SSL_CTX_SET1_CURVES(3)

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

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);

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. Currently supported groups for TLSv1.3 are 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 depending on TLS role and version:

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 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.

SSL_CTX_set1_groups_list() sets the supported groups for ctx to string 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 TLSv1.3 are P-256, P-384, P-521, X25519, X448, brainpoolP256r1tls13, brainpoolP384r1tls13, brainpoolP512r1tls13, ffdhe2048, ffdhe3072, ffdhe4096, ffdhe6144 and 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 "?" 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 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.

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() 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 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.

SEE ALSO
ssl(7), SSL_CTX_add_extra_chain_cert(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.

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 2013-2024 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>.