To Generate a One Time Password or Unique Identification URL

A one-time password (OTP) is a password that is valid for only one login session or transaction, on a computer system or other digital device. For more details refer this.


Randomly pick characters from our all possibilities and generate a string of the desired length from it. OTPs are generally 6-7 characters long and randomness in 6-7 characters almost guarantees a secure way of logging in.


  • OTPs are widely used in websites like- Facebook, Google Sign-in, Wifi – accessing, Railways Portal Login etc.
  • Even the GeeksforGeeks IDE has a unique string for all the codes compiled through it. For example, has the unique string – “Ks84Ck” at the end which is unique for this code only.

How it gets generated ?
Well it is a great possibility that they uses the same algorithm as an OTP is generated. If by chance (very rare) the unique string generated is already been generated before and has been associated with a different code then another random string is used.

As per now it seems that only six character strings are generated randomly for a unique identification of all codes. A time will come when all the possible six character strings may get exhausted. So yes, even the web-related stuffs also heavily relies on randomness.

Probability of collision of two OTPs

  • The length of OTP is 6 and the set size of all possible characters in the OTP is 62. So the total number of possible sets of the pair of OTPs are 6212.
  • Some of them are – [{aaaaaa, aaaaaa}, {aaaaaa, aaaaab},…..{456789, 456788}, {456789, 456789}]
  • But the possible sets of equal pair of OTPs are:626. Some of them are – [{aaaaaa, aaaaaa}, {aaaaab, aaaaab},…..{456788, 456788}, {456789, 456789}]
  • Hence the probability of collision of two OTPs is:
    626 / 6212 = 1 / 626 = 1 / 56800235584 = 1.7605561-11

So the probability of two OTPs colliding are as less probable as the existence of your life on earth (Ratio of the number of years you will live to the number of years from the start of the universe and everything in existence).So yes,OTPs are way more secure than static passwords !






// A C/C++ Program to generate OTP (One Time Password)
using namespace std;
// A Function to generate a unique OTP everytime
string generateOTP(int len)
    // All possible characters of my OTP
    string str = "abcdefghijklmnopqrstuvwxyzABCD"
    int n = str.length();
    // String to hold my OTP
    string OTP;
    for (int i=1; i<=len; i++)
        OTP.push_back(str[rand() % n]);
// Driver Program to test above functions
int main()
    // For different values each time we run the code
    // Declare the length of OTP
    int len = 6;
    printf("Your OTP is - %s", generateOTP(len).c_str());


Output (May be different for every run):

Your OTP is - 8qOtzy

Time Complexity: O(N), where N = number of characters in our OTP

Auxiliary Space: Apart from the string having all possible characters we require O(N) space to hold the OTP, where N = number of characters in our OTP


This article is contributed by Rachit Belwariar. If you like GeeksforGeeks and would like to contribute, you can also write an article using or mail your article to See your article appearing on the GeeksforGeeks main page and help other Geeks.

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Improved By : nidhi_biet

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