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In modern web development, ensuring the security of user passwords is crucial. As technology evolves, new encryption algorithms continue to emerge, while older ones may become insecure or unsuitable over time. Developers often need to replace password hashing algorithms without affecting the user experience. PHP provides the password_needs_rehash() function, which makes the transition to new hashing algorithms much simpler and more efficient.
password_needs_rehash() is a built-in PHP function used to check whether a stored password needs to be rehashed. It determines whether the stored hash matches the requirements of a new hashing algorithm. Typically, when the hashing algorithm changes, developers can use this function to automatically update the password hash during user login, without requiring the user to reset their password manually.
The function is defined as follows:
<span><span><span class="hljs-keyword">bool</span></span><span> </span><span><span class="hljs-title function_ invoke__">password_needs_rehash</span></span><span> ( </span><span><span class="hljs-keyword">string</span></span><span> </span><span><span class="hljs-variable">$hash</span></span><span> , </span><span><span class="hljs-keyword">int</span></span><span> </span><span><span class="hljs-variable">$algo</span></span><span> , </span><span><span class="hljs-keyword">array</span></span><span> </span><span><span class="hljs-variable">$options</span></span><span> )
</span></span>$hash: The stored password hash.
$algo: The new hashing algorithm (e.g., PASSWORD_BCRYPT, PASSWORD_ARGON2I, etc.).
$options: Optional configuration options, usually an array containing algorithm parameters, such as bcrypt’s cost factor.
Returns true if the password needs to be rehashed, and false if no rehash is required.
When a user logs in, first verify the entered password against the stored hash using the old hashing algorithm. At the same time, check whether the stored hash needs to be rehashed. If so, rehash the password with the new algorithm and update the database.
After successfully verifying the password, use password_needs_rehash() to check whether the current stored hash meets the new algorithm’s requirements. If rehashing is needed, use the new algorithm and update the hash in the database.
Suppose we previously used the PASSWORD_BCRYPT algorithm and now want to upgrade to PASSWORD_ARGON2I.
<span><span><span class="hljs-comment">// Get the user input password</span></span><span>
</span><span><span class="hljs-variable">$user_input_password</span></span><span> = </span><span><span class="hljs-variable">$_POST</span></span><span>[</span><span><span class="hljs-string">'password'</span></span><span>];
<p></span>// Get the stored hash from the database<br>
$stored_hash = getUserHashFromDatabase($user_id);</p>
<p>// Verify the password using the old algorithm<br>
if (password_verify($user_input_password, $stored_hash)) {</p>
</span><span><span class="hljs-keyword">if</span></span><span> (</span><span><span class="hljs-title function_ invoke__">password_needs_rehash</span></span><span>(</span><span><span class="hljs-variable">$stored_hash</span></span><span>, PASSWORD_ARGON2I, [</span><span><span class="hljs-string">'memory_cost'</span></span><span> => </span><span><span class="hljs-number">1</span></span><span> << </span><span><span class="hljs-number">17</span></span><span>, </span><span><span class="hljs-string">'time_cost'</span></span><span> => </span><span><span class="hljs-number">4</span></span>, </span><span><span class="hljs-string">'threads'</span></span><span> => </span><span><span class="hljs-number">2</span></span>])) {
</span><span><span class="hljs-comment">// Rehash the password with the new algorithm</span></span><span>
</span><span><span class="hljs-variable">$new_hash</span></span><span> = </span><span><span class="hljs-title function_ invoke__">password_hash</span></span><span>(</span><span><span class="hljs-variable">$user_input_password</span></span><span>, PASSWORD_ARGON2I, [</span><span><span class="hljs-string">'memory_cost'</span></span><span> => </span><span><span class="hljs-number">1</span></span> << </span><span><span class="hljs-number">17</span></span><span>, </span><span><span class="hljs-string">'time_cost'</span></span><span> => </span><span><span class="hljs-number">4</span></span>, </span><span><span class="hljs-string">'threads'</span></span><span> => </span><span><span class="hljs-number">2</span></span>]);
</span><span><span class="hljs-comment">// Update the hash in the database</span></span><span>
</span><span><span class="hljs-title function_ invoke__">updateUserHashInDatabase</span></span><span>(</span><span><span class="hljs-variable">$user_id</span></span><span>, </span><span><span class="hljs-variable">$new_hash</span></span><span>);
}
</span><span><span class="hljs-comment">// Login successful, proceed</span></span><span>
</span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-string">"Login successful!"</span></span><span>;
} else {
// Incorrect password
echo "Incorrect password!";
}
password_verify(): Verifies whether the user’s input matches the stored hash.
password_needs_rehash(): After verification, checks if the stored hash meets the new algorithm’s requirements. If rehashing is needed, proceed with the update.
password_hash(): Rehashes the password using the new algorithm.
updateUserHashInDatabase(): Saves the new hash back into the database.
Using password_needs_rehash() to replace password hashing algorithms has the key advantage that users don’t need to intervene. The system automatically upgrades the hash during login. This provides several benefits:
Seamless upgrade: Users only need to log in once, and their password hash will automatically be updated without resetting their password.
Improved security: Gradually transitioning to stronger algorithms increases overall system security and reduces risks from outdated methods.
High compatibility: Old hashes remain valid during the transition, ensuring backward compatibility and preventing issues for existing users.
Switching password hashing algorithms is an essential step toward better security, but it must be implemented with care to maintain a smooth user experience. PHP’s password_needs_rehash() function allows developers to migrate from older algorithms to newer ones seamlessly during user logins. This improves security while keeping the login experience uninterrupted.
