Weak RNG: predictable tokens

November 10, 2025 · 1 min read · #crypto #random #tokens

Session, password-reset, CSRF, API tokens: all must be unpredictable. A non-cryptographic PRNG (rand(), mt_rand(), Math.random()) is predictable after a few outputs.

The problem

mt_rand() has a reconstructable state; if the seed derives from time (srand(time())), the space is tiny. A reset token like that is brute-forced offline.

// WRONG
$token = md5(uniqid(rand(), true));
// RIGHT
$token = bin2hex(random_bytes(32));

Defence

Use a CSPRNG: random_bytes (PHP), secrets (Python), crypto.randomBytes (Node), /dev/urandom. At least 128 bits of entropy. Compare tokens in constant time and expire them.

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