=pod

=head1 NAME

EC_GROUP_get_ecparameters,
EC_GROUP_get_ecpkparameters,
EC_GROUP_new_from_params,
EC_GROUP_to_params,
EC_GROUP_new_from_ecparameters,
EC_GROUP_new_from_ecpkparameters,
EC_GROUP_new,
EC_GROUP_free,
EC_GROUP_clear_free,
EC_GROUP_new_curve_GFp,
EC_GROUP_new_curve_GF2m,
EC_GROUP_new_by_curve_name_ex,
EC_GROUP_new_by_curve_name,
EC_GROUP_set_curve,
EC_GROUP_get_curve,
EC_GROUP_set_curve_GFp,
EC_GROUP_get_curve_GFp,
EC_GROUP_set_curve_GF2m,
EC_GROUP_get_curve_GF2m,
EC_get_builtin_curves,
OSSL_EC_curve_nid2name,
EC_curve_nid2nist,
EC_curve_nist2nid -
Functions for creating and destroying EC_GROUP objects

=head1 SYNOPSIS

 #include <openssl/ec.h>

 EC_GROUP *EC_GROUP_new_from_params(const OSSL_PARAM params[],
                                    OSSL_LIB_CTX *libctx, const char *propq);
 OSSL_PARAM *EC_GROUP_to_params(const EC_GROUP *group, OSSL_LIB_CTX *libctx,
                                const char *propq, BN_CTX *bnctx);
 EC_GROUP *EC_GROUP_new_from_ecparameters(const ECPARAMETERS *params);
 EC_GROUP *EC_GROUP_new_from_ecpkparameters(const ECPKPARAMETERS *params);
 void EC_GROUP_free(EC_GROUP *group);

 EC_GROUP *EC_GROUP_new_curve_GFp(const BIGNUM *p, const BIGNUM *a,
                                  const BIGNUM *b, BN_CTX *ctx);
 EC_GROUP *EC_GROUP_new_curve_GF2m(const BIGNUM *p, const BIGNUM *a,
                                   const BIGNUM *b, BN_CTX *ctx);
 EC_GROUP *EC_GROUP_new_by_curve_name_ex(OSSL_LIB_CTX *libctx, const char *propq,
                                         int nid);
 EC_GROUP *EC_GROUP_new_by_curve_name(int nid);

 int EC_GROUP_set_curve(EC_GROUP *group, const BIGNUM *p, const BIGNUM *a,
                        const BIGNUM *b, BN_CTX *ctx);
 int EC_GROUP_get_curve(const EC_GROUP *group, BIGNUM *p, BIGNUM *a, BIGNUM *b,
                        BN_CTX *ctx);

 ECPARAMETERS *EC_GROUP_get_ecparameters(const EC_GROUP *group,
                                         ECPARAMETERS *params);
 ECPKPARAMETERS *EC_GROUP_get_ecpkparameters(const EC_GROUP *group,
                                             ECPKPARAMETERS *params);

 size_t EC_get_builtin_curves(EC_builtin_curve *r, size_t nitems);
 const char *OSSL_EC_curve_nid2name(int nid);
 const char *EC_curve_nid2nist(int nid);
 int EC_curve_nist2nid(const char *name);

The following functions have been deprecated since OpenSSL 3.0, and can be
hidden entirely by defining B<OPENSSL_API_COMPAT> with a suitable version value,
see L<openssl_user_macros(7)>:

 EC_GROUP *EC_GROUP_new(const EC_METHOD *meth);
 void EC_GROUP_clear_free(EC_GROUP *group);

 int EC_GROUP_set_curve_GFp(EC_GROUP *group, const BIGNUM *p,
                            const BIGNUM *a, const BIGNUM *b, BN_CTX *ctx);
 int EC_GROUP_get_curve_GFp(const EC_GROUP *group, BIGNUM *p,
                            BIGNUM *a, BIGNUM *b, BN_CTX *ctx);
 int EC_GROUP_set_curve_GF2m(EC_GROUP *group, const BIGNUM *p,
                             const BIGNUM *a, const BIGNUM *b, BN_CTX *ctx);
 int EC_GROUP_get_curve_GF2m(const EC_GROUP *group, BIGNUM *p,
                             BIGNUM *a, BIGNUM *b, BN_CTX *ctx);

=head1 DESCRIPTION

Within the library there are two forms of elliptic curve that are of interest.
The first form is those defined over the prime field Fp. The elements of Fp are
the integers 0 to p-1, where p is a prime number. This gives us a revised
elliptic curve equation as follows:

y^2 mod p = x^3 +ax + b mod p

The second form is those defined over a binary field F2^m where the elements of
the field are integers of length at most m bits. For this form the elliptic
curve equation is modified to:

y^2 + xy = x^3 + ax^2 + b (where b != 0)

Operations in a binary field are performed relative to an
B<irreducible polynomial>. All such curves with OpenSSL use a trinomial or a
pentanomial for this parameter.

Although deprecated since OpenSSL 3.0 and should no longer be used,
a new curve can be constructed by calling EC_GROUP_new(), using the
implementation provided by I<meth> (see L<EC_GFp_simple_method(3)>) and
associated with the library context I<ctx> (see L<OSSL_LIB_CTX(3)>).
The I<ctx> parameter may be NULL in which case the default library context is
used.
It is then necessary to call EC_GROUP_set_curve() to set the curve parameters.
Applications should instead use one of the other EC_GROUP_new_* constructors.

EC_GROUP_new_from_params() creates a group with parameters specified by I<params>.
The library context I<libctx> (see L<OSSL_LIB_CTX(3)>) and property query string
I<propq> are used to fetch algorithms from providers.
I<params> may be either a list of explicit params or a named group,
The values for I<ctx> and I<propq> may be NULL.
The I<params> that can be used are described in
L<B<EVP_PKEY-EC>(7)|EVP_PKEY-EC(7)/Common EC parameters>.

EC_GROUP_to_params creates an OSSL_PARAM array with the corresponding parameters
describing the given EC_GROUP. The resulting parameters may contain parameters
describing a named or explicit curve depending on the EC_GROUP.
The library context I<libctx> (see L<OSSL_LIB_CTX(3)>) and property query string
I<propq> are used to fetch algorithms from providers.
I<bnctx> is an optional preallocated BN_CTX (to save the overhead of allocating
and freeing the structure in a loop).
The values for I<libctx>, I<propq> and I<bnctx> may be NULL.
The caller is responsible for freeing the OSSL_PARAM pointer returned.

EC_GROUP_new_from_ecparameters() will create a group from the
specified I<params> and
EC_GROUP_new_from_ecpkparameters() will create a group from the specific PK
I<params>.

EC_GROUP_set_curve() sets the curve parameters I<p>, I<a> and I<b>. For a curve
over Fp I<p> is the prime for the field. For a curve over F2^m I<p> represents
the irreducible polynomial - each bit represents a term in the polynomial.
Therefore, there will either be three or five bits set dependent on whether the
polynomial is a trinomial or a pentanomial.
In either case, I<a> and I<b> represents the coefficients a and b from the
relevant equation introduced above.

EC_group_get_curve() obtains the previously set curve parameters.

EC_GROUP_set_curve_GFp() and EC_GROUP_set_curve_GF2m() are synonyms for
EC_GROUP_set_curve(). They are defined for backwards compatibility only and
should not be used.

EC_GROUP_get_curve_GFp() and EC_GROUP_get_curve_GF2m() are synonyms for
EC_GROUP_get_curve(). They are defined for backwards compatibility only and
should not be used.

The functions EC_GROUP_new_curve_GFp() and EC_GROUP_new_curve_GF2m() are
shortcuts for calling EC_GROUP_new() and then the EC_GROUP_set_curve() function.
An appropriate default implementation method will be used.

Whilst the library can be used to create any curve using the functions described
above, there are also a number of predefined curves that are available. In order
to obtain a list of all of the predefined curves, call the function
EC_get_builtin_curves(). The parameter I<r> should be an array of
EC_builtin_curve structures of size I<nitems>. The function will populate the
I<r> array with information about the built-in curves. If I<nitems> is less than
the total number of curves available, then the first I<nitems> curves will be
returned. Otherwise the total number of curves will be provided. The return
value is the total number of curves available (whether that number has been
populated in I<r> or not). Passing a NULL I<r>, or setting I<nitems> to 0 will
do nothing other than return the total number of curves available.
The EC_builtin_curve structure is defined as follows:

 typedef struct {
        int nid;
        const char *comment;
        } EC_builtin_curve;

Each EC_builtin_curve item has a unique integer id (I<nid>), and a human
readable comment string describing the curve.

In order to construct a built-in curve use the function
EC_GROUP_new_by_curve_name_ex() and provide the I<nid> of the curve to
be constructed, the associated library context to be used in I<ctx> (see
L<OSSL_LIB_CTX(3)>) and any property query string in I<propq>. The I<ctx> value
may be NULL in which case the default library context is used. The I<propq>
value may also be NULL.

EC_GROUP_new_by_curve_name() is the same as
EC_GROUP_new_by_curve_name_ex() except that the default library context
is always used along with a NULL property query string.

EC_GROUP_free() frees the memory associated with the EC_GROUP.
If I<group> is NULL nothing is done.

EC_GROUP_clear_free() is deprecated: it was meant to destroy any sensitive data
held within the EC_GROUP and then free its memory, but since all the data stored
in the EC_GROUP is public anyway, this function is unnecessary.
Its use can be safely replaced with EC_GROUP_free().
If I<group> is NULL nothing is done.

OSSL_EC_curve_nid2name() converts a curve I<nid> into the corresponding name.

EC_curve_nid2nist() converts a curve I<nid> into the corresponding NIST name.
NIST names are standardized curve names defined by NIST, such as "P-192", "P-224",
"P-256", "P-384", and "P-521" for prime field curves, and "B-163", "B-233",
"B-283", "B-409", "B-571", "K-163", "K-233", "K-283", "K-409", and "K-571" for
binary field curves.
If the curve specified by I<nid> does not have a NIST name, this function returns
NULL.

EC_curve_nist2nid() converts a NIST curve name I<name> to the corresponding NID.
The I<name> argument should be a string containing a NIST curve name such as
"P-256" or "B-163".
If the name does not correspond to a known NIST curve, this function returns
NID_undef.

=head1 RETURN VALUES

All EC_GROUP_new* functions return a pointer to the newly constructed group, or
NULL on error.

EC_get_builtin_curves() returns the number of built-in curves that are
available.

EC_GROUP_set_curve_GFp(), EC_GROUP_get_curve_GFp(), EC_GROUP_set_curve_GF2m(),
EC_GROUP_get_curve_GF2m() return 1 on success or 0 on error.

OSSL_EC_curve_nid2name() returns a character string constant, or NULL on error.

EC_curve_nid2nist() returns a pointer to a constant string containing the NIST
name for the specified curve, or NULL if the curve does not have a NIST name.

EC_curve_nist2nid() returns the NID of the curve with the given NIST name, or
NID_undef if the name does not correspond to a known NIST curve.

=head1 SEE ALSO

L<crypto(7)>, L<EC_GROUP_copy(3)>,
L<EC_POINT_new(3)>, L<EC_POINT_add(3)>, L<EC_KEY_new(3)>,
L<EC_GFp_simple_method(3)>, L<d2i_ECPKParameters(3)>,
L<OSSL_LIB_CTX(3)>, L<EVP_PKEY-EC(7)>

=head1 HISTORY

EC_GROUP_to_params() was added in OpenSSL 3.2.

=over 2

=item *

EC_GROUP_new() was deprecated in OpenSSL 3.0.

EC_GROUP_new_by_curve_name_ex() and EC_GROUP_new_from_params() were
added in OpenSSL 3.0.

=item *

EC_GROUP_clear_free() was deprecated in OpenSSL 3.0; use EC_GROUP_free()
instead.

=item *

 EC_GROUP_set_curve_GFp(), EC_GROUP_get_curve_GFp(),
 EC_GROUP_set_curve_GF2m() and EC_GROUP_get_curve_GF2m() were deprecated in
 OpenSSL 3.0; use EC_GROUP_set_curve() and EC_GROUP_get_curve() instead.

=back

=head1 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
L<https://www.openssl.org/source/license.html>.

=cut
