Problem - Kevin Tafuro

anyway. You can use any of the sources we have discussed elsewhere in this chapter
for seeding the OpenSSLPRNG. Multiple API functions are available that allow seed
information to be passed to the PRNG.
One such function is RAND_seed( ), which allows you to pass in arbitrary data that
should be completely full of entropy. It has the following signature:
void RAND_seed(const void *buf, int num);
This function has the following arguments:
buf
Buffer containing the entropy to seed the PRNG.
num
Length of the seed buffer in bytes.
If you have data that you believe contains entropy but does not come close to one bit
of entropy per bit of data, you can call RAND_add( ), which is similar to RAND_seed( )
except that it allows you to provide an indication of how many bits of entropy the
data has:
void RAND_add(const void *buf, int num, double entropy);
If you want to seed from a device or some other file (usually, you only want to use a
stored seed), you can use the function RAND_load_file( ), which will read the
requested number of bytes from the file. Because there is no way to determine how
much entropy is contained in the data, OpenSSLassumes that the data it reads from
the file is purely entropic.
int RAND_load_file(const char *filename, long max_bytes);
If –1 is specified as the length parameter to this function, it reads the entire file. This
function returns the number of bytes read. The function can be used to read from the
/dev/random and /dev/urandom devices on Unix systems that have them, but you
must make sure that you don’t specify –1 for the number of bytes to read from these
files; otherwise, the function will never return!
To implement PRNG state saving with OpenSSL, you can use RAND_write_file( ),
which writes out a representation of the PRNG’s internal state that can be used to
reseed the PRNG when needed (e.g., after a reboot):
int RAND_write_file(const char *filename);
If there is any sort of error, RAND_write_file( ) will return –1. Note that the system
may write a seed file without enough entropy, in which case it will also return –1.
Otherwise, this function returns the number of bytes written to the seed file.
To obtain pseudo-random data from the PRNG, use the function RAND_bytes( ):
int RAND_bytes(unsigned char *buf, int num);
604 | Chapter 11: Random Numbers
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