EC_Group

This class represents a set of elliptic curve parameters. Only curves over prime fields are supported.

class EC_Group

static bool EC_Group::supports_named_group(std::string_view name)

Check if the named group is supported.

static bool EC_Group::supports_application_specific_group()

Check if application specific groups are supported.

EC_Group::from_OID(const OID &oid)

Initialize an EC_Group using an OID referencing the curve parameters.

EC_Group::from_name(std::string_view name)

Initialize an EC_Group using a name (such as “secp256r1”)

The curve may not be available, based on the build configuration. If this is the case this function will throw Not_Implemented.

EC_Group::from_PEM(std::string_view pem)

Initialize an EC_Group using a PEM encoded parameter block

EC_Group(const OID &oid, const BigInt &p, const BigInt &a, const BigInt &b, const BigInt &base_x, const BigInt &base_y, const BigInt &order)

Create an application specific elliptic curve.

Warning

Using application specific curves may be hazardous to your health.

This constructor imposes the following restrictions:

• The prime must be between 192 and 512 bits, and a multiple of 32 bits.

• As a special extension regarding the above restriction, the prime may alternately be 521 bits, in which case it must be exactly 2**521-1. It can also be 239 bits, in which case it must be the X9.63 239-bit prime.

• The prime must be congruent to 3 modulo 4

• The group order must have identical bitlength to the prime

• No cofactor is allowed

• An object identifier must be specified

EC_Group(const BigInt &p, const BigInt &a, const BigInt &b, const BigInt &base_x, const BigInt &base_y, const BigInt &order, const BigInt &cofactor, const OID &oid = OID())

This is a deprecated alternative interface for creating application specific elliptic curves.

This does not impose the same restrictions regarding use of arbitrary sized groups, use of a cofactor, etc, and the object identifier is optional.

Warning

If you are using this constructor, and cannot use the non-deprecated constructor due to the restrictions it places on the curve parameters, be aware that this constructor will be dropped in Botan 4. Please open an issue on Github describing your usecase.

EC_Group(std::span<const uint8_t> ber_encoding)

Initialize an EC_Group by decoding a DER encoded parameter block.

std::vector<uint8_t> DER_encode() const

Return the DER encoding of this group.

std::vector<uint8_t> DER_encode(EC_Group_Encoding form) const

Return the DER encoding of this group. This variant is deprecated, but allows the curve to be encoded using the explicit (vs OID) encoding. All support for explicitly encoded elliptic curves is deprecated and will be removed in Botan4.

std::string PEM_encode() const

Return the PEM encoding of this group (base64 of DER encoding plus header/trailer).

const BigInt &get_p() const

Return the prime modulus as a BigInt

const BigInt &get_a() const

Return the a parameter of the elliptic curve equation as a BigInt

const BigInt &get_b() const

Return the b parameter of the elliptic curve equation as a BigInt

const BigInt &get_g_x() const

Return the x coordinate of the base point element as a BigInt

const BigInt &get_g_y() const

Return the y coordinate of the base point element as a BigInt

const BigInt &get_order() const

Return the order of the group generated by the base point as a BigInt

const BigInt &get_cofactor() const

Return the cofactor of the curve. In most cases this will be 1.

Warning

In Botan4 all support for elliptic curves group with a cofactor > 1 will be removed.

const OID &get_curve_oid() const

Return the OID used to identify this curve. May be empty.

Note

Botan4 will remove the ability to create elliptic curves without an object identifier.

bool verify_group(RandomNumberGenerator &rng, bool strong = false) const

Attempt to verify the group seems valid.

static const std::set<std::string> &known_named_groups()

Return a list of known groups, ie groups for which EC_Group::from_name(name) will succeed.