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.

Applications

• OTPs are widely used in websites like- Facebook, Google Sign-in, Wifi – accessing, Railways Portal Login etc.

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 !


Implementation

// A C/C++ Program to generate OTP (One Time Password) #include<bits/stdc++.h> using namespace std;   // A Function to generate a unique OTP everytime string generateOTP(int len) {     // All possible characters of my OTP     string str = "abcdefghijklmnopqrstuvwxyzABCD"                "EFGHIJKLMNOPQRSTUVWXYZ0123456789";     int n = str.length();       // String to hold my OTP     string OTP;       for (int i=1; i<=len; i++)         OTP.push_back(str[rand() % n]);       return(OTP); }   // Driver Program to test above functions int main() {     // For different values each time we run the code     srand(time(NULL));       // Delare the length of OTP     int len = 6;     printf("Your OTP is - %s", generateOTP(len).c_str());       return(0); }

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.


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