSSL allows two modes of ticket operation in TLSv1.3: stateful

and stateless. Stateless tickets work the same way as in TLSv1.2 and below.

Stateful tickets mimic the session id behaviour available in TLSv1.2 and below.

The session information is cached on the server and the session id is wrapped up

in a ticket and sent back to the client. When the client wishes to resume, it

presents a ticket in the same way as for stateless tickets. The server can then

extract the session id from the ticket and retrieve the session information from

its cache.

By default OpenSSL will use stateless tickets. The SSL_OP_NO_TICKET option will

cause stateless tickets to not be issued. In TLSv1.2 and below this means no

ticket gets sent to the client at all. In TLSv1.3 a stateful ticket will be

sent. This is a server-side option only.

In TLSv1.3 it is possible to suppress all tickets (stateful and stateless) from

being sent by calling L<SSL_CTX_set_num_tickets(3)> or

L<SSL_set_num_tickets(3)>.

=item SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION

Allow legacy insecure renegotiation between OpenSSL and unpatched clients or

servers. See the B<SECURE RENEGOTIATION> section for more details.

=item SSL_OP_LEGACY_SERVER_CONNECT

Allow legacy insecure renegotiation between OpenSSL and unpatched servers

B<only>: this option is currently set by default. See the

B<SECURE RENEGOTIATION> section for more details.

=item SSL_OP_NO_ENCRYPT_THEN_MAC

Normally clients and servers will transparently attempt to negotiate the

RFC7366 Encrypt-then-MAC option on TLS and DTLS connection.

If this option is set, Encrypt-then-MAC is disabled. Clients will not

propose, and servers will not accept the extension.

=item SSL_OP_NO_RENEGOTIATION

Disable all renegotiation in TLSv1.2 and earlier. Do not send HelloRequest

messages, and ignore renegotiation requests via ClientHello.

=item SSL_OP_ALLOW_NO_DHE_KEX

In TLSv1.3 allow a non-(ec)dhe based key exchange mode on resumption. This means

that there will be no forward secrecy for the resumed session.

=item SSL_OP_PRIORITIZE_CHACHA

When SSL_OP_CIPHER_SERVER_PREFERENCE is set, temporarily reprioritize

ChaCha20-Poly1305 ciphers to the top of the server cipher list if a

ChaCha20-Poly1305 cipher is at the top of the client cipher list. This helps

those clients (e.g. mobile) use ChaCha20-Poly1305 if that cipher is anywhere

in the server cipher list; but still allows other clients to use AES and other

ciphers. Requires B<SSL_OP_CIPHER_SERVER_PREFERENCE>.

=item SSL_OP_ENABLE_MIDDLEBOX_COMPAT

If set then dummy Change Cipher Spec (CCS) messages are sent in TLSv1.3. This

has the effect of making TLSv1.3 look more like TLSv1.2 so that middleboxes that

do not understand TLSv1.3 will not drop the connection. Regardless of whether

this option is set or not CCS messages received from the peer will always be

ignored in TLSv1.3. This option is set by default. To switch it off use

SSL_clear_options(). A future version of OpenSSL may not set this by default.

=item SSL_OP_NO_ANTI_REPLAY

By default, when a server is configured for early data (i.e., max_early_data > 0),

OpenSSL will switch on replay protection. See L<SSL_read_early_data(3)> for a

description of the replay protection feature. Anti-replay measures are required

to comply with the TLSv1.3 specification. Some applications may be able to

mitigate the replay risks in other ways and in such cases the built in OpenSSL

functionality is not required. Those applications can turn this feature off by

setting this option. This is a server-side opton only. It is ignored by

clients.

=back

The following options no longer have any effect but their identifiers are

retained for compatibility purposes:

=over 4

=item SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG

=item SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER

=item SSL_OP_SSLEAY_080_CLIENT_DH_BUG

=item SSL_OP_TLS_D5_BUG

=item SSL_OP_TLS_BLOCK_PADDING_BUG

=item SSL_OP_MSIE_SSLV2_RSA_PADDING

=item SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG

=item SSL_OP_MICROSOFT_SESS_ID_BUG

=item SSL_OP_NETSCAPE_CHALLENGE_BUG

=item SSL_OP_PKCS1_CHECK_1

=item SSL_OP_PKCS1_CHECK_2

=item SSL_OP_SINGLE_DH_USE

=item SSL_OP_SINGLE_ECDH_USE

=item SSL_OP_EPHEMERAL_RSA

=back

=head1 SECURE RENEGOTIATION

OpenSSL always attempts to use secure renegotiation as

described in RFC5746. This counters the prefix attack described in

CVE-2009-3555 and elsewhere.

This attack has far reaching consequences which application writers should be

aware of. In the description below an implementation supporting secure

renegotiation is referred to as I<patched>. A server not supporting secure

renegotiation is referred to as I<unpatched>.

The following sections describe the operations permitted by OpenSSL's secure

renegotiation implementation.

=head2 Patched client and server

Connections and renegotiation are always permitted by OpenSSL implementations.

=head2 Unpatched client and patched OpenSSL server

The initial connection succeeds but client renegotiation is denied by the

server with a B<no_renegotiation> warning alert if TLS v1.0 is used or a fatal

B<handshake_failure> alert in SSL v3.0.

If the patched OpenSSL server attempts to renegotiate a fatal

B<handshake_failure> alert is sent. This is because the server code may be

unaware of the unpatched nature of the client.

If the option B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then

renegotiation B<always> succeeds.

=head2 Patched OpenSSL client and unpatched server.

If the option B<SSL_OP_LEGACY_SERVER_CONNECT> or

B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then initial connections

and renegotiation between patched OpenSSL clients and unpatched servers

succeeds. If neither option is set then initial connections to unpatched

servers will fail.

The option B<SSL_OP_LEGACY_SERVER_CONNECT> is currently set by default even

though it has security implications: otherwise it would be impossible to

connect to unpatched servers (i.e. all of them initially) and this is clearly

not acceptable. Renegotiation is permitted because this does not add any

additional security issues: during an attack clients do not see any

renegotiations anyway.

As more servers become patched the option B<SSL_OP_LEGACY_SERVER_CONNECT> will

B<not> be set by default in a future version of OpenSSL.

OpenSSL client applications wishing to ensure they can connect to unpatched

servers should always B<set> B<SSL_OP_LEGACY_SERVER_CONNECT>

OpenSSL client applications that want to ensure they can B<not> connect to

unpatched servers (and thus avoid any security issues) should always B<clear>

B<SSL_OP_LEGACY_SERVER_CONNECT> using SSL_CTX_clear_options() or

SSL_clear_options().

The difference between the B<SSL_OP_LEGACY_SERVER_CONNECT> and

B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> options is that

B<SSL_OP_LEGACY_SERVER_CONNECT> enables initial connections and secure

renegotiation between OpenSSL clients and unpatched servers B<only>, while

B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> allows initial connections

and renegotiation between OpenSSL and unpatched clients or servers.

=head1 RETURN VALUES

SSL_CTX_set_options() and SSL_set_options() return the new options bitmask

after adding B<options>.

SSL_CTX_clear_options() and SSL_clear_options() return the new options bitmask

after clearing B<options>.

SSL_CTX_get_options() and SSL_get_options() return the current bitmask.

SSL_get_secure_renegotiation_support() returns 1 is the peer supports

secure renegotiation and 0 if it does not.

=head1 SEE ALSO

L<ssl(7)>, L<SSL_new(3)>, L<SSL_clear(3)>,

L<SSL_CTX_set_tmp_dh_callback(3)>,

L<SSL_CTX_set_min_proto_version(3)>,

L<dhparam(1)>

=head1 HISTORY

The attempt to always try to use secure renegotiation was added in

OpenSSL 0.9.8m.

The B<SSL_OP_PRIORITIZE_CHACHA> and B<SSL_OP_NO_RENEGOTIATION> options

were added in OpenSSL 1.1.1.

=head1 COPYRIGHT

Copyright 2001-2018 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the OpenSSL license (the "License").  You may not use

this file except in compliance with the License.  You can obtain a copy

in the file LICENSE in the source distribution or at

L<https://www.openssl.org/source/license.html>.

=cut