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RAND(7)				    OpenSSL			       RAND(7)

       RAND - the OpenSSL random generator

       Random numbers are a vital part of cryptography,	they are needed	to
       provide unpredictability	for tasks like key generation, creating	salts,
       and many	more.  Software-based generators must be seeded	with external
       randomness before they can be used as a cryptographically-secure
       pseudo-random number generator (CSPRNG).	 The availability of common
       hardware	with special instructions and modern operating systems,	which
       may use items such as interrupt jitter and network packet timings, can
       be reasonable sources of	seeding	material.

       OpenSSL comes with a default implementation of the RAND API which is
       based on	the deterministic random bit generator (DRBG) model as
       described in [NIST SP 800-90A Rev. 1]. The default random generator
       will initialize automatically on	first use and will be fully functional
       without having to be initialized	('seeded') explicitly.	It seeds and
       reseeds itself automatically using trusted random sources provided by
       the operating system.

       As a normal application developer, you do not have to worry about any
       details,	just use RAND_bytes(3) to obtain random	data.  Having said
       that, there is one important rule to obey: Always check the error
       return value of RAND_bytes(3) and do not	take randomness	for granted.
       Although	(re-)seeding is	automatic, it can fail because no trusted
       random source is	available or the trusted source(s) temporarily fail to
       provide sufficient random seed material.	 In this case the CSPRNG
       enters an error state and ceases	to provide output, until it is able to
       recover from the	error by reseeding itself.  For	more details on
       reseeding and error recovery, see RAND_DRBG(7).

       For values that should remain secret, you can use RAND_priv_bytes(3)
       instead.	 This method does not provide 'better' randomness, it uses the
       same type of CSPRNG.  The intention behind using	a dedicated CSPRNG
       exclusively for private values is that none of its output should	be
       visible to an attacker (e.g., used as salt value), in order to reveal
       as little information as	possible about its internal state, and that a
       compromise of the "public" CSPRNG instance will not affect the secrecy
       of these	private	values.

       In the rare case	where the default implementation does not satisfy your
       special requirements, there are two options:

       o Replace the default RAND method by your own RAND method using

       o Modify	the default settings of	the OpenSSL RAND method	by modifying
	 the security parameters of the	underlying DRBG, which is described in
	 detail	in RAND_DRBG(7).

       Changing	the default random generator or	its default parameters should
       be necessary only in exceptional	cases and is not recommended, unless
       you have	a profound knowledge of	cryptographic principles and
       understand the implications of your changes.

       RAND_add(3), RAND_bytes(3), RAND_priv_bytes(3),
       RAND_get_rand_method(3),	RAND_set_rand_method(3), RAND_OpenSSL(3),

       Copyright 2018-2019 The OpenSSL Project Authors.	All Rights Reserved.

       Licensed	under the OpenSSL license (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

1.1.1k				  2021-03-25			       RAND(7)


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