Aaron Skonnard mentionsthat

When you take the custom authentication route and write a UsernameTokenManager (UTM), your implementation of AuthenticateToken must return the same secret (e.g., password) used on the client side to generate the hash/signature, depending on which option you use.

As he correctly points out, this makes security experts cringe and hide under the bed (see Keith Brown’s cringing response where he proposes a solution).

The big issue is that your UsernameTokenManager needs access to the original cleartext password. But like any good security conscious developer, you don’t store passwords as cleartext, do you? (I sure hope not. Bad security conscious developer. Bad!). Hopefully you do something along the likes of what Keith suggests in his MSDN column. For each user, he stores a randomly generated salt value and a hash of the cleartext password combined with salt value. The salt value is unique per user.

Keith points out that the secret returned by the AuthenticateToken method doesn’t have to be the actual cleartext password. It just has to match the secret sent by the client. So if you store your passwords as an SHA1 hash, your client just needs to hash the password before creating the UsernameToken.

However, if you store your password as an SHA1 hash of the cleartext password + salt value, you’re going to have to do a little more work. Your client isn’t going to know the salt value for every user, so your client needs a way to discover that. This may require calling a separate web method just to query for the salt value given a user name. Service clients would be required to store that value (probably on a “session” basis) and use it when calling methods on the main web service.

Below is some sample code for doing just that. This assumes that user passwords are stored as described in the aforementioned article using salt and hash (no eggs, but do bring the ketchup). (My apologies for the ugly formatting, I didn’t want the code to be too wide)

//Make an initial web service call to get the 
//the salt value for the user "haacked".  
//This should be stored by the client so its 
//not called for every method of our main service.
MyServiceWse proxy = new MyServiceWse();

//In order to get the salt value, a special account
//"saltAdmin" is used to call GetSalt().  This account
//only has access to this method.
//This also requires that the client app knows the;
//saltAdmin's salt value up front.
string adminPassword = GetAdminPassword(); 
//implementation not shown.

UsernameToken adminToken 
    = new UsernameToken("saltAdmin", adminPassword
                    , PasswordOption.SendHashed);

string username = "haacked";
string salt = proxy.GetSalt(username);

// Hash password and salt.
string pw = "Password"; //assume this came from the user.
SHA1CryptoServiceProvider hashProvider 
    = new SHA1CryptoServiceProvider();

byte[] inputBuffer = Encoding.Unicode.GetBytes(pw + salt);
byte[] result = hashProvider.ComputeHash(inputBuffer);
string hashedPassword = Convert.ToBase64String(result);
//Set up the user's token.
//Notice we the hashed password instead of the cleartext one.
UsernameToken token 
    = new UsernameToken(username, hashedPassword
                    , PasswordOption.SendHashed);


//Make the actual service call.

The AuthenitcateToken method of your custom UsernameTokenManager class can now just return the hashed password value for the calling user from your data store and everything will work just fine and security experts can come out from under the bed.