How to Compare Bcrypt Hashes: A Comprehensive Guide to Secure Password Verification

Introduction

In today’s digital landscape, secure password storage and verification are critical for protecting user data. Bcrypt is a popular algorithm used to hash passwords securely. However, one common question developers have is: “How can I compare two bcrypt hashes?” In this comprehensive guide, we will explore why direct comparison of bcrypt hashes is not advisable and outline the correct method for verifying passwords using bcrypt’s built-in functions.

What Is Bcrypt?

Bcrypt is a password-hashing function designed to be computationally intensive to thwart brute-force attacks. It uses a unique salt for each password and applies multiple rounds of hashing. This makes bcrypt highly resistant to rainbow table and dictionary attacks.

Key Features of Bcrypt

• Salting: Each bcrypt hash includes a random salt, making it unique even when the same password is hashed multiple times.
• Adaptive Cost Factor: Bcrypt allows developers to adjust the computational cost, which can be increased as hardware capabilities improve.
• Security: Due to its design, bcrypt remains one of the most secure methods for password hashing.

Why Directly Comparing Bcrypt Hashes Doesn’t Work

Because bcrypt generates a new salt every time a password is hashed, even two identical passwords will result in different hash outputs. For example:

String hash1 = BCrypt.hashpw("mySecretPassword", BCrypt.gensalt());
String hash2 = BCrypt.hashpw("mySecretPassword", BCrypt.gensalt());

Even though "mySecretPassword" is the same in both cases, hash1 and hash2 will be different due to the random salts. Therefore, a direct equality check like:

if (hash1.equals(hash2)) {
    // This will almost always be false.
}

will not verify that the same password was used.

The Role of Salt in Bcrypt

• Unique Hashes: The salt ensures that each hash is unique, even for identical plaintext inputs.
• Security Benefit: This uniqueness prevents attackers from using precomputed tables (rainbow tables) to reverse-engineer passwords.

The Correct Approach: Verifying Passwords with Bcrypt

Instead of comparing two bcrypt hashes, the proper method is to verify a plaintext password against an existing hash. Bcrypt libraries provide functions such as checkpw (in Java’s jBCrypt) to securely compare a plaintext password with a stored hash.

How It Works

1. Hash Creation: When a user sets a password, bcrypt creates a hash using a random salt. The salt is embedded within the hash.
2. Password Verification: When the user logs in, the plaintext password is hashed using the salt extracted from the stored hash. The library then compares the computed hash with the stored hash.

If they match, the password is correct.

Java Example Using jBCrypt

import org.mindrot.jbcrypt.BCrypt;

public class PasswordVerifier {

    /**
     * Verifies whether the candidate password matches the stored bcrypt hash.
     *
     * @param candidatePassword The plaintext password entered by the user.
     * @param storedHash The bcrypt hash stored in your database.
     * @return true if the password matches; false otherwise.
     */
    public static boolean verifyPassword(String candidatePassword, String storedHash) {
        return BCrypt.checkpw(candidatePassword, storedHash);
    }

    public static void main(String[] args) {
        // Simulate storing a hashed password (e.g., during user registration)
        String storedHash = BCrypt.hashpw("mySecretPassword", BCrypt.gensalt());

        // Candidate password provided by the user during login
        String candidatePassword = "mySecretPassword";

        // Verify the candidate password against the stored hash
        if (verifyPassword(candidatePassword, storedHash)) {
            System.out.println("Password matches! Access granted.");
        } else {
            System.out.println("Invalid password! Access denied.");
        }
    }
}

Key Points to Note

• Never store plaintext passwords: Always store only the bcrypt hash.
• Use the library’s verification function: Functions like BCrypt.checkpw are designed to handle the salt and hash comparison internally.
• Avoid direct hash-to-hash comparisons: Since every bcrypt hash is unique due to its salt, comparing two hashes directly will lead to false negatives.

Best Practices for Secure Password Storage and Verification

1. Always Hash Passwords: Never store or transmit plaintext passwords.
2. Use a Strong Hashing Algorithm: Bcrypt is widely recommended due to its adaptive nature and built-in salting.
3. Keep Libraries Up-to-Date: Ensure you are using the latest version of your bcrypt library to benefit from security patches.
4. Implement Account Lockouts: To further protect against brute-force attacks, consider adding account lockout mechanisms after several failed login attempts.
5. Regularly Review Security Practices: Stay updated on best practices in password security and adjust your systems as necessary.

Common Misconceptions About Bcrypt Hash Comparison

Misconception #1: “Two hashes of the same password should be identical.”

• Reality: Each bcrypt hash is different due to the use of random salts.

Misconception #2: “I can compare hashes if I remove the salt.”

• Reality: Removing the salt is not recommended, as it compromises the security of the hash and defeats the purpose of using bcrypt.

Conclusion

When it comes to comparing bcrypt hashes, direct comparison is not the way to go. Instead, always use the provided verification functions to compare a plaintext password with a stored bcrypt hash. This method leverages the embedded salt and the secure design of bcrypt, ensuring that your password verification process remains robust and secure.

By following the guidelines and best practices outlined in this article, you can confidently implement secure password verification in your applications. Whether you are developing a small website or a large-scale application, understanding and correctly applying bcrypt will help you protect your users’ sensitive information.

By adhering to these best practices and leveraging the right tools, you ensure that your application remains secure against common attacks, giving your users peace of mind and protecting your digital assets.