Why Do password_hash() and password_verify() Functions in PHP Produce Different Results?

Password Verification Discrepancy in PHP's password_hash() and password_verify() Functions

In PHP, the password_hash() and password_verify() functions are commonly used for securely handling and verifying user passwords. However, certain scenarios can lead to unexpected discrepancies in password matching.

Problem Statement

You have observed a discrepancy in password matching when using password_hash() to encrypt passwords and password_verify() to check them. You have noticed that the result of password_verify() does not align with the original unencrypted password.

Understanding the Discrepancy

The discrepancy occurs due to the nature of hashing algorithms. Hashing involves converting a plain text input into a fixed-length output (known as a hash) that is unique and unpredictable. This process is irreversible, meaning that it is computationally infeasible to retrieve the original input from the hash.

When you use password_hash() to encrypt a password, it generates a hash using a bcrypt algorithm. This encrypted hash is then stored in the database. When a user attempts to log in, the provided password is hashed again using password_hash() and compared to the stored hash.

Resolving the Discrepancy

To ensure correct password verification, it is crucial to use the same algorithm and configuration that were utilized when the password was initially hashed. Here are the steps you need to take:

1. Verify Algorithm and Configuration: Confirm that the algorithm used in password_hash() in both the registration and login scripts matches the one associated with the stored password. By default, password_hash() uses bcrypt, but you can specify other algorithms by providing the appropriate cost factor.

<code class="php">$password = password_hash($pwd, PASSWORD_DEFAULT); // Using default bcrypt algorithm</code>

2. Consistent Hashing Parameters: If you have customized the hashing parameters, such as the cost factor or the salt, ensure that they are consistent in both the registration and login scripts. Any variation in these parameters can lead to different hashes, even for the same input password.
3. Use Secure Comparisons: When using password_verify() to compare passwords, avoid using equality comparisons (== or ===). Instead, use the password_verify() function specifically designed for this purpose, as it can handle timing attacks that could potentially reveal password patterns.

<code class="php">if (password_verify($pwd, $password)) {
    // Password matches
}</code>

4. Consider Prepared Statements: To prevent SQL injection attacks, consider using prepared statements when querying the database to retrieve the hashed password.

By following these steps, you can ensure that the password_hash() and password_verify() functions work correctly, providing reliable and secure password handling and verification.

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