Generate WordPress salt

I am in need of a function that automatically generates and returns salts for WordPress wp-config.php (Don’t link me to their API, I’m looking for offline solution).

Does WordPress core has this function defined somewhere?
If it doesn’t, can these salts be generated randomly or are there any specific rules for creating them?

Edit:
This is what I ended up with:

    $keys = array('AUTH_KEY', 'SECURE_AUTH_KEY', 'LOGGED_IN_KEY',
              'NONCE_KEY', 'AUTH_SALT', 'SECURE_AUTH_SALT', 'LOGGED_IN_SALT', 'NONCE_SALT');
    $salts="";
    foreach ($keys as $key) {
        $salt = wp_generate_password(64, true, true);
        $spaces = str_repeat(' ',16 - strlen($key));
        $salts .= "define('{$key}',{$spaces}'{$salt}');\n";
    }

And the output is string:

define('AUTH_KEY',        'f@5^8(OyZLS%+~LNQ6,w(Zpn`3X#A}o4t2LQuqc=x4vn+ b}xYM>TlSwB`|E;}PA');
define('SECURE_AUTH_KEY', 'vEEF@-c_`VO!d{s)_Nv^uS,)eg9{;60.$nU370/9E1z}O#iu)wkPrw8sh[TqGdC;');
define('LOGGED_IN_KEY',   '9:z95.y<_LhUnGlH>6%)/-szx8Dwj{z`#mz-C%taXoD:KK86k(?K-f{w]U5w(41v');
define('NONCE_KEY',       '?YP2djRHOn7[4n[p(KwmX#u.#^s3Fel%AKu@Ac,(L$1DM^@6NNk@x&B/w2/<an:@');
define('AUTH_SALT',       '$r>m{8@l?xDv<^uNz^.|<Am2}J3q(OMAS<dLB({66M)zy2ufOP8$x*{:US|7PL4x');
define('SECURE_AUTH_SALT','Nn4uU#rIe}7CaXw=Z?pk82Cqo8{ALC6McPHYq_G{><]_YWhHlEuk?`tJ6G[)D$)A');
define('LOGGED_IN_SALT',  'YLiGuP$DPKP-F3UGw(0#E0L1w;HO0L_Hkt6.(*92t*B6Mclq*`{OO[xM$3)]^9yi');
define('NONCE_SALT',      '{hh3bpLu$b:e8-uXiCx(3FaK3Q4[`/Mji}~<.cz8W#_a0[O!{h;Fm{^c]p>./RF{');

Does WordPress core has this function defined somewhere?

While I haven’t used it, you are probably looking for wp_salt or wp_generate_password. wp_salt is located in wp-includes/pluggable.php.

can these salts be generated randomly

Yes, of course.

are there any specific rules for creating them

There is no specific rule. The generic rule is to create long, random and complicated password. By default, WordPress generates them using wp_generate_password (wp_generate_password (64, true, true)). This function accepts three parameters (all are optional). The first param is the length (default value is 12), the second is to use standard special characters (boolean value, default is true), and the third is to use other special characters (boolean value, default is false).