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RAND_DRBG_set_callbacks(3) OpenSSL RAND_DRBG_set_callbacks(3)
NAME
RAND_DRBG_set_callbacks, RAND_DRBG_get_entropy_fn,
RAND_DRBG_cleanup_entropy_fn, RAND_DRBG_get_nonce_fn,
RAND_DRBG_cleanup_nonce_fn - set callbacks for reseeding
LIBRARY
libcrypto, -lcrypto
SYNOPSIS
#include <openssl/rand_drbg.h>
int RAND_DRBG_set_callbacks(RAND_DRBG *drbg,
RAND_DRBG_get_entropy_fn get_entropy,
RAND_DRBG_cleanup_entropy_fn cleanup_entropy,
RAND_DRBG_get_nonce_fn get_nonce,
RAND_DRBG_cleanup_nonce_fn cleanup_nonce);
Callback Functions
typedef size_t (*RAND_DRBG_get_entropy_fn)(
RAND_DRBG *drbg,
unsigned char **pout,
int entropy,
size_t min_len, size_t max_len,
int prediction_resistance);
typedef void (*RAND_DRBG_cleanup_entropy_fn)(
RAND_DRBG *drbg,
unsigned char *out, size_t outlen);
typedef size_t (*RAND_DRBG_get_nonce_fn)(
RAND_DRBG *drbg,
unsigned char **pout,
int entropy,
size_t min_len, size_t max_len);
typedef void (*RAND_DRBG_cleanup_nonce_fn)(
RAND_DRBG *drbg,
unsigned char *out, size_t outlen);
DESCRIPTION
RAND_DRBG_set_callbacks() sets the callbacks for obtaining fresh
entropy and the nonce when reseeding the given drbg. The callback
functions are implemented and provided by the caller. Their parameter
lists need to match the function prototypes above.
Setting the callbacks is allowed only if the DRBG has not been
initialized yet. Otherwise, the operation will fail. To change the
settings for one of the three shared DRBGs it is necessary to call
RAND_DRBG_uninstantiate() first.
The get_entropy() callback is called by the drbg when it requests fresh
random input. It is expected that the callback allocates and fills a
random buffer of size min_len <= size <= max_len (in bytes) which
contains at least entropy bits of randomness. The
prediction_resistance flag indicates whether the reseeding was
triggered by a prediction resistance request.
The buffer's address is to be returned in *pout and the number of
collected randomness bytes as return value.
If the callback fails to acquire at least entropy bits of randomness,
it must indicate an error by returning a buffer length of 0.
If prediction_resistance was requested and the random source of the
DRBG does not satisfy the conditions requested by [NIST SP 800-90C],
then it must also indicate an error by returning a buffer length of 0.
See NOTES section for more details.
The cleanup_entropy() callback is called from the drbg to clear and
free the buffer allocated previously by get_entropy(). The values out
and outlen are the random buffer's address and length, as returned by
the get_entropy() callback.
The get_nonce() and cleanup_nonce() callbacks are used to obtain a
nonce and free it again. A nonce is only required for instantiation
(not for reseeding) and only in the case where the DRBG uses a
derivation function. The callbacks are analogous to get_entropy() and
cleanup_entropy(), except for the missing prediction_resistance flag.
If the derivation function is disabled, then no nonce is used for
instantiation, and the get_nonce() and cleanup_nonce() callbacks can be
omitted by setting them to NULL.
RETURN VALUES
RAND_DRBG_set_callbacks() return 1 on success, and 0 on failure
NOTES
It is important that cleanup_entropy() and cleanup_nonce() clear the
buffer contents safely before freeing it, in order not to leave
sensitive information about the DRBG's state in memory.
A request for prediction resistance can only be satisfied by pulling
fresh entropy from one of the approved entropy sources listed in
section 5.5.2 of [NIST SP 800-90C]. Since the default implementation
of the get_entropy callback does not have access to such an approved
entropy source, a request for prediction resistance will always fail.
In other words, prediction resistance is currently not supported yet by
the DRBG.
The derivation function is disabled during initialization by calling
the RAND_DRBG_set() function with the RAND_DRBG_FLAG_CTR_NO_DF flag.
For more information on the derivation function and when it can be
omitted, see [NIST SP 800-90A Rev. 1]. Roughly speaking it can be
omitted if the random source has "full entropy", i.e., contains 8 bits
of entropy per byte.
Even if a nonce is required, the get_nonce() and cleanup_nonce()
callbacks can be omitted by setting them to NULL. In this case the DRBG
will automatically request an extra amount of entropy (using the
get_entropy() and cleanup_entropy() callbacks) which it will utilize
for the nonce, following the recommendations of [NIST SP 800-90A Rev.
1], section 8.6.7.
SEE ALSO
RAND_DRBG_new(3), RAND_DRBG_reseed(3), RAND_DRBG(7)
HISTORY
The RAND_DRBG functions were added in OpenSSL 1.1.1.
COPYRIGHT
Copyright 2017-2020 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
<https://www.openssl.org/source/license.html>.
1.1.1i 2020-12-10 RAND_DRBG_set_callbacks(3)