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I'm trying to correctly do a per user and site wide salt for my passwords. Here's what I've got:

require('../../salt.php'); //this is above the web root and provides $salt variable
$pw = mysql_real_escape_string($_POST['pw']);
$per_user_salt = uniqid(mt_rand());
$site_salt = $salt //from salt.php that was required on first line
$combine = $pw . $per_user_salt . $site_salt;
$pw_to_put_in_db = hash("sha512", $combine);
Is this right? Thanks
                This isn't really an answer to whether you're doing it right.  (1) No need to use mysql_real_escape_string.  The resultant hash() would change any invalid SQL to a letter/number.  (2) Ensure you're storing your $per_user_salt in a safe, secure way to retrieve it when your users are trying to login with their password.
– Charles Sprayberry
                Jun 25, 2011 at 17:09
                @Charles Sprayberry, for storing - do I need to do anything different than INSERT INTO users (per_user_salt) VALUES ($per_user_salt) ?
– Andrew Samuelsen
                Jun 25, 2011 at 17:15
                @andy Well for example if you use md5($_POST['pw']) as 'salt' you have a salt that is user depending but does not have to be stored in the database. Your hash would be hash("sha512", $pw . md5($pw) . $site_salt);, which is about as secure as a random generated number (as long as you don't tell anyone your algorithm). The big upside is, that hacking your database (for example using injection) will not result in the per_user_salt being shown, as it is not stored.
– KilZone
                Jun 25, 2011 at 17:55
                @andy The whole point of the 'salt' is to avoid rainbow tables to be used to de-hash something into a password, so anything you do before hashing makes it more secure. Both options you have are more secure, since the $combine becomes more complex, but I like your last option best. However, in the end it comes down to personal preference. Just remember that your salt-trick is a form of security-through-obscurity, the less people know about how you make your $combine the better. Try to come up with something uncommon, like the two you just posted and you'll be fine.
– KilZone
                Jun 25, 2011 at 18:02
often people use a unique salt concatenated with the password, then use hmac method to add the sitewide hashing key:
http://www.php.net/manual/en/function.hash-hmac.php
$password = hash_hmac('sha512', $password . $salt, $sitewide_key);
                by the way, your unique salt will have to be the same every time you hash the password for comparison. this means you will have to store the unique salt for that user somewhere. sometimes people use a string based on the username for this reason. $salt = substr(md5($username), 1); or whatever depending on your security level. it makes database lookups much easier.
– dqhendricks
                Jun 25, 2011 at 18:27
                if your algo is going to be public knowledge, a good tip may be to use your sitewide key in the generation of your salt as well. $salt = md5($username . $sitewide_key);
– dqhendricks
                Jun 25, 2011 at 18:39
$pw = $_POST['pw'];
$per_user_salt = uniqid(mt_rand());
$site_salt = $salt //from salt.php that was required on first line
$combine = $pw . $per_user_salt . $site_salt;
$pw_to_put_in_db = hash(sha512, $combine);
dont have to use md5 sha512 is secure enough it self
Use crypt, it's available in all languages and your password hashes will be useable by other programs as well:
$hash = crypt("secret", "$6$randomsalt$");
Based on comments here is what I'm going to do:
Change my $combine to something that is unique per user but not stored in db. So something like: $combine = $pw . md5($pw) . 'PoniesAreMagical' . $site_salt . md5($pw);, etc etc etc... Thanks for the help...
So - for those of you trying to figure out how to do this for the first time (like me)... its all about the algorithm... make something obscure, unique, difficult to figure out; because if someone wants to get into your system, they are going to have to figure this out. Thanks to all for awesome comments.
                i don't think this is the best option, and  i disagree with @killzone in the comments to your question. the idea of a salt is to prevent rainbow table attacks with user-unique salts. even with knowledge of your algorithm it should be impossible to create rainbow tables. if you use md5($w) as salt anybody knowing the internals of your algorithm can easily create a rainbowtable for your complete user base — after all he has all the inputs he needs: a password, the hash of the password (easily computed), the "ponies"-constant, and the site-salt. security through obscurity never really works!
– knittl
                Jul 1, 2011 at 8:28
                best and simplest is just to create a random salt per user and store it in the db. this way the salt is unique to each user and cannot be derived from other inputs (like the password) — so it does not help the bad guys if they get to know your algorithm used
– knittl
                Jul 1, 2011 at 8:30
        
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