Peter Dimov wrote:

For the default seed, you might want to look into combining several sources of entropy (unpredictable values) such as

- time(0) - clock() - the values of an unsigned char[] array that is allocated on the stack and not initialized - the addresses of some variables or functions - the address of a newly allocated object - anything else that you can come up with

Here's one implementation of this. Feel free to use it: #include "sha1.hpp" #include <cstdlib> #include <ctime> #include <cstring> #include <cstdio> #include <memory> namespace boost { namespace detail { inline unsigned * sha1_random_digest_state() { static unsigned state[ 5 ]; return state; } inline void sha1_random_digest( unsigned (&rd) [ 5 ] ) { SHA1 sha; unsigned * ps = sha1_random_digest_state(); unsigned state[ 5 ]; std::memcpy( state, ps, sizeof( state ) ); // harmless data race sha.Input( (unsigned char const*)state, sizeof( state ) ); sha.Input( (unsigned char const*)&ps, sizeof( ps ) ); { std::time_t tm = std::time( 0 ); sha.Input( (unsigned char const*)&tm, sizeof( tm ) ); } { std::clock_t ck = std::clock(); sha.Input( (unsigned char const*)&ck, sizeof( ck ) ); } { unsigned rn[] = { std::rand(), std::rand(), std::rand() }; sha.Input( (unsigned char const*)rn, sizeof( rn ) ); } { unsigned char buffer[ 20 ]; if( std::FILE * f = std::fopen( "/dev/urandom", "rb" ) ) { std::fread( buffer, 1, 20, f ); std::fclose( f ); } // using an uninitialized buffer[] if fopen fails // intentional, we rely on its contents being random sha.Input( buffer, sizeof( buffer ) ); } { unsigned * p = new unsigned; sha.Input( (unsigned char const*)p, sizeof( *p ) ); sha.Input( (unsigned char const*)&p, sizeof( p ) ); delete p; } sha.Input( (unsigned char const*)rd, sizeof( rd ) ); unsigned digest[ 5 ]; sha.Result( digest ); for( int i = 0; i < 5; ++i ) { // harmless data race ps[ i ] ^= digest[ i ]; rd[ i ] ^= digest[ i ]; } } } // namespace detail } // namespace boost #include <iostream> #include <iomanip> static void print_digest( unsigned const (&digest) [ 5 ] ) { std::cout << std::hex << std::setfill('0') << std::uppercase << std::setw( 8 ) << digest[ 0 ]; for( int i = 1; i < 5; ++i ) { std::cout << '-' << std::setw( 8 ) << digest[ i ]; } std::cout << std::endl; } int main() { unsigned rd[ 5 ]; for( int i = 0; i < 4; ++i ) { boost::detail::sha1_random_digest( rd ); print_digest( rd ); } }