=pod

=head1 NAME

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_set_ecdh_auto, SSL_set_ecdh_auto - EC supported curve functions

=head1 SYNOPSIS

 #include <openssl/ssl.h>

 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_set_ecdh_auto(SSL_CTX *ctx, int onoff);
 int SSL_set_ecdh_auto(SSL *s, int onoff);

=head1 DESCRIPTION

SSL_CTX_set1_curves() sets the supported curves for B<ctx> to B<clistlen>
curves in the array B<clist>. The array consist of all NIDs of curves in
preference order. For a TLS client the curves are used directly in the
supported curves extension. For a TLS server the curves are used to 
determine the set of shared curves.

SSL_CTX_set1_curves_list() sets the supported curves for B<ctx> to
string B<list>. The string is a colon separated list of curve NIDs or
names, for example "P-521:P-384:P-256".

SSL_set1_curves() and SSL_set1_curves_list() are similar except they set
supported curves for the SSL structure B<ssl>.

SSL_get1_curves() returns the set of supported curves sent by a client
in the supported curves extension. It returns the total number of 
supported curves. The B<curves> parameter can be B<NULL> to simply
return the number of curves for memory allocation purposes. The
B<curves> array is in the form of a set of curve NIDs in preference
order. It can return zero if the client did not send a supported curves
extension.

SSL_get_shared_curve() returns shared curve B<n> for a server-side
SSL B<ssl>. If B<n> is -1 then the total number of shared curves is
returned, which may be zero. Other than for diagnostic purposes,
most applications will only be interested in the first shared curve
so B<n> is normally set to zero. If the value B<n> is out of range,
NID_undef is returned.

SSL_CTX_set_ecdh_auto() and SSL_set_ecdh_auto() set automatic curve
selection for server B<ctx> or B<ssl> to B<onoff>. If B<onoff> is 1 then 
the highest preference curve is automatically used for ECDH temporary
keys used during key exchange.

All these functions are implemented as macros.

=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.

The functions SSL_CTX_set_ecdh_auto() and SSL_set_ecdh_auto() can be used to
make a server always choose the most appropriate curve for a client. If set
it will override any temporary ECDH parameters set by a server. Previous
versions of OpenSSL could effectively only use a single ECDH curve set
using a function such as SSL_CTX_set_ecdh_tmp(). Newer applications should
just call:

 SSL_CTX_set_ecdh_auto(ctx, 1);

and they will automatically support ECDH using the most appropriate shared
curve.

=head1 RETURN VALUES

SSL_CTX_set1_curves(), SSL_CTX_set1_curves_list(), SSL_set1_curves(),
SSL_set1_curves_list(), SSL_CTX_set_ecdh_auto() and SSL_set_ecdh_auto()
return 1 for success and 0 for failure.

SSL_get1_curves() returns the number of curves, which may be zero.

SSL_get_shared_curve() returns the NID of shared curve B<n> or NID_undef if there
is no shared curve B<n>; or the total number of shared curves if B<n>
is -1.

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

=head1 SEE ALSO

L<SSL_CTX_add_extra_chain_cert(3)|SSL_CTX_add_extra_chain_cert(3)>

=head1 HISTORY

These functions were first added to OpenSSL 1.0.2.

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