Problem - Kevin Tafuro

out
Buffer into which the random data will be placed.
l
Number of random bytes to be placed into the output buffer.
If your hash function produces n-bit outputs and has no practical weaknesses, do not
use the generator after you run the MAC more than 2n/2 times. For example, with
SHA1, this generator should be not be a problem for at least 280 × 20 bytes. In practice,
you probably will not have to worry about this issue.
To bind this cryptographic pseudo-random number generator to the API in Recipe
11.2, you can use a single global generator context that you seed in spc_rand_
init( ), requiring you to get a secure seed. Once that is done (assuming the generator
variable is a statically allocated global variable named spc_prng), you can simply
implement spc_rand( ) as follows:
unsigned char *spc_rand(unsigned char *buf, size_t l) {
return spc_bcprng_rand(&spc_prng, buf, l);
}
Note that, although we don’t show it in the previous code, you should ensure that
the generator is initialized before giving output.
See Also
Recipes 5.2, 5.5, 5.9, 5.23, 6.10, 11.2, 11.6, 11.8, 11.16
11.6 Reseeding a Pseudo-Random Number
Generator
Problem
You have an application-level pseudo-random number generator such as the ones
presented in Recipe 11.5, and you want to reseed it, either because you have new
entropy to mix in or because you would like to prevent against backtracking attacks.
Solution
Create a new seed by getting a sufficient number of bytes from the generator to seed
the generator. If mixing in entropy, compress the entropy down to the seed size if
necessary, as discussed in Recipe 11.16, then XOR the compressed seed with the
generator output. Finally, reseed the generator with the resulting value.
Reseeding a Pseudo-Random Number Generator | 591
This is the Title of the Book, eMatter Edition
Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.