RAND_BYTES(3ossl) OpenSSL RAND_BYTES(3ossl)
NAME
RAND_bytes, RAND_priv_bytes, RAND_bytes_ex, RAND_priv_bytes_ex,
RAND_pseudo_bytes, RAND_set1_random_provider - generate random data
SYNOPSIS
#include <openssl/rand.h>
int RAND_bytes(unsigned char *buf, int num);
int RAND_priv_bytes(unsigned char *buf, int num);
int RAND_bytes_ex(OSSL_LIB_CTX *ctx, unsigned char *buf, size_t num,
unsigned int strength);
int RAND_priv_bytes_ex(OSSL_LIB_CTX *ctx, unsigned char *buf, size_t num,
unsigned int strength);
int RAND_set1_random_provider(OSSL_LIB_CTX *ctx, OSSL_PROVIDER *p);
The following function has been deprecated since OpenSSL 1.1.0, and can
be hidden entirely by defining OPENSSL_API_COMPAT with a suitable version
value, see openssl_user_macros(7):
int RAND_pseudo_bytes(unsigned char *buf, int num);
DESCRIPTION
RAND_bytes(3) generates num random bytes using a cryptographically secure
pseudo random generator (CSPRNG) and stores them in buf. buf MUST NOT be
NULL.
RAND_bytes(3). It is intended
to be used for generating values that should remain private. If using the
default RAND_METHOD, this function uses a separate "private" PRNG
instance so that a compromise of the "public" PRNG instance will not
affect the secrecy of these private values, as described in RAND(7) and
EVP_RAND(7).
RAND_bytes(3) and
RAND_priv_bytes() except that they both take additional strength and ctx
parameters. The bytes generated will have a security strength of at least
strength bits. The DRBG used for the operation is the public or private
DRBG associated with the specified ctx. The parameter can be NULL, in
which case the default library context is used (see OSSL_LIB_CTX(3). If
the default RAND_METHOD has been changed then for compatibility reasons
the RAND_METHOD will be used in preference and the DRBG of the library
context ignored.
RAND_set1_random_provider() specifies a provider, prov, which will be
used by the library context ctx for all of the generate calls above
instead of the built-in DRBGs and entropy source. Pass NULL for the
provider to disable the random provider functionality. In this case, the
built-in DRBGs and entropy source will be used. This call should not be
considered thread safe.
NOTES
By default, the OpenSSL CSPRNG supports a security level of 256 bits,
provided it was able to seed itself from a trusted entropy source. On
all major platforms supported by OpenSSL (including the Unix-like
platforms and Windows), OpenSSL is configured to automatically seed the
CSPRNG on first use using the operating systems's random generator.
If the entropy source fails or is not available, the CSPRNG will enter an
error state and refuse to generate random bytes. For that reason, it is
important to always check the error return value of RAND_bytes(3) and
RAND_priv_bytes() and not take randomness for granted.
On other platforms, there might not be a trusted entropy source available
or OpenSSL might have been explicitly configured to use different entropy
sources. If you are in doubt about the quality of the entropy source,
don't hesitate to ask your operating system vendor or post a question on
GitHub or the openssl-users mailing list.
RETURN VALUES
RAND_bytes(3) and RAND_priv_bytes() return 1 on success, -1 if not
supported by the current RAND method, or 0 on other failure. The error
code can be obtained by ERR_get_error(3).
RAND_set1_random_provider() returns 1 on success and 0 on failure.
SEE ALSO
RAND_bytes(3), RAND_priv_bytes(3), ERR_get_error(3),
RAND(7), EVP_RAND(7)
HISTORY
RAND_bytes(3)
instead.
o The RAND_priv_bytes() function was added in OpenSSL 1.1.1.
o The RAND_bytes_ex() and RAND_priv_bytes_ex() functions were added in
OpenSSL 3.0
o The RAND_set1_random_provider() function was added in OpenSSL 3.5
COPYRIGHT
Copyright 2000-2026 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>.
3.6.5 2026-09-29 RAND_BYTES(3ossl)
openssl 3.6.5 - Generated Sat Oct 3 08:32:36 CDT 2026