Find Square Root under Modulo p | (When p is product of two primes in the form 4*i + 3)

Given an integer N and an integer P denoting the product of two primes, the task is to find all possible Square Root of N under modulo P if it exists. It is given that P is the product of p1 and p2, where p1 and p2 are prime numbers of the form 4i + 3 where i is any integer. Some examples of such prime numbers are (7, 11, 19, 23, 31).

Examples:

Input: N = 67, P = 77
Output: 23 65 12 54
Explanation:
All possible answers are (23, 65, 12, 54)

Input: N = 188, P = 437
Output: 25 44 393 412
Explanation:

All possible answers are (25, 44, 393, 412)



Naive Approach:
The simplest approach to solve this problem is to consider all numbers from 2 to P – 1 and for every number, check if it is the square root of N under modulo P. If found to be true, then print that number and break.
Time Complexity: O(P)
Auxiliary Space: O(1)

Efficient Approach:
To optimize the above approach, the idea is to use Prime Factorization to obtain two factors p1 and p2 and then find square root of both using Square root under mod P. Then use Chinese Remainder Theorem to find the square root modulo p1 * p2. Each set will contain two equations (as there are two square roots each for modulo p1 and p2). Combining them all, four sets of equations are obtained which gives the four possible answers.
Follow the steps below to solve this problem:

  • Factorize the number P by using Prime Factorization. Factors will be two primes p1 and p2.
  • Find square root modulo primes p1 and p2.
  • Two sets for answers is found, one for each prime. Each set containing two numbers.
  • So there will be 4 set of equations if one number from each set is chosen.
  • Perform Chinese Remainder Theorem to find the square root modulo p1 * p2 and print them.

Below is the implementation of the above approach where BigInteger class in Java is used to handle very large numbers:

Java

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// Java program for the above approach
import java.math.*;
import java.util.*;
  
class SqrtMod {
  
    // Function to find the modulo inverse
    // of a with respect to m
    static BigInteger modInverse(BigInteger a,
                                 BigInteger m)
    {
  
        BigInteger m0 = m;
        BigInteger y = new BigInteger("0"),
                   x = new BigInteger("1");
  
        if (m.compareTo(new BigInteger("1")) == 0)
            return new BigInteger("0");
  
        // Perform Extended Euclid Algorithm
        while (a.compareTo(new BigInteger("1")) > 0) {
  
            if (m.toString().equals("0"))
                break;
            BigInteger q = a.divide(m);
  
            BigInteger t = m;
  
            // M is remainder now, process
            // Extended Euclid Algorithm
            m = a.mod(m);
            a = t;
  
            t = y;
            y = x.subtract(q.multiply(y));
            x = t;
        }
  
        // Make x positive
        while (x.compareTo(new BigInteger("0")) < 0)
            x = x.add(m0);
        return x;
    }
  
    // Function to apply the Chinese
    // Remainder Theorem
    static String CRT(BigInteger num[],
                      BigInteger rem[],
                      int k)
    {
  
        BigInteger prod = new BigInteger("1");
  
        // Find product of all numbers
        for (int i = 0; i < k; i++)
            prod = prod.multiply(num[i]);
  
        BigInteger result = new BigInteger("0");
  
        // Apply the above formula
        for (int i = 0; i < k; i++) {
            BigInteger pp = prod.divide(num[i]);
            result = result.add(rem[i]
                                    .multiply(modInverse(pp,
                                                         num[i]))
                                    .multiply(pp));
        }
  
        // Return result
        return result.mod(prod).toString();
    }
  
    // Function to perform modular
    // exponentiation
    static BigInteger powMod(BigInteger base1,
                             BigInteger exponent,
                             BigInteger modulus)
    {
  
        BigInteger result = new BigInteger("1");
  
        base1 = base1.mod(modulus);
  
        while (exponent
                   .compareTo(new BigInteger("0"))
               > 0) {
  
            if (exponent
                    .mod(new BigInteger("2"))
                    .equals(new BigInteger("1")))
  
                result = (result.multiply(base1))
                             .mod(modulus);
  
            exponent = exponent
                           .divide(new BigInteger("2"));
  
            base1 = base1
                        .multiply(base1)
                        .mod(modulus);
        }
  
        // Return the result
        return result;
    }
  
    // Function that returns square root
    // of n under modulo p if exists
    static BigInteger squareRoot(
        BigInteger n,
        BigInteger p)
    {
  
        // For Invalid Input
        if (!p.mod(new BigInteger("4"))
                 .equals(new BigInteger("3"))) {
  
            System.out.print("Invalid Input");
            return new BigInteger("-1");
        }
  
        n = n.mod(p);
  
        // Try "+(n^((p + 1)/4))"
        BigInteger x = powMod(n,
                              (p.add(new BigInteger("1")))
                                  .divide(new BigInteger("4")),
                              p);
  
        if ((x.multiply(x)).mod(p).equals(n)) {
            return x;
        }
  
        // Try "-(n ^ ((p + 1)/4))"
        x = p.subtract(x);
        if ((x.multiply(x)).mod(p).equals(n)) {
            return x;
        }
  
        // If none of above two work,
        // then sqrt doesn't exist
        return new BigInteger("-1");
    }
  
    // Function to find the ceiling
    // of square root of a number
    public static BigInteger
    sqrtC(BigInteger x)
    {
  
        // If number < zero
        if (x.compareTo(BigInteger.ZERO) < 0) {
            return new BigInteger("-1");
        }
  
        // If number is zero or 1
        if (x == BigInteger.ZERO
            || x == BigInteger.ONE) {
            return x;
        }
  
        BigInteger two
            = BigInteger.valueOf(2L);
  
        BigInteger y;
  
        // Iterate to find the square root
        for (y = x.divide(two);
             y.compareTo(x.divide(y)) > 0;
             y = ((x.divide(y)).add(y)).divide(two))
            ;
        if (x.compareTo(y.multiply(y)) == 0) {
            return y;
        }
        else {
            return y.add(BigInteger.ONE);
        }
    }
  
    // Function to factorise number P
    static BigInteger[] factorise(BigInteger p)
    {
  
        BigInteger ans[] = new BigInteger[2];
        BigInteger b = new BigInteger("2");
  
        BigInteger ONE = new BigInteger("1");
        BigInteger ZERO = new BigInteger("0");
  
        // Iterate over [2, sqrt(N)]
        for (; b.compareTo(
                   sqrtC(p).add(ONE))
               < 0;
             b = b.add(ONE)) {
  
            // Check if mod is zero
            if (p.mod(b).equals(ZERO)) {
                ans[0] = b;
                ans[1] = p.divide(b);
            }
        }
  
        // Return the ans
        return ans;
    }
  
    // Function to find the all possible
    // squareRoot of a under modulo m
    public static void
    solve(BigInteger a, BigInteger m)
    {
  
        // Find factorization of m
        BigInteger s[] = factorise(m);
        BigInteger ZERO = new BigInteger("0");
  
        // If number P is not product
        // of two primes
        if (!s[0].multiply(s[1]).equals(m)) {
  
            System.out.println("Not a product "
                               + "of two primes");
        }
  
        // Find the numbers
        else {
  
            BigInteger s1[] = new BigInteger[4];
            BigInteger a1[] = new BigInteger[4];
            BigInteger a2[] = new BigInteger[2];
  
            // Find squareRoot
            a1[0] = squareRoot(a.mod(s[0]), s[0]);
            a1[1] = squareRoot(a.mod(s[1]), s[1]);
  
            a1[2] = a1[0].multiply(new BigInteger("-1"));
            a1[3] = a1[1].multiply(new BigInteger("-1"));
  
            // Compare to Zero
            while (a1[2].compareTo(ZERO) < 0)
                a1[2] = a1[2].add(s[0]);
            while (a1[3].compareTo(ZERO) < 0)
                a1[3] = a1[3].add(s[1]);
            s1[0] = s[0];
            s1[1] = s[1];
            s1[2] = s[0];
            s1[3] = s[1];
  
            // Condition for no solution
            if (a1[0].equals(new BigInteger("-1"))
                || a1[1].equals(new BigInteger("-1"))) {
  
                System.out.println("No Solution");
            }
  
            // Else print all possible answers
            else {
  
                // Perform Chinese Remainder
                // Theorm and print numbers
                System.out.print(
                    CRT(s, a1, 2) + " ");
  
                a2[0] = a1[0];
                a2[1] = a1[3];
  
                System.out.print(
                    CRT(s, a2, 2) + " ");
  
                a2[0] = a1[2];
                a2[1] = a1[1];
  
                System.out.print(
                    CRT(s, a2, 2) + " ");
  
                a2[0] = a1[2];
                a2[1] = a1[3];
  
                System.out.print(
                    CRT(s, a2, 2) + " ");
            }
        }
    }
  
    // Driver Code
    public static void
        main(String[] args) throws Exception
    {
  
        // Given Number N
        BigInteger N = new BigInteger("188");
  
        // Given product of Prime Number
        BigInteger P = new BigInteger("437");
  
        // Function Call
        solve(N, P);
    }
}

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Output:

25 44 393 412

Time Complexity: O(√P + logN)
Auxiliary Space: O(1)

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