/// <summary>
/// Mask generation function.
/// </summary>
/// <param name="seed">Seed</param>
/// <param name="maskLen">Length of generated mask</param>
/// <param name="hashLength">Length of the hash produced by the supplied hash provider</param>
/// <param name="hashProvider">Hash provider to use in mask generation</param>
/// <returns>Generated mask of specified length</returns>
internal static byte[] OAEPMGF( byte[] seed, int maskLen, int hashLength, HashProviders.IHashProvider hashProvider )
{
byte[] result = new byte[maskLen];
//Determine how many interations we have to do. We'll be appending
//m_hLen (hash length) bytes for every iteration, so the size of the generated byte array
//will be m_hLen * iNum (number of iterations).
int iNum = (int)Math.Floor( (double)( maskLen / hashLength ) ) + 1;
//Mask that will be truncated to create the final
//resulting mask returned by this function.
byte[] bytLongMask = new byte[( iNum * hashLength )];
byte[] bytAppend = new byte[4];
byte[] bytTmp = null;
int iPadLen = 0;
byte[] bytSeedHash = new byte[hashLength];
//Padded pseudorandom seed to be hashed.
byte[] bytPadSeed = new byte[( seed.Length + 4 )];
seed.CopyTo( bytPadSeed, 0 );
for( int i = 0; i <= iNum - 1; i++ ) {
//Convert the iterator to an Octet String byte array
bytTmp = CryptoMathematics.I2OSP( i, 4 );
//Calculate the needed padding zeros, and add
//them to the resulting Array. Result must be 4 bytes long.
iPadLen = bytAppend.Length - bytTmp.Length;
bytTmp.CopyTo( bytAppend, 0 );
//Hash the pseudorandom padded seed and append it to the
//long version of the mask.
bytAppend.CopyTo( bytPadSeed, seed.Length );
bytSeedHash = hashProvider.ComputeHash( bytPadSeed );
bytSeedHash.CopyTo( bytLongMask, i * hashLength );
}
//Copy the first maskLen bytes of bytLongMask to the result
//and return the result.
Array.Copy( bytLongMask, result, maskLen );
return result;
}
