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09066cf2a1
It took me a little while to realize why the test_rand_drbg_reseed test
kept crashing after replacing the RAND_DRBG_{gs}et_ex_data() calls by
RAND_DRBG_{gs}et_callback_data().
The reason was that the ex_data API prohibits modifying the callbacks
or callback data of chained DRBGs and returned an error which was
ignored by the `test_rand_drbg_reseed` test, for good reasons.
The `test_rand_drbg_reseed` test is special in this respect, because
it needs to install callbacks for all DRBGs, in order to intercept
and count the reseeding events.
Since the drbgtest module has access to the internal structures of
the DRBG anyway, the problem could be solved by accessing the members
directly. I added a warning comment in hook_drbg().
[extended tests]
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/10950)
171 lines
6.4 KiB
Plaintext
171 lines
6.4 KiB
Plaintext
=pod
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=head1 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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=head1 SYNOPSIS
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#include <openssl/rand_drbg.h>
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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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int RAND_DRBG_set_callback_data(RAND_DRBG *drbg, void *ctx);
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void *RAND_DRBG_get_callback_data(RAND_DRBG *drbg);
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=head2 Callback Functions
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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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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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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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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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=head1 DESCRIPTION
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RAND_DRBG_set_callbacks() sets the callbacks for obtaining fresh entropy and
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the nonce when reseeding the given B<drbg>.
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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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RAND_DRBG_set_callback_data() 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 RAND_DRBG_get_callback_data().
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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 NOTES section.
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Setting the callbacks or the callback data is allowed only if the DRBG 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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RAND_DRBG_uninstantiate() first.
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The B<get_entropy>() callback is called by the B<drbg> 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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B<min_len> <= size <= B<max_len> (in bytes) which contains at least B<entropy>
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bits of randomness.
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The B<prediction_resistance> flag indicates whether the reseeding was
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triggered by a prediction resistance request.
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The buffer's address is to be returned in *B<pout> and the number of collected
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randomness bytes as return value.
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If the callback fails to acquire at least B<entropy> bits of randomness,
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it must indicate an error by returning a buffer length of 0.
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If B<prediction_resistance> was requested and the random source of the DRBG
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does not satisfy the conditions requested by [NIST SP 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 NOTES section for more details.
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The B<cleanup_entropy>() callback is called from the B<drbg> to to clear and
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free the buffer allocated previously by get_entropy().
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The values B<out> and B<outlen> are the random buffer's address and length,
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as returned by the get_entropy() callback.
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The B<get_nonce>() and B<cleanup_nonce>() 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 DRBG uses a derivation function.
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The callbacks are analogous to get_entropy() and cleanup_entropy(),
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except for the missing prediction_resistance flag.
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If the derivation function is disabled, then no nonce is used for instantiation,
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and the B<get_nonce>() and B<cleanup_nonce>() callbacks can be omitted by
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setting them to NULL.
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=head1 RETURN VALUES
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RAND_DRBG_set_callbacks() returns 1 on success, and 0 on failure.
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RAND_DRBG_set_callback_data() returns 1 on success, and 0 on failure.
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RAND_DRBG_get_callback_data() returns the pointer to the callback data,
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which is NULL if none has been set previously.
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=head1 NOTES
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It is important that B<cleanup_entropy>() and B<cleanup_nonce>() 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 DRBG's state in memory.
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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 [NIST SP 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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The derivation function is disabled during initialization by calling the
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RAND_DRBG_set() function with the RAND_DRBG_FLAG_CTR_NO_DF flag.
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For more information on the derivation function and when it can be omitted,
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see [NIST SP 800-90A Rev. 1]. Roughly speaking it can be omitted if the random
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source has "full entropy", i.e., contains 8 bits of entropy per byte.
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Even if a nonce is required, the B<get_nonce>() and B<cleanup_nonce>()
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callbacks can be omitted by setting them to NULL.
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In this case the DRBG will automatically request an extra amount of entropy
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(using the B<get_entropy>() and B<cleanup_entropy>() callbacks) which it will
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utilize for the nonce, following the recommendations of [NIST SP 800-90A Rev. 1],
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section 8.6.7.
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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 DRBG.
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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 RFC6979, which require passing a specified entropy and nonce
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for instantiating the DRBG.
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=head1 SEE ALSO
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L<RAND_DRBG_new(3)>,
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L<RAND_DRBG_reseed(3)>,
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L<RAND_DRBG(7)>
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=head1 HISTORY
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The RAND_DRBG functions were added in OpenSSL 1.1.1.
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=head1 COPYRIGHT
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Copyright 2017-2019 The OpenSSL Project Authors. All Rights Reserved.
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Licensed under the Apache License 2.0 (the "License"). 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 LICENSE in the source distribution or at
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L<https://www.openssl.org/source/license.html>.
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=cut
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