=pod

=head1 NAME

EVP_RAND-JITTER - The randomness seed source EVP_RAND implementation

=head1 DESCRIPTION

Support for deterministic random number generator seeding through the
B<EVP_RAND> API.

This software seed source produces randomness based on tiny CPU
"jitter" fluctuations.

It is available when OpenSSL is compiled with B<enable-jitter>
option. When available it is listed in B<openssl list
-random-generators> and B<openssl info -seeds>.

=head2 Identity

"JITTER" 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>

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

=back

=head1 NOTES

A context for the seed source can be obtained by calling:

 EVP_RAND *rand = EVP_RAND_fetch(NULL, "JITTER", NULL);
 EVP_RAND_CTX *rctx = EVP_RAND_CTX_new(rand, NULL);

The B<enable-jitter> configuration option was added in OpenSSL 3.4.

=head1 EXAMPLES

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

 /* Create and instantiate a seed source */
 rand = EVP_RAND_fetch(NULL, "JITTER", NULL);
 seed = EVP_RAND_CTX_new(rand, NULL);
 EVP_RAND_instantiate(seed, strength, 0, NULL, 0, NULL);
 EVP_RAND_free(rand);

 /* Feed this into a DRBG */
 rand = EVP_RAND_fetch(NULL, "CTR-DRBG", NULL);
 rctx = EVP_RAND_CTX_new(rand, seed);
 EVP_RAND_free(rand);

 /* Configure the DRBG */
 *p++ = OSSL_PARAM_construct_utf8_string(OSSL_DRBG_PARAM_CIPHER,
                                         SN_aes_256_ctr, 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);
 EVP_RAND_CTX_free(seed);

=head1 SEE ALSO

L<EVP_RAND(3)>,
L<EVP_RAND(3)/PARAMETERS>

=head1 COPYRIGHT

Copyright 2024 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
