what is a way in php to make a random, variable length salt for use in hashing. let's say i want to make a 16-character long salt - how would i do it?
and so this is appropriate for use with a hashing alg like sha256?
hatorade
2010-02-10 08:53:58
@hatorade, an md5 collision doesn't matter, so sha256 won't help. However time() is not a random number, and thats why you got -1. Don't give people secuirty advice when you have no idea what you are doing.
Rook
2010-02-10 09:03:08
what is an md5 collision...?
hatorade
2010-02-10 09:06:20
@hatorade: An MD5 collision is when two values generate the same MD5 checksum.
Johannes Gorset
2010-02-10 09:12:59
A:
There are two prerequisites for a good salt: It must be long, and it must be random. There are many ways to accomplish this. You could use a combination of microtime and rand, for example, but you might go to even greater lengths to ensure that your salt is unique.
While the chance of a collision is neglible, keep in mind that hashing your salt with MD5 or other collision-prone algorithms will reduce the chance that your salt is unique for no reason.
EDIT: Substitute rand() for mt_rand(). As Michael noted, it's better than rand.
Johannes Gorset
2010-02-10 08:55:09
@FRKT: do i need to put a salt for crypt()? the syntax is crypt(password, salt) where salt is optional but it seems to determine which variant of crypt is used and i don't know how much that affects security. http://php.net/manual/en/function.crypt.php
hatorade
2010-02-10 08:59:41
@hatorade: The manual says about the salt parameter: "An optional salt string to base the encryption on. If not provided, one will be randomly generated by PHP each time you call this function."
VolkerK
2010-02-10 09:55:13
A:
Not my answer, exactly, but having read the kerfuffle over your last question as well about MD5 vs. SHA, try this article, it could be what you're after.
Kevin Jones
2010-02-10 08:56:48
+2
A:
My solution:
function unique_md5() {
mt_srand(microtime(true)*100000 + memory_get_usage(true));
return md5(uniqid(mt_rand(), true));
}
XUE Can
2010-02-10 08:57:58
A:
If the mcrypt extension is available you could simply use mcrypt_create_iv(size, source) to create a salt.
$iv = mcrypt_create_iv(16, MCRYPT_DEV_URANDOM);
var_dump($iv);
Since each byte of the "string" can be in the range between 0-255 you need a binary-safe function to save/retrieve it.
VolkerK
2010-02-10 09:06:49
A:
depending on your OS, something like:
$fh=fopen('/dev/urandom','rb');
$salt=fgets($fh,16);
fclose($fh);
Do read up on the behaviour of random and urandom.
While others have correctly pointed out that there some issues with md5 and repeated hashing, for passwords (i.e. relatively short strings) brute force attacks take the same amount of time regardless of how sophisticated the hashing algorithm is.
C.
symcbean
2010-02-10 09:11:09