290 lines
11 KiB
Groff
Executable File
290 lines
11 KiB
Groff
Executable File
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.\" ========================================================================
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.\"
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.IX Title "RAND_DRBG_SET_CALLBACKS 3"
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.TH RAND_DRBG_SET_CALLBACKS 3 "2020-03-02" "3.0.0-dev" "OpenSSL"
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.\" For nroff, turn off justification. Always turn off hyphenation; it makes
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.\" way too many mistakes in technical documents.
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.if n .ad l
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.nh
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.SH "NAME"
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RAND_DRBG_set_callbacks,
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RAND_DRBG_set_callback_data,
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RAND_DRBG_get_callback_data,
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RAND_DRBG_get_entropy_fn,
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RAND_DRBG_cleanup_entropy_fn,
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RAND_DRBG_get_nonce_fn,
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RAND_DRBG_cleanup_nonce_fn
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\&\- set callbacks for reseeding
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.SH "SYNOPSIS"
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.IX Header "SYNOPSIS"
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.Vb 1
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\& #include <openssl/rand_drbg.h>
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\&
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\&
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\& int RAND_DRBG_set_callbacks(RAND_DRBG *drbg,
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\& RAND_DRBG_get_entropy_fn get_entropy,
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\& RAND_DRBG_cleanup_entropy_fn cleanup_entropy,
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\& RAND_DRBG_get_nonce_fn get_nonce,
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\& RAND_DRBG_cleanup_nonce_fn cleanup_nonce);
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\&
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\& int RAND_DRBG_set_callback_data(RAND_DRBG *drbg, void *ctx);
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\&
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\& void *RAND_DRBG_get_callback_data(RAND_DRBG *drbg);
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.Ve
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.SS "Callback Functions"
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.IX Subsection "Callback Functions"
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.Vb 6
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\& typedef size_t (*RAND_DRBG_get_entropy_fn)(
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\& RAND_DRBG *drbg,
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\& unsigned char **pout,
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\& int entropy,
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\& size_t min_len, size_t max_len,
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\& int prediction_resistance);
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\&
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\& typedef void (*RAND_DRBG_cleanup_entropy_fn)(
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\& RAND_DRBG *drbg,
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\& unsigned char *out, size_t outlen);
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\&
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\& typedef size_t (*RAND_DRBG_get_nonce_fn)(
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\& RAND_DRBG *drbg,
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\& unsigned char **pout,
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\& int entropy,
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\& size_t min_len, size_t max_len);
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\&
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\& typedef void (*RAND_DRBG_cleanup_nonce_fn)(
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\& RAND_DRBG *drbg,
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\& unsigned char *out, size_t outlen);
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.Ve
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.SH "DESCRIPTION"
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.IX Header "DESCRIPTION"
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\&\fIRAND_DRBG_set_callbacks()\fR sets the callbacks for obtaining fresh entropy and
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the nonce when reseeding the given \fBdrbg\fR.
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The callback functions are implemented and provided by the caller.
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Their parameter lists need to match the function prototypes above.
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.PP
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\&\fIRAND_DRBG_set_callback_data()\fR can be used to store a pointer to some context
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specific data, which can subsequently be retrieved by the entropy and nonce
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callbacks using \fIRAND_DRBG_get_callback_data()\fR.
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The ownership of the context data remains with the caller, i.e., it is the
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caller's responsibility to keep it available as long as it is needed by the
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callbacks and free it after use.
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For more information about the the callback data see the \s-1NOTES\s0 section.
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.PP
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Setting the callbacks or the callback data is allowed only if the \s-1DRBG\s0 has
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not been initialized yet.
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Otherwise, the operation will fail.
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To change the settings for one of the three shared DRBGs it is necessary to call
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\&\fIRAND_DRBG_uninstantiate()\fR first.
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.PP
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The \fBget_entropy\fR() callback is called by the \fBdrbg\fR when it requests fresh
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random input.
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It is expected that the callback allocates and fills a random buffer of size
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\&\fBmin_len\fR <= size <= \fBmax_len\fR (in bytes) which contains at least \fBentropy\fR
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bits of randomness.
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The \fBprediction_resistance\fR flag indicates whether the reseeding was
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triggered by a prediction resistance request.
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.PP
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The buffer's address is to be returned in *\fBpout\fR and the number of collected
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randomness bytes as return value.
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.PP
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If the callback fails to acquire at least \fBentropy\fR bits of randomness,
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it must indicate an error by returning a buffer length of 0.
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.PP
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If \fBprediction_resistance\fR was requested and the random source of the \s-1DRBG\s0
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does not satisfy the conditions requested by [\s-1NIST\s0 \s-1SP\s0 800\-90C], then
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it must also indicate an error by returning a buffer length of 0.
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See \s-1NOTES\s0 section for more details.
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.PP
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The \fBcleanup_entropy\fR() callback is called from the \fBdrbg\fR to to clear and
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free the buffer allocated previously by \fIget_entropy()\fR.
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The values \fBout\fR and \fBoutlen\fR are the random buffer's address and length,
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as returned by the \fIget_entropy()\fR callback.
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.PP
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The \fBget_nonce\fR() and \fBcleanup_nonce\fR() callbacks are used to obtain a nonce
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and free it again. A nonce is only required for instantiation (not for reseeding)
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and only in the case where the \s-1DRBG\s0 uses a derivation function.
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The callbacks are analogous to \fIget_entropy()\fR and \fIcleanup_entropy()\fR,
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except for the missing prediction_resistance flag.
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.PP
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If the derivation function is disabled, then no nonce is used for instantiation,
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and the \fBget_nonce\fR() and \fBcleanup_nonce\fR() callbacks can be omitted by
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setting them to \s-1NULL\s0.
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.SH "RETURN VALUES"
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.IX Header "RETURN VALUES"
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\&\fIRAND_DRBG_set_callbacks()\fR returns 1 on success, and 0 on failure.
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.PP
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\&\fIRAND_DRBG_set_callback_data()\fR returns 1 on success, and 0 on failure.
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.PP
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\&\fIRAND_DRBG_get_callback_data()\fR returns the pointer to the callback data,
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which is \s-1NULL\s0 if none has been set previously.
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.SH "NOTES"
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.IX Header "NOTES"
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It is important that \fBcleanup_entropy\fR() and \fBcleanup_nonce\fR() clear the buffer
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contents safely before freeing it, in order not to leave sensitive information
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about the \s-1DRBG\s0's state in memory.
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.PP
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A request for prediction resistance can only be satisfied by pulling fresh
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entropy from a live entropy source (section 5.5.2 of [\s-1NIST\s0 \s-1SP\s0 800\-90C]).
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It is up to the user to ensure that a live entropy source is configured
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and is being used.
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.PP
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The derivation function is disabled during initialization by calling the
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\&\fIRAND_DRBG_set()\fR function with the \s-1RAND_DRBG_FLAG_CTR_NO_DF\s0 flag.
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For more information on the derivation function and when it can be omitted,
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see [\s-1NIST\s0 \s-1SP\s0 800\-90A Rev. 1]. Roughly speaking it can be omitted if the random
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source has \*(L"full entropy\*(R", i.e., contains 8 bits of entropy per byte.
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.PP
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Even if a nonce is required, the \fBget_nonce\fR() and \fBcleanup_nonce\fR()
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callbacks can be omitted by setting them to \s-1NULL\s0.
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In this case the \s-1DRBG\s0 will automatically request an extra amount of entropy
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(using the \fBget_entropy\fR() and \fBcleanup_entropy\fR() callbacks) which it will
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utilize for the nonce, following the recommendations of [\s-1NIST\s0 \s-1SP\s0 800\-90A Rev. 1],
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section 8.6.7.
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.PP
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The callback data is a rather specialized feature, because in general the
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random sources don't (and in fact, they must not) depend on any state provided
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by the \s-1DRBG\s0.
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There are however exceptional cases where this feature is useful, most notably
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for implementing known answer tests (KATs) or deterministic signatures like
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those specified in \s-1RFC6979\s0, which require passing a specified entropy and nonce
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for instantiating the \s-1DRBG\s0.
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.SH "SEE ALSO"
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.IX Header "SEE ALSO"
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\&\fIRAND_DRBG_new\fR\|(3),
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\&\fIRAND_DRBG_reseed\fR\|(3),
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\&\s-1\fIRAND_DRBG\s0\fR\|(7)
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.SH "HISTORY"
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.IX Header "HISTORY"
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The \s-1RAND_DRBG\s0 functions were added in OpenSSL 1.1.1.
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.SH "COPYRIGHT"
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.IX Header "COPYRIGHT"
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Copyright 2017\-2019 The OpenSSL Project Authors. All Rights Reserved.
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.PP
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Licensed under the Apache License 2.0 (the \*(L"License\*(R"). You may not use
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this file except in compliance with the License. You can obtain a copy
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in the file \s-1LICENSE\s0 in the source distribution or at
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<https://www.openssl.org/source/license.html>.
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