RAND_DRBG_SET_CALLBACKS(3)

NAME
RAND_DRBG_set_callbacks, RAND_DRBG_set_callback_data, RAND_DRBG_get_callback_data, RAND_DRBG_get_entropy_fn, RAND_DRBG_cleanup_entropy_fn, RAND_DRBG_get_nonce_fn, RAND_DRBG_cleanup_nonce_fn - set callbacks for reseeding

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);

 int RAND_DRBG_set_callback_data(RAND_DRBG *drbg, void *ctx);

 void *RAND_DRBG_get_callback_data(RAND_DRBG *drbg);

  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.

RAND_DRBG_set_callback_data() can be used to store a pointer to some context specific data, which can subsequently be retrieved by the entropy and nonce callbacks using RAND_DRBG_get_callback_data(). The ownership of the context data remains with the caller, i.e., it is the caller's responsibility to keep it available as long as it is needed by the callbacks and free it after use. For more information about the the callback data see the NOTES section.

Setting the callbacks or the callback data 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 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() returns 1 on success, and 0 on failure.

RAND_DRBG_set_callback_data() returns 1 on success, and 0 on failure.

RAND_DRBG_get_callback_data() returns the pointer to the callback data, which is NULL if none has been set previously.

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 a live entropy source (section 5.5.2 of [NIST SP 800-90C]). It is up to the user to ensure that a live entropy source is configured and is being used.

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.

The callback data is a rather specialized feature, because in general the random sources don't (and in fact, they must not) depend on any state provided by the DRBG. There are however exceptional cases where this feature is useful, most notably for implementing known answer tests (KATs) or deterministic signatures like those specified in RFC6979, which require passing a specified entropy and nonce for instantiating the DRBG.

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 2017-2019 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>.