Using Rfc2898DeriveBytes for PBKDF2

Password storage is a large topic in application security. If a security failure occurs, and the database is stolen, the passwords of the users are some of the most important data stored. Given the state of contemporary authentication, they do not need to be stored in plain text, so they should not. A hashed representation of the password, using a contemporary encryption algorithm and process, is the accepted way to store a password in today’s systems. More information can be found in the Password Storage Cheat Sheet.

=Common .NET password storage=

In .NET, the SQL Membership or ASP.NET membership patterns are often used for identity. In a best case scenario passwords aren’t stored by the local application or a well-known and trusted system is used such as AD, Facebook, or ASP.NET Identity. In each of these cases, the password storage is either handled by the subsystem, or not handled by the application at all.

Sometimes, however, there is no choice but to store the password in the application using home grown code. When this is the case, it is upon the software developer to select and use the correct hashing algorithm and process for password storage. Hashing is the process of deriving a unique, repeatable value form a text input and salt. This prevents the storage of the password itself, thus protecting the password if the database is stolen.

Hashes create unique values that cannot be reversed into their source values, however, brute force could potentially lead an attacker to the source values. As such, a key derivation function is often used to increase the work factor needed to create the representative value. Often this key derivitation function is PBKDF2, or the Password-Based Key Derivation Function 2.

=PBKDF2 basics=

PBKDF2 uses a pseudorandom function and a configurable number of iterations to derive a cryptographic key from a password. Because this process is difficult to reverse (similar to a cryptographic hash function) but can also be configured to be slow to compute, key derivation functions are ideally suited for password hashing use cases.

The details of PBKDF2 are openly published, and the goal of this document is not to replicate that information. Generally speaking, the function is one that accepts a pseudorandom function (such as SHA1), a salt, the number of iterations, the length of the resultant hash, and the text to be hashed. The goal is one of ‘key stretching’, making the overall process of generating or reversing the hash harder. The .NET Framework can abstract the details of the algorithm from the developer.

=Implementing PBKDF2 in .NET=

Microsoft’s .NET platform supports PBKDF2 out of the box. Rfc2898DeriveBytes allows a developer to generate a key for a value using PDKDF2 without implementing the algorithm. Using a number of iterations and a salt, a developer can easily implement the key stretching hash then store that data in the database. During registration, rather than storing the password entered by the user, you should store the password and salt. Guids provide a very strong salt, as while they are not cryptographically random, they are guaranteed globally unique. Rfc2898DeriveBytes will generate it's own salt, which is retrievable before save, as we have done here. Rfc2898DeriveBytes will generate a hash and return the derived key with the requested number of iterations. You need to store them both.

Each base hash function sized block of output from PBKDF2 is iterated independently of each other. Using PBKDF2 for password storage, one should never output more bits than the base hash function's size. With PBKDF2-SHA1 this is 160 bits or 20 bytes. Output more bits doesn't make the hash more secure, but it costs the defender a lot more time while not costing the attacker. An attacker will just compare the first hash function sized output saving them the time to generate the reset of the PBKDF2 output.

Here is an example of using the System.Security.Cryptography namespace in a simple method. It returns the salt and key in a pipe delimited string.

Another note is that related to the number of iterations. In the Password Storage Cheat Sheet, it is recommended to make the password generation process as slow as possible without negatively affecting the user experience. Here, we have set the iterations to 10,000 and should review that number every year.

=Using the hash on login=

When a user later logs in, rather than using the password to confirm authentication, you can use the hashing function to generate a hash with the stored salt rather than a generated salt. Then compare the hash with the stored hash.

=Limitations=

The built-in .NET implementation of Rfc2898DeriveBytes limits the user to one psudorandom function - HMAC with SHA-1. This is acceptable in most scenarios today, but in the future, a more complex hashing function may be required.

The .NET Compact Framework does not support Rfc2898DeriveBytes.

=References=
 * Password Storage Cheat Sheet
 * SQL Membership
 * ASP.NET Membership
 * Rfc2898DeriveBytes
 * Salted Password Hashing