SSL_CTX_SET1_CURVES(3ossl)

📦 version 3.5.0, in opensuse-leap 16.0

✨ NAME ✨

okay bestie, welcome to the SSL_CTX_SET1_CURVES(3ossl) page — the EC supported curve functions one. It covers: 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, and SSL_CTX_get0_implemented_groups. Yes, all of these are on the same page. 💅

🦋 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 💖

okay so first things first: for all of the functions below that set the supported groups, there must be at least one group in the list. No empty vibes. 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_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.

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 at 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 attention 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_CIPHER_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_CIPHER_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. 💖

Okay so real talk about group names: a group name can optionally be preceded by any of *, ? or -, in any order — the one exception is that only * is allowed to precede DEFAULT. And the separator characters : and / are only allowed inside the list, never at the very beginning or end. Noted, bestie 💅

SSL_set1_groups() and SSL_set1_groups_list() are the same vibe, except they set supported groups for the SSL structure ssl.

SSL_get1_groups() returns the set of supported groups that a client sent in the supported groups extension, and it returns the total number of supported groups. The groups parameter can be NULL — that's your move when you just want the count for memory allocation purposes. The groups array comes back as a set of group NIDs in preference order. It can return zero if the client didn't send a supported groups extension at all. And 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 the client sent in the supported_groups extension. The *out array of bytes gets populated with the host-byte-order representation of the uint16_t group identifiers, exactly as assigned by IANA 🏛️. The group list comes back in the same order that was received in the ClientHello. And the return value is the number of groups — not the number of bytes written. Don't get it twisted 🫶

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. And 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. iykyk 💜

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's 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. Those 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 get 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. Yes, all of them 🎀

And the curve functions? They're 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. Retro branding, basically 🦋

📝 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. Save yourself the pain fr fr 😭

🔁 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 ✨

Okay so picture this 💭 Assume the server list is P-521:P-256/P-384/X25519:ffdhe2048 and the client list is P-521:*P-384 when connecting to such a server. That means 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 a 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. Iconic behavior honestly 💀

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. Big messages, big energy 🧚

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

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. So like, don't @ me about it 💅

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 <https://www.openssl.org/source/license.html>.

And that's the whole thing bestie, you ate that 🍓 This icon of a page is from OpenSSL version 3.5.0, dated 2026-09-25, and it's the SSL_CTX_SET1_CURVES(3ossl) manual page — go forth and configure your groups 💖✨🎀

📦 where this page comes from