.\"	$OpenBSD: EC_GROUP_copy.3,v 1.7 2016/12/11 14:22:43 schwarze Exp $
.\"	OpenSSL aafbe1cc Jun 12 23:42:08 2013 +0100
.\"
.\" This file was written by Matt Caswell <matt@openssl.org>.
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.Dd $Mdocdate: December 11 2016 $
.Dt EC_GROUP_COPY 3
.Os
.Sh NAME
.Nm EC_GROUP_copy ,
.Nm EC_GROUP_dup ,
.Nm EC_GROUP_method_of ,
.Nm EC_GROUP_set_generator ,
.Nm EC_GROUP_get0_generator ,
.Nm EC_GROUP_get_order ,
.Nm EC_GROUP_get_cofactor ,
.Nm EC_GROUP_set_curve_name ,
.Nm EC_GROUP_get_curve_name ,
.Nm EC_GROUP_set_asn1_flag ,
.Nm EC_GROUP_get_asn1_flag ,
.Nm EC_GROUP_set_point_conversion_form ,
.Nm EC_GROUP_get_point_conversion_form ,
.Nm EC_GROUP_get0_seed ,
.Nm EC_GROUP_get_seed_len ,
.Nm EC_GROUP_set_seed ,
.Nm EC_GROUP_get_degree ,
.Nm EC_GROUP_check ,
.Nm EC_GROUP_check_discriminant ,
.Nm EC_GROUP_cmp ,
.Nm EC_GROUP_get_basis_type ,
.Nm EC_GROUP_get_trinomial_basis ,
.Nm EC_GROUP_get_pentanomial_basis
.Nd manipulate EC_GROUP objects
.Sh SYNOPSIS
.In openssl/ec.h
.In openssl/bn.h
.Ft int
.Fo EC_GROUP_copy
.Fa "EC_GROUP *dst"
.Fa "const EC_GROUP *src"
.Fc
.Ft EC_GROUP *
.Fo EC_GROUP_dup
.Fa "const EC_GROUP *src"
.Fc
.Ft const EC_METHOD *
.Fo EC_GROUP_method_of
.Fa "const EC_GROUP *group"
.Fc
.Ft int
.Fo EC_GROUP_set_generator
.Fa "EC_GROUP *group"
.Fa "const EC_POINT *generator"
.Fa "const BIGNUM *order"
.Fa "const BIGNUM *cofactor"
.Fc
.Ft const EC_POINT *
.Fo EC_GROUP_get0_generator
.Fa "const EC_GROUP *group"
.Fc
.Ft int
.Fo EC_GROUP_get_order
.Fa "const EC_GROUP *group"
.Fa "BIGNUM *order"
.Fa "BN_CTX *ctx"
.Fc
.Ft int
.Fo EC_GROUP_get_cofactor
.Fa "const EC_GROUP *group"
.Fa "BIGNUM *cofactor"
.Fa "BN_CTX *ctx"
.Fc
.Ft void
.Fo EC_GROUP_set_curve_name
.Fa "EC_GROUP *group"
.Fa "int nid"
.Fc
.Ft int
.Fo EC_GROUP_get_curve_name
.Fa "const EC_GROUP *group"
.Fc
.Ft void
.Fo EC_GROUP_set_asn1_flag
.Fa "EC_GROUP *group"
.Fa "int flag"
.Fc
.Ft int
.Fo EC_GROUP_get_asn1_flag
.Fa "const EC_GROUP *group"
.Fc
.Ft void
.Fo EC_GROUP_set_point_conversion_form
.Fa "EC_GROUP *group"
.Fa "point_conversion_form_t form"
.Fc
.Ft point_conversion_form_t
.Fo EC_GROUP_get_point_conversion_form
.Fa "const EC_GROUP *"
.Fc
.Ft unsigned char *
.Fo EC_GROUP_get0_seed
.Fa "const EC_GROUP *x"
.Fc
.Ft size_t
.Fo EC_GROUP_get_seed_len
.Fa "const EC_GROUP *"
.Fc
.Ft size_t
.Fo EC_GROUP_set_seed
.Fa "EC_GROUP *"
.Fa "const unsigned char *"
.Fa "size_t len"
.Fc
.Ft int
.Fo EC_GROUP_get_degree
.Fa "const EC_GROUP *group"
.Fc
.Ft int
.Fo EC_GROUP_check
.Fa "const EC_GROUP *group"
.Fa "BN_CTX *ctx"
.Fc
.Ft int
.Fo EC_GROUP_check_discriminant
.Fa "const EC_GROUP *group"
.Fa "BN_CTX *ctx"
.Fc
.Ft int
.Fo EC_GROUP_cmp
.Fa "const EC_GROUP *a"
.Fa "const EC_GROUP *b"
.Fa "BN_CTX *ctx"
.Fc
.Ft int
.Fo EC_GROUP_get_basis_type
.Fa "const EC_GROUP *"
.Fc
.Ft int
.Fo EC_GROUP_get_trinomial_basis
.Fa "const EC_GROUP *"
.Fa "unsigned int *k"
.Fc
.Ft int
.Fo EC_GROUP_get_pentanomial_basis
.Fa "const EC_GROUP *"
.Fa "unsigned int *k1"
.Fa "unsigned int *k2"
.Fa "unsigned int *k3"
.Fc
.Sh DESCRIPTION
These functions operate on
.Vt EC_GROUP
objects created by the functions described in
.Xr EC_GROUP_new 3 .
.Pp
.Fn EC_GROUP_copy
copies the curve
.Fa src
into
.Fa dst .
Both
.Fa src
and
.Fa dst
must use the same
.Vt EC_METHOD .
.Pp
.Fn EC_GROUP_dup
creates a new
.Vt EC_GROUP
object and copies the content from
.Fa src
to the newly created
.Vt EC_GROUP
object.
.Pp
.Fn EC_GROUP_method_of
obtains the
.Vt EC_METHOD
of
.Fa group .
.Pp
.Fn EC_GROUP_set_generator
sets curve parameters that must be agreed by all participants using
the curve.
These parameters include the
.Fa generator ,
the
.Fa order
and the
.Fa cofactor .
The
.Fa generator
is a well defined point on the curve chosen for cryptographic
operations.
Integers used for point multiplications will be between 0 and
.Fa order No - 1 .
The
.Fa order
multiplied by the
.Fa cofactor
gives the number of points on the curve.
.Pp
.Fn EC_GROUP_get0_generator
returns the generator for the identified
.Fa group .
.Pp
The functions
.Fn EC_GROUP_get_order
and
.Fn EC_GROUP_get_cofactor
populate the provided
.Fa order
and
.Fa cofactor
parameters with the respective order and cofactors for the
.Fa group .
.Pp
The functions
.Fn EC_GROUP_set_curve_name
and
.Fn EC_GROUP_get_curve_name
set and get the NID for the curve, respectively (see
.Xr EC_GROUP_new 3 ) .
If a curve does not have a NID associated with it, then
.Fn EC_GROUP_get_curve_name
will return 0.
.Pp
The asn1_flag value on a curve is used to determine whether there is a
specific ASN.1 OID to describe the curve or not.
If the asn1_flag is 1 then this is a named curve with an associated ASN.1 OID.
If not then asn1_flag is 0.
The functions
.Fn EC_GROUP_get_asn1_flag
and
.Fn EC_GROUP_set_asn1_flag
get and set the status of the asn1_flag for the curve.
If set, then the curve_name must also be set.
.Pp
The point_conversion_form for a curve controls how
.Vt EC_POINT
data is encoded as ASN.1 as defined in X9.62 (ECDSA).
.Vt point_conversion_form_t
is an enum defined as follows:
.Bd -literal
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 0x02  */
	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;
.Ed
.Pp
For
.Dv 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.
.Pp
For any given x coordinate for a point on a curve it is possible to
derive two possible y values.
For
.Dv 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.
.Pp
For
.Dv 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.
.Pp
The functions
.Fn EC_GROUP_set_point_conversion_form
and
.Fn EC_GROUP_get_point_conversion_form
set and get the point_conversion_form for the curve, respectively.
.Pp
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
.Fn 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
.Fn 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
.Fn 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.
.Pp
.Fn 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.
.Pp
The function
.Fn 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.
.Pp
The function
.Fn 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.
.Pp
.Fn EC_GROUP_cmp
compares
.Fa a
and
.Fa b
to determine whether they represent the same curve or not.
.Pp
The functions
.Fn EC_GROUP_get_basis_type ,
.Fn EC_GROUP_get_trinomial_basis ,
and
.Fn EC_GROUP_get_pentanomial_basis
should only be called for curves defined over an F2^m field.
Addition and multiplication operations within an F2^m field are
performed using an irreducible polynomial function f(x).
This function is either a trinomial of the form:
.Pp
.Dl f(x) = x^m + x^k + 1 with m > k >= 1
.Pp
or a pentanomial of the form:
.Pp
.Dl f(x) = x^m + x^k3 + x^k2 + x^k1 + 1 with m > k3 > k2 > k1 >= 1
.Pp
The function
.Fn EC_GROUP_get_basis_type
returns a NID identifying whether a trinomial or pentanomial is in
use for the field.
The function
.Fn EC_GROUP_get_trinomial_basis
must only be called where f(x) is of the trinomial form, and returns
the value of
.Fa k .
Similarly, the function
.Fn EC_GROUP_get_pentanomial_basis
must only be called where f(x) is of the pentanomial form, and
returns the values of
.Fa k1 ,
.Fa k2 ,
and
.Fa k3 .
.Sh RETURN VALUES
The following functions return 1 on success or 0 on error:
.Fn EC_GROUP_copy ,
.Fn EC_GROUP_set_generator ,
.Fn EC_GROUP_check ,
.Fn EC_GROUP_check_discriminant ,
.Fn EC_GROUP_get_trinomial_basis ,
and
.Fn EC_GROUP_get_pentanomial_basis .
.Pp
.Fn EC_GROUP_dup
returns a pointer to the duplicated curve or
.Dv NULL
on error.
.Pp
.Fn EC_GROUP_method_of
returns the
.Vt EC_METHOD
implementation in use for the given curve or
.Dv NULL
on error.
.Pp
.Fn EC_GROUP_get0_generator
returns the generator for the given curve or
.Dv NULL
on error.
.Pp
.Fn EC_GROUP_get_order ,
.Fn EC_GROUP_get_cofactor ,
.Fn EC_GROUP_get_curve_name ,
.Fn EC_GROUP_get_asn1_flag ,
.Fn EC_GROUP_get_point_conversion_form ,
and
.Fn EC_GROUP_get_degree
return the order, cofactor, curve name (NID), ASN.1 flag,
point_conversion_form and degree for the specified curve, respectively.
If there is no curve name associated with a curve then
.Fn EC_GROUP_get_curve_name
returns 0.
.Pp
.Fn EC_GROUP_get0_seed
returns a pointer to the seed that was used to generate the parameter
b, or
.Dv NULL
if the seed is not specified.
.Fn EC_GROUP_get_seed_len
returns the length of the seed or 0 if the seed is not specified.
.Pp
.Fn EC_GROUP_set_seed
returns the length of the seed that has been set.
If the supplied seed is
.Dv NULL
or the supplied seed length is 0, the return value will be 1.
On error 0 is returned.
.Pp
.Fn EC_GROUP_cmp
returns 0 if the curves are equal, 1 if they are not equal,
or -1 on error.
.Pp
.Fn EC_GROUP_get_basis_type
returns the values
.Dv NID_X9_62_tpBasis
or
.Dv NID_X9_62_ppBasis
as defined in
.In openssl/obj_mac.h
for a trinomial or pentanomial, respectively.
Alternatively in the event of an error a 0 is returned.
.Sh SEE ALSO
.Xr d2i_ECPKParameters 3 ,
.Xr EC_GFp_simple_method 3 ,
.Xr EC_GROUP_new 3 ,
.Xr EC_KEY_new 3 ,
.Xr EC_POINT_add 3 ,
.Xr EC_POINT_new 3