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())

Debian                                            July 2, 2023                       TLS_CONFIG_SET_PROTOCOLS(3)