EC_GROUP_COPY(3)

NAME
EC_GROUP_copy, EC_GROUP_dup, EC_GROUP_set_generator, EC_GROUP_get0_generator, EC_GROUP_get_order, EC_GROUP_order_bits, EC_GROUP_get_cofactor, EC_GROUP_set_curve_name, EC_GROUP_get_curve_name, EC_GROUP_set_asn1_flag, EC_GROUP_get_asn1_flag, EC_GROUP_set_point_conversion_form, EC_GROUP_get_point_conversion_form, EC_GROUP_get0_seed, EC_GROUP_get_seed_len, EC_GROUP_set_seed, EC_GROUP_get_degree, EC_GROUP_check, EC_GROUP_check_discriminant, EC_GROUP_cmp, EC_GROUP_get_basis_type - manipulate EC_GROUP objects

SYNOPSIS
#include <openssl/ec.h>
#include <openssl/bn.h>
int
EC_GROUP_copy(EC_GROUP *dst, const EC_GROUP *src);
EC_GROUP *
EC_GROUP_dup(const EC_GROUP *src);
int
EC_GROUP_set_generator(EC_GROUP *group, const EC_POINT *generator, const BIGNUM *order, const BIGNUM *cofactor);
const EC_POINT *
EC_GROUP_get0_generator(const EC_GROUP *group);
int
EC_GROUP_get_order(const EC_GROUP *group, BIGNUM *order, BN_CTX *ctx);
int
EC_GROUP_order_bits(const EC_GROUP *group);
int
EC_GROUP_get_cofactor(const EC_GROUP *group, BIGNUM *cofactor, BN_CTX *ctx);
void
EC_GROUP_set_curve_name(EC_GROUP *group, int nid);
int
EC_GROUP_get_curve_name(const EC_GROUP *group);
void
EC_GROUP_set_asn1_flag(EC_GROUP *group, int flag);
int
EC_GROUP_get_asn1_flag(const EC_GROUP *group);
void
EC_GROUP_set_point_conversion_form(EC_GROUP *group, point_conversion_form_t form);
point_conversion_form_t
EC_GROUP_get_point_conversion_form(const EC_GROUP *);
unsigned char *
EC_GROUP_get0_seed(const EC_GROUP *x);
size_t
EC_GROUP_get_seed_len(const EC_GROUP *);
size_t
EC_GROUP_set_seed(EC_GROUP *, const unsigned char *, size_t len);
int
EC_GROUP_get_degree(const EC_GROUP *group);
int
EC_GROUP_check(const EC_GROUP *group, BN_CTX *ctx);
int
EC_GROUP_check_discriminant(const EC_GROUP *group, BN_CTX *ctx);
int
EC_GROUP_cmp(const EC_GROUP *a, const EC_GROUP *b, BN_CTX *ctx);
int
EC_GROUP_get_basis_type(const EC_GROUP *);

DESCRIPTION
These functions operate on EC_GROUP objects created by the functions described in EC_GROUP_new(3).

EC_GROUP_copy() copies the curve src into dst. Both src and dst must use the same EC_METHOD.

EC_GROUP_dup() creates a new EC_GROUP object and copies the content from src to the newly created EC_GROUP object.

EC_GROUP_set_generator() sets curve parameters that must be agreed by all participants using the curve. These parameters include the generator, the order and the cofactor. The generator is a well defined point on the curve chosen for cryptographic operations. Integers used for point multiplications will be between 0 and order - 1. The order multiplied by the cofactor gives the number of points on the curve.

EC_GROUP_get0_generator() returns the generator for the identified group.

EC_GROUP_get_order() retrieves the order of the group and copies its value into order. It fails if the order of the group is not set or set to zero.

EC_GROUP_get_cofactor() retrieves the cofactor of the group and copies its value into cofactor. It fails if the cofactor of the group is not set or set to zero.

The functions EC_GROUP_set_curve_name() and EC_GROUP_get_curve_name() set and get the NID for the curve, respectively (see EC_GROUP_new(3)). If a curve does not have a NID associated with it, then EC_GROUP_get_curve_name() will return NID_undef.

The asn1_flag value is used to determine whether the curve encoding uses explicit parameters or a named curve using an ASN.1 OID: many applications only support the latter form. If asn1_flag is the default value OPENSSL_EC_NAMED_CURVE, then the named curve form is used and the parameters must have a corresponding named curve NID set. If asn1_flags is OPENSSL_EC_EXPLICIT_CURVE, the parameters are explicitly encoded. The functions EC_GROUP_get_asn1_flag() and EC_GROUP_set_asn1_flag() get and set the status of the asn1_flag for the curve.

The point_conversion_form for a curve controls how EC_POINT data is encoded as ASN.1 as defined in X9.62 (ECDSA). point_conversion_form_t is an enum defined as follows:

    typedef enum {
    	/** the point is encoded as z||x, where the octet z specifies
    	 *   which solution of the quadratic equation y is  */
    	POINT_CONVERSION_COMPRESSED = 2,
    	/** the point is encoded as z||x||y, where z is the octet 0x04  */
    	POINT_CONVERSION_UNCOMPRESSED = 4,
    	/** the point is encoded as z||x||y, where the octet z specifies
             *  which solution of the quadratic equation y is  */
    	POINT_CONVERSION_HYBRID = 6
    } point_conversion_form_t;

For POINT_CONVERSION_UNCOMPRESSED the point is encoded as an octet signifying the UNCOMPRESSED form has been used followed by the octets for x, followed by the octets for y.

For any given x coordinate for a point on a curve it is possible to derive two possible y values. For POINT_CONVERSION_COMPRESSED the point is encoded as an octet signifying that the COMPRESSED form has been used AND which of the two possible solutions for y has been used, followed by the octets for x.

For POINT_CONVERSION_HYBRID the point is encoded as an octet signifying the HYBRID form has been used AND which of the two possible solutions for y has been used, followed by the octets for x, followed by the octets for y.

The functions EC_GROUP_set_point_conversion_form() and EC_GROUP_get_point_conversion_form() set and get the point_conversion_form for the curve, respectively.

ANSI X9.62 (ECDSA standard) defines a method of generating the curve parameter b from a random number. This provides advantages in that a parameter obtained in this way is highly unlikely to be susceptible to special purpose attacks, or have any trapdoors in it. If the seed is present for a curve then the b parameter was generated in a verifiable fashion using that seed. The OpenSSL EC library does not use this seed value but does enable you to inspect it using EC_GROUP_get0_seed(). This returns a pointer to a memory block containing the seed that was used. The length of the memory block can be obtained using EC_GROUP_get_seed_len(). A number of the builtin curves within the library provide seed values that can be obtained. It is also possible to set a custom seed using EC_GROUP_set_seed() and passing a pointer to a memory block, along with the length of the seed. Again, the EC library will not use this seed value, although it will be preserved in any ASN.1 based communications.

EC_GROUP_get_degree() gets the degree of the field. For Fp fields this will be the number of bits in p. For F2^m fields this will be the value m.

The function EC_GROUP_check_discriminant() calculates the discriminant for the curve and verifies that it is valid. For a curve defined over Fp the discriminant is given by the formula 4*a^3 + 27*b^2 whilst for F2^m curves the discriminant is simply b. In either case for the curve to be valid the discriminant must be non-zero.

The function EC_GROUP_check() performs a number of checks on a curve to verify that it is valid. Checks performed include verifying that the discriminant is non-zero; that a generator has been defined; that the generator is on the curve and has the correct order.

EC_GROUP_cmp() compares a and b to determine whether they represent the same curve or not.

EC_GROUP_get_basis_type() always returns 0 and is only provided for compatibility.

RETURN VALUES
The following functions return 1 on success or 0 on error: EC_GROUP_copy(), EC_GROUP_set_generator(), EC_GROUP_check(), and EC_GROUP_check_discriminant().

EC_GROUP_dup() returns a pointer to the duplicated curve or NULL on error.

EC_GROUP_get0_generator() returns the generator for the given curve or NULL on error.

EC_GROUP_get_order() returns 0 if the order is not set or set to zero for the group or if copying into order fails, or 1 otherwise.

EC_GROUP_order_bits() returns the number of bits in the group order.

EC_GROUP_get_cofactor() returns 0 if the cofactor is not set or set to zero for the group or if copying into cofactor fails, or 1 otherwise.

EC_GROUP_get_curve_name() returns the curve name (NID) for the group or NID_undef if no curve name is associated.

EC_GROUP_get_asn1_flag() returns the ASN.1 flag for the specified group.

EC_GROUP_get_point_conversion_form() returns the point_conversion_form for the group.

EC_GROUP_get_degree() returns the degree for the group or 0 if the operation is not supported by the underlying group implementation.

EC_GROUP_get0_seed() returns a pointer to the seed that was used to generate the parameter b, or NULL if the seed is not specified. EC_GROUP_get_seed_len() returns the length of the seed or 0 if the seed is not specified.

EC_GROUP_set_seed() returns the length of the seed that has been set. If the supplied seed is NULL or the supplied seed length is 0, the return value will be 1. On error 0 is returned.

EC_GROUP_cmp() returns 0 if the curves are equal, 1 if they are not equal, or -1 on error.

EC_GROUP_get_basis_type() always returns 0.

SEE ALSO
d2i_ECPKParameters(3), EC_GROUP_new(3), EC_KEY_new(3), EC_POINT_add(3), EC_POINT_new(3)

HISTORY
EC_GROUP_copy(), EC_GROUP_set_generator(), EC_GROUP_get0_generator(), EC_GROUP_get_order(), and EC_GROUP_get_cofactor() first appeared in OpenSSL 0.9.7 and have been available since OpenBSD 3.2.

EC_GROUP_dup(), EC_GROUP_set_curve_name(), EC_GROUP_get_curve_name(), EC_GROUP_set_asn1_flag(), EC_GROUP_get_asn1_flag(), EC_GROUP_set_point_conversion_form(), EC_GROUP_get_point_conversion_form(), EC_GROUP_get0_seed(), EC_GROUP_get_seed_len(), EC_GROUP_set_seed(), EC_GROUP_get_degree(), EC_GROUP_check(), EC_GROUP_check_discriminant(), EC_GROUP_cmp(), and EC_GROUP_get_basis_type() first appeared in OpenSSL 0.9.8 and have been available since OpenBSD 4.5.

EC_GROUP_order_bits() first appeared in OpenSSL 1.1.0 and has been available since OpenBSD 7.0.