EVP_PKEY-DH(7ossl) OpenSSL EVP_PKEY-DH(7ossl)
NAME
EVP_PKEY-DH, EVP_PKEY-DHX, EVP_KEYMGMT-DH, EVP_KEYMGMT-DHX - EVP_PKEY DH
and DHX keytype and algorithm support
DESCRIPTION
For finite field Diffie-Hellman key agreement, two classes of domain
parameters can be used: "safe" domain parameters that are associated with
approved named safe-prime groups, and a class of "FIPS186-type" domain
parameters. FIPS186-type domain parameters should only be used for
backward compatibility with existing applications that cannot be upgraded
to use the approved safe-prime groups.
See EVP_PKEY-FFC(7) for more information about FFC keys.
The DH key type uses PKCS#3 format which saves p and g, but not the q
value. The DHX key type uses X9.42 format which saves the value of q and
this must be used for FIPS186-4. If key validation is required, users
should be aware of the nuances associated with FIPS186-4 style parameters
as discussed in "DH and DHX key validation".
DH and DHX domain parameters
In addition to the common FFC parameters that all FFC keytypes should
support (see "FFC parameters" in EVP_PKEY-FFC(7)) the DHX and DH keytype
implementations support the following:
"group" (OSSL_PKEY_PARAM_GROUP_NAME) <UTF8 string>
Sets or gets a string that associates a DH or DHX named safe prime
group with known values for p, q and g.
The following values can be used by the OpenSSL's default and FIPS
providers: "ffdhe2048", "ffdhe3072", "ffdhe4096", "ffdhe6144",
"ffdhe8192", "modp_2048", "modp_3072", "modp_4096", "modp_6144",
"modp_8192".
The following additional values can also be used by OpenSSL's default
provider: "modp_1536", "dh_1024_160", "dh_2048_224", "dh_2048_256".
DH/DHX named groups can be easily validated since the parameters are
well known. For protocols that only transfer p and g the value of q
can also be retrieved.
DH and DHX additional parameters
"encoded-pub-key" (OSSL_PKEY_PARAM_ENCODED_PUBLIC_KEY) <octet string>
Used for getting and setting the encoding of the DH public key used
in a key exchange message for the TLS protocol. See
EVP_PKEY_set1_encoded_public_key() and
EVP_PKEY_get1_encoded_public_key().
DH additional domain parameters
"safeprime-generator" (OSSL_PKEY_PARAM_DH_GENERATOR) <integer>
Used for DH generation of safe primes using the old safe prime
generator code. The default value is 2. It is recommended to use a
named safe prime group instead, if domain parameter validation is
required.
Randomly generated safe primes are not allowed by FIPS, so setting
this value for the OpenSSL FIPS provider will instead choose a named
safe prime group based on the size of p.
DH and DHX domain parameter / key generation parameters
In addition to the common FFC key generation parameters that all FFC key
types should support (see "FFC key generation parameters" in
EVP_PKEY-FFC(7)) the DH and DHX keytype implementation supports the
following:
"type" (OSSL_PKEY_PARAM_FFC_TYPE) <UTF8 string>
Sets the type of parameter generation. For DH valid values are:
"fips186_4"
"default"
"fips186_2"
These are described in "FFC key generation parameters" in
EVP_PKEY-FFC(7)
"group"
This specifies that a named safe prime name will be chosen using
the "pbits" type.
"generator"
A safe prime generator. See the "safeprime-generator" type above.
This is only valid for DH keys.
"pbits" (OSSL_PKEY_PARAM_FFC_PBITS) <unsigned integer>
Sets the size (in bits) of the prime 'p'.
For "fips186_4" this must be 2048. For "fips186_2" this must be
1024. For "group" this can be any one of 2048, 3072, 4096, 6144 or
8192.
"priv_len" (OSSL_PKEY_PARAM_DH_PRIV_LEN) <integer>
An optional value to set the maximum length of the generated private
key. The default value used if this is not set is the maximum value
of BN_num_bits(q)). The minimum value that this can be set to is 2 *
s. Where s is the security strength of the key which has values of
112, 128, 152, 176 and 200 for key sizes of 2048, 3072, 4096, 6144
and 8192.
DH and DHX key generation FIPS indicator
The following key generation parameter can be retrieved with
EVP_PKEY_CTX_get_params(3) from the OpenSSL FIPS provider:
"fips-indicator" (OSSL_PKEY_PARAM_FIPS_APPROVED_INDICATOR) <integer>
Returns 1 if the operation is FIPS approved, or 0 otherwise. See
"FIPS indicators" in fips_module(7) for more information.
DH and DHX key validation
For keys that are not a named group the FIPS186-4 standard specifies that
the values used for FFC parameter generation are also required for
parameter validation. This means that optional FFC domain parameter
values for seed, pcounter and gindex or hindex may need to be stored for
validation purposes. For DHX the seed and pcounter can be stored in ASN1
data (but the gindex or hindex cannot be stored). It is recommended to
use a DH parameters with named safe prime group instead.
With the OpenSSL FIPS provider, EVP_PKEY_param_check(3) and
EVP_PKEY_param_check_quick(3) behave in the following way: the parameters
are tested if they are either an approved safe prime group OR that the
FFC parameters conform to FIPS186-4 as defined in SP800-56Ar3 Assurances
of Domain-Parameter Validity.
The OpenSSL default provider uses simpler checks that allows there to be
no q value for backwards compatibility, however the
EVP_PKEY_param_check(3) will test the p value for being a prime (and a
safe prime if q is missing) which can take significant time. The
EVP_PKEY_param_check_quick(3) avoids the prime tests.
EVP_PKEY_public_check(3) conforms to SP800-56Ar3 FFC Full Public-Key
Validation.
EVP_PKEY_public_check_quick(3) conforms to SP800-56Ar3 FFC Partial
Public-Key Validation when the key is an approved named safe prime group,
otherwise it is the same as EVP_PKEY_public_check(3).
EVP_PKEY_private_check(3) tests that the private key is in the correct
range according to SP800-56Ar3. The OpenSSL FIPS provider requires the
value of q to be set (note that this is implicitly set for named safe
prime groups). For backwards compatibility the OpenSSL default provider
only requires p to be set.
EVP_PKEY_pairwise_check(3) conforms to SP800-56Ar3 Owner Assurance of
Pair-wise Consistency.
EXAMPLES
An EVP_PKEY context can be obtained by calling:
EVP_PKEY_CTX *pctx = EVP_PKEY_CTX_new_from_name(NULL, "DH", NULL);
A DH key can be generated with a named safe prime group by calling:
int priv_len = 2 * 112;
OSSL_PARAM params[3];
EVP_PKEY *pkey = NULL;
EVP_PKEY_CTX *pctx = EVP_PKEY_CTX_new_from_name(NULL, "DH", NULL);
params[0] = OSSL_PARAM_construct_utf8_string("group", "ffdhe2048", 0);
/* "priv_len" is optional */
params[1] = OSSL_PARAM_construct_int("priv_len", &priv_len);
params[2] = OSSL_PARAM_construct_end();
EVP_PKEY_keygen_init(pctx);
EVP_PKEY_CTX_set_params(pctx, params);
EVP_PKEY_generate(pctx, &pkey);
...
EVP_PKEY_free(pkey);
EVP_PKEY_CTX_free(pctx);
DHX domain parameters can be generated according to FIPS186-4 by calling:
int gindex = 2;
unsigned int pbits = 2048;
unsigned int qbits = 256;
OSSL_PARAM params[6];
EVP_PKEY *param_key = NULL;
EVP_PKEY_CTX *pctx = NULL;
pctx = EVP_PKEY_CTX_new_from_name(NULL, "DHX", NULL);
EVP_PKEY_paramgen_init(pctx);
params[0] = OSSL_PARAM_construct_uint("pbits", &pbits);
params[1] = OSSL_PARAM_construct_uint("qbits", &qbits);
params[2] = OSSL_PARAM_construct_int("gindex", &gindex);
params[3] = OSSL_PARAM_construct_utf8_string("type", "fips186_4", 0);
params[4] = OSSL_PARAM_construct_utf8_string("digest", "SHA256", 0);
params[5] = OSSL_PARAM_construct_end();
EVP_PKEY_CTX_set_params(pctx, params);
EVP_PKEY_generate(pctx, ¶m_key);
EVP_PKEY_print_params(bio_out, param_key, 0, NULL);
...
EVP_PKEY_free(param_key);
EVP_PKEY_CTX_free(pctx);
A DH key can be generated using domain parameters by calling:
EVP_PKEY *key = NULL;
EVP_PKEY_CTX *gctx = EVP_PKEY_CTX_new_from_pkey(NULL, param_key, NULL);
EVP_PKEY_keygen_init(gctx);
EVP_PKEY_generate(gctx, &key);
EVP_PKEY_print_private(bio_out, key, 0, NULL);
...
EVP_PKEY_free(key);
EVP_PKEY_CTX_free(gctx);
To validate FIPS186-4 DHX domain parameters decoded from PEM or DER data,
additional values used during generation may be required to be set into
the key.
EVP_PKEY_todata(), OSSL_PARAM_merge(), and EVP_PKEY_fromdata() are useful
to add these parameters to the original key or domain parameters before
the actual validation. In production code the return values should be
checked.
EVP_PKEY *received_domp = ...; /* parameters received and decoded */
unsigned char *seed = ...; /* and additional parameters received */
size_t seedlen = ...; /* by other means, required */
int gindex = ...; /* for the validation */
int pcounter = ...;
int hindex = ...;
OSSL_PARAM extra_params[4];
OSSL_PARAM *domain_params = NULL;
OSSL_PARAM *merged_params = NULL;
EVP_PKEY_CTX *ctx = NULL, *validate_ctx = NULL;
EVP_PKEY *complete_domp = NULL;
EVP_PKEY_todata(received_domp, OSSL_KEYMGMT_SELECT_DOMAIN_PARAMETERS,
&domain_params);
extra_params[0] = OSSL_PARAM_construct_octet_string("seed", seed, seedlen);
/*
* NOTE: For unverifiable g use "hindex" instead of "gindex"
* extra_params[1] = OSSL_PARAM_construct_int("hindex", &hindex);
*/
extra_params[1] = OSSL_PARAM_construct_int("gindex", &gindex);
extra_params[2] = OSSL_PARAM_construct_int("pcounter", &pcounter);
extra_params[3] = OSSL_PARAM_construct_end();
merged_params = OSSL_PARAM_merge(domain_params, extra_params);
ctx = EVP_PKEY_CTX_new_from_name(NULL, "DHX", NULL);
EVP_PKEY_fromdata_init(ctx);
EVP_PKEY_fromdata(ctx, &complete_domp, OSSL_KEYMGMT_SELECT_ALL,
merged_params);
validate_ctx = EVP_PKEY_CTX_new_from_pkey(NULL, complete_domp, NULL);
if (EVP_PKEY_param_check(validate_ctx) > 0)
/* validation_passed(); */
else
/* validation_failed(); */
OSSL_PARAM_free(domain_params);
OSSL_PARAM_free(merged_params);
EVP_PKEY_CTX_free(ctx);
EVP_PKEY_CTX_free(validate_ctx);
EVP_PKEY_free(complete_domp);
CONFORMING TO
RFC 7919 (TLS ffdhe named safe prime groups)
RFC 3526 (IKE modp named safe prime groups)
RFC 5114 (Additional DH named groups for dh_1024_160", "dh_2048_224" and
"dh_2048_256").
The following sections of SP800-56Ar3:
5.5.1.1 FFC Domain Parameter Selection/Generation
Appendix D: FFC Safe-prime Groups
The following sections of FIPS186-4:
A.1.1.2 Generation of Probable Primes p and q Using an Approved Hash
Function.
A.2.3 Generation of canonical generator g.
A.2.1 Unverifiable Generation of the Generator g.
SEE ALSO
EVP_PKEY-FFC(7), EVP_KEYEXCH-DH(7) EVP_PKEY(3), provider-keymgmt(7),
EVP_KEYMGMT(3), OSSL_PROVIDER-default(7), OSSL_PROVIDER-FIPS(7)
COPYRIGHT
Copyright 2020-2026 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
<https://www.openssl.org/source/license.html>.
3.6.5 2026-09-29 EVP_PKEY-DH(7ossl)
openssl 3.6.5 - Generated Sat Oct 3 09:24:53 CDT 2026