Ubuntu Manpage: SSL_CTX_set_options, SSL_set_options, SSL_CTX_clear_options, SSL_clear

Provided by: libssl-doc_1.1.1-1ubuntu2.1~18.04.23_all

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

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);

DESCRIPTION

       SSL_CTX_set_options() adds the options set via bitmask in options to ctx.  Options already
       set before are not cleared!

       SSL_set_options() adds the options set via bitmask in options to ssl.  Options already set
       before are not cleared!

       SSL_CTX_clear_options() clears the options set via bitmask in options to ctx.

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

       SSL_CTX_get_options() returns the options set for ctx.

       SSL_get_options() returns the options set for ssl.

       SSL_get_secure_renegotiation_support() indicates whether the peer supports secure
       renegotiation.  Note, this is implemented via a macro.

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 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
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 ctx do not affect already created SSL objects. SSL_clear() does not affect the
settings.

The following bug workaround options are available:

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.

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.

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.

SSL_OP_ALL
All of the above bug workarounds plus SSL_OP_LEGACY_SERVER_CONNECT as mentioned below.

It is usually safe to use SSL_OP_ALL to enable the bug workaround options if compatibility
with somewhat broken implementations is desired.

The following modifying options are available:

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.)

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.

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 SSL_CTX_set_min_proto_version(3) and
SSL_CTX_set_max_proto_version(3) instead.

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.

SSL_OP_NO_COMPRESSION
Do not use compression even if it is supported.

SSL_OP_NO_QUERY_MTU
Do not query the MTU. Only affects DTLS connections.

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

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 SSL_CTX_set_num_tickets(3) or SSL_set_num_tickets(3).

SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION
Allow legacy insecure renegotiation between OpenSSL and unpatched clients or servers.
See the SECURE RENEGOTIATION section for more details.

SSL_OP_LEGACY_SERVER_CONNECT
Allow legacy insecure renegotiation between OpenSSL and unpatched servers only: this
option is currently set by default. See the SECURE RENEGOTIATION section for more
details.

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.

SSL_OP_NO_RENEGOTIATION
Disable all renegotiation in TLSv1.2 and earlier. Do not send HelloRequest messages,
and ignore renegotiation requests via ClientHello.

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.

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
SSL_OP_CIPHER_SERVER_PREFERENCE.

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.

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 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.

The following options no longer have any effect but their identifiers are retained for
compatibility purposes:

SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG
SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER
SSL_OP_SSLEAY_080_CLIENT_DH_BUG
SSL_OP_TLS_D5_BUG
SSL_OP_TLS_BLOCK_PADDING_BUG
SSL_OP_MSIE_SSLV2_RSA_PADDING
SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG
SSL_OP_MICROSOFT_SESS_ID_BUG
SSL_OP_NETSCAPE_CHALLENGE_BUG
SSL_OP_PKCS1_CHECK_1
SSL_OP_PKCS1_CHECK_2
SSL_OP_SINGLE_DH_USE
SSL_OP_SINGLE_ECDH_USE
SSL_OP_EPHEMERAL_RSA

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
patched. A server not supporting secure renegotiation is referred to as unpatched.

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

Patched client and server
Connections and renegotiation are always permitted by OpenSSL implementations.

Unpatched client and patched OpenSSL server
The initial connection succeeds but client renegotiation is denied by the server with a
no_renegotiation warning alert if TLS v1.0 is used or a fatal handshake_failure alert in
SSL v3.0.

If the patched OpenSSL server attempts to renegotiate a fatal handshake_failure alert is
sent. This is because the server code may be unaware of the unpatched nature of the
client.

If the option SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION is set then renegotiation always
succeeds.

Patched OpenSSL client and unpatched server.
If the option SSL_OP_LEGACY_SERVER_CONNECT or 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 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 SSL_OP_LEGACY_SERVER_CONNECT will 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 set SSL_OP_LEGACY_SERVER_CONNECT

OpenSSL client applications that want to ensure they can not connect to unpatched servers
(and thus avoid any security issues) should always clear SSL_OP_LEGACY_SERVER_CONNECT
using SSL_CTX_clear_options() or SSL_clear_options().

The difference between the SSL_OP_LEGACY_SERVER_CONNECT and
SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION options is that SSL_OP_LEGACY_SERVER_CONNECT
enables initial connections and secure renegotiation between OpenSSL clients and unpatched
servers only, while SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION allows initial connections
and renegotiation between OpenSSL and unpatched clients or servers.

RETURN VALUES

       SSL_CTX_set_options() and SSL_set_options() return the new options bitmask after adding
       options.

       SSL_CTX_clear_options() and SSL_clear_options() return the new options bitmask after
       clearing 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.

HISTORY

       The attempt to always try to use secure renegotiation was added in Openssl 0.9.8m.

       SSL_OP_PRIORITIZE_CHACHA and SSL_OP_NO_RENEGOTIATION were added in OpenSSL 1.1.1.

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