SSL_CTX_set1_curves.3ssl man page

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


 #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);


SSL_CTX_set1_curves() sets the supported curves for ctx to clistlen curves in the array 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 ctx to string 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 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 curves parameter can be NULL to simply return the number of curves for memory allocation purposes. The 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 n for a server-side SSL ssl. If 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 n is normally set to zero. If the value 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 ctx or ssl to onoff. If 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.


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.

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 n or NID_undef if there is no shared curve n; or the total number of shared curves if n is -1.

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

See Also



These functions were first added to OpenSSL 1.0.2.


2016-09-26 1.0.2j OpenSSL