Google MFA in C#

Google MFA

class Program
    {
        static void Main(string[] args) {
            var secretByets = Base32Encoding.ToBytes("supersecretpassword");
            var input = GetEpoch() / 30;
            var hmac = new HMACSHA1(secretByets);

            var output = hmac.ComputeHash(toBytes(input));
            Console.WriteLine("{0:d6}", calculate(output));

            // print value for the past 30 seconds as well for user convenience
            output = hmac.ComputeHash(toBytes(--input));
            Console.WriteLine("{0:d6}", calculate(output));
        }

        private static uint calculate(byte[] output) {
            // Slim down the value from 20 bytes to 4 bytes. 
            // But, instead of always using the first 4 bytes,
            // use the value of last byte of the 20-bytes 
            // to determine the start index of the 4-bytes we're interested in.
            var index = output[output.Length - 1] & 0x0f;
            var slimmed = new byte[4];
            slimmed[0] = output[index++];
            slimmed[1] = output[index++];
            slimmed[2] = output[index++];
            slimmed[3] = output[index];
            slimmed[0] &= 0x7f; // ignore the most significant bit as per RFC 4226

            var large = ToUint32(slimmed);
            // modulus 1 million to ensure the values are 6 digits or less
            return large % 1000000;
        }
        // convert to/from byte arrays to/from uint using Big-Endian
        // BitConverter class is will use Little-Endian on Windows
        private static byte[] toBytes(UInt64 input) {
            var result = new byte[8];
            var shifts = new UInt16[] {56, 48, 40, 32, 24, 16, 8, 0};
            for (int i = 0; i < shifts.Length; i++) {
                result[i] = (byte)((input >> shifts[i]) & 0xFF);
            }
            return result;
        }

        private static UInt32 ToUint32(byte[] bytes) {
            return (UInt32)((bytes[0] << 24) | (bytes[1] << 16) | (bytes[2] << 8) | (bytes[3]));
        }

        private static UInt64 GetEpoch() {
            var epoch = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
            return (UInt64)((DateTime.UtcNow - epoch).TotalSeconds);
        }
    }