Compute salt with known plaintext and hash?

Question

I have the string that was used in a webapp with md5 hashing to come up with a hash. I also have the hash. But the md5 of the string doesn't match the hash, so I'm guessing there's a salt involved. Is there any software that can help me determine the salt?

Answer 1

In standard hashing functions (e.g., UNIX passwords in /etc/shadow) the salt is stored as part of the hash.

Pass the stored hash value as the salt and you should get the correct result.

The hashed password value in /etc/shadow is actually a $ delimited record. For example, we have this hash of the password 'blarg':

$1$KfcI/JTQ$b5VTf4i9Mnf6QFgLuVZNM0

There are three fields separated by $'s which are

1. The hash function (in this case '1', representing MD5)
2. The salt (which is 'KfcI/JTQ')
3. The hash value (which is 'b5VTf4i9Mnf6QFgLuVZNM0')

If you use mkpasswd several times the hash will change.

$ mkpasswd -m md5 blarg
$1$Gst52IWk$8ARVeSlpkcZOlyKV10Slu/

$ mkpasswd -m md5 blarg
$1$JeqRviA/$GnH/AvGnZEG9wLfJjiaAt1

However, by passing in the salt (i.e., the second field) from the hash value above we can match it against the original hash:

$ mkpasswd -m md5 -S KfcI/JTQ blarg
$1$KfcI/JTQ$b5VTf4i9Mnf6QFgLuVZNM0

You can also pass in the whole password hash (although omitting the hash function).

$ mkpasswd -m md5 -S KfcI/JTQ$b5VTf4i9Mnf6QFgLuVZNM0 blarg
$1$KfcI/JTQ$b5VTf4i9Mnf6QFgLuVZNM0

On Linux the hash type can be controlled in /etc/login.defs. Be very careful when changing this, you can completely lock yourself out of the system. I suggest reading the man page carefully specifically looking at the headings MD5_CRYPT_ENAB and ENCRYPT_METHOD.