ulong xorKey = new ulong( );
                
xorKey &= 0xFF << 8;
xorKey |= (byte) randNum.Next( 8, 64 );

int bitPos  = (int) xorKey & 0xFF;
bitPos     -= bitPos%8;
ulong bitMask = (ulong) 2 >> --bitPos;
bitMask--;

xorKey &= bitMask;
xorKey |= (ulong) keySeed << ++bitPos;

return xorKey;