Problem - Kevin Tafuro

spc_rand( ) by reading from /dev/urandom. Finally, we will implement spc_keygen( )
by reading as much data as possible from /dev/random in a nonblocking fashion, then
falling back to /dev/urandom when /dev/random is dry.
Note that we need to open /dev/random on two file descriptors, one blocking and
one not, so that we may avoid race conditions where spc_keygen( ) expects a function
to be nonblocking but spc_entropy( ) has set the descriptor to blocking in
another thread.
In addition, we assume that the system has sufficient entropy to seed /dev/urandom
properly and /dev/random’s entropy is not reused by /dev/urandom. If you are worried
about either of these assumptions, see the recipes suggested earlier for remedies.
Note that you can expect that /dev/random output is properly postprocessed (whitened)
to remove any patterns that might facilitate analysis in the case that the data
contains less entropy than expected.
This code depends on the spc_make_fd_nonblocking( ) function presented earlier.
#include
#include
#include
#include
#include
static int spc_devrand_fd = -1,
spc_devrand_fd_noblock = -1,
spc_devurand_fd = -1;
void spc_rand_init(void) {
spc_devrand_fd = open("/dev/random", O_RDONLY);
spc_devrand_fd_noblock = open("/dev/random", O_RDONLY);
spc_devurand_fd = open("/dev/urandom", O_RDONLY);
if (spc_devrand_fd = = -1 || spc_devrand_fd_noblock = = -1) {
perror("spc_rand_init failed to open /dev/random");
exit(-1);
}
if (spc_devurand_fd = = -1) {
perror("spc_rand_init failed to open /dev/urandom");
exit(-1);
}
spc_make_fd_nonblocking(spc_devrand_fd_noblock);
}
unsigned char *spc_rand(unsigned char *buf, size_t nbytes) {
ssize_t r;
unsigned char *where = buf;
if (spc_devrand_fd = = -1 && spc_devrand_fd_noblock = = -1 && spc_devurand_fd = = -1)
spc_rand_init( );
while (nbytes) {
if ((r = read(spc_devurand_fd, where, nbytes)) = = -1) {
578 | Chapter 11: Random Numbers
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