DISCLAIMER: This answer was written in 2008.
Since then, PHP has given us password_hash
and password_verify
and, since their introduction, they are the recommended password hashing & checking method.
The theory of the answer is still a good read though.
TL;DR
Don'ts
- Don't limit what characters users can enter for passwords. Only idiots do this.
- Don't limit the length of a password. If your users want a sentence with supercalifragilisticexpialidocious in it, don't prevent them from using it.
- Don't strip or escape HTML and special characters in the password.
- Never store your user's password in plain-text.
- Never email a password to your user except when they have lost theirs, and you sent a temporary one.
- Never, ever log passwords in any manner.
- Never hash passwords with SHA1 or MD5 or even SHA256! Modern crackers can exceed 60 and 180 billion hashes/second (respectively).
- Don't mix bcrypt and with the raw output of hash(), either use hex output or base64_encode it. (This applies to any input that may have a rogue
\0
in it, which can seriously weaken security.)
Dos
- Use scrypt when you can; bcrypt if you cannot.
- Use PBKDF2 if you cannot use either bcrypt or scrypt, with SHA2 hashes.
- Reset everyone's passwords when the database is compromised.
- Implement a reasonable 8-10 character minimum length, plus require at least 1 upper case letter, 1 lower case letter, a number, and a symbol. This will improve the entropy of the password, in turn making it harder to crack. (See the "What makes a good password?" section for some debate.)
Why hash passwords anyway?
The objective behind hashing passwords is simple: preventing malicious access to user accounts by compromising the database. So the goal of password hashing is to deter a hacker or cracker by costing them too much time or money to calculate the plain-text passwords. And time/cost are the best deterrents in your arsenal.
Another reason that you want a good, robust hash on a user accounts is to give you enough time to change all the passwords in the system. If your database is compromised you will need enough time to at least lock the system down, if not change every password in the database.
Jeremiah Grossman, CTO of Whitehat Security, stated on White Hat Security blog after a recent password recovery that required brute-force breaking of his password protection:
Interestingly, in living out this nightmare, I learned A LOT I didn’t know about password cracking, storage, and complexity. I’ve come to appreciate why password storage is ever so much more important than password complexity. If you don’t know how your password is stored, then all you really can depend upon is complexity. This might be common knowledge to password and crypto pros, but for the average InfoSec or Web Security expert, I highly doubt it.
(Emphasis mine.)
What makes a good password anyway?
Entropy. (Not that I fully subscribe to Randall's viewpoint.)
In short, entropy is how much variation is within the password. When a password is only lowercase roman letters, that's only 26 characters. That isn't much variation. Alpha-numeric passwords are better, with 36 characters. But allowing upper and lower case, with symbols, is roughly 96 characters. That's a lot better than just letters. One problem is, to make our passwords memorable we insert patterns—which reduces entropy. Oops!
Password entropy is approximated easily. Using the full range of ascii characters (roughly 96 typeable characters) yields an entropy of 6.6 per character, which at 8 characters for a password is still too low (52.679 bits of entropy) for future security. But the good news is: longer passwords, and passwords with unicode characters, really increase the entropy of a password and make it harder to crack.
There's a longer discussion of password entropy on the Crypto StackExchange site. A good Google search will also turn up a lot of results.
In the comments I talked with @popnoodles, who pointed out that enforcing a password policy of X length with X many letters, numbers, symbols, etc, can actually reduce entropy by making the password scheme more predictable. I do agree. Randomess, as truly random as possible, is always the safest but least memorable solution.
So far as I've been able to tell, making the world's best password is a Catch-22. Either its not memorable, too predictable, too short, too many unicode characters (hard to type on a Windows/Mobile device), too long, etc. No password is truly good enough for our purposes, so we must protect them as though they were in Fort Knox.
Best practices
Bcrypt and scrypt are the current best practices. Scrypt will be better than bcrypt in time, but it hasn't seen adoption as a standard by Linux/Unix or by webservers, and hasn't had in-depth reviews of its algorithm posted yet. But still, the future of the algorithm does look promising. If you are working with Ruby there is an scrypt gem that will help you out, and Node.js now has its own scrypt package. You can use Scrypt in PHP either via the Scrypt extension or the Libsodium extension (both are available in PECL).
I highly suggest reading the documentation for the crypt function if you want to understand how to use bcrypt, or finding yourself a good wrapper or use something like PHPASS for a more legacy implementation. I recommend a minimum of 12 rounds of bcrypt, if not 15 to 18.
I changed my mind about using bcrypt when I learned that bcrypt only uses blowfish's key schedule, with a variable cost mechanism. The latter lets you increase the cost to brute-force a password by increasing blowfish's already expensive key schedule.
Average practices
I almost can't imagine this situation anymore. PHPASS supports PHP 3.0.18 through 5.3, so it is usable on almost every installation imaginable—and should be used if you don't know for certain that your environment supports bcrypt.
But suppose that you cannot use bcrypt or PHPASS at all. What then?
Try an implementation of PDKBF2 with the maximum number of rounds that your environment/application/user-perception can tolerate. The lowest number I'd recommend is 2500 rounds. Also, make sure to use hash_hmac() if it is available to make the operation harder to reproduce.
Future Practices
Coming in PHP 5.5 is a full password protection library that abstracts away any pains of working with bcrypt. While most of us are stuck with PHP 5.2 and 5.3 in most common environments, especially shared hosts, @ircmaxell has built a compatibility layer for the coming API that is backward compatible to PHP 5.3.7.
Cryptography Recap & Disclaimer
The computational power required to actually crack a hashed password doesn't exist. The only way for computers to "crack" a password is to recreate it and simulate the hashing algorithm used to secure it. The speed of the hash is linearly related to its ability to be brute-forced. Worse still, most hash algorithms can be easily parallelized to perform even faster. This is why costly schemes like bcrypt and scrypt are so important.
You cannot possibly foresee all threats or avenues of attack, and so you must make your best effort to protect your users up front. If you do not, then you might even miss the fact that you were attacked until it's too late... and you're liable. To avoid that situation, act paranoid to begin with. Attack your own software (internally) and attempt to steal user credentials, or modify other user's accounts or access their data. If you don't test the security of your system, then you cannot blame anyone but yourself.
Lastly: I am not a cryptographer. Whatever I've said is my opinion, but I happen to think it's based on good ol' common sense ... and lots of reading. Remember, be as paranoid as possible, make things as hard to intrude as possible, and then, if you are still worried, contact a white-hat hacker or cryptographer to see what they say about your code/system.
Here's how I do it.
I decided to use IPrincipal instead of IIdentity because it means I don't have to implement both IIdentity and IPrincipal.
Create the interface
interface ICustomPrincipal : IPrincipal
{
int Id { get; set; }
string FirstName { get; set; }
string LastName { get; set; }
}
CustomPrincipal
public class CustomPrincipal : ICustomPrincipal
{
public IIdentity Identity { get; private set; }
public bool IsInRole(string role) { return false; }
public CustomPrincipal(string email)
{
this.Identity = new GenericIdentity(email);
}
public int Id { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
}
CustomPrincipalSerializeModel - for serializing custom information into userdata field in FormsAuthenticationTicket object.
public class CustomPrincipalSerializeModel
{
public int Id { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
}
LogIn method - setting up a cookie with custom information
if (Membership.ValidateUser(viewModel.Email, viewModel.Password))
{
var user = userRepository.Users.Where(u => u.Email == viewModel.Email).First();
CustomPrincipalSerializeModel serializeModel = new CustomPrincipalSerializeModel();
serializeModel.Id = user.Id;
serializeModel.FirstName = user.FirstName;
serializeModel.LastName = user.LastName;
JavaScriptSerializer serializer = new JavaScriptSerializer();
string userData = serializer.Serialize(serializeModel);
FormsAuthenticationTicket authTicket = new FormsAuthenticationTicket(
1,
viewModel.Email,
DateTime.Now,
DateTime.Now.AddMinutes(15),
false,
userData);
string encTicket = FormsAuthentication.Encrypt(authTicket);
HttpCookie faCookie = new HttpCookie(FormsAuthentication.FormsCookieName, encTicket);
Response.Cookies.Add(faCookie);
return RedirectToAction("Index", "Home");
}
Global.asax.cs - Reading cookie and replacing HttpContext.User object, this is done by overriding PostAuthenticateRequest
protected void Application_PostAuthenticateRequest(Object sender, EventArgs e)
{
HttpCookie authCookie = Request.Cookies[FormsAuthentication.FormsCookieName];
if (authCookie != null)
{
FormsAuthenticationTicket authTicket = FormsAuthentication.Decrypt(authCookie.Value);
JavaScriptSerializer serializer = new JavaScriptSerializer();
CustomPrincipalSerializeModel serializeModel = serializer.Deserialize<CustomPrincipalSerializeModel>(authTicket.UserData);
CustomPrincipal newUser = new CustomPrincipal(authTicket.Name);
newUser.Id = serializeModel.Id;
newUser.FirstName = serializeModel.FirstName;
newUser.LastName = serializeModel.LastName;
HttpContext.Current.User = newUser;
}
}
Access in Razor views
@((User as CustomPrincipal).Id)
@((User as CustomPrincipal).FirstName)
@((User as CustomPrincipal).LastName)
and in code:
(User as CustomPrincipal).Id
(User as CustomPrincipal).FirstName
(User as CustomPrincipal).LastName
I think the code is self-explanatory. If it isn't, let me know.
Additionally to make the access even easier you can create a base controller and override the returned User object (HttpContext.User):
public class BaseController : Controller
{
protected virtual new CustomPrincipal User
{
get { return HttpContext.User as CustomPrincipal; }
}
}
and then, for each controller:
public class AccountController : BaseController
{
// ...
}
which will allow you to access custom fields in code like this:
User.Id
User.FirstName
User.LastName
But this will not work inside views. For that you would need to create a custom WebViewPage implementation:
public abstract class BaseViewPage : WebViewPage
{
public virtual new CustomPrincipal User
{
get { return base.User as CustomPrincipal; }
}
}
public abstract class BaseViewPage<TModel> : WebViewPage<TModel>
{
public virtual new CustomPrincipal User
{
get { return base.User as CustomPrincipal; }
}
}
Make it a default page type in Views/web.config:
<pages pageBaseType="Your.Namespace.BaseViewPage">
<namespaces>
<add namespace="System.Web.Mvc" />
<add namespace="System.Web.Mvc.Ajax" />
<add namespace="System.Web.Mvc.Html" />
<add namespace="System.Web.Routing" />
</namespaces>
</pages>
and in views, you can access it like this:
@User.FirstName
@User.LastName
Best Answer
What should I do to protect myself?
[Update 2010-09-29]
Microsoft security bulletin
KB Article with reference to the fix
ScottGu has links for the downloads
[Update 2010-09-25]
While we are waiting for the fix, yesterday ScottGu postet an update on how to add an extra step to protect your sites with a custom URLScan rule.
Basically make sure you provide a custom error page so that an attacker is not exposed to internal .Net errors, which you always should anyways in release/production mode.
Additionally add a random time sleep in the error page to prevent the attacker from timing the responses for added attack information.
In web.config
This will redirect any error to a custom page returned with a 200 status code. This way an attacker cannot look at the error code or error information for information needed for further attacks.
It is also safe to set
customErrors mode="RemoteOnly"
, as this will redirect "real" clients. Only browsing from localhost will show internal .Net errors.The important part is to make sure that all errors are configured to return the same error page. This requires you to explicitly set the
defaultRedirect
attribute on the<customErrors>
section and ensure that no per-status codes are set.What's at stake?
If an attacker manage to use the mentioned exploit, he/she can download internal files from within your web application. Typically web.config is a target and may contain sensitive information like login information in a database connection string, or even link to an automouted sql-express database which you don't want someone to get hold of. But if you are following best practice you use Protected Configuration to encrypt all sensitive data in your web.config.
Links to references
Read Microsoft's official comment about the vulnerability at http://www.microsoft.com/technet/security/advisory/2416728.mspx. Specifically the "Workaround" part for implementation details on this issue.
Also some information on ScottGu's blog, including a script to find vulnerable ASP.Net apps on your web server.
For an explanation on "Understanding Padding Oracle Attacks", read @sri's answer.
Comments to the article:
In order for the attack to work the following must be true:
So, if you return human readable error messages in your app like "Something went wrong, please try again" then you should be pretty safe. Reading a bit on the comments on the article also gives valuable information.
That way a hijacked cookie can only be used to retrieve a session which most likely is no longer present or invalidated.
It will be interesting to see what is actually presented at the Ekoparty conference, but right now I'm not too worried about this vulnerability.