The Impact of Iteration Count (Iterations) in the hash_pbkdf2 Function on Security

gitbox 2025-07-01

In PHP, the hash_pbkdf2 function is a commonly used cryptographic tool for encrypting and storing passwords. It is based on the PBKDF2 (Password-Based Key Derivation Function 2) algorithm, which uses repeated hashing to make cryptographic computations more complex, thereby increasing password security. A key parameter of the hash_pbkdf2 function is iterations, which refers to the number of times the hashing operation is repeated. This parameter directly affects the computational complexity of the password hash and the overall security of the system.

What is Iteration Count?

Iteration count refers to the number of times a hash function performs operations on the same data. In the PBKDF2 algorithm, each additional iteration results in a new hashing operation on the password and salt, increasing the computational cost for an attacker trying to brute force the password.

The function prototype is as follows:

<span><span><span class="hljs-keyword">string</span></span><span> </span><span><span class="hljs-title function_ invoke__">hash_pbkdf2</span></span><span>(
    </span><span><span class="hljs-keyword">string</span></span><span> </span><span><span class="hljs-variable">$algo</span></span><span>,
    </span><span><span class="hljs-keyword">string</span></span><span> </span><span><span class="hljs-variable">$password</span></span><span>,
    </span><span><span class="hljs-keyword">string</span></span><span> </span><span><span class="hljs-variable">$salt</span></span><span>,
    </span><span><span class="hljs-keyword">int</span></span><span> </span><span><span class="hljs-variable">$iterations</span></span><span>,
    </span><span><span class="hljs-keyword">int</span></span><span> </span><span><span class="hljs-variable">$length</span></span><span> = </span><span><span class="hljs-number">0</span></span><span>,
    </span><span><span class="hljs-keyword">bool</span></span><span> </span><span><span class="hljs-variable">$binary</span></span><span> = </span><span><span class="hljs-literal">false</span></span><span>
)
</span></span>

Here, $iterations is the parameter that controls the iteration count for hashing.

Security Impact

1. Increased Cracking Cost

The direct effect of increasing the iteration count is a higher computational cost. For legitimate users, logging in might take only a few extra milliseconds, but for attackers attempting a large-scale brute force attack, each additional iteration significantly increases the computational effort required. For example, with 1,000 iterations, an attacker can try 1,000 passwords per second; if the iteration count is increased to 100,000, they can only try 10 passwords per second, dramatically reducing the attack efficiency.

2. Mitigating Hardware-Accelerated Attacks

Modern attackers often use GPU, FPGA, or ASIC devices for parallel cracking. Due to PBKDF2’s serial nature, it can somewhat mitigate the impact of these hardware-accelerated attacks. By increasing the iteration count, the advantage of these devices can be reduced, making it much harder to crack passwords even with powerful hardware.

3. Preventing Dictionary Attacks

Dictionary attacks rely on precomputed hash lists to match passwords. Since PBKDF2 combines the iteration mechanism with the use of a salt ($salt), even if the same password is used, the resulting hash will differ because of the unique salt. Higher iteration counts slow down the dictionary generation process, further weakening the feasibility of this type of attack.

Tradeoff Between Performance and Security

While higher iteration counts improve security, they cannot be increased indefinitely. Each time a server handles a login request, it must perform the corresponding hashing operation, and an excessively high iteration count can cause significant performance bottlenecks, potentially even creating an attack surface for denial-of-service (DoS) attacks. Therefore, selecting an appropriate iteration count is crucial.

According to OWASP (Open Web Application Security Project) recommendations, by 2025, the minimum recommended iteration count is 310,000. This number should be adjusted annually to keep pace with advancements in computer hardware and maintain a balance with the computing capabilities of attackers.

Practical Recommendations

Use the currently recommended minimum iteration count, such as 310,000 or higher;
Regularly assess the system's performance capacity and fine-tune based on actual hardware;
Password policies should be combined with hashing strategies, such as strong password requirements and multi-factor authentication;
Ensure the use of unique and sufficiently random salts to prevent rainbow table attacks.

Conclusion

hash_pbkdf2’s iteration count is one of the key factors in protecting password security. Properly configuring this parameter not only significantly enhances the system’s ability to resist brute-force attacks but also helps prevent various forms of cryptographic attacks. When designing authentication systems, it is essential to fully understand and properly utilize this parameter in order to build a more robust security defense.

hash_pbkdf2