How do I write/put together a secure login in PHP? The website developer guide said I shouldn't roll my own, so referring to samples available via Google is useless.
How do you pros do it? Lets say you're building a world-class app in rails, would the same libraries / techniques be usable here?
Thanks
In Rails, one would generally use a pre-existing library. Authentication is easy to do wrong, and the problem's been solved so many times that it's rarely worth the effort to solve it again. If you are interested in writing your own implementation, then I'll describe how modern authentication works.
The naive method of authenticating a user is to store their password in a database and compare it to the password the user submits. This is simple but unbelievably insecure. Anyone who can read your database can view anyone's password. Even if you put in database access controls, you (and your users) are vulnerable to anyone who hacks around them.
Proper form is to use a cryptographic hash function to process the password when it is chosen and then every time it is submitted. A good hash function is practically irreversible -- you can't take a hash and turn it back into a password. So when the user logs in, you take the submitted password, hash it, and compare it to the hash in the database. This way, you never store the password itself. On the downside, if the user forgets their password, you have to reset it rather than send it to them.
Even this, however, is vulnerable to certain attacks. If an attacker gets hold of your password hashes, and knows how you hash your passwords, then he can make a dictionary attack: he simply takes every word in the dictionary and hashes that word, keeping it with the original. This data structure is called a rainbow table. Then, if any of the dictionary word hashes match a password hash, the attacker can conclude that the password is the dictionary word that hashes to that password. In short, an attacker who can read your database can still log in to accounts with weak passwords.
The solution is that before a password is hashed, it is combined (usually concatenated or xor'd) with a value called the salt which is unique to each user. It may be randomly generated, or it may be an account creation timestamp or some such. Then, an attacker cannot use a rainbow table because every password is essentially hashed slightly differently; he would have to create a separate rainbow table for every single distinct salt (practically for each account), which would be prohibitively computationally expensive.
I will echo the advice of the other answerers: this is not simple stuff, and you don't need to do it because it's been done before, and if you do it yourself you stand a very good chance of making a mistake and inadvertently compromising your system's security. But if, for whatever reason, you really, really want to write one yourself, I hope that I have provided an (incomplete!) outline of how it's done.
The Zend Framework has an 'Auth' module which would be a good place to start. Or, if your site will be hosting an install of WordPress or PHPBB, there are ways of leveraging those technologies' authentication modules to sign in to other pages of a site.
One thing to look at when you are trying to authenticate is what is your real goal.
For example, on SO I use my google login, and that works, as they just need to know who I am, and they can trust that Google has an idea. So, if that model will work for you, then look at using OpenID, as there are various tools for that.
If you must do your own, then there will be various tests to ensure that it is secure, again, depending on how paranoid you want to be.
I will end my response here as Thom did a fantastic response.
by Soulmerge:
I think the accepted answer in your other question states it pretty well. Hash the passwords with a salt. Other than that, there are some security ideas on the transport layer:
md5($_POST['postedPwHash'] . $salt))
a good method to somewhat secure the client-server transaction (if no ssl is available) is to use a one-time random key to create a unique hash from the credentials, then only send that unique hash to the server. the server then compares this hash to its own generated hash instead of comparing it to the real credentials. this would provide a good defense against the man-in-the-middle attack. the downside is that to do this the user must have JS enabled (at least i dont know of a good method to encrypt client-side data without it). this means that you will need a sufficient fallback when it isn't on. you can even create the form in JS to make sure its enabled.
this library is a simple library i wrote once that does the procedure i described, though it probably needs some improvements.
note that this is in addition to using "salting" methods and other server-side security measures. it is also quite vulnerable to dictionary attacks as the entire hashing process is by definition procedural, predictable and visible to the user (as JS always is).
My answer is "Don't do it"
This is a very complex area, full of potential security gotcha's. If you are not an expert in this field, then you are really just asking for trouble and problems down the road.
I would recommend looking at getting an existing solution to do. Sadly I don't know any that I would be happy to recommend, other than openid. I'm sure you will get some good suggestions here though...
