How to Securely Hash Passwords in .NET Using PBKDF2?

Use current best practices to securely hash passwords in .NET

When storing passwords in a database, it is critical to protect sensitive information using hashing algorithms. Encryption methods are not suitable for this purpose. The best native password hashing algorithm in .NET is PBKDF2 (Password-Based Key Derivation Function 2).

Step-by-step guide to password hashing using PBKDF2

Step 1: Generate salt value

The salt is a random value used to make the hash of each password unique. Create a salt value using a cryptographic PRNG (pseudo-random number generator):

<code class="language-csharp">byte[] salt;
new RNGCryptoServiceProvider().GetBytes(salt = new byte[16]);</code>

Step 2: Create PBKDF2 object and calculate hash value

Instantiate the Rfc2898DeriveBytes class and specify the password, salt, and desired number of iterations (~100,000 recommended):

<code class="language-csharp">var pbkdf2 = new Rfc2898DeriveBytes(password, salt, 100000);
byte[] hash = pbkdf2.GetBytes(20);</code>

Step 3: Combine salt and cipher bytes for storage

Combine the salt and password bytes to create a single string for database storage:

<code class="language-csharp">byte[] hashBytes = new byte[36];
Array.Copy(salt, 0, hashBytes, 0, 16);
Array.Copy(hash, 0, hashBytes, 16, 20);</code>

Step 4: Convert to Base64 string for storage

Encode the combined bytes into a Base64 string for storage in the database:

<code class="language-csharp">string savedPasswordHash = Convert.ToBase64String(hashBytes);</code>

Step 5: Verify the password entered by the user

To verify that the password entered by the user matches the stored hash:

• Get the stored password hash from the database.
• Extract the salt and hash bytes from the stored hash value.
• Calculate a new hash using the password entered by the user and the extracted salt.
• Compare the calculated hash with the stored hash. If they match, the password is valid.

<code class="language-csharp">string savedPasswordHash = DBContext.GetUser(u => u.UserName == user).Password;

byte[] hashBytes = Convert.FromBase64String(savedPasswordHash);
byte[] salt = new byte[16];
Array.Copy(hashBytes, 0, salt, 0, 16);

var pbkdf2 = new Rfc2898DeriveBytes(password, salt, 100000);
byte[] hash = pbkdf2.GetBytes(20);

for (int i=0; i < 20; i++)
    if (hashBytes[i+16] != hash[i])
        throw new UnauthorizedAccessException();</code>

Note: The number of iterations can be adjusted based on the performance requirements of the application. A generally recommended minimum value is 10,000.

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