=pod

=head1 NAME

EVP_PKEY-DSA, EVP_KEYMGMT-DSA, EVP_PKEY-DH, EVP_KEYMGMT-DH
- EVP_PKEY DSA and DH keytype and algorithm support

=head1 DESCRIPTION

The B<DSA> and B<DH> keytypes are implemented in OpenSSL's default and FIPS
providers.
The implementations support the basic DSA and DH keys, containing the public
and private keys I<pub> and I<priv> as well as the three main domain parameters
I<p>, I<q> and I<g>.

Finite field cryptography (FFC) is a method of implementing discrete logarithm
cryptography using finite field mathematics. DSA is an example of FFC and
Diffie-Hellman key establishment algorithms specified in SP800-56A can also be
implemented as FFC.

For B<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 "FIPS 186-type" domain parameters. FIPS 186-type domain
parameters should only be used for backward compatibility with existing
applications that cannot be upgraded to use the approved safe-prime groups.

For B<DSA> (and B<DH> 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 I<seed>, I<pcounter>
and I<gindex> may need to be stored for validation purposes.
For B<DH> the I<seed> and I<pcounter> can be stored in ASN1 data
(but the I<gindex> is not). For B<DSA> however, these fields are not stored in
the ASN1 data so they need to be stored externally if validation is required.

=head2 Common DH parameters

=over 4

=item "group" (B<OSSL_PKEY_PARAM_FFC_GROUP>) <UTF8 string>

A string that associates a B<DH> named safe prime group with known values for
I<p>, I<q> and I<g>.

The following values can be used by the default and OpenSSL's 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 the default provider:
"modp_1536", "dh_1024_160", "dh_2048_224", "dh_2048_256".

DH named groups can be easily validated since the parameters are well known.
For protocols that only transfer I<p> and I<g> the value of I<q> can also be
retrieved.

=item "safeprime-generator" (B<OSSL_PKEY_PARAM_FFC_GENERATOR>) <integer>

Used for DH generation of safe primes using the old generator code.
It is recommended to use a named safe prime group instead, if domain parameter
validation is required. The default value is 2.

These are not named safe prime groups so setting this value for the OpenSSL FIPS
provider will instead choose a named safe prime group based on the size of I<p>.

=back

=head2 Common DSA & DH parameters

In addition to the common parameters that all keytypes should support (see
L<provider-keymgmt(7)/Common parameters>), the B<DSA> and B<DH> keytype
implementations support the following.

=over 4

=item "pub" (B<OSSL_PKEY_PARAM_PUB_KEY>) <unsigned integer>

The public key value.

=item "priv" (B<OSSL_PKEY_PARAM_PRIV_KEY>) <unsigned integer>

The private key value.

=item "p" (B<OSSL_PKEY_PARAM_FFC_P>) <unsigned integer>

A DSA or Diffie-Hellman prime "p" value.

=item "q" (B<OSSL_PKEY_PARAM_FFC_Q>) <unsigned integer>

A DSA or Diffie-Hellman prime "q" value.

=item "g" (B<OSSL_PKEY_PARAM_FFC_G>) <unsigned integer>

A DSA or Diffie-Hellman generator "g" value.

=item "seed" (B<OSSL_PKEY_PARAM_FFC_SEED>) <octet string>

An optional domain parameter I<seed> value used during generation and validation
of I<p>, I<q> and canonical I<g>.
For validation this needs to set the I<seed> that was produced during generation.

=item "gindex" (B<OSSL_PKEY_PARAM_FFC_GINDEX>) <integer>

Sets the index to use for canonical generation and verification of the generator
I<g>.
Set this to a positive value from 0..FF to use this mode. This I<gindex> can
then be reused during key validation to verify the value of I<g>. If this value
is not set or is -1 then unverifiable generation of the generator I<g> will be
used.

=item "pcounter" (B<OSSL_PKEY_PARAM_FFC_PCOUNTER>) <integer>

An optional domain parameter I<counter> value that is output during generation
of I<p>. This value must be saved if domain parameter validation is required.

=item "hindex" (B<OSSL_PKEY_PARAM_FFC_H>) <integer>

For unverifiable generation of the generator I<g> this value is output during 
generation of I<g>. Its value is the first integer larger than one that
satisfies g = h^j mod p (where g != 1 and "j" is the cofactor).

=item "j" (B<OSSL_PKEY_PARAM_FFC_COFACTOR>) <unsigned integer>

An optional informational cofactor parameter that should equal (p - 1) / q.

=back


=head2 DSA / DH key generation (FFC) parameters

The following Key Generation types are available for the built-in FFC algorithms:

=over 4

=item "type" (B<OSSL_PKEY_PARAM_FFC_TYPE>) <utf8_string>

Sets the type of parameter generation. For DH Valid values are:

=over 4

=item "fips186_4"

The current standard. This is the default value.

=item "default"

This is an alias to use the latest implemented standard.
It is currently set to "fips186_4".

=item "group"

This specifies that a named safe prime name will be chosen using the "pbits"
type.

=item "fips186_2"

The old standard that should only be used for legacy purposes.

=item "generator"

A safe prime generator. See the "safeprime-generator" type.

=back

For DSA valid values are one of "default", "fips186_4" or "fips186_2" as
described above.

=item "pbits" (B<OSSL_PKEY_PARAM_FFC_PBITS>) <unsigned integer>

Sets the size (in bits) of the prime 'p'.

For "fips186_4" this must be 2048 for DH, and either of 2048 or 3072 for DSA.
For "fips186_2" this must be 1024.
For "group" this can be any one of 2048, 3072, 4096, 6144 or 8192.

=item "qbits" (B<OSSL_PKEY_PARAM_FFC_QBITS>) <unsigned integer>

Sets the size (in bits) of the prime 'q'.

For "fips186_4" this can be either 224 or 256.
For "fips186_2" this has a size of 160.

=item "digest" (B<OSSL_PKEY_PARAM_FFC_DIGEST>)  <utf8_string>

Sets the Digest algorithm to be used as part of the Key Generation Function
associated with the given Key Generation I<ctx>.
This must also be set for key validation.

=item "properties" (B<OSSL_PKEY_PARAM_FFC_DIGEST_PROPS>) <utf8_string>

Sets properties to be used upon look up of the implementation for the selected
Digest algorithm for the Key Generation Function associated with the given key
generation I<ctx>. This may also be set for key validation.

=item "seed" (B<OSSL_PKEY_PARAM_FFC_SEED>) <octet string>

For "fips186_4" or "fips186_2" generation this sets the I<seed> data to use
instead of generating a random seed internally. This should be used for
testing purposes only. This will either produce fixed values for the generated
parameters OR it will fail if the seed did not generate valid primes.

=item "group" (B<OSSL_PKEY_PARAM_FFC_GROUP>) <UTF8 string>

=item "safeprime-generator" (B<OSSL_PKEY_PARAM_FFC_GENERATOR>) <integer>

=item "gindex" (B<OSSL_PKEY_PARAM_FFC_GINDEX>) <integer>

=item "pcounter" (B<OSSL_PKEY_PARAM_FFC_PCOUNTER>) <integer>

=item "hindex" (B<OSSL_PKEY_PARAM_FFC_H>) <integer>

These types are described above.

=back


=head1 CONFORMING TO

=over 4

=item RFC 7919 (TLS ffdhe named safe prime groups)

=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").

=back

The following sections of SP800-56Ar3:

=over 4

=item 5.5.1.1 FFC Domain Parameter Selection/Generation

=item Appendix D: FFC Safe-prime Groups

=back

The following sections of FIPS 186-4:

=over 4

=item A.1.1.2 Generation of Probable Primes p and q Using an Approved Hash Function.

=item A.2.3 Generation of canonical generator g.

=item A.2.1 Unverifiable Generation of the Generator g.

=back

=head1 SEE ALSO

L<EVP_KEYMGMT(3)>, L<EVP_PKEY(3)>, L<provider-keymgmt(7)>,
L<OSSL_PROVIDER-default(7)>, L<OSSL_PROVIDER-FIPS(7)>,
L<EVP_SIGNATURE-DSA(7)>, L<EVP_KEYEXCH-DH(7)>

=head1 COPYRIGHT

Copyright 2020 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
L<https://www.openssl.org/source/license.html>.

=cut