Problem - Kevin Tafuro

while (xl-- && i < SPC_BLOCK_SZ)
prng->ctr[i++] = *x++;
prng->ix = 0;
prng->kl = kl;
SPC_BCPRNG_UNLOCK( );
}
unsigned char *spc_bcprng_rand(SPC_BCPRNG_CTX *prng, unsigned char *buf, size_t l) {
unsigned char *p;
SPC_BCPRNG_LOCK( );
for (p = buf; prng->ix && l; l--) {
*p++ = prng->lo[prng->ix++];
prng->ix %= SPC_BLOCK_SZ;
}
while (l >= SPC_BLOCK_SZ) {
SPC_DO_ENCRYPT(&(prng->ks), prng->ctr, p);
spc_increment_counter(prng);
p += SPC_BLOCK_SZ;
l -= SPC_BLOCK_SZ;
}
if (l) {
SPC_DO_ENCRYPT(&(prng->ks), prng->ctr, prng->lo);
spc_increment_counter(prng);
prng->ix = l;
while (l--) p[l] = prng->lo[l];
}
SPC_BCPRNG_UNLOCK( );
return buf;
}
If your block cipher has 64-bit blocks and has no practical weaknesses, do not use
this generator for more than 235 bytes of output (232 block cipher calls). If the cipher
has 128-bit blocks, do not exceed 268 bytes of output (264 block cipher calls). If using
a 128-bit block cipher, it is generally acceptable not to check for this condition, as
you generally would not reasonably expect to ever use that many bytes of output.
To bind this cryptographic PRNG 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’s 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 you should probably be sure to check that the generator is seeded before
calling spc_bcprng_rand( ).
586 | Chapter 11: Random Numbers
This is the Title of the Book, eMatter Edition
Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.