Problem - Kevin Tafuro

1. Figure out how big a seed you need. At the very least, you need a seed that is as
many bits in length as bits of entropy you think are in the generator. Generally,
this will be at least as large as the key size of the underlying primitive (or the output
size when using a one-way hash function instead of a cipher).
2. If you need to introduce new entropy, properly compress the data containing
entropy. In particular, you must transform the data into a seed of the proper
size, with minimal loss of entropy. One easy way to do that is to process the
string with a cryptographic hash function (truncating the hash output to the
desired length, if necessary). Then XOR the compressed entropy with the seed
output by the generator.
3. Take the value and use it to reseed the generator. If you are using a counterbased
generator, you can either reset the counter or choose not to do so. In fact,
it is preferable to take a bit of extra output from the generator so that the
counter can be set to a random value.
For example, using the block cipher–based PRNG from Recipe 11.5, here is a function
that reseeds the generator, given new, uncompressed data containing entropy:
void spc_bcprng_reseed(SPC_BCPRNG_CTX *prng, unsigned char *new_data, size_t l) {
size_t i;
unsigned char m[SPC_MAX_KEYLEN + SPC_BLOCK_SZ];
SPC_BCPRNG_LOCK( );
if (prng->kl > SPC_MAX_KEYLEN) prng->kl = SPC_MAX_KEYLEN;
spc_bcprng_rand(prng, m, prng->kl + SPC_BLOCK_SZ);
while (l > prng->kl) {
for (i = 0; i < prng->kl; i++) m[i] ^= *new_data++;
l -= prng->kl;
spc_bcprng_init(prng, m, prng->kl, m + prng->kl, SPC_BLOCK_SZ);
spc_bcprng_rand(prng, m, prng->kl + SPC_BLOCK_SZ);
}
for (i = 0; i kl, m + prng->kl, SPC_BLOCK_SZ);
SPC_BCPRNG_UNLOCK( );
}
To handle compression of the data that contains entropy, we avoid using a hash
function. Instead, we break the data up into chunks no larger than the required seed
size, and reseed multiple times until we have run out of data. This is an entropy-preserving
way of processing the data that does not require the use of a cryptographic
hash function.
See Also
Recipes 11.5, 11.16
Reseeding a Pseudo-Random Number Generator | 593
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