EVP_RAND-HMAC-DRBG(7)

EVP_RAND-HMAC-DRBG(7) | OpenSSL | EVP_RAND-HMAC-DRBG(7)

# NAME

EVP_RAND-HMAC-DRBG - The HMAC DRBG EVP_RAND implementation

# DESCRIPTION

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

## Identity

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

## Supported parameters

The supported parameters are:

"state" (OSSL_RAND_PARAM_STATE) <integer>
"strength" (OSSL_RAND_PARAM_STRENGTH) <unsigned integer>
"max_request" (OSSL_RAND_PARAM_MAX_REQUEST) <unsigned integer>
"reseed_requests" (OSSL_DRBG_PARAM_RESEED_REQUESTS) <unsigned integer>
"reseed_time_interval" (OSSL_DRBG_PARAM_RESEED_TIME_INTERVAL) <integer>
"min_entropylen" (OSSL_DRBG_PARAM_MIN_ENTROPYLEN) <unsigned integer>
"max_entropylen" (OSSL_DRBG_PARAM_MAX_ENTROPYLEN) <unsigned integer>
"min_noncelen" (OSSL_DRBG_PARAM_MIN_NONCELEN) <unsigned integer>
"max_noncelen" (OSSL_DRBG_PARAM_MAX_NONCELEN) <unsigned integer>
"max_perslen" (OSSL_DRBG_PARAM_MAX_PERSLEN) <unsigned integer>
"max_adinlen" (OSSL_DRBG_PARAM_MAX_ADINLEN) <unsigned integer>
"reseed_counter" (OSSL_DRBG_PARAM_RESEED_COUNTER) <unsigned integer>
"properties" (OSSL_DRBG_PARAM_PROPERTIES) <UTF8 string>
"mac" (OSSL_DRBG_PARAM_MAC) <UTF8 string>
"digest" (OSSL_DRBG_PARAM_DIGEST) <UTF8 string>
  These parameters work as described in "PARAMETERS" in EVP_RAND(3).
"fips-indicator" (OSSL_DRBG_PARAM_FIPS_APPROVED_INDICATOR) <integer>
"digest-check" (OSSL_DRBG_PARAM_FIPS_DIGEST_CHECK) <integer>
  These parameters work as described in "PARAMETERS" in provider-rand(7).

# NOTES

When using the FIPS provider, only these digests are permitted (as per 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.

SHA-1
SHA2-256
SHA2-512
SHA3-256
SHA3-512

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);

# EXAMPLES

     EVP_RAND *rand;
     EVP_RAND_CTX *rctx;
     unsigned char bytes[100];
     OSSL_PARAM params[3], *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_MAC, SN_hmac, 0);
     *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);

# CONFORMING TO

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

# SEE ALSO

EVP_RAND(3), "PARAMETERS" in EVP_RAND(3), openssl-fipsinstall(1)

# HISTORY

OpenSSL 3.1.1 introduced the -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 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>).

# 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 <https://www.openssl.org/source/license.html>.

2026-04-07 | 3.5.6