=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, SSL_CTX_get0_implemented_groups
- 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);

 int SSL_CTX_get0_implemented_groups(SSL_CTX *ctx, int all,
                                     STACK_OF(OPENSSL_CSTRING) *names);

=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 B<NID> value. However, support for some groups may be added by
external providers. In this case there will be no B<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 B<NIDs> of supported groups.
The supported groups for B<TLSv1.3> include:
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 based on the TLS version, and
whether the groups are used in a client or server.

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.

For a TLS 1.2 server, the groups determine the selected group. If
B<SSL_OP_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.

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 and predicted keyshares for a subset
of groups.
A server choosing a group outside the client's predicted subset incurs an extra
roundtrip.
However, in some situations, the most preferred group may not be predicted.

When groups are specified via SSL_CTX_set1_groups() as a list of B<NID>
values, OpenSSL considers all supported groups in I<clist> to be comparable in
security and prioritises 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.
It is therefore recommended to use SSL_CTX_set1_groups_list() instead, making
it possible to specify group tuples as described below.

SSL_CTX_set1_groups_list() sets the supported groups for B<ctx> to
string I<list>. In contrast to SSL_CTX_set1_groups(), the names of the
groups, rather than their B<NIDs>, are used.

The commands below list the available groups for TLS 1.2 and TLS 1.3,
respectively:

    $ openssl list -tls1_2 -tls-groups
    $ openssl list -tls1_3 -tls-groups

Each group can be either the B<NIST> name (e.g. B<P-256>), some other commonly
used name where applicable (e.g. B<X25519>, B<ffdhe2048>) or an OpenSSL OID name
(e.g. B<prime256v1>).
Group names are case-insensitive in OpenSSL 3.5 and later.
The preferred group names are those defined by
L<IANA|https://www.iana.org/assignments/tls-parameters/tls-parameters.xhtml#tls-parameters-8>.

The I<list> can be used to define several group tuples of comparable security
levels, and can specify which predicted key shares should be sent by a client.
Group tuples are used by OpenSSL TLS servers to decide whether to request a
stronger keyshare than those predicted by sending a Hello Retry Request
(B<HRR>) even if some of the predicted groups are supported.
OpenSSL clients ignore tuple boundaries, and pay attenion only to the overall
order of I<list> elements and which groups are selected as predicted keyshares
as described below.

The specified list elements can optionally be ignored if not implemented
(listing unknown groups otherwise results in error).
It is also possible to specify the built-in default set of groups, and to
explicitly remove a group from that list.

In its simplest legacy form, the string I<list> is just a colon separated list
of group names, for example "P-521:P-384:P-256:X25519:ffdhe2048".
The first group listed will in this case be used as the sole predicted
B<key_share> sent by a client in a TLSv1.3 B<ClientHello>.
The list should be in order of preference with the most preferred group first.

A more expressive syntax supports definition of group tuples of comparable
security by separating them from each other with C</> characters.

The predicted keyshares to be sent by clients can be explicitly specified by
adding a C<*> prefix to the associated group name.
These C<*> prefixes are ignored by servers.

If a group name is prefixed with the C<?> character, it will be ignored if an
implementation is missing.
Otherwise, listing an unknown group name will cause a failure to parse the
I<list>.
Note that whether a group is known or not may depend on the OpenSSL version,
how OpenSSL was compiled and/or which providers are loaded.
Make sure you have the correct spelling of the group name and when in doubt
prefix it with a C<?> to handle configurations in which it might nevertheless
be unknown.

If a group name is prefixed with the C<-> character, it will be removed from
the list of groups specified up to that point.
It can be added again if specified later.
Removal of groups that have not been included earlier in the list is silently
ignored.

The pseudo group name C<DEFAULT> can be used to select the OpenSSL built-in
default list of groups.
Prepending one or more groups to C<DEFAULT> using only C<:> separators prepends those
groups to the built-in default list's first tuple.
Additional tuples can be prepended by use of the C</> separator.
Appending a set of groups to C<DEFAULT> using only C<:> separators appends those
groups to the built-in default list's last tuple.
Additional tuples can be appended by use of the C</> separator.

The B<DEFAULT> list selects B<X25519MLKEM768> as one of the predicted keyshares.
In rare cases this can lead to failures or timeouts because the resulting
larger TLS Client Hello message may no longer fit in a single TCP segment and
firewall software may erroneously disrupt the TLS handshake.
If this is an issue or concern, prepending C<?X25519MLKEM768:> without a C<*>
prefix leads to its occurrence in the default list to be ignored as a duplicate,
and along with that also the keyshare prediction.
The group will then only be selected by servers that specifically expect it,
after a Hello Retry Request (HRR).
Servers that specifically prefer B<X25519MLKEM768>, are much less likely to be
found behind problematic firewalls.

The following string I<list> for example defines three tuples when used on the
server-side, and triggers the generation of three key shares when used on the
client-side: P-521:*P-256/*P-384/*X25519:P-384:ffdhe2048.

For a TLS 1.3 client, all the groups in the string I<list> are added to the
supported groups extension of a C<ClientHello>, in the order in which they are listed,
thereby interpreting tuple separators as group separators. The extension's
preference order, to be evaluated by the server, is determined by the
order of the elements in the array, see below.

If a group name is preceded by C<*>, a key share will be sent for this group.
When preceding C<DEFAULT> with C<*>, a key share will be sent for the first group
of the OpenSSL built-in default list of groups. If no C<*> is used anywhere in the list,
a single key share for the leftmost valid group is sent. A maximum of 4 key shares
are supported. Example: "P-521:*P-256/*P-384" will add P-521, P-256 and P-384 to the
supported groups extension in a C<ClientHello> and will send key shares for P-256 and P-384.

For a TLS 1.3 server, the groups in the string I<list> will be used to determine which group
is used for the key agreement. The preference order of the group tuples is determined
by the order of the tuples in the array, and the preference order of the groups within
a group tuple is determined by the order of the groups in the tuple. Server preference
can be enforced by setting B<SSL_OP_SERVER_PREFERENCE> using
B<SSL_set_options> (default: client preference).

The server will select the group to be used for a key agreement using the following
pseudo-code algorithm:

 FOR each group tuple
     IF client preference (= default)
         FOR each client key-share group
             IF current key-share group is also part of current group tuple: SH, return success
         FOR each client supported groups
             IF current supported group is also part of current group tuple: HRR, return success
     ELSE (= server preference = with SSL_OP_SERVER_PREFERENCE option set)
         FOR each group in current tuple
             IF current group is also part of client key-share groups: SH, return success
         FOR each group in current tuple
             IF current group is also part of client supported groups: HRR, return success
 return failure

 with : SH:  Server hello with current group
        HRR: Server retry request with current group

Hence, if a client supports a group in a server group tuple, but does not send a key
share for this group, a Hello Retry Request (HRR) is triggered, asking the client
to send a new Hello message with a more preferred keyshare. See examples below.

A group name can optionally be preceded by any of C<*>, C<?> or C<->, in any order, with
the exception that only C<*> is allowed to precede C<DEFAULT>. Separator characters
C<:> and C</> are only allowed inside the I<list> and not at the very beginning or end.

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().

SSL_CTX_get0_implemented_groups() populates a stack with the names of TLS
groups that are compatible with the TLS version of the B<ctx> argument.
The returned names are references to internal constants and must not be
modified or freed.  When B<all> is nonzero, the returned list includes not
only the preferred IANA names of the groups, but also any associated aliases.
If the SSL_CTX is version-flexible, the groups will be those compatible
with any configured minimum and maximum protocol versions.
The B<names> stack should be allocated by the caller and be empty, the
matching group names are appended to the provided stack.
The B<-tls-groups> and B<-all-tls-groups> options of the
L<openssl list|openssl-list(1)> command output these lists for either
TLS 1.2 or TLS 1.3 (by default).

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(),
SSL_set1_groups_list(), and SSL_CTX_get0_implemented_groups() 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 EXAMPLES

Assume the server I<list> is "P-521:P-256/P-384/X25519:ffdhe2048" and client
I<list> is "P-521:*P-384" when connecting to such a server, meaning that the
client supports C<P-521> but does not send a key share for this group to the
server, and the client supports C<P-384> including key share for this group.
With both server and client preference, an HRR will be triggered for C<P-521>
despite the availability of a key share for P-384, which overlaps with a lower
priority server-side tuple.

As a separate example, consider a server I<list> "A:B/C:D/E:F". Listed in order
of highest preference to least, 3 group tuples are created: "A:B", "C:D", and
"E:F". Here are some examples of a client I<list> where setting server/client
preference will not change the outcome:

- "A:D:*F": Both prefer "A", but the server didn't receive a keyshare for the
most-preferred tuple in which there's at least one group supported by both.
Therefore, an HRR is triggered for "A".

- "B:*C": Both prefer "B" from the first group tuple "A:B", so an HRR is
triggered for "B".

- "C:*F": Both prefer "C" from the second group tuple "C:D", so an HRR is
triggered for "C".

- "C:*D": Even though both prefer "C" over "D", the server will accept
the key share for "D". Within a tuple, existing keyshares trump preference
order.

- "*C:*D": The server accepts the "C" key share.

- "F": Even though it is not prepended with a "*", the client will send a key
share for "F". The server will then accept the key share for "F".

- "*E:C:A": The server prefers "A" from the "A:B" group tuple, so an HRR is
triggered for "A".

- "*E:B:*A": The server uses the key share for "A".

Here are some examples where setting server/client preference will change the
result:

- "*D:*C"
  - Client preference: The server uses the key share for "D".
  - Server preference: The server uses the key share for "C".

- "B:A:*C"
  - Client preference: The server triggers an HRR for "B". For the server,
"A" and "B" are considered comparable in security. But because the client
prefers "B", the server will trigger an HRR for "B".
  - Server preference: The server triggers an HRR for "A".

=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.

Support for B<ML-KEM> was added in OpenSSL 3.5.

OpenSSL 3.5 also introduces support for three I<hybrid> ECDH PQ key exchange
TLS groups: B<X25519MLKEM768>, B<SecP256r1MLKEM768> and
B<SecP384r1MLKEM1024>.
They offer CPU performance comparable to the associated ECDH group, though at
the cost of significantly larger key exchange messages.
The third group, B<SecP384r1MLKEM1024> is substantially more CPU-intensive,
largely as a result of the high CPU cost of ECDH for the underlying B<P-384>
group.
Also its key exchange messages at close to 1700 bytes are larger than the
roughly 1200 bytes for the first two groups.

As of OpenSSL 3.5 key exchange group names are case-insensitive.

B<SSL_CTX_get0_implemented_groups> was first implemented in OpenSSL 3.5.

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 2013-2026 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
