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/* hmac_prng.h - TinyCrypt interface to an HMAC-PRNG implementation. */
/*
* Copyright (C) 2015 by Intel Corporation, All Rights Reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are met:
*
* - Redistributions of source code must retain the above copyright notice,
* this list of conditions and the following disclaimer.
*
* - Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* - Neither the name of Intel Corporation nor the names of its contributors
* may be used to endorse or promote products derived from this software
* without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
* DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
* SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
* CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
* OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*
* hmac_prng.h -- Interface to an HMAC-PRNG implementation.
*
* Overview: A pseudo-random number generator (PRNG) generates a sequence
* of numbers that have a distribution close to the one expected
* for a sequence of truly random numbers. The NIST Special
* Publication 800-90A specifies several mechanisms to generate
* sequences of pseudo random numbers, including the HMAC-PRNG one
* which is based on HMAC. TinyCrypt implements HMAC-PRNG with
* certain modifications from the NIST SP 800-90A spec.
*
* Security: A cryptographically secure PRNG depends on the existence of an
* entropy source to provide a truly random seed as well as the
* security of the primitives used as the building blocks (HMAC and
* SHA256, for TinyCrypt).
*
* The NIST SP 800-90A standard tolerates a null personalization,
* while TinyCrypt requires a non-null personalization. This is
* because a personalization string (the host name concatenated
* with a time stamp, for example) is easily computed and might be
* the last line of defense against failure of the entropy source.
*
* Requires: - SHA-256
* - HMAC
*
* Usage: 1) call hmac_prng_init to set the HMAC key and process the
* personalization data.
*
* 2) call hmac_prng_reseed to process the seed and additional
* input.
*
* 3) call hmac_prng_generate to out put the pseudo-random data.
*/
#ifndef __HMAC_PRNG_H__
#define __HMAC_PRNG_H__
#include "sha256.h"
#include "hmac.h"
struct hmac_prng_struct {
struct hmac_state_struct h; /* the HMAC instance for this PRNG */
uint8_t key[SHA256_DIGEST_SIZE]; /* the PRNG key */
uint8_t v[SHA256_DIGEST_SIZE]; /* PRNG state */
uint32_t countdown; /* calls to hmac_prng_generate left before re-seed */
};
typedef struct hmac_prng_struct *HmacPrng_t;
/*
* HMAC-PRNG initialization procedure.
*
* Assumes: - personalization != NULL.
* The personalization is a platform unique string (e.g., the host
* name) and is the last line of defense against failure of the
* entropy source.
*
* Effects: Initializes prng with personalization, disables
* hmac_prng_generate, and returns 1.
*
* Exceptions: Returns 0 if:
* prng == NULL,
* personalization == NULL,
* plen > MAX_PLEN.
*
* Note: NIST SP 800-90A specifies 3 items as seed material during
* initialization: entropy seed, personalization, and an optional
* nonce. TinyCrypts requires instead a non-null personalization
* (which is easily computed) and indirectly requires an entropy
* seed (since the reseed function is mandatorily called after
* init).
*
*/
int32_t hmac_prng_init (
HmacPrng_t prng, /* IN/OUT -- the PRNG state to initialize */
const uint8_t *personalization, /* IN -- personalization string */
uint32_t plen); /* IN -- personalization length in bytes */
/*
* HMAC-PRNG reseed procedure.
*
* Assumes: - hmac_prng_init has been called for prng
* - seed has sufficient entropy.
*
* Effects: Mixes seed into prng, enables hmac_prng_generate, and returns 1
*
* Exceptions: Returns 0 if:
* prng == NULL,
* seed == NULL,
* seedlen < MIN_SLEN,
* seendlen > MAX_SLEN,
* additional_input != (const uint8_t *) 0 && additionallen == 0,
* additional_input != (const uint8_t *) 0 && additionallen > MAX_ALEN.
*/
int32_t hmac_prng_reseed (
HmacPrng_t prng, /* IN/OUT -- the PRNG state */
const uint8_t *seed, /* IN -- entropy to mix into the prng */
uint32_t seedlen, /* IN -- length of seed in bytes */
const uint8_t *additional_input,/* IN -- additional input to the prng */
uint32_t additionallen); /* IN -- additional input length in bytes */
/*
* HMAC-PRNG generate procedure.
*
* Assumes: - hmac_prng_init has been called for prng.
*
* Effects: Generates outlen pseudo-random bytes into out buffer, updates
* prng, and returns 1
*
* Exceptions: Returns -1 if:
* a reseed is needed
*
* Returns 0 if:
* out == NULL,
* prng == NULL,
* outlen == 0,
* outlen >= MAX_OUT.
*/
int32_t hmac_prng_generate (
uint8_t *out, /* IN/OUT -- buffer to receive output */
uint32_t outlen, /* IN -- size of out buffer in bytes */
HmacPrng_t prng); /* IN/OUT -- the PRNG state */
#endif
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