$NetBSD: RAND_DRBG_set_callbacks.3,v 1.3 2020/12/10 00:33:12 christos Exp $

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Title "RAND_DRBG_set_callbacks 3" For nroff, turn off justification. Always turn off hyphenation; it makes
way too many mistakes in technical documents.
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 libcrypto, -lcrypto Header "SYNOPSIS" .Vb 1 #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); .Ve Subsection "Callback Functions" .Vb 6 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); .Ve Header "DESCRIPTION" \fBRAND_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 \s-1DRBG\s0 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 \fBRAND_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 \fBmin_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 \s-1DRBG\s0 does not satisfy the conditions requested by [\s-1NIST SP 800-90C\s0], then it must also indicate an error by returning a buffer length of 0. See \s-1NOTES\s0 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 \s-1DRBG\s0 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 \s-1NULL.\s0

Header "RETURN VALUES" \fBRAND_DRBG_set_callbacks() return 1 on success, and 0 on failure Header "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 \s-1DRBG\s0'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 [\s-1NIST SP 800-90C\s0]. 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 \s-1DRBG.\s0

The derivation function is disabled during initialization by calling the \fBRAND_DRBG_set() function with the \s-1RAND_DRBG_FLAG_CTR_NO_DF\s0 flag. For more information on the derivation function and when it can be omitted, see [\s-1NIST SP 800-90A\s0 Rev. 1]. Roughly speaking it can be omitted if the random source has \*(L"full entropy\*(R", 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 \s-1NULL.\s0 In this case the \s-1DRBG\s0 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 [\s-1NIST SP 800-90A\s0 Rev. 1], section 8.6.7.

Header "SEE ALSO" \fBRAND_DRBG_new\|(3), \fBRAND_DRBG_reseed\|(3), \s-1RAND_DRBG\s0\|(7) Header "HISTORY" The \s-1RAND_DRBG\s0 functions were added in OpenSSL 1.1.1. Header "COPYRIGHT" Copyright 2017-2020 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the OpenSSL license (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>.