How to generate a good salt - Is my function secure enough?

Question

Here's the function I'm using to generate random salts:

function generateRandomString($nbLetters){     $randString="";     $charUniverse="abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";     for($i=0; $i<$nbLetters; $i++){        $randInt=rand(0,61);         $randChar=$charUniverse[$randInt];         $randString=$randomString.$randChar;     }     return $randomString; } 

This is for a non commercial website. It's only used to generate the salt (to be stored in the db and used along with the user submitted pw for hashing).

Is this appropriate? Should I use a larger subset of characters, and if so is there an easy way to do that in PHP?

Answer 1

If you are hashing passwords, you should use a modern hashing algorithm that does not require you to generate your own salt. Using weak hashing algorithms presents a danger to both you and your users. My original answer was written eight years ago. Times have changed, and password hashing is a lot easier now.

You should always use built in functions to hash/check passwords. Using your own algorithms at any point introduces a huge amount of unnecessary risk.

For PHP, consider using password_hash(), with the PASSWORD_BCRYPT algorithm. There is no need to provide your own salt.

Below is my original answer, for posterity:

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Warning: The following implementation does not produce an unpredictable salt, as per the documentation for uniqid.

From the php sha1 page:

$salt = uniqid(mt_rand(), true); 

This looks simpler, and more effective (since each is unique) than what you have proposed.

Answer 2

If you're on Linux, /dev/urandom is probably your best source of randomness. It's supplied by the OS itself, so it's guaranteed to be much more reliable than any PHP built-in function.

$fp = fopen('/dev/urandom', 'r'); $randomString = fread($fp, 32); fclose($fp); 

This will give you 32 bytes of random blob. You'll probably want to pass this through something like base64_encode() to make it legible. No need to juggle characters yourself.

Edit 2014: In PHP 5.3 and above, openssl_random_pseudo_bytes() is the easiest way to get a bunch of random bytes. On *nix systems, it uses /dev/urandom behind the scenes. On Windows systems, it uses a different algorithm that is built into the OpenSSL library.

Related: https://security.stackexchange.com/questions/26206

Related: should i use urandom or openssl_random_pseudo_bytes?