SSL_CTX_SET1_CURVES(3ossl) OpenSSL SSL_CTX_SET1_CURVES(3ossl)
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
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);
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 ctx to glistlen
groups in the array glist. The array consist of all NIDs of supported
groups. The supported groups for TLSv1.3 include:
NID_X9_62_prime256v1, NID_secp384r1, NID_secp521r1, NID_X25519,
NID_X448, NID_brainpoolP256r1tls13, NID_brainpoolP384r1tls13,
NID_brainpoolP512r1tls13, NID_ffdhe2048, NID_ffdhe3072, NID_ffdhe4096,
NID_ffdhe6144, and 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
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 NID
values, OpenSSL considers all supported groups in 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 ctx to string
list. In contrast to SSL_CTX_set1_groups(), the names of the groups,
rather than their 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 NIST name (e.g. P-256), some other
commonly used name where applicable (e.g. X25519, ffdhe2048) or an
OpenSSL OID name (e.g. prime256v1). Group names are case-insensitive
in OpenSSL 3.5 and later. The preferred group names are those defined
by IANA <https://www.iana.org/assignments/tls-parameters/tls-
parameters.xhtml#tls-parameters-8>.
The 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 (HRR) even if some of the predicted
groups are supported. OpenSSL clients ignore tuple boundaries, and pay
attenion only to the overall order of 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 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
key_share sent by a client in a TLSv1.3 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 "/"
characters.
The predicted keyshares to be sent by clients can be explicitly
specified by adding a "*" prefix to the associated group name. These
"*" prefixes are ignored by servers.
If a group name is prefixed with the "?" character, it will be ignored
if an implementation is missing. Otherwise, listing an unknown group
name will cause a failure to parse the 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
"?" to handle configurations in which it might nevertheless be unknown.
If a group name is prefixed with the "-" 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 "DEFAULT" can be used to select the OpenSSL
built-in default list of groups. Prepending one or more groups to
"DEFAULT" using only ":" separators prepends those groups to the
built-in default list's first tuple. Additional tuples can be
prepended by use of the "/" separator. Appending a set of groups to
"DEFAULT" using only ":" separators appends those groups to the
built-in default list's last tuple. Additional tuples can be appended
by use of the "/" separator.
The DEFAULT list selects 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
"?X25519MLKEM768:" without a "*" 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 X25519MLKEM768, are much less likely
to be found behind problematic firewalls.
The following string 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 list are added to
the supported groups extension of a "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 "*", a key share will be sent for this
group. When preceding "DEFAULT" with "*", a key share will be sent for
the first group of the OpenSSL built-in default list of groups. If no
"*" 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 "ClientHello" and will send key shares for P-256
and P-384.
For a TLS 1.3 server, the groups in the string 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 SSL_OP_SERVER_PREFERENCE using
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 "*", "?" or "-", in
any order, with the exception that only "*" is allowed to precede
"DEFAULT". Separator characters ":" and "/" are only allowed inside the
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 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 groups parameter can be NULL to simply return the
number of groups for memory allocation purposes. The 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 *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 n for a
server-side SSL ssl. If 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
n is normally set to zero. If the value 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
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 ctx
argument. The returned names are references to internal constants and
must not be modified or freed. When 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 names stack should be allocated by the caller
and be empty, the matching group names are appended to the provided
stack. The -tls-groups and -all-tls-groups options of the openssl list
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.
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.
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 n or NID_undef
if there is no shared group n; or the total number of shared groups if
n is -1.
When called on a client 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.
EXAMPLES
Assume the server list is "P-521:P-256/P-384/X25519:ffdhe2048" and
client list is "P-521:*P-384" when connecting to such a server, meaning
that the client supports "P-521" but does not send a key share for this
group to the server, and the client supports "P-384" including key
share for this group. With both server and client preference, an HRR
will be triggered for "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 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 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".
SEE ALSO
ssl(7), SSL_CTX_add_extra_chain_cert(3), SSL_get0_group_name(3)
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 ML-KEM was added in OpenSSL 3.5.
OpenSSL 3.5 also introduces support for three hybrid ECDH PQ key
exchange TLS groups: X25519MLKEM768, SecP256r1MLKEM768 and
SecP384r1MLKEM1024. They offer CPU performance comparable to the
associated ECDH group, though at the cost of significantly larger key
exchange messages. The third group, SecP384r1MLKEM1024 is
substantially more CPU-intensive, largely as a result of the high CPU
cost of ECDH for the underlying 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.
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
all supported groups as comparable in security.
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
<https://www.openssl.org/source/license.html>.
3.6.2 2026-04-07 SSL_CTX_SET1_CURVES(3ossl)
openssl 3.6.2 - Generated Sun May 3 09:12:04 CDT 2026