Question

This is the code I use to “hash” (or derive key as called in the PBKDF2 implementation of the PKCS standard) passwords strings with the Rfc2898DeriveBytes class provided in .NET:

int saltSize = 256;
int iterations = 1000;
int keySize = 20;  // The parameter I'm not sure of

var deriveBytes = new Rfc2898DeriveBytes("mypassword", saltSize, iterations);
byte[] salt = deriveBytes.Salt;
byte[] key = deriveBytes.GetBytes(keySize);

Now, I understand that the salt size doesn’t matter much (as long as it is enough to ensure that random salts will be unique), but what about the key size? Does a longer key provides more security against attacks?

(Notes:
1. Performance matters do not import for me here, it is obvious that a longer salt or a longer key will take more time for GetBytes to return a value.
2. I want to use this “hash” to store them in a database, not to use them afterwards in a encryption scheme)

Answer 1

Generally you use PKCS#5 v2 / RFC2898 to create a symmetric key from a user password. The size is important because it must match the required size of the symmetric algorithm you’ll be using.

aes.Key = deriveBytes.GetBytes (16); // 16 * 8 = 128 bits

However you seems to be looking at keeping an hash of passwords, not for a key, so the size is not as important in your specific case. You can safely fix it to the hash size (20 bytes for SHA1) if you want a specific value.

General note (for people where performance matters): using PKCS#5 v2 (or older) will take a lot longer (iteration count) than using a salted hash or an HMAC.