=pod

=head1 NAME

EVP_RAND-HMAC-DRBG - The HMAC DRBG EVP_RAND implementation

=head1 DESCRIPTION

Support for the HMAC deterministic random bit generator through the
B<EVP_RAND> API.

=head2 Identity

"HMAC-DRBG" is the name for this implementation; it can be used with the
EVP_RAND_fetch() function.

=head2 Supported parameters

The supported parameters are:

=over 4

=item "state" (B<OSSL_RAND_PARAM_STATE>) <integer>

=item "strength" (B<OSSL_RAND_PARAM_STRENGTH>) <unsigned integer>

=item "max_request" (B<OSSL_RAND_PARAM_MAX_REQUEST>) <unsigned integer>

=item "reseed_requests" (B<OSSL_DRBG_PARAM_RESEED_REQUESTS>) <unsigned integer>

=item "reseed_time_interval" (B<OSSL_DRBG_PARAM_RESEED_TIME_INTERVAL>) <integer>

=item "min_entropylen" (B<OSSL_DRBG_PARAM_MIN_ENTROPYLEN>) <unsigned integer>

=item "max_entropylen" (B<OSSL_DRBG_PARAM_MAX_ENTROPYLEN>) <unsigned integer>

=item "min_noncelen" (B<OSSL_DRBG_PARAM_MIN_NONCELEN>) <unsigned integer>

=item "max_noncelen" (B<OSSL_DRBG_PARAM_MAX_NONCELEN>) <unsigned integer>

=item "max_perslen" (B<OSSL_DRBG_PARAM_MAX_PERSLEN>) <unsigned integer>

=item "max_adinlen" (B<OSSL_DRBG_PARAM_MAX_ADINLEN>) <unsigned integer>

=item "reseed_counter" (B<OSSL_DRBG_PARAM_RESEED_COUNTER>) <unsigned integer>

=item "properties" (B<OSSL_DRBG_PARAM_PROPERTIES>) <UTF8 string>

=item "digest" (B<OSSL_DRBG_PARAM_DIGEST>) <UTF8 string>

These parameters work as described in L<EVP_RAND(3)/PARAMETERS>.

=item "fips-indicator" (B<OSSL_DRBG_PARAM_FIPS_APPROVED_INDICATOR>) <integer>

=item "digest-check" (B<OSSL_DRBG_PARAM_FIPS_DIGEST_CHECK>) <integer>

These parameters work as described in L<provider-rand(7)/PARAMETERS>.

=back

=head1 NOTES

When using the FIPS provider, only these digests are permitted (as per
L<FIPS 140-3 IG D.R|https://csrc.nist.gov/CSRC/media/Projects/cryptographic-module-validation-program/documents/fips%20140-3/FIPS%20140-3%20IG.pdf>):

The default HMAC-DRBG implementation attempts to fetch the required internal
algorithms from the provider they are built into (eg the default provider)
regardless of the properties provided. Should the provider not implement
the required algorithms then properties will be used to find a different
implementation.

=over 4

=item SHA-1

=item SHA2-256

=item SHA2-512

=item SHA3-256

=item SHA3-512

=back

A context for HMAC DRBG can be obtained by calling:

 EVP_RAND *rand = EVP_RAND_fetch(NULL, "HMAC-DRBG", NULL);
 EVP_RAND_CTX *rctx = EVP_RAND_CTX_new(rand, NULL);

=head1 EXAMPLES

 EVP_RAND *rand;
 EVP_RAND_CTX *rctx;
 unsigned char bytes[100];
 OSSL_PARAM params[2], *p = params;
 unsigned int strength = 128;

 rand = EVP_RAND_fetch(NULL, "HMAC-DRBG", NULL);
 rctx = EVP_RAND_CTX_new(rand, NULL);
 EVP_RAND_free(rand);

 *p++ = OSSL_PARAM_construct_utf8_string(OSSL_DRBG_PARAM_DIGEST, SN_sha256, 0);
 *p = OSSL_PARAM_construct_end();
 EVP_RAND_instantiate(rctx, strength, 0, NULL, 0, params);

 EVP_RAND_generate(rctx, bytes, sizeof(bytes), strength, 0, NULL, 0);

 EVP_RAND_CTX_free(rctx);

=head1 CONFORMING TO

NIST SP 800-90A and SP 800-90B

=head1 SEE ALSO

L<EVP_RAND(3)>,
L<EVP_RAND(3)/PARAMETERS>,
L<openssl-fipsinstall(1)>


=head1 HISTORY

OpenSSL 3.1.1 introduced the B<-no_drbg_truncated_digests> option to
fipsinstall which restricts the permitted digests when using the FIPS
provider in a complaint manner.  For details refer to
L<FIPS 140-3 IG D.R|https://csrc.nist.gov/CSRC/media/Projects/cryptographic-module-validation-program/documents/fips%20140-3/FIPS%20140-3%20IG.pdf>).

=head1 COPYRIGHT

Copyright 2020-2025 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
