C# PasswordDeriveBytes Confusion

Question

I have following code in C#

PasswordDeriveBytes DerivedPassword = new PasswordDeriveBytes(Password, SaltValueBytes, HashAlgorithm, PasswordIterations);
byte[] KeyBytes = DerivedPassword.GetBytes(32);

I am using "SHA1" hashing algorithm.

According to SHA1 definition, its generate 160 bits (20 bytes) key. My question is how GetBytes method get 32 bytes from DerivedPassword, what algorithm used behind GetBytes method?

Answer 1

Microsoft's implementation of original PKCS#5 (aka PBKDF1) include insecure extensions to provide more bytes than the hash function can provide (see bug reports here and here).

Even if it was not buggy you should avoid undocumented, proprietary extensions to standards (or you might never be able to decrypt your data in the future - at least not outside Windows.)

I strongly suggest you to use the newer Rfc2898DeriveBytes which implements PBKDF2 (PKCS#5 v2) which is available since .NET 2.0.