EVP_PKEY-EC(7SSL) OpenSSL EVP_PKEY-EC(7SSL)
NAME
EVP_PKEY-EC, EVP_KEYMGMT-EC - EVP_PKEY EC keytype and algorithm support
DESCRIPTION
The EC keytype is implemented in OpenSSL's default provider.
Common EC parameters
The normal way of specifying domain parameters for an EC curve is via
the curve name "group". For curves with no curve name, explicit
parameters can be used that specify "field-type", "p", "a", "b",
"generator" and "order". Explicit parameters are supported for
backwards compatibility reasons, but they are not compliant with
multiple standards (including RFC5915) which only allow named curves.
The following KeyGen/Gettable/Import/Export types are available for the
built-in EC algorithm:
"group" (OSSL_PKEY_PARAM_GROUP_NAME) <UTF8 string>
The curve name.
"field-type" (OSSL_PKEY_PARAM_EC_FIELD_TYPE) <UTF8 string>
The value should be either "prime-field" or "characteristic-two-
field", which correspond to prime field Fp and binary field F2^m.
"p" (OSSL_PKEY_PARAM_EC_P) <unsigned integer>
For a curve over Fp p is the prime for the field. For a curve over
F2^m 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.
"a" (OSSL_PKEY_PARAM_EC_A) <unsigned integer>
"b" (OSSL_PKEY_PARAM_EC_B) <unsigned integer>
"seed" (OSSL_PKEY_PARAM_EC_SEED) <octet string>
a and b represents the coefficients of the curve For Fp: y^2 mod
p = x^3 +ax + b mod p OR For F2^m: y^2 + xy = x^3 + ax^2 + b
seed is an optional value that is for information purposes only.
It represents the random number seed used to generate the
coefficient b from a random number.
"generator" (OSSL_PKEY_PARAM_EC_GENERATOR) <octet string>
"order" (OSSL_PKEY_PARAM_EC_ORDER) <unsigned integer>
"cofactor" (OSSL_PKEY_PARAM_EC_COFACTOR) <unsigned integer>
The generator is a well defined point on the curve chosen for
cryptographic operations. The encoding conforms with Sec. 2.3.3 of
the SECG SEC 1 ("Elliptic Curve Cryptography") standard. See
EC_POINT_oct2point(). Integers used for point multiplications will
be between 0 and order - 1. cofactor is an optional value. order
multiplied by the cofactor gives the number of points on the curve.
"decoded-from-explicit"
(OSSL_PKEY_PARAM_EC_DECODED_FROM_EXPLICIT_PARAMS) <integer>
Gets a flag indicating whether the key or parameters were decoded
from explicit curve parameters. Set to 1 if so or 0 if a named
curve was used.
"use-cofactor-flag" (OSSL_PKEY_PARAM_USE_COFACTOR_ECDH) <integer>
Enable Cofactor DH (ECC CDH) if this value is 1, otherwise it uses
normal EC DH if the value is zero. The cofactor variant multiplies
the shared secret by the EC curve's cofactor (note for some curves
the cofactor is 1).
See also EVP_KEYEXCH-ECDH(7) for the related
OSSL_EXCHANGE_PARAM_EC_ECDH_COFACTOR_MODE parameter that can be set
on a per-operation basis.
"encoding" (OSSL_PKEY_PARAM_EC_ENCODING) <UTF8 string>
Set the format used for serializing the EC group parameters. Valid
values are "explicit" or "named_curve". The default value is
"named_curve".
"point-format" (OSSL_PKEY_PARAM_EC_POINT_CONVERSION_FORMAT) <UTF8
string>
Sets or gets the point_conversion_form for the key. For a
description of point_conversion_forms please see EC_POINT_new(3).
Valid values are "uncompressed" or "compressed". The default value
is "uncompressed".
"group-check" (OSSL_PKEY_PARAM_EC_GROUP_CHECK_TYPE) <UTF8 string>
Sets or Gets the type of group check done when
EVP_PKEY_param_check() is called. Valid values are "default",
"named" and "named-nist". The "named" type checks that the domain
parameters match the inbuilt curve parameters, "named-nist" is
similar but also checks that the named curve is a nist curve. The
"default" type does domain parameter validation for the OpenSSL
default provider, but is equivalent to "named-nist" for the OpenSSL
FIPS provider.
"include-public" (OSSL_PKEY_PARAM_EC_INCLUDE_PUBLIC) <integer>
Setting this value to 0 indicates that the public key should not be
included when encoding the private key. The default value of 1 will
include the public key.
"pub" (OSSL_PKEY_PARAM_PUB_KEY) <octet string>
The public key value in encoded EC point format conforming to Sec.
2.3.3 and 2.3.4 of the SECG SEC 1 ("Elliptic Curve Cryptography")
standard. This parameter is used when importing or exporting the
public key value with the EVP_PKEY_fromdata() and EVP_PKEY_todata()
functions.
Note, in particular, that the choice of point compression format
used for encoding the exported value via EVP_PKEY_todata() depends
on the underlying provider implementation. Before OpenSSL 3.0.8,
the implementation of providers included with OpenSSL always opted
for an encoding in compressed format, unconditionally. Since
OpenSSL 3.0.8, the implementation has been changed to honor the
OSSL_PKEY_PARAM_EC_POINT_CONVERSION_FORMAT parameter, if set, or to
default to uncompressed format.
"priv" (OSSL_PKEY_PARAM_PRIV_KEY) <unsigned integer>
The private key value.
"encoded-pub-key" (OSSL_PKEY_PARAM_ENCODED_PUBLIC_KEY) <octet string>
Used for getting and setting the encoding of an EC public key. The
public key is expected to be a point conforming to Sec. 2.3.4 of
the SECG SEC 1 ("Elliptic Curve Cryptography") standard.
"qx" (OSSL_PKEY_PARAM_EC_PUB_X) <unsigned integer>
Used for getting the EC public key X component.
"qy" (OSSL_PKEY_PARAM_EC_PUB_Y) <unsigned integer>
Used for getting the EC public key Y component.
"default-digest" (OSSL_PKEY_PARAM_DEFAULT_DIGEST) <UTF8 string>
Getter that returns the default digest name. (Currently returns
"SHA256" as of OpenSSL 3.0).
The following Gettable types are also available for the built-in EC
algorithm:
"basis-type" (OSSL_PKEY_PARAM_EC_CHAR2_TYPE) <UTF8 string>
Supports the values "tpBasis" for a trinomial or "ppBasis" for a
pentanomial. This field is only used for a binary field F2^m.
"m" (OSSL_PKEY_PARAM_EC_CHAR2_M) <integer>
"tp" (OSSL_PKEY_PARAM_EC_CHAR2_TP_BASIS) <integer>
"k1" (OSSL_PKEY_PARAM_EC_CHAR2_PP_K1) <integer>
"k2" (OSSL_PKEY_PARAM_EC_CHAR2_PP_K2) <integer>
"k3" (OSSL_PKEY_PARAM_EC_CHAR2_PP_K3) <integer>
These fields are only used for a binary field F2^m. m is the
degree of the binary field.
tp is the middle bit of a trinomial so its value must be in the
range m > tp > 0.
k1, k2 and k3 are used to get the middle bits of a pentanomial such
that m > k3 > k2 > k1 > 0
EC key validation
For EC keys, EVP_PKEY_param_check(3) behaves in the following way: For
the OpenSSL default provider it uses either EC_GROUP_check(3) or
EC_GROUP_check_named_curve(3) depending on the flag
EC_FLAG_CHECK_NAMED_GROUP. The OpenSSL FIPS provider uses
EC_GROUP_check_named_curve(3) in order to conform to SP800-56Ar3
Assurances of Domain-Parameter Validity.
For EC keys, EVP_PKEY_param_check_quick(3) is equivalent to
EVP_PKEY_param_check(3).
For EC keys, EVP_PKEY_public_check(3) and
EVP_PKEY_public_check_quick(3) conform to SP800-56Ar3 ECC Full Public-
Key Validation and ECC Partial Public-Key Validation respectively.
For EC Keys, EVP_PKEY_private_check(3) and EVP_PKEY_pairwise_check(3)
conform to SP800-56Ar3 Private key validity and Owner Assurance of
Pair-wise Consistency respectively.
EXAMPLES
An EVP_PKEY context can be obtained by calling:
EVP_PKEY_CTX *pctx =
EVP_PKEY_CTX_new_from_name(NULL, "EC", NULL);
An EVP_PKEY ECDSA or ECDH key can be generated with a "P-256" named
group by calling:
pkey = EVP_EC_gen("P-256");
or like this:
EVP_PKEY *key = NULL;
OSSL_PARAM params[2];
EVP_PKEY_CTX *gctx =
EVP_PKEY_CTX_new_from_name(NULL, "EC", NULL);
EVP_PKEY_keygen_init(gctx);
params[0] = OSSL_PARAM_construct_utf8_string(OSSL_PKEY_PARAM_GROUP_NAME,
"P-256", 0);
params[1] = OSSL_PARAM_construct_end();
EVP_PKEY_CTX_set_params(gctx, params);
EVP_PKEY_generate(gctx, &key);
EVP_PKEY_print_private(bio_out, key, 0, NULL);
...
EVP_PKEY_free(key);
EVP_PKEY_CTX_free(gctx);
An EVP_PKEY EC CDH (Cofactor Diffie-Hellman) key can be generated with
a "K-571" named group by calling:
int use_cdh = 1;
EVP_PKEY *key = NULL;
OSSL_PARAM params[3];
EVP_PKEY_CTX *gctx =
EVP_PKEY_CTX_new_from_name(NULL, "EC", NULL);
EVP_PKEY_keygen_init(gctx);
params[0] = OSSL_PARAM_construct_utf8_string(OSSL_PKEY_PARAM_GROUP_NAME,
"K-571", 0);
/*
* This curve has a cofactor that is not 1 - so setting CDH mode changes
* the behaviour. For many curves the cofactor is 1 - so setting this has
* no effect.
*/
params[1] = OSSL_PARAM_construct_int(OSSL_PKEY_PARAM_USE_COFACTOR_ECDH,
&use_cdh);
params[2] = OSSL_PARAM_construct_end();
EVP_PKEY_CTX_set_params(gctx, params);
EVP_PKEY_generate(gctx, &key);
EVP_PKEY_print_private(bio_out, key, 0, NULL);
...
EVP_PKEY_free(key);
EVP_PKEY_CTX_free(gctx);
SEE ALSO
EVP_EC_gen(3), EVP_KEYMGMT(3), EVP_PKEY(3), provider-keymgmt(7),
EVP_SIGNATURE-ECDSA(7), EVP_KEYEXCH-ECDH(7)
COPYRIGHT
Copyright 2020-2023 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
<https://www.openssl.org/source/license.html>.
3.0.13 2025-09-18 EVP_PKEY-EC(7SSL)
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