NAME

     tls_config_set_protocols, tls_config_parse_protocols, tls_config_set_alpn,
     tls_config_set_ciphers, tls_config_set_dheparams, tls_config_set_ecdhecurves,
     tls_config_prefer_ciphers_client, tls_config_prefer_ciphers_server — TLS protocol and cipher
     selection

SYNOPSIS

     #include <tls.h>

     int
     tls_config_set_protocols(struct tls_config *config, uint32_t protocols);

     int
     tls_config_parse_protocols(uint32_t *protocols, const char *protostr);

     int
     tls_config_set_alpn(struct tls_config *config, const char *alpn);

     int
     tls_config_set_ciphers(struct tls_config *config, const char *ciphers);

     int
     tls_config_set_dheparams(struct tls_config *config, const char *params);

     int
     tls_config_set_ecdhecurves(struct tls_config *config, const char *curves);

     void
     tls_config_prefer_ciphers_client(struct tls_config *config);

     void
     tls_config_prefer_ciphers_server(struct tls_config *config);

DESCRIPTION

     These functions modify a configuration by setting parameters.  The configuration options
     apply to both clients and servers, unless noted otherwise.

     tls_config_set_protocols() specifies which versions of the TLS protocol may be used.
     Possible values are the bitwise OR of:

           TLS_PROTOCOL_TLSv1_2
           TLS_PROTOCOL_TLSv1_3

     Additionally, the values TLS_PROTOCOL_TLSv1 (TLSv1.2, TLSv1.3), TLS_PROTOCOLS_ALL (all
     supported protocols) and TLS_PROTOCOLS_DEFAULT (TLSv1.2 and TLSv1.3) may be used.

     The tls_config_parse_protocols() utility function parses a protocol string and returns the
     corresponding value via the protocols argument.  This value can then be passed to the
     tls_config_set_protocols() function.  The protocol string is a comma or colon separated list
     of keywords.  Valid keywords are:

           tlsv1.2
           tlsv1.3
           all      (all supported protocols)
           default  (an alias for secure)
           legacy   (an alias for all)
           secure   (currently TLSv1.2 and TLSv1.3)

     If a value has a negative prefix (in the form of a leading exclamation mark) then it is
     removed from the list of available protocols, rather than being added to it.

     tls_config_set_alpn() sets the ALPN protocols that are supported.  The alpn string is a
     comma separated list of protocols, in order of preference.

     tls_config_set_ciphers() sets the list of ciphers that may be used.  Lists of ciphers are
     specified by name, and the permitted names are:

           secure (or alias default)
           compat
           legacy
           insecure (or alias all)

     Alternatively, libssl cipher strings can be specified.  See the CIPHERS section of
     openssl(1) for further information.

     tls_config_set_dheparams() specifies the parameters that will be used during Diffie-Hellman
     Ephemeral (DHE) key exchange.  Possible values are:

           none
           auto
           legacy

     In auto mode, the key size for the ephemeral key is automatically selected based on the size
     of the private key being used for signing.  In legacy mode, 1024 bit ephemeral keys are
     used.  The default value is none, which disables DHE key exchange.

     tls_config_set_ecdhecurves() specifies the names of the elliptic curves that may be used
     during Elliptic Curve Diffie-Hellman Ephemeral (ECDHE) key exchange.  This is a comma
     separated list, given in order of preference.  The special value of "default" will use the
     default curves (currently X25519, P-256 and P-384).  This function replaces
     tls_config_set_ecdhecurve(), which is deprecated.

     tls_config_prefer_ciphers_client() prefers ciphers in the client's cipher list when
     selecting a cipher suite (server only).  This is considered to be less secure than
     preferring the server's list.

     tls_config_prefer_ciphers_server() prefers ciphers in the server's cipher list when
     selecting a cipher suite (server only).  This is considered to be more secure than
     preferring the client's list and is the default.

RETURN VALUES

     These functions return 0 on success or -1 on error.

SEE ALSO

     tls_config_ocsp_require_stapling(3), tls_config_set_session_id(3), tls_config_verify(3),
     tls_init(3), tls_load_file(3)

HISTORY

     tls_config_set_ciphers() appeared in OpenBSD 5.6 and got its final name in OpenBSD 5.7.

     tls_config_set_protocols(), tls_config_parse_protocols(), tls_config_set_dheparams(), and
     tls_config_set_ecdhecurve() appeared in OpenBSD 5.7, tls_config_prefer_ciphers_client() and
     tls_config_prefer_ciphers_server() in OpenBSD 5.9, and tls_config_set_alpn() in OpenBSD 6.1.

AUTHORS

     Joel Sing <jsing@openbsd.org> with contributions from
     Ted Unangst <tedu@openbsd.org> (tls_config_set_ciphers()) and
     Reyk Floeter <reyk@openbsd.org> (tls_config_set_ecdhecurve())