Alexander Gromnitsky's Blog

What to do when you don't have /dev/urandom

Latest update: 2023-07-11 19:39:17

Imagine it's 1983, and you're at Bell Labs in desperate need of random data. Where would you get it?

Don Mitchell (who later defected to Microsoft Research) has you covered--utilise file access/modification time from /bin/ls and such. A snipped from /usr/src/cmd/descrypt/randblock.c, v8 Reseach UNIX:

char *volatile_file[] = {
	"/bin/echo",
	"/bin/sh",
	"/bin/cat",
	"/bin/ls",
	"/bin/stty",
	"/bin/date",
	"/bin/cat",
	"/bin/cc",
	"/etc/motd",
	"/etc/utmp",
	"/dev/kmem",
	"",
};

Block
rand_block()
{
	register i;
	Block block;
	struct stat buf;
	extern long time();

	block.left  ^= 0x9662f394;
	block.right ^= 0x9f17c55f;
	key_setup(&block, 0);
	block.right = NEXT(getpid() + (getuid() << 16));
	block.left = NEXT(time((long *)0));
	des(&block);
	for (i = 0; strlen(volatile_file[i]); i++) {
		if (stat(volatile_file[i], &buf) == -1)
			continue;
		block.right ^= NEXT(buf.st_atime);
		block.left ^= NEXT(buf.st_mtime);
		des(&block);
	}
	return block;
}

The condition in the for-loop also qualifies as a little gem.