Is it possible for 2 messages to hash to the same value using a cryptographic hash function?

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Cryptographic Hash is a Hash function that takes random size input and yields a fixed-size output. It is easy to calculate but challenging to retrieve original data. It is strong and difficult to duplicate the same hash with unique inputs and is a one-way function so revert is not possible. Hashing is also known by different names such as Digest, Message Digest, Checksum, etc.

Properties Of Cryptography Hash Function

The ideal cryptographic hash function has the following main properties:

1. Deterministic: This means that the same message always results in the same hash.
2. Quick: It is quick to compute the hash value for any given message.
3. Avalanche Effect: This means that every minor change in the message results in a major change in the hash value.
4. One-Way Function: You cannot reverse the cryptographic hash function to get to the data.
5. Collision Resistance: It is infeasible to find two different messages that produce the same hash value.

Cracking Hash

We often hear the term Cracking a Hash, there are a couple of ways to do that: 

• Find an algorithm to generate a collision between two hashes. The more advance the algorithm is, the more difficult it is to crack the hash.
• Another way is to find an algorithm to identify a unique and different input that will produce a given hash. It is similar to a collision, but instead of colliding, we are focusing on finding the input using an algorithm.
• Some common hashes we still use today that are considered “cracked” from a cryptographic point of view are MD5(Message-Digest Algorithm) and SHA-1(Secure Hash Algorithm 1). Keep in mind that these are technically broken Hashes and never use for your security purposes.

How to create a Cryptographic Hash

• Create a random salt value using SecureRandom class, SecureRandom class generates strong random values. The engineNextBytes(byte[] bytes) method is used to generate a user-specified number of random bytes.
• Convert two sets of bytes into one using ByteArrayOutputStream class and creating it to ByteArray.
• Create an instance of a messageDigest passing SHA2_ALGORITHM which returns a hash of the given input value.
• UUID is used to generate a random number that is converted to string and passed as input.
• The returned object can be converted to a hex binary format using DatatypeConverter.

Java

package java_cryptography;

import java.io.ByteArrayOutputStream;

import java.security.MessageDigest;

import java.util.UUID;

import javax.xml.bind.DatatypeConverter;

import sun.security.provider.SecureRandom;

public class Hashing {

    private static final String SHA2_ALGORITHM

        = "SHA-256";

    public static byte[] Creating_Random_Salt()

    {

        byte[] salt = new byte[16];

        SecureRandom secure_random

            = new SecureRandom();

        secure_random.engineNextBytes(salt);

        return salt;

    }

    public static byte[] Creating_SHA2_Hash(

        String input, byte[] salt) throws Exception

    {

        ByteArrayOutputStream byte_Stream

            = new ByteArrayOutputStream();

        byte_Stream.write(salt);

        byte_Stream.write(input.getBytes());

        byte[] valueToHash

            = byte_Stream.toByteArray();

        MessageDigest messageDigest

            = MessageDigest

                  .getInstance(SHA2_ALGORITHM);

        return messageDigest

            .digest(valueToHash);

    }

    public static void main(String args[])

        throws Exception

    {

        byte[] salt = Creating_Random_Salt();

        System.out.println(

            "SALT_VALUE: "

            + DatatypeConverter.printHexBinary(salt));

        String valueToHash

            = UUID.randomUUID().toString();

        byte[] hash2

            = Creating_SHA2_Hash(valueToHash, salt);

        byte[] hash2

            = Creating_SHA2_Hash(valueToHash, salt);

        System.out.println(

            "HASH1_VALUE: "

            + DatatypeConverter

                  .printHexBinary(hash2));

        System.out.println(

            "HASH2_VALUE: "

            + DatatypeConverter

                  .printHexBinary(hash2));

    }

}

Note: Salt is a random value added to the input data(passwords) to defend against pre-computed hash attacks such as Rainbow tables.

Output:

SALT_VALUE: A96BB94B1FDACDD9B5FDDFFF2E173366 HASH1_VALUE: 53C77F310EEBCBDA585E9458BCA02715555624D9838190AC7DB5F7FA424C8429 HASH2_VALUE: 53C77F310EEBCBDA585E9458BCA02715555624D9838190AC7DB5F7FA424C8429

How to create Cryptographic Hashing Passwords

As we have seen how to generate a Hash now, let us use Bcrypt to hash a password. Do not use broken Hashing algorithms for Hashing Passwords. Bcrypt is a password Hashing function based on Blowfish Cipher.

Approach:

• Pass the password to hashpw function which is in Bcrypt class which can also generate the salt by itself and returns a string.
• Verify if the password hash and password are really matching using the checkpw() function. It returns a Boolean value.

Code:

Java

package java_cryptography;

import java.util.Scanner;

import org.springframework

    .security

    .crypto

    .bcrypt

    .BCrypt;

public class Hashing {

    private static Scanner sc;

    public static String Password_Hash(

        String password)

    {

        return BCrypt.hashpw(

            password, BCrypt.gensalt());

    }

    public static boolean Verify_Password(

        String password,

        String hashed_password)

    {

        return BCrypt.checkpw(

            password, hashed_password);

    }

    public static void main(

        String args[]) throws Exception

    {

        sc = new Scanner(System.in);

        System.out.println(

            "Enter the password: ");

        String p = sc.nextLine();

        String passwordHash

            = Password_Hash(p);

        System.out.println(

            "Hashed-password: "

            + passwordHash);

        System.out.println(

            "Verification: "

            + Verify_Password(

                  p, passwordHash));

    }

}

Output:

Input: Enter the password: GEEKS FOR GEEKS Output: Hashed-password: $2a$10$u6MFjykfR76nHGfhYYzjjOOe1I3EY.YxpQY4vKRHpKRCqz7w69RTa Verification: true

Hash Uses

• Digital signatures.
• Digital fingerprints.
• Logging sensitive data.
• Saving passwords.