hmac_prng.h File Reference

Interface to an HMAC-PRNG implementation. More...

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Functions
int	tc_hmac_prng_init (TCHmacPrng_t prng, const uint8_t *personalization, unsigned int plen)
	HMAC-PRNG initialization procedure Initializes prng with personalization, disables tc_hmac_prng_generate.
int	tc_hmac_prng_reseed (TCHmacPrng_t prng, const uint8_t *seed, unsigned int seedlen, const uint8_t *additional_input, unsigned int additionallen)
	HMAC-PRNG reseed procedure Mixes seed into prng, enables tc_hmac_prng_generate.
int	tc_hmac_prng_generate (uint8_t *out, unsigned int outlen, TCHmacPrng_t prng)
	HMAC-PRNG generate procedure Generates outlen pseudo-random bytes into out buffer, updates prng.

Detailed Description

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 tc_hmac_prng_init to set the HMAC key and process the personalization data.

2) call tc_hmac_prng_reseed to process the seed and additional input.

3) call tc_hmac_prng_generate to out put the pseudo-random data.

Function Documentation

tc_hmac_prng_generate()

int tc_hmac_prng_generate	(	uint8_t *	out,
		unsigned int	outlen,
		TCHmacPrng_t	prng
	)

HMAC-PRNG generate procedure Generates outlen pseudo-random bytes into out buffer, updates prng.

Returns

returns TC_CRYPTO_SUCCESS (1) returns TC_HMAC_PRNG_RESEED_REQ (-1) if a reseed is needed returns TC_CRYPTO_FAIL (0) if: out == NULL, prng == NULL, outlen == 0, outlen >= MAX_OUT

Note

Assumes tc_hmac_prng_init has been called for prng

Parameters

out	IN/OUT – buffer to receive output
outlen	IN – size of out buffer in bytes
prng	IN/OUT – the PRNG state

tc_hmac_prng_init()

int tc_hmac_prng_init	(	TCHmacPrng_t	prng,
		const uint8_t *	personalization,
		unsigned int	plen
	)

HMAC-PRNG initialization procedure Initializes prng with personalization, disables tc_hmac_prng_generate.

Returns

returns TC_CRYPTO_SUCCESS (1) returns TC_CRYPTO_FAIL (0) if: prng == NULL, personalization == NULL, plen > MAX_PLEN

Note

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

Warning

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)

Parameters

prng	IN/OUT – the PRNG state to initialize
personalization	IN – personalization string
plen	IN – personalization length in bytes

tc_hmac_prng_reseed()

int tc_hmac_prng_reseed	(	TCHmacPrng_t	prng,
		const uint8_t *	seed,
		unsigned int	seedlen,
		const uint8_t *	additional_input,
		unsigned int	additionallen
	)

HMAC-PRNG reseed procedure Mixes seed into prng, enables tc_hmac_prng_generate.

Returns

returns TC_CRYPTO_SUCCESS (1) returns TC_CRYPTO_FAIL (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

Note

Assumes:- tc_hmac_prng_init has been called for prng

• seed has sufficient entropy.

Parameters

prng	IN/OUT – the PRNG state
seed	IN – entropy to mix into the prng
seedlen	IN – length of seed in bytes
additional_input	IN – additional input to the prng
additionallen	IN – additional input length in bytes