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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:

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 <[email protected]> with contributions from
Ted Unangst <[email protected]> (tls_config_set_ciphers()) and
Reyk Floeter <[email protected]> (tls_config_set_ecdhecurve())