Why does PHP crypt() prepend the salt to the hash?

问题:

I am looking into building a login system and after reading the php manual when you pass a 2 digit salt to the crypt() function it returns a hash string, and the first 2 digits of the string are the salt that you used.

example:

$salt = "kr";
echo crypt("mysecret",$salt); //returns "kreOI.F7eOQMY"

My first thought was, wouldn't this help someone who is trying to reverse your hash?

I looked up salt on wikipedia which said:

For best security, the salt value is kept secret.

So I do not understand why then would the crypt function return all hashes prepended with the salt value used?

Is there a reason for this? Should this be a security concern?

回答1:

The crypt() function is obsolete. It was used to hash passwords for old-style Unix systems, before shadow password support came along. The salt was there to increase the complexity of brute forcing the password. However, since the salt was randomly generated by the password subsystem, it had to be stored in the clear so any future password actions would work. If the salt had been embedded into the password before crypting, there would be no practical way to verify a password - you'd have to try every single possible salt whenever a password check was done - highly impractical. So, the salt was prepended to the crypted password, so you could pull it out again for future use.

crypted password: xxabcdefghijklmn
                  ^^- salt
                    ^^^^^^^^^^^^^^-- crypted pw

if ('xx' + crypt('xx' + password) == 'crypted string') then
     password is ok
endif

These days, crypt() is the security equivalent of a cereal box decoder ring. There for historical purposes, and low-security "who cares if it's cracked into" storage. For any modern password usage, you'd be better off with more modern hashes, such as sha1/sha256/md5. And even md5 is considered broken these days, sha1 has cracks around the edges, and (last I checked) sha256 is still secure.

回答2:

The author of the Wikipedia article is conflating salt with the idea of search space, implying salt is a way to deter brute force attacks. Security is not improved by confusing these ideas; someone who can't recognize and delineate these two issues is not a credible guide.

The purpose of salt is to thwart pre-computed lookup tables (like a Rainbow table). Salt prevents an attacker from trading "space" for "time." Every bit of salt doubles the storage requirements for a table; a two byte salt makes a big (65536 times) difference, but eight bytes would require non-existent "yottabyte" storage devices for lookup tables.

The salt has to be stored somewhere. If you had an effective way to keep a "secret", why not use it to store the password and forget about hashing altogether? No; if you want real security, you need to design the system to be safe from a brute-force attack even though the attacker knows the salt. The article's presumption that the salt can be kept secret should be treated as false.

Brute force attacks are best prevented by key-strengthening (applying the hash function thousands of times), and password selection rules (a minimum length, digits, special characters).

Assuming that the salt cannot be kept secret encourages better key-strengthening and password selection, and this leads to more secure system.

回答3:

I should comment that Crypt is not as bad as Marc B makes it sound, and may in fact be the easiest way to good hashes, as long as you don't rely on its weaker schemes like MD5.

See:

How do you use bcrypt for hashing passwords in PHP?

http://uk.php.net/manual/en/function.crypt.php

http://www.openwall.com/phpass/

回答4:

Yes, the salt is supposed to be kept secret, but then so is the password hash. It's perfectly acceptable for them to be kept equally secret in the same place. To check a password against the hash, you have to combine the salt with the password and then check it against the hash. So, any user or process with the right to see the password hash should also have the right to see the salt, since the password hash by itself is not useful for checking passwords (unless you're going to brute-force the salt).

The purpose of the salt is so that if two different users have the same password, they'll hash to different things. This also means that dictionary attacks are much more complex because you can't just hash all likely passwords and then check them against a list of user password hashes to find multiple user's passwords. Instead you have to try passwords for an individual salt to find one user's password or try all combinations of likely passwords with multiple salts in order to find hits. But knowledge of the salt, by itself, doesn't mean you can reverse the password hash. It just means that you can do a dictionary attack on the password hash.

If you can find a way to keep the salt more secure than the hash value, it certainly wouldn't be a bad thing, but it's hard to see how this is feasible when any program which needs access to one needs access to both.

回答5:

The salt is appended to the has so that you will know which salt to use when you get the password and want to see if it matches the hash. The idea here is to use a different salt for every password so that someone cannot precompute a hash table.

You could also append a second salt to every password (the same for all) and not tell anyone what it is.

回答6:

PHP crypt inherits this behaviour from the UNIX crypt() function, which was used for generating password hashes in the UNIX passwd file. It's necessary to store the salt somewhere, or you can't verify later that the password is correct. For the passwd file, the simple behaviour was just to prepend the salt (always two characters) to the start of the crypted password, which makes it simple to store it in a single field.

The statement that the salt value should be kept secret is open to misinterpretation. For best practice you should not publish your salts, in the same way that you should not publish your password hashes. Giving an attacker the hashes and salts makes it easy for them to run a brute-force attack without generating suspicious traffic to your system. However, the system should still be secure even if an attacker can see both salt and hashed password.

In fact, there's nowhere you can store the hash that couldn't, in principle, be compromised by a hacker in exactly the same way as the hashed passwords. If the password-checking code can access it, then you have to assume that someone who's compromised the system could get access to it as well.