=pod

=head1 NAME

SSL_CTX_set1_groups, SSL_CTX_set1_groups_list, SSL_set1_groups,
SSL_set1_groups_list, SSL_get1_groups, SSL_get0_iana_groups,
SSL_get_shared_group, SSL_get_negotiated_group, SSL_CTX_set1_curves,
SSL_CTX_set1_curves_list, SSL_set1_curves, SSL_set1_curves_list,
SSL_get1_curves, SSL_get_shared_curve
- EC supported curve functions

=head1 SYNOPSIS

 #include <openssl/ssl.h>

 int SSL_CTX_set1_groups(SSL_CTX *ctx, int *glist, int glistlen);
 int SSL_CTX_set1_groups_list(SSL_CTX *ctx, char *list);

 int SSL_set1_groups(SSL *ssl, int *glist, int glistlen);
 int SSL_set1_groups_list(SSL *ssl, char *list);

 int SSL_get1_groups(SSL *ssl, int *groups);
 int SSL_get0_iana_groups(SSL *ssl, uint16_t **out);
 int SSL_get_shared_group(SSL *s, int n);
 int SSL_get_negotiated_group(SSL *s);

 int SSL_CTX_set1_curves(SSL_CTX *ctx, int *clist, int clistlen);
 int SSL_CTX_set1_curves_list(SSL_CTX *ctx, char *list);

 int SSL_set1_curves(SSL *ssl, int *clist, int clistlen);
 int SSL_set1_curves_list(SSL *ssl, char *list);

 int SSL_get1_curves(SSL *ssl, int *curves);
 int SSL_get_shared_curve(SSL *s, int n);

=head1 DESCRIPTION

For all of the functions below that set the supported groups there must be at
least one group in the list. A number of these functions identify groups via a
unique integer NID value. However, support for some groups may be added by
external providers. In this case there will be no NID assigned for the group.
When setting such groups applications should use the "list" form of these
functions (i.e. SSL_CTX_set1_groups_list() and SSL_set1_groups_list).

SSL_CTX_set1_groups() sets the supported groups for B<ctx> to B<glistlen>
groups in the array B<glist>. The array consist of all NIDs of supported groups.
Currently supported groups for B<TLSv1.3> are B<NID_X9_62_prime256v1>,
B<NID_secp384r1>, B<NID_secp521r1>, B<NID_X25519>, B<NID_X448>,
B<NID_brainpoolP256r1tls13>, B<NID_brainpoolP384r1tls13>,
B<NID_brainpoolP512r1tls13>, B<NID_ffdhe2048>, B<NID_ffdhe3072>,
B<NID_ffdhe4096>, B<NID_ffdhe6144> and B<NID_ffdhe8192>.
OpenSSL will use this array in different ways depending on TLS role and version:

=over 4

=item For a TLS client, the groups are used directly in the supported groups
extension. The extension's preference order, to be evaluated by the server, is
determined by the order of the elements in the array.

=item For a TLS 1.2 server, the groups determine the selected group. If
B<SSL_OP_CIPHER_SERVER_PREFERENCE> is set, the order of the elements in the
array determines the selected group. Otherwise, the order is ignored and the
client's order determines the selection.

=item For a TLS 1.3 server, the groups determine the selected group, but
selection is more complex. A TLS 1.3 client sends both a group list as well as a
predicted subset of groups. Choosing a group outside the predicted subset incurs
an extra roundtrip. However, in some situations, the most preferred group may
not be predicted. OpenSSL considers all supported groups to be comparable in
security and prioritizes avoiding roundtrips above either client or server
preference order. If an application uses an external provider to extend OpenSSL
with, e.g., a post-quantum algorithm, this behavior may allow a network attacker
to downgrade connections to a weaker algorithm.

=back

SSL_CTX_set1_groups_list() sets the supported groups for B<ctx> to
string B<list>. The string is a colon separated list of group names, for example
"P-521:P-384:P-256:X25519:ffdhe2048". The groups are used as in
SSL_CTX_set1_groups(), described above. Currently supported groups for
B<TLSv1.3> are B<P-256>, B<P-384>, B<P-521>, B<X25519>, B<X448>,
B<brainpoolP256r1tls13>, B<brainpoolP384r1tls13>, B<brainpoolP512r1tls13>,
B<ffdhe2048>, B<ffdhe3072>, B<ffdhe4096>, B<ffdhe6144> and B<ffdhe8192>. Support
for other groups may be added by external providers, however note the discussion
on TLS 1.3 selection criteria above. If a group name is preceded with the C<?>
character, it will be ignored if an implementation is missing.

SSL_set1_groups() and SSL_set1_groups_list() are similar except they set
supported groups for the SSL structure B<ssl>.

SSL_get1_groups() returns the set of supported groups sent by a client
in the supported groups extension. It returns the total number of
supported groups. The B<groups> parameter can be B<NULL> to simply
return the number of groups for memory allocation purposes. The
B<groups> array is in the form of a set of group NIDs in preference
order. It can return zero if the client did not send a supported groups
extension. If a supported group NID is unknown then the value is set to the
bitwise OR of TLSEXT_nid_unknown (0x1000000) and the id of the group.

SSL_get0_iana_groups() retrieves the list of groups sent by the
client in the supported_groups extension.  The B<*out> array of bytes
is populated with the host-byte-order representation of the uint16_t group
identifiers, as assigned by IANA.  The group list is returned in the same order
that was received in the ClientHello.  The return value is the number of groups,
not the number of bytes written.

SSL_get_shared_group() returns the NID of the shared group B<n> for a
server-side SSL B<ssl>. If B<n> is -1 then the total number of shared groups is
returned, which may be zero. Other than for diagnostic purposes,
most applications will only be interested in the first shared group
so B<n> is normally set to zero. If the value B<n> is out of range,
NID_undef is returned. If the NID for the shared group is unknown then the value
is set to the bitwise OR of TLSEXT_nid_unknown (0x1000000) and the id of the
group.

SSL_get_negotiated_group() returns the NID of the negotiated group used for
the handshake key exchange process.  For TLSv1.3 connections this typically
reflects the state of the current connection, though in the case of PSK-only
resumption, the returned value will be from a previous connection.  For earlier
TLS versions, when a session has been resumed, it always reflects the group
used for key exchange during the initial handshake (otherwise it is from the
current, non-resumption, connection).  This can be called by either client or
server. If the NID for the shared group is unknown then the value is set to the
bitwise OR of TLSEXT_nid_unknown (0x1000000) and the id of the group. See also
L<SSL_get0_group_name(3)> which returns the name of the negotiated group
directly and is generally preferred over SSL_get_negotiated_group().

All these functions are implemented as macros.

The curve functions are synonyms for the equivalently named group functions and
are identical in every respect. They exist because, prior to TLS1.3, there was
only the concept of supported curves. In TLS1.3 this was renamed to supported
groups, and extended to include Diffie Hellman groups. The group functions
should be used in preference.

=head1 NOTES

If an application wishes to make use of several of these functions for
configuration purposes either on a command line or in a file it should
consider using the SSL_CONF interface instead of manually parsing options.

=head1 RETURN VALUES

SSL_CTX_set1_groups(), SSL_CTX_set1_groups_list(), SSL_set1_groups() and
SSL_set1_groups_list(), return 1 for success and 0 for failure.

SSL_get1_groups() returns the number of groups, which may be zero.

SSL_get0_iana_groups() returns the number of (uint16_t) groups, which may be zero.

SSL_get_shared_group() returns the NID of shared group B<n> or NID_undef if there
is no shared group B<n>; or the total number of shared groups if B<n>
is -1.

When called on a client B<ssl>, SSL_get_shared_group() has no meaning and
returns -1.

SSL_get_negotiated_group() returns the NID of the negotiated group used for
key exchange, or NID_undef if there was no negotiated group.

=head1 SEE ALSO

L<ssl(7)>,
L<SSL_CTX_add_extra_chain_cert(3)>,
L<SSL_get0_group_name(3)>

=head1 HISTORY

The curve functions were added in OpenSSL 1.0.2. The equivalent group
functions were added in OpenSSL 1.1.1. The SSL_get_negotiated_group() function
was added in OpenSSL 3.0.0.

Support for ignoring unknown groups in SSL_CTX_set1_groups_list() and
SSL_set1_groups_list() was added in OpenSSL 3.3.

Earlier versions of this document described the list as a preference order.
However, OpenSSL's behavior as a TLS 1.3 server is to consider I<all>
supported groups as comparable in security.

=head1 COPYRIGHT

Copyright 2013-2025 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (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
