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.\" ========================================================================
.\"
.IX Title "RAND_BYTES 3ossl"
.TH RAND_BYTES 3ossl "2024-09-03" "3.1.7+quic" "OpenSSL"
.\" For nroff, turn off justification. Always turn off hyphenation; it makes
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.nh
.SH "NAME"
RAND_bytes, RAND_priv_bytes, RAND_bytes_ex, RAND_priv_bytes_ex,
RAND_pseudo_bytes \- generate random data
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #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);
.Ve
.PP
The following function has been deprecated since OpenSSL 1.1.0, and can be
hidden entirely by defining \fB\s-1OPENSSL_API_COMPAT\s0\fR with a suitable version value,
see \fBopenssl_user_macros\fR\|(7):
.PP
.Vb 1
\& int RAND_pseudo_bytes(unsigned char *buf, int num);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\fBRAND_bytes()\fR generates \fBnum\fR random bytes using a cryptographically
secure pseudo random generator (\s-1CSPRNG\s0) and stores them in \fBbuf\fR.
.PP
\&\fBRAND_priv_bytes()\fR has the same semantics as \fBRAND_bytes()\fR. It is intended to
be used for generating values that should remain private. If using the
default \s-1RAND_METHOD,\s0 this function uses a separate \*(L"private\*(R" \s-1PRNG\s0
instance so that a compromise of the \*(L"public\*(R" \s-1PRNG\s0 instance will not
affect the secrecy of these private values, as described in \s-1\fBRAND\s0\fR\|(7)
and \s-1\fBEVP_RAND\s0\fR\|(7).
.PP
\&\fBRAND_bytes_ex()\fR and \fBRAND_priv_bytes_ex()\fR are the same as \fBRAND_bytes()\fR and
\&\fBRAND_priv_bytes()\fR except that they both take additional \fIstrength\fR and
\&\fIctx\fR parameters. The bytes generated will have a security strength of at
least \fIstrength\fR bits.
The \s-1DRBG\s0 used for the operation is the public or private \s-1DRBG\s0 associated with
the specified \fIctx\fR. The parameter can be \s-1NULL,\s0 in which case
the default library context is used (see \s-1\fBOSSL_LIB_CTX\s0\fR\|(3).
If the default \s-1RAND_METHOD\s0 has been changed then for compatibility reasons the
\&\s-1RAND_METHOD\s0 will be used in preference and the \s-1DRBG\s0 of the library context
ignored.
.SH "NOTES"
.IX Header "NOTES"
By default, the OpenSSL \s-1CSPRNG\s0 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 \s-1CSPRNG\s0 on first use
using the operating systems's random generator.
.PP
If the entropy source fails or is not available, the \s-1CSPRNG\s0 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 \fBRAND_bytes()\fR and \fBRAND_priv_bytes()\fR and
not take randomness for granted.
.PP
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.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
\&\fBRAND_bytes()\fR and \fBRAND_priv_bytes()\fR
return 1 on success, \-1 if not supported by the current
\&\s-1RAND\s0 method, or 0 on other failure. The error code can be
obtained by \fBERR_get_error\fR\|(3).
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBRAND_add\fR\|(3),
\&\fBRAND_bytes\fR\|(3),
\&\fBRAND_priv_bytes\fR\|(3),
\&\fBERR_get_error\fR\|(3),
\&\s-1\fBRAND\s0\fR\|(7),
\&\s-1\fBEVP_RAND\s0\fR\|(7)
.SH "HISTORY"
.IX Header "HISTORY"
.IP "\(bu" 2
\&\fBRAND_pseudo_bytes()\fR was deprecated in OpenSSL 1.1.0; use \fBRAND_bytes()\fR instead.
.IP "\(bu" 2
The \fBRAND_priv_bytes()\fR function was added in OpenSSL 1.1.1.
.IP "\(bu" 2
The \fBRAND_bytes_ex()\fR and \fBRAND_priv_bytes_ex()\fR functions were added in OpenSSL 3.0
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright 2000\-2022 The OpenSSL Project Authors. All Rights Reserved.
.PP
Licensed under the Apache License 2.0 (the \*(L"License\*(R"). You may not use
this file except in compliance with the License. You can obtain a copy
in the file \s-1LICENSE\s0 in the source distribution or at
<https://www.openssl.org/source/license.html>.
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