Does Convert.ToBase64String return the same length as it's source byte array?

Question

I don't know if i am asking a silly question but I want to know whether Convert.ToBase64String function in .NET returns the same length as it's source byte size or is it different? I wanted to try out the article from MSDN itself How To: Use Forms Authentication with SQL Server 2000 to hash my password but I found out that the function they used to create salt string is returning 3 more length than it is supposed to return. To clarify here is the code in that article.

private static string CreateSalt(int size)
{
   // Generate a cryptographic random number using the cryptographic
   // service provider
   RNGCryptoServiceProvider rng = new RNGCryptoServiceProvider();
   byte[] buff = new byte[size];
   rng.GetBytes(buff);
   // Return a Base64 string representation of the random number
   return Convert.ToBase64String(buff);
}

Answer 1

No, Base64 returns 4 bytes output for 3 bytes input, rounded up (padded with =) to the next 4-byte boundary.

int outputLength = ((inputLength+2)/3)*4

That's because it only uses 6 bits (basically a number 0-63) per byte in order to only use ASCII chars which are not control chars and in the 7-bit range. Therefore, you get 3*8 => 4*6 bits when encoding data with Base64.

Answer 2

base-64 rarely returns a string of the same length as the input. Essentially, it is only using 6 of the available 8 bits, so large messages (in particular) will require an extra 1/3 volume. There are a few packing bytes (usually "=") at the end to make the message unambiguous.