Is time() a good salt?

Short answer:

No, time() is not a good salt.

Long answer:

^{copied from my answer to Salt Generation and open source software}

What is a salt?

A salt is a random set of bytes of a fixed length that is added to the input of a hash algorithm.

Why is salting (or seeding) a hash useful?

Adding a random salt to a hash ensures that the same password will produce many different hashes. The salt is usually stored in the database, together with the result of the hash function. Salting a hash is good for a number of reasons:

1. Salting greatly increases the difficulty/cost of precomputated attacks (including rainbow tables)
2. Salting makes sure that the same password does not result in the same hash. This makes sure you cannot determine if two users have the same password. And, even more important, you cannot determine if the same person uses the same password across different systems.
3. Salting increases the complexity of passwords, thereby greatly decreasing the effectiveness of both Dictionary- and Birthday attacks. (This is only true if the salt is stored separate from the hash).
4. Proper salting greatly increases the storage need for precomputation attacks, up to the point where they are no longer practical. (8 character case-sensitive alpha-numeric passwords with 16 bit salt, hashed to a 128 bit value, would take up just under 200 exabytes without rainbow reduction).

There is no need for the salt to be secret.

A salt is not a secret key, instead a salt 'works' by making the hash function specific to each instance. With salted hash, there is not one hash function, but one for every possible salt value. This prevent the attacker from attacking N hashed passwords for less than N times the cost of attacking one password. This is the point of the salt.
A "secret salt" is not a salt, it is called a "key", and it means that you are no longer computing a hash, but a Message Authentication Code (MAC). Computing MAC is tricky business (much trickier than simply slapping together a key and a value into a hash function) and it is a very different subject altogether.

The salt must be random for every instance in which it is used. This ensures that an attacker has to attack every salted hash separately.
If you rely on your salt (or salting algorithm) being secret, you enter the realms of Security Through Obscurity (won't work). Most probably, you do not get additional security from the salt secrecy; you just get the warm fuzzy feeling of security. So instead of making your system more secure, it just distracts you from reality.

So, why does the salt have to be random?

Technically, the salt should be unique. The point of the salt is to be distinct for each hashed password. This is meant worldwide. Since there is no central organization which distributes unique salts on demand, we have to rely on the next best thing, which is random selection with an unpredictable random generator, preferably within a salt space large enough to make collisions improbable (two instances using the same salt value).

It is tempting to try to derive a salt from some data which is "presumably unique", such as the user ID, but such schemes often fail due to some nasty details:

1. If you use for example the user ID, some bad guys, attacking distinct systems, may just pool their resources and create precomputed tables for user IDs 1 to 50. A user ID is unique system-wide but not worldwide.

2. The same applies to the username: there is one "root" per Unix system, but there are many roots in the world. A rainbow table for "root" would be worth the effort, since it could be applied to millions of systems. Worse yet, there are also many "bob" out there, and many do not have sysadmin training: their passwords could be quite weak.

3. Uniqueness is also temporal. Sometimes, users change their password. For each new password, a new salt must be selected. Otherwise, an attacker obtained the hash of the old password and the hash of the new could try to attack both simultaneously.

Using a random salt obtained from a cryptographically secure, unpredictable PRNG may be some kind of overkill, but at least it provably protects you against all those hazards. It's not about preventing the attacker from knowing what an individual salt is, it's about not giving them the big, fat target that will be used on a substantial number of potential targets. Random selection makes the targets as thin as is practical.

In conclusion:

Use a random, evenly distributed, high entropy salt. Use a new salt whenever you create a new password or change a password. Store the salt along with the hashed password. Favor big salts (at least 10 bytes, preferably 16 or more).

A salt does not turn a bad password into a good password. It just makes sure that the attacker will at least pay the dictionary attack price for each bad password he breaks.

Usefull sources:
stackoverflow.com: Non-random salt for password hashes
Bruce Schneier: Practical Cryptography (book)
Matasano Security: Enough with the Rainbow Tables
usenix.org: Unix crypt used salt since 1976
owasp.org: Why add salt
openwall.com: Salts

Disclaimer:
I'm not a security expert. (Although this answer was reviewed by Thomas Pornin)
If any of the security professionals out there find something wrong, please do comment or edit this wiki answer.

As for what seems to be a good source for your random salt
^{Also read: What is the most secure seed for random number generation?}
In the absence of dedicated, hardware based, random generators, the best way of obtaining random data is to ask the operating system (on Linux, this is called /dev/random or /dev/urandom [both have advantages and problems, choose your poison]; on Windows, call CryptGenRandom())

If for some reason you do not have access to the above mentioned sources of random, in PHP you could use the following function:
^{From the source of phpass v0.3}

<?php/** * Generate pseudo random bits * @copyright: public domain * @link http://www.openwall.com/phpass/ * @param int $length number of bits to generate * @return string A string with the hexadecimal number * @note don't try to improve this, you will likely just ruin it */function random_bits($entropy) {    $entropy /= 8;    $state = uniqid();    $str = '';    for ($i = 0; $i < $entropy; $i += 16) {        $state = md5(microtime().$state);        $str .= md5($state, true);    }    $str = unpack('H*', substr($str, 0, $entropy));    // for some weird reason, on some machines 32 bits binary data comes out as 65! hex characters!?    // so, added the substr    return substr(str_pad($str[1], $entropy*2, '0'), 0, $entropy*2);}?>

Updated

It's not a really good salt, but probably good enough to defeat all but the most determined and resourceful attackers. The requirements for a good salt are:

• Different for each user
• long enough (at the very least alphanumeric 8 characters) to make the concatenation of salt and (potentially weak) password too long for a brute force attack.

time() values are not really long enough, since they have 10 characters, but only digits.

Also, sometimes two users may get the same value when they are created within the same second. But that's only a problem if you have situations where many users are automatically created within the same second.

In any case, far more important than a perfect salt is using a good hash function, and SHA512 is one of the best we have available right now.

This post may veer a little too far away from your original question, but I hope you find it useful;

Security is about raising barriers and hurdles; defence in depth. There is no truly secure hashing solution, just ones that are hard to break. It's like putting in a burglar alarm and window locks in your house - make your site less attractive to break into than someone else's.

Salt for a crypt algorithm is only a small part of the security problem. A single salt simply means that there is one less thing to figure out when trying to break the password for multiple users. A low-entropy salt (such as the server's time) makes it a little bit harder, and a high-entropy salt makes it harder still. Which of these to use, and whether it's something you need to worry about primarily depends upon both the sensitivity of the data you're protecting, but also what other security measures you have in place. A site that just gives a personalised weather forecast for a selected city obviously has less sensitive data than one which has your home address, mother's maiden name, date of birth and other info which could be used for identification purposes.

So here's the rub; a high entropy salt is still a bad salt if it's easily obtainable.

In the real world, storing a salt in the database (random or not) is probably less secure than using a constant salt and burying it away from private eyes in a file inaccessible via the web browser. Whilst a unique and high entropy salt is harder to guess, if you've allowed root login from any server on MySql and set the password to 'password' it doesn't really matter! Constrast how easy it is to crack the database versus getting a valid login to your server - which is possibly more difficult to do discretely as you can put fail2ban and a plethora of other attack vector watchers in place depending upon your setup.

You can combine the two approaches by storing the location of a file containing a user-specific salt in the database, rather than the salt itself. Whether having to crack both the file system and the database is warranted depends whether the sensitivity of the data you are trying to protect warrants this overhead.

Another, alternative, recommendation from security experts is to store the username in a separate database (and ideally different technology) to the password, and reference between the two using a UUID. E.g. use both MySQL and SQLite. This means that both databases have to be cracked (and is also why, to go down a separate rabbit hole for the sake of an example, you should not store user details and credit card numbers in the same database since one is of no use without the other).

Note that Algorithms like SHA-512 and Blowfish can return the salt as part of their hash. Be careful with these as if you store the complete hash you give away the algorithm, which means there's two less thing for the hackers to figure out (the salt also gives away the algorithm).

Make sure you enforce strong passwords and usernames, so dictionary attacks will fail; I know of dictionaries for all 6-alphanumeric combinations of username/ password entries for MD5 and I suspect that there are more than this available for all sorts of algorithms. With the explosion of low-cost cloud and CPGPU computing, the size and complexity of available dictionaries is going to explode.

Ultimately, the most secure way is never to programatically generate a salt but require a user to enter it along with their username and password over a SSL link (so can't be snooped), but never store it. This is the approach taken by credit card companies; i.e. the 3-digit CSV security key on your credit card which you have to enter each and every time you buy online, since it should never be stored in any database. If you really want to generate the salt, send it to them separately (e.g. via SMS message or Email) and still make them enter it manually each time. With this approach, although more secure, you need to contrast the complexity against whether users will just stop using the site as you've made it too difficult for them to be bothered with it.

All of the above still relies on the fact that you also have protection in place against session hijacking, cross-site scripting, etc., etc. The world's strongest password algorithm is irrelevant if all I need to do is to calculate a valid PHPSESSID for a logged-in user and hijack it!

I am not a security expert, but have read up on this as much as I reasonably can do. The fact that there are so many books on the subject indicates how big the answer to your question really is.

A couple of really great books you might like to try which I've found invaluable are;

Web Application Vulnerabilities Detect, Exploit, Prevent - ISBN-13: 978-1-59749-209-6

Preventing Web Attacks with Apache - ISBN-13: 978-0-321-32128-2