In PHP, the function mt_rand() is used to generate pseudo-random numbers, but the randomness is deterministic, meaning that the sequence of numbers it produces is predictable if the seed value is known. The behavior of mt_rand() can be influenced by the seed set via the mt_srand() function.
Key Points:
mt_rand()generates random numbers based on a seed value.- If the seed value is known (e.g., a combination of email and constant value), the sequence of numbers generated by
mt_rand()is predictable. mt_srand()is used to set the seed for the random number generator. If no seed is provided, PHP will automatically use a default seed based on the system time or other predictable factors.- Deterministic Seed: If you use the same seed (such as the same email and constant value),
mt_rand()will always generate the same sequence of random numbers, including the same invite code.
Why Is This a Problem?
- Predictable Seed: If an attacker knows the email and constant value, they can easily predict the random number generated by
mt_rand(), and thus predict the invite code. - Weak Seeding: Since the seed is based on easily accessible information (like the email and a constant), this makes the random number generation weak and vulnerable to attacks.
How to Exploit This:
- Brute Force or Reverse-Engineer the Seed: If an attacker knows the email and can guess or brute-force the constant value, they can calculate the seed value and predict the random number sequence.
- Base64-encoded Invite Codes: The invite code generated from
mt_rand()is base64 encoded for easy transmission, but the encoding doesn’t add any additional security. If the seed is known, the code can be easily decoded.
Conclusion:
mt_rand()is not secure for cryptographic purposes because of its predictability.- For secure randomness, use PHP’s
random_int(), which provides cryptographically secure random numbers that are not based on predictable seeds.
Example:
mt_srand(100); // Set a known seed
echo mt_rand(); // Output: 62415
echo mt_rand(); // Output: 144100