Problem - Kevin Tafuro

Using a stream cipher as a generator
As we mentioned, stream ciphers are themselves pseudo-random number generators,
where the key (and the initialization vector, if appropriate) constitutes the seed.
If you are planning to use such a cipher, we strongly recommend the SNOW 2.0
cipher, discussed in Recipe 5.2.
Because of the popularity of the RC4 cipher, we expect that people will prefer to use
RC4, even though it does not look as good as SNOW. The RC4 stream cipher does
make an acceptable pseudo-random number generator, and it is incredibly fast if you
do not rekey frequently (that is particularly useful if you expect to need a heck of a
lot of numbers). If you do rekey frequently to avoid backtracking attacks, a block
cipher–based approach may be faster; time it to make sure.
RC4 requires a little bit of work to use properly, given a standard API. First, most
APIs want you to pass in data to encrypt. Because you want only the raw keystream,
you must always pass in zeros. Second, be sure to use RC4 in a secure manner, as
discussed in Recipe 5.23.
If your RC4 implementation has the API discussed in Recipe 5.23, seeding it as a
pseudo-random number generator is the same as keying the algorithm. RC4 can
accept keys up to 256 bytes in length.
Because of limitations in RC4, you should throw away the first 256
bytes of RC4 output, as discussed in Recipe 5.23.
After encrypting 256 bytes and throwing the results away, you can then, given an
RC4 context, get random data by encrypting zeros. Assuming the RC4 API from Recipe
5.23 and assuming you have a context statically allocated in a global variable
named spc_prng, here’s a binding of RC4 to the spc_rand( ) function that we introduced
in Recipe 11.2:
/* NOTE: This code should be augmented to reseed after each request
/* for pseudo-random data, as discussed in Recipe 11.6
/*
#ifndef WIN32
#include
static pthread_mutex_t spc_rc4rng_mutex = PTHREAD_MUTEX_INITIALIZER;
#define SPC_RC4RNG_LOCK( ) pthread_mutex_lock(&spc_rc4rng_mutex)
#define SPC_RC4RNG_UNLOCK( ) pthread_mutex_unlock(&spc_rc4rng_mutex)
#else
#include
static HANDLE hSpcRC4RNGMutex;
#define SPC_RC4RNG_LOCK( ) WaitForSingleObject(hSpcRC4RNGMutex, INFINITE)
Using an Application-Level Generator | 587
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