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Always turn off hyphenation; it makes .\" way too many mistakes in technical documents. .if n .ad l .nh .SH "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 .SH "SYNOPSIS" .IX Header "SYNOPSIS" .Vb 1 \& #include \& \& \& 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); .Ve .SS "Callback Functions" .IX 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 .SH "DESCRIPTION" .IX Header "DESCRIPTION" \&\fBRAND_DRBG_set_callbacks()\fR sets the callbacks for obtaining fresh entropy and the nonce when reseeding the given \fBdrbg\fR. The callback functions are implemented and provided by the caller. Their parameter lists need to match the function prototypes above. .PP \&\fBRAND_DRBG_set_callback_data()\fR can be used to store a pointer to some context specific data, which can subsequently be retrieved by the entropy and nonce callbacks using \fBRAND_DRBG_get_callback_data()\fR. 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 \s-1NOTES\s0 section. .PP Setting the callbacks or the callback data 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()\fR first. .PP The \fBget_entropy\fR() callback is called by the \fBdrbg\fR when it requests fresh random input. It is expected that the callback allocates and fills a random buffer of size \&\fBmin_len\fR <= size <= \fBmax_len\fR (in bytes) which contains at least \fBentropy\fR bits of randomness. The \fBprediction_resistance\fR flag indicates whether the reseeding was triggered by a prediction resistance request. .PP The buffer's address is to be returned in *\fBpout\fR and the number of collected randomness bytes as return value. .PP If the callback fails to acquire at least \fBentropy\fR bits of randomness, it must indicate an error by returning a buffer length of 0. .PP If \fBprediction_resistance\fR 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. .PP The \fBcleanup_entropy\fR() callback is called from the \fBdrbg\fR to to clear and free the buffer allocated previously by \fBget_entropy()\fR. The values \fBout\fR and \fBoutlen\fR are the random buffer's address and length, as returned by the \fBget_entropy()\fR callback. .PP The \fBget_nonce\fR() and \fBcleanup_nonce\fR() 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 \fBget_entropy()\fR and \fBcleanup_entropy()\fR, except for the missing prediction_resistance flag. .PP If the derivation function is disabled, then no nonce is used for instantiation, and the \fBget_nonce\fR() and \fBcleanup_nonce\fR() callbacks can be omitted by setting them to \s-1NULL.\s0 .SH "RETURN VALUES" .IX Header "RETURN VALUES" \&\fBRAND_DRBG_set_callbacks()\fR returns 1 on success, and 0 on failure. .PP \&\fBRAND_DRBG_set_callback_data()\fR returns 1 on success, and 0 on failure. .PP \&\fBRAND_DRBG_get_callback_data()\fR returns the pointer to the callback data, which is \s-1NULL\s0 if none has been set previously. .SH "NOTES" .IX Header "NOTES" It is important that \fBcleanup_entropy\fR() and \fBcleanup_nonce\fR() clear the buffer contents safely before freeing it, in order not to leave sensitive information about the \s-1DRBG\s0's state in memory. .PP A request for prediction resistance can only be satisfied by pulling fresh entropy from a live entropy source (section 5.5.2 of [\s-1NIST SP 800\-90C\s0]). It is up to the user to ensure that a live entropy source is configured and is being used. .PP The derivation function is disabled by calling the \fBRAND_DRBG_new_ex()\fR 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", that is, it contains 8 bits of entropy per byte. In a \s-1FIPS\s0 context, the derivation function can never be omitted. .PP Even if a nonce is required, the \fBget_nonce\fR() and \fBcleanup_nonce\fR() 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 \fBget_entropy\fR() and \fBcleanup_entropy\fR() callbacks) which it will utilize for the nonce, following the recommendations of [\s-1NIST SP 800\-90A\s0 Rev. 1], section 8.6.7. .PP 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 \s-1DRBG.\s0 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 \s-1RFC6979,\s0 which require passing a specified entropy and nonce for instantiating the \s-1DRBG.\s0 .SH "SEE ALSO" .IX Header "SEE ALSO" \&\fBRAND_DRBG_new\fR\|(3), \&\fBRAND_DRBG_reseed\fR\|(3), \&\s-1\fBRAND_DRBG\s0\fR\|(7) .SH "HISTORY" .IX Header "HISTORY" The \s-1RAND_DRBG\s0 functions were added in OpenSSL 1.1.1. .SH "COPYRIGHT" .IX Header "COPYRIGHT" Copyright 2017\-2020 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 .