TOTP: Time-Based One-Time Password Algorithm http://tools.ietf.org/html/rfc6238#

convert hash to N-digit string.cs

public static string GeneratePassword(string secret, long iterationNumber, int digits = 6)
{
    byte[] counter = BitConverter.GetBytes(iterationNumber);

    if (BitConverter.IsLittleEndian)
        Array.Reverse(counter);

    byte[] key = Encoding.ASCII.GetBytes(secret);

    HMACSHA1 hmac = new HMACSHA1(key, true);

    byte[] hash = hmac.ComputeHash(counter);

    int offset = hash[hash.Length - 1] & 0xf;

    int binary =
        ((hash[offset] & 0x7f) << 24)
        | ((hash[offset + 1] & 0xff) << 16)
        | ((hash[offset + 2] & 0xff) << 8)
        | (hash[offset + 3] & 0xff);

    int password = binary % (int)Math.Pow(10, digits); // 6 digits

    return password.ToString(new string('0', digits));
}

totp.cs

using System;
using System.Security.Cryptography;
using System.Text;
					
public class Program
{
	
		public static string FormatOTP(byte[] hmac)
    {
	int offset =  hmac[19] & 0xf ;
	int bin_code = (hmac[offset]   & 0x7f) << 24
		| (hmac[offset+1] & 0xff) << 16
		| (hmac[offset+2] & 0xff) <<  8
		| (hmac[offset+3] & 0xff) ;
	int Code_Digits = bin_code % 10000000;
	
	int OTP = Code_Digits * 10;

	return string.Format("{0:d08}", OTP);
    }
	
	public static void Main()
	{
		 
		int TTL = 10;
		
		Console.WriteLine("Hello World");
		
		//Get current unix time
		TimeSpan CurrTime = (DateTime.UtcNow - new DateTime(1990, 1, 1)); 
		
		var sha1 = SHA1.Create();
		
		string secret = "aasaadsdasd"; //totaly rndom
		
		string mail = "mymail@mail.com";
		
		int counter = ((int)CurrTime.TotalSeconds) / TTL; 
		
		string rawSeq = counter.ToString() + secret + mail;
		
		
		Console.WriteLine(" == RAW data to generate == ");
		Console.WriteLine(rawSeq);
		
		byte[] hashBytes = sha1.ComputeHash(Encoding.UTF8.GetBytes(rawSeq));
		
		
		string rawHash = BitConverter.ToString(hashBytes);
		
		Console.WriteLine(" == RAW hash             == ");
		Console.WriteLine(rawHash);
		
		Console.WriteLine(FormatOTP(hashBytes));
		
	}
}