Why string hashed with its hash as salt returns the hash?

Question

I was reading up on password security in PHP and I stumbled upon an interesting statement:

Hashing the password with its hash as the salt returns the same hash

Without much thinking, I went on php.net and found that is says the same thing.

Let's look at an example:

crypt("test", "test"); -> teH0wLIpW0gyQ
crypt("test", "teH0wLIpW0gyQ"); -> teH0wLIpW0gyQ

I can totally understand that crypt in PHP generates a one-way hash of the given string.

1. What I don't understand is how can we obtain the same hash output using two completely different salts?
2. Does it mean there are possibly other salts that could possibly give me the same hash?

Follow up

Thank you all for your pointers. I can see now that the default behavior is to use only the first two characters of the salt, which totally answers all my questions. Feels like a silly thing, but...

Answer 1

This is done on purpose. Your crypt function, when second argument consists of letters and digits, uses only two first characters of "salt" for encrypting, and those two characters are placed at the beginning of the result. So,

crypt("test", "test");        -> teH0wLIpW0gyQ
crypt("test", "te");          -> teH0wLIpW0gyQ
crypt("test", "tea");         -> teH0wLIpW0gyQ
crypt("test", "temperature"); -> teH0wLIpW0gyQ
etc.

This is done for easy password correctness checking, so that
crypt($password, crypt($password, $salt)) == crypt($password, $salt)

Answer 2

1‍. What I don't understand is how can we obtain the same hash output using two completely different salts?

Although you provided different salts to the crypt function, it uses the same salt internally, i. e., te. This is due to how crypt is implemented:

Standard DES-based hash with a two character salt from the alphabet "./0-9A-Za-z".

So even if you provide a salt longer than 2 characters it will only take the first two.

And as crypt’s output contains the used salt prepended to the calculated hash, using a crypt hash as salt results in exactly the same output. And that’s just perfect as then the following can be used to verify a stored password:

crypt($password, $hash) === $hash

2‍. Does it mean there are possibly other salts that could possibly give me the same hash?

Yes. This does also apply to other crypt algorithms like bcrypt.