man > EVP_KEYMGMT-DHX(7ssl)

EVP_PKEY-DH(7SSL)                              OpenSSL                             EVP_PKEY-DH(7SSL)

NAME
       EVP_PKEY-DH, EVP_PKEY-DHX, EVP_KEYMGMT-DH, EVP_KEYMGMT-DHX - EVP_PKEY DH and DHX keytype and
       algorithm support

DESCRIPTION
       For DH FFC 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 key validation".

   DH and DHX domain parameters
       In addition to the common FCC 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 key validation
       For DHX that is 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  named  safe  prime  group
       instead.

       For  DH keys, EVP_PKEY_param_check(3) behaves in the following way: The OpenSSL FIPS provider
       tests if the parameters 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.

       For DH keys, EVP_PKEY_param_check_quick(3) is equivalent to EVP_PKEY_param_check(3).

       For DH keys, EVP_PKEY_public_check(3) conforms to SP800-56Ar3 FFC Full Public-Key Validation.

       For DH keys, EVP_PKEY_public_check_quick(3) conforms to SP800-56Ar3  FFC  Partial  Public-Key
       Validation when the DH key is an approved named safe prime group, otherwise it is the same as
       EVP_PKEY_public_check(3).

       For  DH  Keys,  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  set  for  named  safe prime groups).  For backwards compatibility the OpenSSL
       default provider only requires p to be set.

       For DH keys, 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, &param_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 2020-2022 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                                       2026-07-29                            EVP_PKEY-DH(7SSL)