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Title "EVP_PKEY-DH 7" For nroff, turn off justification. Always turn off hyphenation; it makes
way too many mistakes in technical documents.
EVP_PKEY-DH, EVP_PKEY-DHX, EVP_KEYMGMT-DH, EVP_KEYMGMT-DHX \- EVP_PKEY DH and DHX keytype and algorithm support Header "DESCRIPTION" For \s-1DH\s0 \s-1FFC\s0 key agreement, two classes of domain parameters can be used: \*(L"safe\*(R" domain parameters that are associated with approved named safe-prime groups, and a class of \*(L"FIPS186-type\*(R" 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 \s-1EVP_PKEY-FFC\s0\|(7) for more information about \s-1FFC\s0 keys.

The \s-1DH\s0 key type uses PKCS#3 format which saves p and g, but not the \fIq value. The \s-1DHX\s0 key type uses X9.42 format which saves the value of q and this must be used for \s-1FIPS186-4.\s0 If key validation is required, users should be aware of the nuances associated with \s-1FIPS186-4\s0 style parameters as discussed in \*(L"\s-1DH\s0 key validation\*(R".

Subsection "DH and DHX domain parameters" In addition to the common \s-1FCC\s0 parameters that all \s-1FFC\s0 keytypes should support (see \*(L"\s-1FFC\s0 parameters\*(R" in \s-1EVP_PKEY-FFC\s0\|(7)) the \s-1DHX\s0 and \s-1DH\s0 keytype implementations support the following: Item "group (OSSL_PKEY_PARAM_GROUP_NAME) <UTF8 string>" Sets or gets a string that associates a \s-1DH\s0 or \s-1DHX\s0 named safe prime group with known values for p, q and g. .Sp The following values can be used by the OpenSSL's default and \s-1FIPS\s0 providers: \*(L"ffdhe2048\*(R", \*(L"ffdhe3072\*(R", \*(L"ffdhe4096\*(R", \*(L"ffdhe6144\*(R", \*(L"ffdhe8192\*(R", \*(L"modp_2048\*(R", \*(L"modp_3072\*(R", \*(L"modp_4096\*(R", \*(L"modp_6144\*(R", \*(L"modp_8192\*(R". .Sp The following additional values can also be used by OpenSSL's default provider: \*(L"modp_1536\*(R", \*(L"dh_1024_160\*(R", \*(L"dh_2048_224\*(R", \*(L"dh_2048_256\*(R". .Sp \s-1DH/DHX\s0 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. Subsection "DH and DHX additional parameters" Item "encoded-pub-key (OSSL_PKEY_PARAM_ENCODED_PUBLIC_KEY) <octet string>" Used for getting and setting the encoding of the \s-1DH\s0 public key used in a key exchange message for the \s-1TLS\s0 protocol. See EVP_PKEY_set1_encoded_public_key() and EVP_PKEY_get1_encoded_public_key(). Subsection "DH additional domain parameters" Item "safeprime-generator (OSSL_PKEY_PARAM_DH_GENERATOR) <integer>" Used for \s-1DH\s0 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. .Sp Randomly generated safe primes are not allowed by \s-1FIPS,\s0 so setting this value for the OpenSSL \s-1FIPS\s0 provider will instead choose a named safe prime group based on the size of p. Subsection "DH and DHX domain parameter / key generation parameters" In addition to the common \s-1FFC\s0 key generation parameters that all \s-1FFC\s0 key types should support (see \*(L"\s-1FFC\s0 key generation parameters\*(R" in \s-1EVP_PKEY-FFC\s0\|(7)) the \fB\s-1DH\s0 and \s-1DHX\s0 keytype implementation supports the following: Item "type (OSSL_PKEY_PARAM_FFC_TYPE) <UTF8 string>" Sets the type of parameter generation. For \s-1DH\s0 valid values are: Item "pbits (OSSL_PKEY_PARAM_FFC_PBITS) <unsigned integer>" Sets the size (in bits) of the prime 'p'. .Sp For \*(L"fips186_4\*(R" this must be 2048. For \*(L"fips186_2\*(R" this must be 1024. For \*(L"group\*(R" this can be any one of 2048, 3072, 4096, 6144 or 8192. Item "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. Subsection "DH key validation" For \s-1DHX\s0 that is not a named group the \s-1FIPS186-4\s0 standard specifies that the values used for \s-1FFC\s0 parameter generation are also required for parameter validation. This means that optional \s-1FFC\s0 domain parameter values for \fIseed, pcounter and gindex or hindex may need to be stored for validation purposes. For \s-1DHX\s0 the seed and pcounter can be stored in \s-1ASN1\s0 data (but the gindex or hindex cannot be stored). It is recommended to use a named safe prime group instead.

For \s-1DH\s0 keys, EVP_PKEY_param_check\|(3) behaves in the following way: The OpenSSL \s-1FIPS\s0 provider tests if the parameters are either an approved safe prime group \s-1OR\s0 that the \s-1FFC\s0 parameters conform to \s-1FIPS186-4\s0 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 \s-1DH\s0 keys, EVP_PKEY_param_check_quick\|(3) is equivalent to \fBEVP_PKEY_param_check\|(3).

For \s-1DH\s0 keys, EVP_PKEY_public_check\|(3) conforms to SP800-56Ar3 \s-1FFC\s0 Full Public-Key Validation.

For \s-1DH\s0 keys, EVP_PKEY_public_check_quick\|(3) conforms to SP800-56Ar3 \s-1FFC\s0 Partial Public-Key Validation when the \s-1DH\s0 key is an approved named safe prime group, otherwise it is the same as \fBEVP_PKEY_public_check\|(3).

For \s-1DH\s0 Keys, EVP_PKEY_private_check\|(3) tests that the private key is in the correct range according to SP800-56Ar3. The OpenSSL \s-1FIPS\s0 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 \s-1DH\s0 keys, EVP_PKEY_pairwise_check\|(3) conforms to SP800-56Ar3 Owner Assurance of Pair-wise Consistency.

Header "EXAMPLES" An \s-1EVP_PKEY\s0 context can be obtained by calling:

.Vb 1 EVP_PKEY_CTX *pctx = EVP_PKEY_CTX_new_from_name(NULL, "DH", NULL); .Ve

A \s-1DH\s0 key can be generated with a named safe prime group by calling:

.Vb 4 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); .Ve

\fB\s-1DHX\s0 domain parameters can be generated according to \s-1FIPS186-4\s0 by calling:

.Vb 6 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); .Ve

A \s-1DH\s0 key can be generated using domain parameters by calling:

.Vb 2 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); .Ve

To validate \s-1FIPS186-4\s0 \s-1DHX\s0 domain parameters decoded from \s-1PEM\s0 or \fB\s-1DER\s0 data, additional values used during generation may be required to be set into the key.

\fBEVP_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.

.Vb 11 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); .Ve

Header "CONFORMING TO" Item "RFC 7919 (TLS ffdhe named safe prime groups)"

0

Item "RFC 3526 (IKE modp named safe prime groups)" Item "RFC 5114 (Additional DH named groups for dh_1024_160, dh_2048_224 and dh_2048_256"")."

The following sections of SP800-56Ar3:

Item "5.5.1.1 FFC Domain Parameter Selection/Generation"

0

Item "Appendix D: FFC Safe-prime Groups"

The following sections of \s-1FIPS186-4:\s0

Item "A.1.1.2 Generation of Probable Primes p and q Using an Approved Hash Function."

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Item "A.2.3 Generation of canonical generator g." Item "A.2.1 Unverifiable Generation of the Generator g."

Header "SEE ALSO" \s-1EVP_PKEY-FFC\s0\|(7), \s-1EVP_KEYEXCH-DH\s0\|(7) \s-1EVP_PKEY\s0\|(3), \fBprovider-keymgmt\|(7), \s-1EVP_KEYMGMT\s0\|(3), \fBOSSL_PROVIDER-default\|(7), \s-1OSSL_PROVIDER-FIPS\s0\|(7) Header "COPYRIGHT" Copyright 2020-2022 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the \*(L"License\*(R"). You may not use this file except in compliance with the License. You can obtain a copy in the file \s-1LICENSE\s0 in the source distribution or at <https://www.openssl.org/source/license.html>.