NAME

       RAND_bytes, RAND_priv_bytes, RAND_bytes_ex, RAND_priv_bytes_ex, RAND_pseudo_bytes -
       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);

       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() generates num random bytes using a cryptographically secure pseudo random
       generator (CSPRNG) and stores them in buf.

       RAND_priv_bytes() has the same semantics as RAND_bytes().  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_ex() and RAND_priv_bytes_ex() are the same as RAND_bytes() and
       RAND_priv_bytes() except that they both take additional strength and ctx parameters. The
       bytes genreated 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.

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() 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() 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).

SEE ALSO

       RAND_add(3), RAND_bytes(3), RAND_priv_bytes(3), ERR_get_error(3), RAND(7), EVP_RAND(7)

HISTORY

       • RAND_pseudo_bytes() was deprecated in OpenSSL 1.1.0; use RAND_bytes() instead.

       • The RAND_priv_bytes() function was added in OpenSSL 1.1.1.

       • The RAND_bytes_ex() and RAND_priv_bytes_ex() functions were added in OpenSSL 3.0

COPYRIGHT

       Copyright 2000-2021 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>.