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.