#include <openssl/rand.h> int RAND_egd(const char *path); int RAND_egd_bytes(const char *path, int bytes); int RAND_query_egd_bytes(const char *path, unsigned char *buf, int bytes);
RAND_egd_bytes() queries the entropy gathering daemon EGD on socket path. It queries bytes bytes and uses RAND_add(3) to seed the OpenSSL built-in PRNG. This function is more flexible than RAND_egd(). When only one secret key must be generated, it is not necessary to request the full amount 255 bytes from the EGD socket. This can be advantageous, since the amount of entropy that can be retrieved from EGD over time is limited.
RAND_query_egd_bytes() performs the actual query of the EGD daemon on socket path. If buf is given, bytes bytes are queried and written into buf. If buf is NULL, bytes bytes are queried and used to seed the OpenSSL built-in PRNG using RAND_add(3).
EGD is available from http://www.lothar.com/tech/crypto/ ("perl Makefile.PL; make; make install" to install). It is run as egd path, where path is an absolute path designating a socket. When RAND_egd() is called with that path as an argument, it tries to read random bytes that EGD has collected. RAND_egd() retrieves entropy from the daemon using the daemon's ``non-blocking read'' command which shall be answered immediately by the daemon without waiting for additional entropy to be collected. The write and read socket operations in the communication are blocking.
Alternatively, the EGD-interface compatible daemon PRNGD can be used. It is available from http://prngd.sourceforge.net/ . PRNGD does employ an internal PRNG itself and can therefore never run out of entropy.
OpenSSL automatically queries EGD when entropy is requested via RAND_bytes() or the status is checked via RAND_status() for the first time, if the socket is located at /var/run/egd-pool, /dev/egd-pool or /etc/egd-pool.
RAND_egd_bytes() is available since OpenSSL 0.9.6.
RAND_query_egd_bytes() is available since OpenSSL 0.9.7.
The automatic query of /var/run/egd-pool et al was added in OpenSSL 0.9.7.