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Need a sample code to create OTP - random number using System.Security.Cryptography.

Google has a lot of answers for you. But first of all you have to decide the hash you want to use, the password length, compexity, if you need to comply with a standard, and so on. But this is a good start:

public static int CalculateHotp(byte[] key, byte[] counter)
    var hmacsha1 = new HMACSHA1(key);
    byte[] hmac_result = hmacsha1.ComputeHash(counter);
    int offset = hmac_result[19] & 0x0f;
    int bin_code = (hmac_result[offset]  & 0x7f) << 24
                   | (hmac_result[offset+1] & 0xff) << 16
                   | (hmac_result[offset+2] & 0xff) <<  8
                   | (hmac_result[offset+3] & 0xff);
    int hotp = bin_code % 1000000;
    return hotp;

(copied from here[^])

using System;
using System.Security.Cryptography;
using System.Text;

namespace OTATest
    static class OTAGenerator
        private static char[] pwdCharArray = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789`~!@#$%^&*()-_=+[]{}\\|;:'\",<.>/?".ToCharArray();

        public static string GenerateOTA(int keyLength = 10)
            using (RNGCryptoServiceProvider rngCsp = new RNGCryptoServiceProvider())
                StringBuilder sBuilder = new StringBuilder();
                byte[] key = new byte[1];

                for (int i = 0; i < keyLength; i++)
                    while (key[0] > pwdCharArray.Length);

                return sBuilder.ToString();
R.SIVAA 25-Jun-12 6:39am
Thanks Zoltan. The above code is good to generate the encryption for a given word. But I really need to generate random numbers, not based on any input. I wanto use these random numbers for OTP(One Time Password). To be particular, want to use this "System.Security.Cryptography.RandomNumberGenerator".
Zoltán Zörgő 25-Jun-12 8:16am
See my update
public decimal Rand()
        byte[] Salt = new byte[8];
        decimal result = 0;
        foreach (byte b in Salt)
        {      result = result * 255 + b;  }
        while (result > 100)
        {      result /= 10;
        return result;

Hope this works...

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